Revert r103213. It broke several sections of live website.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103219 91177308-0d34-0410-b5e6-96231b3b80d8
author: mike-m <mikem.llvm@gmail.com> 2010-05-07 00:28:04 +0000
committer: mike-m <mikem.llvm@gmail.com> 2010-05-07 00:28:04 +0000
commit: e2c3a49c8029ebd9ef530101cc24c66562e3dff5 (patch)
tree: 91bf9600cc8df90cf99751a8f8bafc317cffc91e /docs/tutorial
parent: c10b5afbe8138b0fdf3af4ed3e1ddf96cf3cb4cb (diff)
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Tutorial Introduction and the Lexer</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Tutorial Introduction and the Lexer</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 1
+  <ol>
+    <li><a href="#intro">Tutorial Introduction</a></li>
+    <li><a href="#language">The Basic Language</a></li>
+    <li><a href="#lexer">The Lexer</a></li>
+  </ol>
+</li>
+<li><a href="LangImpl2.html">Chapter 2</a>: Implementing a Parser and AST</li>
+</ul>
+
+<div class="doc_author">
+  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Tutorial Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to the "Implementing a language with LLVM" tutorial.  This tutorial
+runs through the implementation of a simple language, showing how fun and
+easy it can be.  This tutorial will get you up and started as well as help to
+build a framework you can extend to other languages.  The code in this tutorial
+can also be used as a playground to hack on other LLVM specific things.
+</p>
+
+<p>
+The goal of this tutorial is to progressively unveil our language, describing
+how it is built up over time.  This will let us cover a fairly broad range of
+language design and LLVM-specific usage issues, showing and explaining the code
+for it all along the way, without overwhelming you with tons of details up
+front.</p>
+
+<p>It is useful to point out ahead of time that this tutorial is really about
+teaching compiler techniques and LLVM specifically, <em>not</em> about teaching
+modern and sane software engineering principles.  In practice, this means that
+we'll take a number of shortcuts to simplify the exposition.  For example, the
+code leaks memory, uses global variables all over the place, doesn't use nice
+design patterns like <a
+href="http://en.wikipedia.org/wiki/Visitor_pattern">visitors</a>, etc... but it
+is very simple.  If you dig in and use the code as a basis for future projects,
+fixing these deficiencies shouldn't be hard.</p>
+
+<p>I've tried to put this tutorial together in a way that makes chapters easy to
+skip over if you are already familiar with or are uninterested in the various
+pieces.  The structure of the tutorial is:
+</p>
+
+<ul>
+<li><b><a href="#language">Chapter #1</a>: Introduction to the Kaleidoscope
+language, and the definition of its Lexer</b> - This shows where we are going
+and the basic functionality that we want it to do.  In order to make this
+tutorial maximally understandable and hackable, we choose to implement 
+everything in C++ instead of using lexer and parser generators.  LLVM obviously
+works just fine with such tools, feel free to use one if you prefer.</li>
+<li><b><a href="LangImpl2.html">Chapter #2</a>: Implementing a Parser and
+AST</b> - With the lexer in place, we can talk about parsing techniques and
+basic AST construction.  This tutorial describes recursive descent parsing and
+operator precedence parsing.  Nothing in Chapters 1 or 2 is LLVM-specific,
+the code doesn't even link in LLVM at this point. :)</li>
+<li><b><a href="LangImpl3.html">Chapter #3</a>: Code generation to LLVM IR</b> -
+With the AST ready, we can show off how easy generation of LLVM IR really 
+is.</li>
+<li><b><a href="LangImpl4.html">Chapter #4</a>: Adding JIT and Optimizer
+Support</b> - Because a lot of people are interested in using LLVM as a JIT,
+we'll dive right into it and show you the 3 lines it takes to add JIT support.
+LLVM is also useful in many other ways, but this is one simple and "sexy" way
+to shows off its power. :)</li>
+<li><b><a href="LangImpl5.html">Chapter #5</a>: Extending the Language: Control
+Flow</b> - With the language up and running, we show how to extend it with
+control flow operations (if/then/else and a 'for' loop).  This gives us a chance
+to talk about simple SSA construction and control flow.</li>
+<li><b><a href="LangImpl6.html">Chapter #6</a>: Extending the Language: 
+User-defined Operators</b> - This is a silly but fun chapter that talks about
+extending the language to let the user program define their own arbitrary
+unary and binary operators (with assignable precedence!).  This lets us build a
+significant piece of the "language" as library routines.</li>
+<li><b><a href="LangImpl7.html">Chapter #7</a>: Extending the Language: Mutable
+Variables</b> - This chapter talks about adding user-defined local variables
+along with an assignment operator.  The interesting part about this is how
+easy and trivial it is to construct SSA form in LLVM: no, LLVM does <em>not</em>
+require your front-end to construct SSA form!</li>
+<li><b><a href="LangImpl8.html">Chapter #8</a>: Conclusion and other useful LLVM
+tidbits</b> - This chapter wraps up the series by talking about potential
+ways to extend the language, but also includes a bunch of pointers to info about
+"special topics" like adding garbage collection support, exceptions, debugging,
+support for "spaghetti stacks", and a bunch of other tips and tricks.</li>
+
+</ul>
+
+<p>By the end of the tutorial, we'll have written a bit less than 700 lines of 
+non-comment, non-blank, lines of code.  With this small amount of code, we'll
+have built up a very reasonable compiler for a non-trivial language including
+a hand-written lexer, parser, AST, as well as code generation support with a JIT
+compiler.  While other systems may have interesting "hello world" tutorials,
+I think the breadth of this tutorial is a great testament to the strengths of
+LLVM and why you should consider it if you're interested in language or compiler
+design.</p>
+
+<p>A note about this tutorial: we expect you to extend the language and play
+with it on your own.  Take the code and go crazy hacking away at it, compilers
+don't need to be scary creatures - it can be a lot of fun to play with
+languages!</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="language">The Basic Language</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>This tutorial will be illustrated with a toy language that we'll call
+"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>" (derived 
+from "meaning beautiful, form, and view").
+Kaleidoscope is a procedural language that allows you to define functions, use
+conditionals, math, etc.  Over the course of the tutorial, we'll extend
+Kaleidoscope to support the if/then/else construct, a for loop, user defined
+operators, JIT compilation with a simple command line interface, etc.</p>
+
+<p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
+64-bit floating point type (aka 'double' in C parlance).  As such, all values
+are implicitly double precision and the language doesn't require type
+declarations.  This gives the language a very nice and simple syntax.  For
+example, the following simple example computes <a 
+href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>
+
+<div class="doc_code">
+<pre>
+# Compute the x'th fibonacci number.
+def fib(x)
+  if x &lt; 3 then
+    1
+  else
+    fib(x-1)+fib(x-2)
+
+# This expression will compute the 40th number.
+fib(40)
+</pre>
+</div>
+
+<p>We also allow Kaleidoscope to call into standard library functions (the LLVM
+JIT makes this completely trivial).  This means that you can use the 'extern'
+keyword to define a function before you use it (this is also useful for mutually
+recursive functions).  For example:</p>
+
+<div class="doc_code">
+<pre>
+extern sin(arg);
+extern cos(arg);
+extern atan2(arg1 arg2);
+
+atan2(sin(.4), cos(42))
+</pre>
+</div>
+
+<p>A more interesting example is included in Chapter 6 where we write a little
+Kaleidoscope application that <a href="LangImpl6.html#example">displays 
+a Mandelbrot Set</a> at various levels of magnification.</p>
+
+<p>Lets dive into the implementation of this language!</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="lexer">The Lexer</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>When it comes to implementing a language, the first thing needed is
+the ability to process a text file and recognize what it says.  The traditional
+way to do this is to use a "<a 
+href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
+to break the input up into "tokens".  Each token returned by the lexer includes
+a token code and potentially some metadata (e.g. the numeric value of a number).
+First, we define the possibilities:
+</p>
+
+<div class="doc_code">
+<pre>
+// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
+// of these for known things.
+enum Token {
+  tok_eof = -1,
+
+  // commands
+  tok_def = -2, tok_extern = -3,
+
+  // primary
+  tok_identifier = -4, tok_number = -5,
+};
+
+static std::string IdentifierStr;  // Filled in if tok_identifier
+static double NumVal;              // Filled in if tok_number
+</pre>
+</div>
+
+<p>Each token returned by our lexer will either be one of the Token enum values
+or it will be an 'unknown' character like '+', which is returned as its ASCII
+value.  If the current token is an identifier, the <tt>IdentifierStr</tt>
+global variable holds the name of the identifier.  If the current token is a
+numeric literal (like 1.0), <tt>NumVal</tt> holds its value.  Note that we use
+global variables for simplicity, this is not the best choice for a real language
+implementation :).
+</p>
+
+<p>The actual implementation of the lexer is a single function named
+<tt>gettok</tt>. The <tt>gettok</tt> function is called to return the next token
+from standard input.  Its definition starts as:</p>
+
+<div class="doc_code">
+<pre>
+/// gettok - Return the next token from standard input.
+static int gettok() {
+  static int LastChar = ' ';
+
+  // Skip any whitespace.
+  while (isspace(LastChar))
+    LastChar = getchar();
+</pre>
+</div>
+
+<p>
+<tt>gettok</tt> works by calling the C <tt>getchar()</tt> function to read
+characters one at a time from standard input.  It eats them as it recognizes
+them and stores the last character read, but not processed, in LastChar.  The
+first thing that it has to do is ignore whitespace between tokens.  This is 
+accomplished with the loop above.</p>
+
+<p>The next thing <tt>gettok</tt> needs to do is recognize identifiers and
+specific keywords like "def".  Kaleidoscope does this with this simple loop:</p>
+
+<div class="doc_code">
+<pre>
+  if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
+    IdentifierStr = LastChar;
+    while (isalnum((LastChar = getchar())))
+      IdentifierStr += LastChar;
+
+    if (IdentifierStr == "def") return tok_def;
+    if (IdentifierStr == "extern") return tok_extern;
+    return tok_identifier;
+  }
+</pre>
+</div>
+
+<p>Note that this code sets the '<tt>IdentifierStr</tt>' global whenever it
+lexes an identifier.  Also, since language keywords are matched by the same
+loop, we handle them here inline.  Numeric values are similar:</p>
+
+<div class="doc_code">
+<pre>
+  if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
+    std::string NumStr;
+    do {
+      NumStr += LastChar;
+      LastChar = getchar();
+    } while (isdigit(LastChar) || LastChar == '.');
+
+    NumVal = strtod(NumStr.c_str(), 0);
+    return tok_number;
+  }
+</pre>
+</div>
+
+<p>This is all pretty straight-forward code for processing input.  When reading
+a numeric value from input, we use the C <tt>strtod</tt> function to convert it
+to a numeric value that we store in <tt>NumVal</tt>.  Note that this isn't doing
+sufficient error checking: it will incorrectly read "1.23.45.67" and handle it as
+if you typed in "1.23".  Feel free to extend it :).  Next we handle comments:
+</p>
+
+<div class="doc_code">
+<pre>
+  if (LastChar == '#') {
+    // Comment until end of line.
+    do LastChar = getchar();
+    while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp;&amp; LastChar != '\r');
+    
+    if (LastChar != EOF)
+      return gettok();
+  }
+</pre>
+</div>
+
+<p>We handle comments by skipping to the end of the line and then return the
+next token.  Finally, if the input doesn't match one of the above cases, it is
+either an operator character like '+' or the end of the file.  These are handled
+with this code:</p>
+
+<div class="doc_code">
+<pre>
+  // Check for end of file.  Don't eat the EOF.
+  if (LastChar == EOF)
+    return tok_eof;
+  
+  // Otherwise, just return the character as its ascii value.
+  int ThisChar = LastChar;
+  LastChar = getchar();
+  return ThisChar;
+}
+</pre>
+</div>
+
+<p>With this, we have the complete lexer for the basic Kaleidoscope language
+(the <a href="LangImpl2.html#code">full code listing</a> for the Lexer is
+available in the <a href="LangImpl2.html">next chapter</a> of the tutorial).
+Next we'll <a href="LangImpl2.html">build a simple parser that uses this to 
+build an Abstract Syntax Tree</a>.  When we have that, we'll include a driver
+so that you can use the lexer and parser together.
+</p>
+
+<a href="LangImpl2.html">Next: Implementing a Parser and AST</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/LangImpl2.html b/docs/tutorial/LangImpl2.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Implementing a Parser and AST</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Implementing a Parser and AST</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 2
+  <ol>
+    <li><a href="#intro">Chapter 2 Introduction</a></li>
+    <li><a href="#ast">The Abstract Syntax Tree (AST)</a></li>
+    <li><a href="#parserbasics">Parser Basics</a></li>
+    <li><a href="#parserprimexprs">Basic Expression Parsing</a></li>
+    <li><a href="#parserbinops">Binary Expression Parsing</a></li>
+    <li><a href="#parsertop">Parsing the Rest</a></li>
+    <li><a href="#driver">The Driver</a></li>
+    <li><a href="#conclusions">Conclusions</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="LangImpl3.html">Chapter 3</a>: Code generation to LLVM IR</li>
+</ul>
+
+<div class="doc_author">
+  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 2 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 2 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  This chapter shows you how to use the lexer, built in 
+<a href="LangImpl1.html">Chapter 1</a>, to build a full <a
+href="http://en.wikipedia.org/wiki/Parsing">parser</a> for
+our Kaleidoscope language.  Once we have a parser, we'll define and build an <a 
+href="http://en.wikipedia.org/wiki/Abstract_syntax_tree">Abstract Syntax 
+Tree</a> (AST).</p>
+
+<p>The parser we will build uses a combination of <a 
+href="http://en.wikipedia.org/wiki/Recursive_descent_parser">Recursive Descent
+Parsing</a> and <a href=
+"http://en.wikipedia.org/wiki/Operator-precedence_parser">Operator-Precedence 
+Parsing</a> to parse the Kaleidoscope language (the latter for 
+binary expressions and the former for everything else).  Before we get to
+parsing though, lets talk about the output of the parser: the Abstract Syntax
+Tree.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="ast">The Abstract Syntax Tree (AST)</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>The AST for a program captures its behavior in such a way that it is easy for
+later stages of the compiler (e.g. code generation) to interpret.  We basically
+want one object for each construct in the language, and the AST should closely
+model the language.  In Kaleidoscope, we have expressions, a prototype, and a
+function object.  We'll start with expressions first:</p>
+
+<div class="doc_code">
+<pre>
+/// ExprAST - Base class for all expression nodes.
+class ExprAST {
+public:
+  virtual ~ExprAST() {}
+};
+
+/// NumberExprAST - Expression class for numeric literals like "1.0".
+class NumberExprAST : public ExprAST {
+  double Val;
+public:
+  NumberExprAST(double val) : Val(val) {}
+};
+</pre>
+</div>
+
+<p>The code above shows the definition of the base ExprAST class and one
+subclass which we use for numeric literals.  The important thing to note about
+this code is that the NumberExprAST class captures the numeric value of the
+literal as an instance variable. This allows later phases of the compiler to
+know what the stored numeric value is.</p>
+
+<p>Right now we only create the AST,  so there are no useful accessor methods on
+them.  It would be very easy to add a virtual method to pretty print the code,
+for example.  Here are the other expression AST node definitions that we'll use
+in the basic form of the Kaleidoscope language:
+</p>
+
+<div class="doc_code">
+<pre>
+/// VariableExprAST - Expression class for referencing a variable, like "a".
+class VariableExprAST : public ExprAST {
+  std::string Name;
+public:
+  VariableExprAST(const std::string &amp;name) : Name(name) {}
+};
+
+/// BinaryExprAST - Expression class for a binary operator.
+class BinaryExprAST : public ExprAST {
+  char Op;
+  ExprAST *LHS, *RHS;
+public:
+  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
+    : Op(op), LHS(lhs), RHS(rhs) {}
+};
+
+/// CallExprAST - Expression class for function calls.
+class CallExprAST : public ExprAST {
+  std::string Callee;
+  std::vector&lt;ExprAST*&gt; Args;
+public:
+  CallExprAST(const std::string &amp;callee, std::vector&lt;ExprAST*&gt; &amp;args)
+    : Callee(callee), Args(args) {}
+};
+</pre>
+</div>
+
+<p>This is all (intentionally) rather straight-forward: variables capture the
+variable name, binary operators capture their opcode (e.g. '+'), and calls
+capture a function name as well as a list of any argument expressions.  One thing 
+that is nice about our AST is that it captures the language features without 
+talking about the syntax of the language.  Note that there is no discussion about 
+precedence of binary operators, lexical structure, etc.</p>
+
+<p>For our basic language, these are all of the expression nodes we'll define.
+Because it doesn't have conditional control flow, it isn't Turing-complete;
+we'll fix that in a later installment.  The two things we need next are a way
+to talk about the interface to a function, and a way to talk about functions
+themselves:</p>
+
+<div class="doc_code">
+<pre>
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its name, and its argument names (thus implicitly the number
+/// of arguments the function takes).
+class PrototypeAST {
+  std::string Name;
+  std::vector&lt;std::string&gt; Args;
+public:
+  PrototypeAST(const std::string &amp;name, const std::vector&lt;std::string&gt; &amp;args)
+    : Name(name), Args(args) {}
+};
+
+/// FunctionAST - This class represents a function definition itself.
+class FunctionAST {
+  PrototypeAST *Proto;
+  ExprAST *Body;
+public:
+  FunctionAST(PrototypeAST *proto, ExprAST *body)
+    : Proto(proto), Body(body) {}
+};
+</pre>
+</div>
+
+<p>In Kaleidoscope, functions are typed with just a count of their arguments.
+Since all values are double precision floating point, the type of each argument
+doesn't need to be stored anywhere.  In a more aggressive and realistic
+language, the "ExprAST" class would probably have a type field.</p>
+
+<p>With this scaffolding, we can now talk about parsing expressions and function
+bodies in Kaleidoscope.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="parserbasics">Parser Basics</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Now that we have an AST to build, we need to define the parser code to build
+it.  The idea here is that we want to parse something like "x+y" (which is
+returned as three tokens by the lexer) into an AST that could be generated with
+calls like this:</p>
+
+<div class="doc_code">
+<pre>
+  ExprAST *X = new VariableExprAST("x");
+  ExprAST *Y = new VariableExprAST("y");
+  ExprAST *Result = new BinaryExprAST('+', X, Y);
+</pre>
+</div>
+
+<p>In order to do this, we'll start by defining some basic helper routines:</p>
+
+<div class="doc_code">
+<pre>
+/// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
+/// token the parser is looking at.  getNextToken reads another token from the
+/// lexer and updates CurTok with its results.
+static int CurTok;
+static int getNextToken() {
+  return CurTok = gettok();
+}
+</pre>
+</div>
+
+<p>
+This implements a simple token buffer around the lexer.  This allows 
+us to look one token ahead at what the lexer is returning.  Every function in
+our parser will assume that CurTok is the current token that needs to be
+parsed.</p>
+
+<div class="doc_code">
+<pre>
+
+/// Error* - These are little helper functions for error handling.
+ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
+PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
+FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
+</pre>
+</div>
+
+<p>
+The <tt>Error</tt> routines are simple helper routines that our parser will use
+to handle errors.  The error recovery in our parser will not be the best and
+is not particular user-friendly, but it will be enough for our tutorial.  These
+routines make it easier to handle errors in routines that have various return
+types: they always return null.</p>
+
+<p>With these basic helper functions, we can implement the first
+piece of our grammar: numeric literals.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="parserprimexprs">Basic Expression
+ Parsing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>We start with numeric literals, because they are the simplest to process.
+For each production in our grammar, we'll define a function which parses that
+production.  For numeric literals, we have:
+</p>
+
+<div class="doc_code">
+<pre>
+/// numberexpr ::= number
+static ExprAST *ParseNumberExpr() {
+  ExprAST *Result = new NumberExprAST(NumVal);
+  getNextToken(); // consume the number
+  return Result;
+}
+</pre>
+</div>
+
+<p>This routine is very simple: it expects to be called when the current token
+is a <tt>tok_number</tt> token.  It takes the current number value, creates 
+a <tt>NumberExprAST</tt> node, advances the lexer to the next token, and finally
+returns.</p>
+
+<p>There are some interesting aspects to this.  The most important one is that
+this routine eats all of the tokens that correspond to the production and
+returns the lexer buffer with the next token (which is not part of the grammar
+production) ready to go.  This is a fairly standard way to go for recursive
+descent parsers.  For a better example, the parenthesis operator is defined like
+this:</p>
+
+<div class="doc_code">
+<pre>
+/// parenexpr ::= '(' expression ')'
+static ExprAST *ParseParenExpr() {
+  getNextToken();  // eat (.
+  ExprAST *V = ParseExpression();
+  if (!V) return 0;
+  
+  if (CurTok != ')')
+    return Error("expected ')'");
+  getNextToken();  // eat ).
+  return V;
+}
+</pre>
+</div>
+
+<p>This function illustrates a number of interesting things about the 
+parser:</p>
+
+<p>
+1) It shows how we use the Error routines.  When called, this function expects
+that the current token is a '(' token, but after parsing the subexpression, it
+is possible that there is no ')' waiting.  For example, if the user types in
+"(4 x" instead of "(4)", the parser should emit an error.  Because errors can
+occur, the parser needs a way to indicate that they happened: in our parser, we
+return null on an error.</p>
+
+<p>2) Another interesting aspect of this function is that it uses recursion by
+calling <tt>ParseExpression</tt> (we will soon see that <tt>ParseExpression</tt> can call
+<tt>ParseParenExpr</tt>).  This is powerful because it allows us to handle 
+recursive grammars, and keeps each production very simple.  Note that
+parentheses do not cause construction of AST nodes themselves.  While we could
+do it this way, the most important role of parentheses are to guide the parser
+and provide grouping.  Once the parser constructs the AST, parentheses are not
+needed.</p>
+
+<p>The next simple production is for handling variable references and function
+calls:</p>
+
+<div class="doc_code">
+<pre>
+/// identifierexpr
+///   ::= identifier
+///   ::= identifier '(' expression* ')'
+static ExprAST *ParseIdentifierExpr() {
+  std::string IdName = IdentifierStr;
+  
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '(') // Simple variable ref.
+    return new VariableExprAST(IdName);
+  
+  // Call.
+  getNextToken();  // eat (
+  std::vector&lt;ExprAST*&gt; Args;
+  if (CurTok != ')') {
+    while (1) {
+      ExprAST *Arg = ParseExpression();
+      if (!Arg) return 0;
+      Args.push_back(Arg);
+
+      if (CurTok == ')') break;
+
+      if (CurTok != ',')
+        return Error("Expected ')' or ',' in argument list");
+      getNextToken();
+    }
+  }
+
+  // Eat the ')'.
+  getNextToken();
+  
+  return new CallExprAST(IdName, Args);
+}
+</pre>
+</div>
+
+<p>This routine follows the same style as the other routines.  (It expects to be
+called if the current token is a <tt>tok_identifier</tt> token).  It also has
+recursion and error handling.  One interesting aspect of this is that it uses
+<em>look-ahead</em> to determine if the current identifier is a stand alone
+variable reference or if it is a function call expression.  It handles this by
+checking to see if the token after the identifier is a '(' token, constructing
+either a <tt>VariableExprAST</tt> or <tt>CallExprAST</tt> node as appropriate.
+</p>
+
+<p>Now that we have all of our simple expression-parsing logic in place, we can
+define a helper function to wrap it together into one entry point.  We call this
+class of expressions "primary" expressions, for reasons that will become more
+clear <a href="LangImpl6.html#unary">later in the tutorial</a>.  In order to
+parse an arbitrary primary expression, we need to determine what sort of
+expression it is:</p>
+
+<div class="doc_code">
+<pre>
+/// primary
+///   ::= identifierexpr
+///   ::= numberexpr
+///   ::= parenexpr
+static ExprAST *ParsePrimary() {
+  switch (CurTok) {
+  default: return Error("unknown token when expecting an expression");
+  case tok_identifier: return ParseIdentifierExpr();
+  case tok_number:     return ParseNumberExpr();
+  case '(':            return ParseParenExpr();
+  }
+}
+</pre>
+</div>
+
+<p>Now that you see the definition of this function, it is more obvious why we
+can assume the state of CurTok in the various functions.  This uses look-ahead
+to determine which sort of expression is being inspected, and then parses it
+with a function call.</p>
+
+<p>Now that basic expressions are handled, we need to handle binary expressions.
+They are a bit more complex.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="parserbinops">Binary Expression
+ Parsing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Binary expressions are significantly harder to parse because they are often
+ambiguous.  For example, when given the string "x+y*z", the parser can choose
+to parse it as either "(x+y)*z" or "x+(y*z)".  With common definitions from
+mathematics, we expect the later parse, because "*" (multiplication) has
+higher <em>precedence</em> than "+" (addition).</p>
+
+<p>There are many ways to handle this, but an elegant and efficient way is to
+use <a href=
+"http://en.wikipedia.org/wiki/Operator-precedence_parser">Operator-Precedence 
+Parsing</a>.  This parsing technique uses the precedence of binary operators to
+guide recursion.  To start with, we need a table of precedences:</p>
+
+<div class="doc_code">
+<pre>
+/// BinopPrecedence - This holds the precedence for each binary operator that is
+/// defined.
+static std::map&lt;char, int&gt; BinopPrecedence;
+
+/// GetTokPrecedence - Get the precedence of the pending binary operator token.
+static int GetTokPrecedence() {
+  if (!isascii(CurTok))
+    return -1;
+    
+  // Make sure it's a declared binop.
+  int TokPrec = BinopPrecedence[CurTok];
+  if (TokPrec &lt;= 0) return -1;
+  return TokPrec;
+}
+
+int main() {
+  // Install standard binary operators.
+  // 1 is lowest precedence.
+  BinopPrecedence['&lt;'] = 10;
+  BinopPrecedence['+'] = 20;
+  BinopPrecedence['-'] = 20;
+  BinopPrecedence['*'] = 40;  // highest.
+  ...
+}
+</pre>
+</div>
+
+<p>For the basic form of Kaleidoscope, we will only support 4 binary operators
+(this can obviously be extended by you, our brave and intrepid reader).  The
+<tt>GetTokPrecedence</tt> function returns the precedence for the current token,
+or -1 if the token is not a binary operator.  Having a map makes it easy to add
+new operators and makes it clear that the algorithm doesn't depend on the
+specific operators involved, but it would be easy enough to eliminate the map
+and do the comparisons in the <tt>GetTokPrecedence</tt> function.  (Or just use
+a fixed-size array).</p>
+
+<p>With the helper above defined, we can now start parsing binary expressions.
+The basic idea of operator precedence parsing is to break down an expression
+with potentially ambiguous binary operators into pieces.  Consider ,for example,
+the expression "a+b+(c+d)*e*f+g".  Operator precedence parsing considers this
+as a stream of primary expressions separated by binary operators.  As such,
+it will first parse the leading primary expression "a", then it will see the
+pairs [+, b] [+, (c+d)] [*, e] [*, f] and [+, g].  Note that because parentheses
+are primary expressions, the binary expression parser doesn't need to worry
+about nested subexpressions like (c+d) at all. 
+</p>
+
+<p>
+To start, an expression is a primary expression potentially followed by a
+sequence of [binop,primaryexpr] pairs:</p>
+
+<div class="doc_code">
+<pre>
+/// expression
+///   ::= primary binoprhs
+///
+static ExprAST *ParseExpression() {
+  ExprAST *LHS = ParsePrimary();
+  if (!LHS) return 0;
+  
+  return ParseBinOpRHS(0, LHS);
+}
+</pre>
+</div>
+
+<p><tt>ParseBinOpRHS</tt> is the function that parses the sequence of pairs for
+us.  It takes a precedence and a pointer to an expression for the part that has been
+parsed so far.   Note that "x" is a perfectly valid expression: As such, "binoprhs" is
+allowed to be empty, in which case it returns the expression that is passed into
+it. In our example above, the code passes the expression for "a" into
+<tt>ParseBinOpRHS</tt> and the current token is "+".</p>
+
+<p>The precedence value passed into <tt>ParseBinOpRHS</tt> indicates the <em>
+minimal operator precedence</em> that the function is allowed to eat.  For
+example, if the current pair stream is [+, x] and <tt>ParseBinOpRHS</tt> is
+passed in a precedence of 40, it will not consume any tokens (because the
+precedence of '+' is only 20).  With this in mind, <tt>ParseBinOpRHS</tt> starts
+with:</p>
+
+<div class="doc_code">
+<pre>
+/// binoprhs
+///   ::= ('+' primary)*
+static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+  // If this is a binop, find its precedence.
+  while (1) {
+    int TokPrec = GetTokPrecedence();
+    
+    // If this is a binop that binds at least as tightly as the current binop,
+    // consume it, otherwise we are done.
+    if (TokPrec &lt; ExprPrec)
+      return LHS;
+</pre>
+</div>
+
+<p>This code gets the precedence of the current token and checks to see if if is
+too low.  Because we defined invalid tokens to have a precedence of -1, this 
+check implicitly knows that the pair-stream ends when the token stream runs out
+of binary operators.  If this check succeeds, we know that the token is a binary
+operator and that it will be included in this expression:</p>
+
+<div class="doc_code">
+<pre>
+    // Okay, we know this is a binop.
+    int BinOp = CurTok;
+    getNextToken();  // eat binop
+    
+    // Parse the primary expression after the binary operator.
+    ExprAST *RHS = ParsePrimary();
+    if (!RHS) return 0;
+</pre>
+</div>
+
+<p>As such, this code eats (and remembers) the binary operator and then parses
+the primary expression that follows.  This builds up the whole pair, the first of
+which is [+, b] for the running example.</p>
+
+<p>Now that we parsed the left-hand side of an expression and one pair of the 
+RHS sequence, we have to decide which way the expression associates.  In
+particular, we could have "(a+b) binop unparsed"  or "a + (b binop unparsed)".
+To determine this, we look ahead at "binop" to determine its precedence and 
+compare it to BinOp's precedence (which is '+' in this case):</p>
+
+<div class="doc_code">
+<pre>
+    // If BinOp binds less tightly with RHS than the operator after RHS, let
+    // the pending operator take RHS as its LHS.
+    int NextPrec = GetTokPrecedence();
+    if (TokPrec &lt; NextPrec) {
+</pre>
+</div>
+
+<p>If the precedence of the binop to the right of "RHS" is lower or equal to the
+precedence of our current operator, then we know that the parentheses associate
+as "(a+b) binop ...".  In our example, the current operator is "+" and the next 
+operator is "+", we know that they have the same precedence.  In this case we'll
+create the AST node for "a+b", and then continue parsing:</p>
+
+<div class="doc_code">
+<pre>
+      ... if body omitted ...
+    }
+    
+    // Merge LHS/RHS.
+    LHS = new BinaryExprAST(BinOp, LHS, RHS);
+  }  // loop around to the top of the while loop.
+}
+</pre>
+</div>
+
+<p>In our example above, this will turn "a+b+" into "(a+b)" and execute the next
+iteration of the loop, with "+" as the current token.  The code above will eat, 
+remember, and parse "(c+d)" as the primary expression, which makes the
+current pair equal to [+, (c+d)].  It will then evaluate the 'if' conditional above with 
+"*" as the binop to the right of the primary.  In this case, the precedence of "*" is
+higher than the precedence of "+" so the if condition will be entered.</p>
+
+<p>The critical question left here is "how can the if condition parse the right
+hand side in full"?  In particular, to build the AST correctly for our example,
+it needs to get all of "(c+d)*e*f" as the RHS expression variable.  The code to
+do this is surprisingly simple (code from the above two blocks duplicated for
+context):</p>
+
+<div class="doc_code">
+<pre>
+    // If BinOp binds less tightly with RHS than the operator after RHS, let
+    // the pending operator take RHS as its LHS.
+    int NextPrec = GetTokPrecedence();
+    if (TokPrec &lt; NextPrec) {
+      <b>RHS = ParseBinOpRHS(TokPrec+1, RHS);
+      if (RHS == 0) return 0;</b>
+    }
+    // Merge LHS/RHS.
+    LHS = new BinaryExprAST(BinOp, LHS, RHS);
+  }  // loop around to the top of the while loop.
+}
+</pre>
+</div>
+
+<p>At this point, we know that the binary operator to the RHS of our primary
+has higher precedence than the binop we are currently parsing.  As such, we know
+that any sequence of pairs whose operators are all higher precedence than "+"
+should be parsed together and returned as "RHS".  To do this, we recursively
+invoke the <tt>ParseBinOpRHS</tt> function specifying "TokPrec+1" as the minimum
+precedence required for it to continue.  In our example above, this will cause
+it to return the AST node for "(c+d)*e*f" as RHS, which is then set as the RHS
+of the '+' expression.</p>
+
+<p>Finally, on the next iteration of the while loop, the "+g" piece is parsed
+and added to the AST.  With this little bit of code (14 non-trivial lines), we
+correctly handle fully general binary expression parsing in a very elegant way.
+This was a whirlwind tour of this code, and it is somewhat subtle.  I recommend
+running through it with a few tough examples to see how it works.
+</p>
+
+<p>This wraps up handling of expressions.  At this point, we can point the
+parser at an arbitrary token stream and build an expression from it, stopping
+at the first token that is not part of the expression.  Next up we need to
+handle function definitions, etc.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="parsertop">Parsing the Rest</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+The next thing missing is handling of function prototypes.  In Kaleidoscope,
+these are used both for 'extern' function declarations as well as function body
+definitions.  The code to do this is straight-forward and not very interesting
+(once you've survived expressions):
+</p>
+
+<div class="doc_code">
+<pre>
+/// prototype
+///   ::= id '(' id* ')'
+static PrototypeAST *ParsePrototype() {
+  if (CurTok != tok_identifier)
+    return ErrorP("Expected function name in prototype");
+
+  std::string FnName = IdentifierStr;
+  getNextToken();
+  
+  if (CurTok != '(')
+    return ErrorP("Expected '(' in prototype");
+  
+  // Read the list of argument names.
+  std::vector&lt;std::string&gt; ArgNames;
+  while (getNextToken() == tok_identifier)
+    ArgNames.push_back(IdentifierStr);
+  if (CurTok != ')')
+    return ErrorP("Expected ')' in prototype");
+  
+  // success.
+  getNextToken();  // eat ')'.
+  
+  return new PrototypeAST(FnName, ArgNames);
+}
+</pre>
+</div>
+
+<p>Given this, a function definition is very simple, just a prototype plus
+an expression to implement the body:</p>
+
+<div class="doc_code">
+<pre>
+/// definition ::= 'def' prototype expression
+static FunctionAST *ParseDefinition() {
+  getNextToken();  // eat def.
+  PrototypeAST *Proto = ParsePrototype();
+  if (Proto == 0) return 0;
+
+  if (ExprAST *E = ParseExpression())
+    return new FunctionAST(Proto, E);
+  return 0;
+}
+</pre>
+</div>
+
+<p>In addition, we support 'extern' to declare functions like 'sin' and 'cos' as
+well as to support forward declaration of user functions.  These 'extern's are just
+prototypes with no body:</p>
+
+<div class="doc_code">
+<pre>
+/// external ::= 'extern' prototype
+static PrototypeAST *ParseExtern() {
+  getNextToken();  // eat extern.
+  return ParsePrototype();
+}
+</pre>
+</div>
+
+<p>Finally, we'll also let the user type in arbitrary top-level expressions and
+evaluate them on the fly.  We will handle this by defining anonymous nullary
+(zero argument) functions for them:</p>
+
+<div class="doc_code">
+<pre>
+/// toplevelexpr ::= expression
+static FunctionAST *ParseTopLevelExpr() {
+  if (ExprAST *E = ParseExpression()) {
+    // Make an anonymous proto.
+    PrototypeAST *Proto = new PrototypeAST("", std::vector&lt;std::string&gt;());
+    return new FunctionAST(Proto, E);
+  }
+  return 0;
+}
+</pre>
+</div>
+
+<p>Now that we have all the pieces, let's build a little driver that will let us
+actually <em>execute</em> this code we've built!</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="driver">The Driver</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>The driver for this simply invokes all of the parsing pieces with a top-level
+dispatch loop.  There isn't much interesting here, so I'll just include the
+top-level loop.  See <a href="#code">below</a> for full code in the "Top-Level
+Parsing" section.</p>
+
+<div class="doc_code">
+<pre>
+/// top ::= definition | external | expression | ';'
+static void MainLoop() {
+  while (1) {
+    fprintf(stderr, "ready&gt; ");
+    switch (CurTok) {
+    case tok_eof:    return;
+    case ';':        getNextToken(); break;  // ignore top-level semicolons.
+    case tok_def:    HandleDefinition(); break;
+    case tok_extern: HandleExtern(); break;
+    default:         HandleTopLevelExpression(); break;
+    }
+  }
+}
+</pre>
+</div>
+
+<p>The most interesting part of this is that we ignore top-level semicolons.
+Why is this, you ask?  The basic reason is that if you type "4 + 5" at the
+command line, the parser doesn't know whether that is the end of what you will type
+or not.  For example, on the next line you could type "def foo..." in which case
+4+5 is the end of a top-level expression.  Alternatively you could type "* 6",
+which would continue the expression.  Having top-level semicolons allows you to
+type "4+5;", and the parser will know you are done.</p> 
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="conclusions">Conclusions</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>With just under 400 lines of commented code (240 lines of non-comment, 
+non-blank code), we fully defined our minimal language, including a lexer,
+parser, and AST builder.  With this done, the executable will validate 
+Kaleidoscope code and tell us if it is grammatically invalid.  For
+example, here is a sample interaction:</p>
+
+<div class="doc_code">
+<pre>
+$ <b>./a.out</b>
+ready&gt; <b>def foo(x y) x+foo(y, 4.0);</b>
+Parsed a function definition.
+ready&gt; <b>def foo(x y) x+y y;</b>
+Parsed a function definition.
+Parsed a top-level expr
+ready&gt; <b>def foo(x y) x+y );</b>
+Parsed a function definition.
+Error: unknown token when expecting an expression
+ready&gt; <b>extern sin(a);</b>
+ready&gt; Parsed an extern
+ready&gt; <b>^D</b>
+$ 
+</pre>
+</div>
+
+<p>There is a lot of room for extension here.  You can define new AST nodes,
+extend the language in many ways, etc.  In the <a href="LangImpl3.html">next
+installment</a>, we will describe how to generate LLVM Intermediate
+Representation (IR) from the AST.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for this and the previous chapter.  
+Note that it is fully self-contained: you don't need LLVM or any external
+libraries at all for this.  (Besides the C and C++ standard libraries, of
+course.)  To build this, just compile with:</p>
+
+<div class="doc_code">
+<pre>
+   # Compile
+   g++ -g -O3 toy.cpp 
+   # Run
+   ./a.out 
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<div class="doc_code">
+<pre>
+#include &lt;cstdio&gt;
+#include &lt;cstdlib&gt;
+#include &lt;string&gt;
+#include &lt;map&gt;
+#include &lt;vector&gt;
+
+//===----------------------------------------------------------------------===//
+// Lexer
+//===----------------------------------------------------------------------===//
+
+// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
+// of these for known things.
+enum Token {
+  tok_eof = -1,
+
+  // commands
+  tok_def = -2, tok_extern = -3,
+
+  // primary
+  tok_identifier = -4, tok_number = -5
+};
+
+static std::string IdentifierStr;  // Filled in if tok_identifier
+static double NumVal;              // Filled in if tok_number
+
+/// gettok - Return the next token from standard input.
+static int gettok() {
+  static int LastChar = ' ';
+
+  // Skip any whitespace.
+  while (isspace(LastChar))
+    LastChar = getchar();
+
+  if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
+    IdentifierStr = LastChar;
+    while (isalnum((LastChar = getchar())))
+      IdentifierStr += LastChar;
+
+    if (IdentifierStr == "def") return tok_def;
+    if (IdentifierStr == "extern") return tok_extern;
+    return tok_identifier;
+  }
+
+  if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
+    std::string NumStr;
+    do {
+      NumStr += LastChar;
+      LastChar = getchar();
+    } while (isdigit(LastChar) || LastChar == '.');
+
+    NumVal = strtod(NumStr.c_str(), 0);
+    return tok_number;
+  }
+
+  if (LastChar == '#') {
+    // Comment until end of line.
+    do LastChar = getchar();
+    while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp;&amp; LastChar != '\r');
+    
+    if (LastChar != EOF)
+      return gettok();
+  }
+  
+  // Check for end of file.  Don't eat the EOF.
+  if (LastChar == EOF)
+    return tok_eof;
+
+  // Otherwise, just return the character as its ascii value.
+  int ThisChar = LastChar;
+  LastChar = getchar();
+  return ThisChar;
+}
+
+//===----------------------------------------------------------------------===//
+// Abstract Syntax Tree (aka Parse Tree)
+//===----------------------------------------------------------------------===//
+
+/// ExprAST - Base class for all expression nodes.
+class ExprAST {
+public:
+  virtual ~ExprAST() {}
+};
+
+/// NumberExprAST - Expression class for numeric literals like "1.0".
+class NumberExprAST : public ExprAST {
+  double Val;
+public:
+  NumberExprAST(double val) : Val(val) {}
+};
+
+/// VariableExprAST - Expression class for referencing a variable, like "a".
+class VariableExprAST : public ExprAST {
+  std::string Name;
+public:
+  VariableExprAST(const std::string &amp;name) : Name(name) {}
+};
+
+/// BinaryExprAST - Expression class for a binary operator.
+class BinaryExprAST : public ExprAST {
+  char Op;
+  ExprAST *LHS, *RHS;
+public:
+  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
+    : Op(op), LHS(lhs), RHS(rhs) {}
+};
+
+/// CallExprAST - Expression class for function calls.
+class CallExprAST : public ExprAST {
+  std::string Callee;
+  std::vector&lt;ExprAST*&gt; Args;
+public:
+  CallExprAST(const std::string &amp;callee, std::vector&lt;ExprAST*&gt; &amp;args)
+    : Callee(callee), Args(args) {}
+};
+
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its name, and its argument names (thus implicitly the number
+/// of arguments the function takes).
+class PrototypeAST {
+  std::string Name;
+  std::vector&lt;std::string&gt; Args;
+public:
+  PrototypeAST(const std::string &amp;name, const std::vector&lt;std::string&gt; &amp;args)
+    : Name(name), Args(args) {}
+  
+};
+
+/// FunctionAST - This class represents a function definition itself.
+class FunctionAST {
+  PrototypeAST *Proto;
+  ExprAST *Body;
+public:
+  FunctionAST(PrototypeAST *proto, ExprAST *body)
+    : Proto(proto), Body(body) {}
+  
+};
+
+//===----------------------------------------------------------------------===//
+// Parser
+//===----------------------------------------------------------------------===//
+
+/// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
+/// token the parser is looking at.  getNextToken reads another token from the
+/// lexer and updates CurTok with its results.
+static int CurTok;
+static int getNextToken() {
+  return CurTok = gettok();
+}
+
+/// BinopPrecedence - This holds the precedence for each binary operator that is
+/// defined.
+static std::map&lt;char, int&gt; BinopPrecedence;
+
+/// GetTokPrecedence - Get the precedence of the pending binary operator token.
+static int GetTokPrecedence() {
+  if (!isascii(CurTok))
+    return -1;
+  
+  // Make sure it's a declared binop.
+  int TokPrec = BinopPrecedence[CurTok];
+  if (TokPrec &lt;= 0) return -1;
+  return TokPrec;
+}
+
+/// Error* - These are little helper functions for error handling.
+ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
+PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
+FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
+
+static ExprAST *ParseExpression();
+
+/// identifierexpr
+///   ::= identifier
+///   ::= identifier '(' expression* ')'
+static ExprAST *ParseIdentifierExpr() {
+  std::string IdName = IdentifierStr;
+  
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '(') // Simple variable ref.
+    return new VariableExprAST(IdName);
+  
+  // Call.
+  getNextToken();  // eat (
+  std::vector&lt;ExprAST*&gt; Args;
+  if (CurTok != ')') {
+    while (1) {
+      ExprAST *Arg = ParseExpression();
+      if (!Arg) return 0;
+      Args.push_back(Arg);
+
+      if (CurTok == ')') break;
+
+      if (CurTok != ',')
+        return Error("Expected ')' or ',' in argument list");
+      getNextToken();
+    }
+  }
+
+  // Eat the ')'.
+  getNextToken();
+  
+  return new CallExprAST(IdName, Args);
+}
+
+/// numberexpr ::= number
+static ExprAST *ParseNumberExpr() {
+  ExprAST *Result = new NumberExprAST(NumVal);
+  getNextToken(); // consume the number
+  return Result;
+}
+
+/// parenexpr ::= '(' expression ')'
+static ExprAST *ParseParenExpr() {
+  getNextToken();  // eat (.
+  ExprAST *V = ParseExpression();
+  if (!V) return 0;
+  
+  if (CurTok != ')')
+    return Error("expected ')'");
+  getNextToken();  // eat ).
+  return V;
+}
+
+/// primary
+///   ::= identifierexpr
+///   ::= numberexpr
+///   ::= parenexpr
+static ExprAST *ParsePrimary() {
+  switch (CurTok) {
+  default: return Error("unknown token when expecting an expression");
+  case tok_identifier: return ParseIdentifierExpr();
+  case tok_number:     return ParseNumberExpr();
+  case '(':            return ParseParenExpr();
+  }
+}
+
+/// binoprhs
+///   ::= ('+' primary)*
+static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+  // If this is a binop, find its precedence.
+  while (1) {
+    int TokPrec = GetTokPrecedence();
+    
+    // If this is a binop that binds at least as tightly as the current binop,
+    // consume it, otherwise we are done.
+    if (TokPrec &lt; ExprPrec)
+      return LHS;
+    
+    // Okay, we know this is a binop.
+    int BinOp = CurTok;
+    getNextToken();  // eat binop
+    
+    // Parse the primary expression after the binary operator.
+    ExprAST *RHS = ParsePrimary();
+    if (!RHS) return 0;
+    
+    // If BinOp binds less tightly with RHS than the operator after RHS, let
+    // the pending operator take RHS as its LHS.
+    int NextPrec = GetTokPrecedence();
+    if (TokPrec &lt; NextPrec) {
+      RHS = ParseBinOpRHS(TokPrec+1, RHS);
+      if (RHS == 0) return 0;
+    }
+    
+    // Merge LHS/RHS.
+    LHS = new BinaryExprAST(BinOp, LHS, RHS);
+  }
+}
+
+/// expression
+///   ::= primary binoprhs
+///
+static ExprAST *ParseExpression() {
+  ExprAST *LHS = ParsePrimary();
+  if (!LHS) return 0;
+  
+  return ParseBinOpRHS(0, LHS);
+}
+
+/// prototype
+///   ::= id '(' id* ')'
+static PrototypeAST *ParsePrototype() {
+  if (CurTok != tok_identifier)
+    return ErrorP("Expected function name in prototype");
+
+  std::string FnName = IdentifierStr;
+  getNextToken();
+  
+  if (CurTok != '(')
+    return ErrorP("Expected '(' in prototype");
+  
+  std::vector&lt;std::string&gt; ArgNames;
+  while (getNextToken() == tok_identifier)
+    ArgNames.push_back(IdentifierStr);
+  if (CurTok != ')')
+    return ErrorP("Expected ')' in prototype");
+  
+  // success.
+  getNextToken();  // eat ')'.
+  
+  return new PrototypeAST(FnName, ArgNames);
+}
+
+/// definition ::= 'def' prototype expression
+static FunctionAST *ParseDefinition() {
+  getNextToken();  // eat def.
+  PrototypeAST *Proto = ParsePrototype();
+  if (Proto == 0) return 0;
+
+  if (ExprAST *E = ParseExpression())
+    return new FunctionAST(Proto, E);
+  return 0;
+}
+
+/// toplevelexpr ::= expression
+static FunctionAST *ParseTopLevelExpr() {
+  if (ExprAST *E = ParseExpression()) {
+    // Make an anonymous proto.
+    PrototypeAST *Proto = new PrototypeAST("", std::vector&lt;std::string&gt;());
+    return new FunctionAST(Proto, E);
+  }
+  return 0;
+}
+
+/// external ::= 'extern' prototype
+static PrototypeAST *ParseExtern() {
+  getNextToken();  // eat extern.
+  return ParsePrototype();
+}
+
+//===----------------------------------------------------------------------===//
+// Top-Level parsing
+//===----------------------------------------------------------------------===//
+
+static void HandleDefinition() {
+  if (ParseDefinition()) {
+    fprintf(stderr, "Parsed a function definition.\n");
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleExtern() {
+  if (ParseExtern()) {
+    fprintf(stderr, "Parsed an extern\n");
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleTopLevelExpression() {
+  // Evaluate a top-level expression into an anonymous function.
+  if (ParseTopLevelExpr()) {
+    fprintf(stderr, "Parsed a top-level expr\n");
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+/// top ::= definition | external | expression | ';'
+static void MainLoop() {
+  while (1) {
+    fprintf(stderr, "ready&gt; ");
+    switch (CurTok) {
+    case tok_eof:    return;
+    case ';':        getNextToken(); break;  // ignore top-level semicolons.
+    case tok_def:    HandleDefinition(); break;
+    case tok_extern: HandleExtern(); break;
+    default:         HandleTopLevelExpression(); break;
+    }
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// Main driver code.
+//===----------------------------------------------------------------------===//
+
+int main() {
+  // Install standard binary operators.
+  // 1 is lowest precedence.
+  BinopPrecedence['&lt;'] = 10;
+  BinopPrecedence['+'] = 20;
+  BinopPrecedence['-'] = 20;
+  BinopPrecedence['*'] = 40;  // highest.
+
+  // Prime the first token.
+  fprintf(stderr, "ready&gt; ");
+  getNextToken();
+
+  // Run the main "interpreter loop" now.
+  MainLoop();
+
+  return 0;
+}
+</pre>
+</div>
+<a href="LangImpl3.html">Next: Implementing Code Generation to LLVM IR</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
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+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
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diff --git a/docs/tutorial/LangImpl3.html b/docs/tutorial/LangImpl3.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Implementing code generation to LLVM IR</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Code generation to LLVM IR</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 3
+  <ol>
+    <li><a href="#intro">Chapter 3 Introduction</a></li>
+    <li><a href="#basics">Code Generation Setup</a></li>
+    <li><a href="#exprs">Expression Code Generation</a></li>
+    <li><a href="#funcs">Function Code Generation</a></li>
+    <li><a href="#driver">Driver Changes and Closing Thoughts</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="LangImpl4.html">Chapter 4</a>: Adding JIT and Optimizer 
+Support</li>
+</ul>
+
+<div class="doc_author">
+  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 3 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 3 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  This chapter shows you how to transform the <a 
+href="LangImpl2.html">Abstract Syntax Tree</a>, built in Chapter 2, into LLVM IR.
+This will teach you a little bit about how LLVM does things, as well as
+demonstrate how easy it is to use.  It's much more work to build a lexer and
+parser than it is to generate LLVM IR code. :)
+</p>
+
+<p><b>Please note</b>: the code in this chapter and later require LLVM 2.2 or
+later.  LLVM 2.1 and before will not work with it.  Also note that you need
+to use a version of this tutorial that matches your LLVM release: If you are
+using an official LLVM release, use the version of the documentation included
+with your release or on the <a href="http://llvm.org/releases/">llvm.org 
+releases page</a>.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="basics">Code Generation Setup</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+In order to generate LLVM IR, we want some simple setup to get started.  First
+we define virtual code generation (codegen) methods in each AST class:</p>
+
+<div class="doc_code">
+<pre>
+/// ExprAST - Base class for all expression nodes.
+class ExprAST {
+public:
+  virtual ~ExprAST() {}
+  <b>virtual Value *Codegen() = 0;</b>
+};
+
+/// NumberExprAST - Expression class for numeric literals like "1.0".
+class NumberExprAST : public ExprAST {
+  double Val;
+public:
+  NumberExprAST(double val) : Val(val) {}
+  <b>virtual Value *Codegen();</b>
+};
+...
+</pre>
+</div>
+
+<p>The Codegen() method says to emit IR for that AST node along with all the things it
+depends on, and they all return an LLVM Value object. 
+"Value" is the class used to represent a "<a 
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Static Single
+Assignment (SSA)</a> register" or "SSA value" in LLVM.  The most distinct aspect
+of SSA values is that their value is computed as the related instruction
+executes, and it does not get a new value until (and if) the instruction
+re-executes.  In other words, there is no way to "change" an SSA value.  For
+more information, please read up on <a 
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Static Single
+Assignment</a> - the concepts are really quite natural once you grok them.</p>
+
+<p>Note that instead of adding virtual methods to the ExprAST class hierarchy,
+it could also make sense to use a <a
+href="http://en.wikipedia.org/wiki/Visitor_pattern">visitor pattern</a> or some
+other way to model this.  Again, this tutorial won't dwell on good software
+engineering practices: for our purposes, adding a virtual method is
+simplest.</p>
+
+<p>The
+second thing we want is an "Error" method like we used for the parser, which will
+be used to report errors found during code generation (for example, use of an
+undeclared parameter):</p>
+
+<div class="doc_code">
+<pre>
+Value *ErrorV(const char *Str) { Error(Str); return 0; }
+
+static Module *TheModule;
+static IRBuilder&lt;&gt; Builder(getGlobalContext());
+static std::map&lt;std::string, Value*&gt; NamedValues;
+</pre>
+</div>
+
+<p>The static variables will be used during code generation.  <tt>TheModule</tt>
+is the LLVM construct that contains all of the functions and global variables in
+a chunk of code.  In many ways, it is the top-level structure that the LLVM IR
+uses to contain code.</p>
+
+<p>The <tt>Builder</tt> object is a helper object that makes it easy to generate
+LLVM instructions.  Instances of the <a 
+href="http://llvm.org/doxygen/IRBuilder_8h-source.html"><tt>IRBuilder</tt></a> 
+class template keep track of the current place to insert instructions and has
+methods to create new instructions.</p>
+
+<p>The <tt>NamedValues</tt> map keeps track of which values are defined in the
+current scope and what their LLVM representation is.  (In other words, it is a
+symbol table for the code).  In this form of Kaleidoscope, the only things that
+can be referenced are function parameters.  As such, function parameters will
+be in this map when generating code for their function body.</p>
+
+<p>
+With these basics in place, we can start talking about how to generate code for
+each expression.  Note that this assumes that the <tt>Builder</tt> has been set
+up to generate code <em>into</em> something.  For now, we'll assume that this
+has already been done, and we'll just use it to emit code.
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="exprs">Expression Code Generation</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Generating LLVM code for expression nodes is very straightforward: less
+than 45 lines of commented code for all four of our expression nodes.  First
+we'll do numeric literals:</p>
+
+<div class="doc_code">
+<pre>
+Value *NumberExprAST::Codegen() {
+  return ConstantFP::get(getGlobalContext(), APFloat(Val));
+}
+</pre>
+</div>
+
+<p>In the LLVM IR, numeric constants are represented with the
+<tt>ConstantFP</tt> class, which holds the numeric value in an <tt>APFloat</tt>
+internally (<tt>APFloat</tt> has the capability of holding floating point
+constants of <em>A</em>rbitrary <em>P</em>recision).  This code basically just
+creates and returns a <tt>ConstantFP</tt>.  Note that in the LLVM IR
+that constants are all uniqued together and shared.  For this reason, the API
+uses the "foo::get(...)" idiom instead of "new foo(..)" or "foo::Create(..)".</p>
+
+<div class="doc_code">
+<pre>
+Value *VariableExprAST::Codegen() {
+  // Look this variable up in the function.
+  Value *V = NamedValues[Name];
+  return V ? V : ErrorV("Unknown variable name");
+}
+</pre>
+</div>
+
+<p>References to variables are also quite simple using LLVM.  In the simple version
+of Kaleidoscope, we assume that the variable has already been emitted somewhere
+and its value is available.  In practice, the only values that can be in the
+<tt>NamedValues</tt> map are function arguments.  This
+code simply checks to see that the specified name is in the map (if not, an 
+unknown variable is being referenced) and returns the value for it.  In future
+chapters, we'll add support for <a href="LangImpl5.html#for">loop induction 
+variables</a> in the symbol table, and for <a 
+href="LangImpl7.html#localvars">local variables</a>.</p>
+
+<div class="doc_code">
+<pre>
+Value *BinaryExprAST::Codegen() {
+  Value *L = LHS-&gt;Codegen();
+  Value *R = RHS-&gt;Codegen();
+  if (L == 0 || R == 0) return 0;
+  
+  switch (Op) {
+  case '+': return Builder.CreateAdd(L, R, "addtmp");
+  case '-': return Builder.CreateSub(L, R, "subtmp");
+  case '*': return Builder.CreateMul(L, R, "multmp");
+  case '&lt;':
+    L = Builder.CreateFCmpULT(L, R, "cmptmp");
+    // Convert bool 0/1 to double 0.0 or 1.0
+    return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
+                                "booltmp");
+  default: return ErrorV("invalid binary operator");
+  }
+}
+</pre>
+</div>
+
+<p>Binary operators start to get more interesting.  The basic idea here is that
+we recursively emit code for the left-hand side of the expression, then the 
+right-hand side, then we compute the result of the binary expression.  In this
+code, we do a simple switch on the opcode to create the right LLVM instruction.
+</p>
+
+<p>In the example above, the LLVM builder class is starting to show its value.  
+IRBuilder knows where to insert the newly created instruction, all you have to
+do is specify what instruction to create (e.g. with <tt>CreateAdd</tt>), which
+operands to use (<tt>L</tt> and <tt>R</tt> here) and optionally provide a name
+for the generated instruction.</p>
+
+<p>One nice thing about LLVM is that the name is just a hint.  For instance, if
+the code above emits multiple "addtmp" variables, LLVM will automatically
+provide each one with an increasing, unique numeric suffix.  Local value names
+for instructions are purely optional, but it makes it much easier to read the
+IR dumps.</p>
+
+<p><a href="../LangRef.html#instref">LLVM instructions</a> are constrained by
+strict rules: for example, the Left and Right operators of
+an <a href="../LangRef.html#i_add">add instruction</a> must have the same
+type, and the result type of the add must match the operand types.  Because
+all values in Kaleidoscope are doubles, this makes for very simple code for add,
+sub and mul.</p>
+
+<p>On the other hand, LLVM specifies that the <a 
+href="../LangRef.html#i_fcmp">fcmp instruction</a> always returns an 'i1' value
+(a one bit integer).  The problem with this is that Kaleidoscope wants the value to be a 0.0 or 1.0 value.  In order to get these semantics, we combine the fcmp instruction with
+a <a href="../LangRef.html#i_uitofp">uitofp instruction</a>.  This instruction
+converts its input integer into a floating point value by treating the input
+as an unsigned value.  In contrast, if we used the <a 
+href="../LangRef.html#i_sitofp">sitofp instruction</a>, the Kaleidoscope '&lt;'
+operator would return 0.0 and -1.0, depending on the input value.</p>
+
+<div class="doc_code">
+<pre>
+Value *CallExprAST::Codegen() {
+  // Look up the name in the global module table.
+  Function *CalleeF = TheModule-&gt;getFunction(Callee);
+  if (CalleeF == 0)
+    return ErrorV("Unknown function referenced");
+  
+  // If argument mismatch error.
+  if (CalleeF-&gt;arg_size() != Args.size())
+    return ErrorV("Incorrect # arguments passed");
+
+  std::vector&lt;Value*&gt; ArgsV;
+  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
+    ArgsV.push_back(Args[i]-&gt;Codegen());
+    if (ArgsV.back() == 0) return 0;
+  }
+  
+  return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp");
+}
+</pre>
+</div>
+
+<p>Code generation for function calls is quite straightforward with LLVM.  The
+code above initially does a function name lookup in the LLVM Module's symbol
+table.  Recall that the LLVM Module is the container that holds all of the
+functions we are JIT'ing.  By giving each function the same name as what the
+user specifies, we can use the LLVM symbol table to resolve function names for
+us.</p>
+
+<p>Once we have the function to call, we recursively codegen each argument that
+is to be passed in, and create an LLVM <a href="../LangRef.html#i_call">call
+instruction</a>.  Note that LLVM uses the native C calling conventions by
+default, allowing these calls to also call into standard library functions like
+"sin" and "cos", with no additional effort.</p>
+
+<p>This wraps up our handling of the four basic expressions that we have so far
+in Kaleidoscope.  Feel free to go in and add some more.  For example, by 
+browsing the <a href="../LangRef.html">LLVM language reference</a> you'll find
+several other interesting instructions that are really easy to plug into our
+basic framework.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="funcs">Function Code Generation</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Code generation for prototypes and functions must handle a number of
+details, which make their code less beautiful than expression code
+generation, but allows us to  illustrate some important points.  First, lets
+talk about code generation for prototypes: they are used both for function 
+bodies and external function declarations.  The code starts with:</p>
+
+<div class="doc_code">
+<pre>
+Function *PrototypeAST::Codegen() {
+  // Make the function type:  double(double,double) etc.
+  std::vector&lt;const Type*&gt; Doubles(Args.size(),
+                                   Type::getDoubleTy(getGlobalContext()));
+  FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
+                                       Doubles, false);
+  
+  Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
+</pre>
+</div>
+
+<p>This code packs a lot of power into a few lines.  Note first that this 
+function returns a "Function*" instead of a "Value*".  Because a "prototype"
+really talks about the external interface for a function (not the value computed
+by an expression), it makes sense for it to return the LLVM Function it
+corresponds to when codegen'd.</p>
+
+<p>The call to <tt>FunctionType::get</tt> creates
+the <tt>FunctionType</tt> that should be used for a given Prototype.  Since all
+function arguments in Kaleidoscope are of type double, the first line creates
+a vector of "N" LLVM double types.  It then uses the <tt>Functiontype::get</tt>
+method to create a function type that takes "N" doubles as arguments, returns
+one double as a result, and that is not vararg (the false parameter indicates
+this).  Note that Types in LLVM are uniqued just like Constants are, so you
+don't "new" a type, you "get" it.</p>
+
+<p>The final line above actually creates the function that the prototype will
+correspond to.  This indicates the type, linkage and name to use, as well as which
+module to insert into.  "<a href="../LangRef.html#linkage">external linkage</a>"
+means that the function may be defined outside the current module and/or that it
+is callable by functions outside the module.  The Name passed in is the name the
+user specified: since "<tt>TheModule</tt>" is specified, this name is registered
+in "<tt>TheModule</tt>"s symbol table, which is used by the function call code
+above.</p>
+
+<div class="doc_code">
+<pre>
+  // If F conflicted, there was already something named 'Name'.  If it has a
+  // body, don't allow redefinition or reextern.
+  if (F-&gt;getName() != Name) {
+    // Delete the one we just made and get the existing one.
+    F-&gt;eraseFromParent();
+    F = TheModule-&gt;getFunction(Name);
+</pre>
+</div>
+
+<p>The Module symbol table works just like the Function symbol table when it
+comes to name conflicts: if a new function is created with a name was previously
+added to the symbol table, it will get implicitly renamed when added to the
+Module.  The code above exploits this fact to determine if there was a previous
+definition of this function.</p>
+
+<p>In Kaleidoscope, I choose to allow redefinitions of functions in two cases:
+first, we want to allow 'extern'ing a function more than once, as long as the
+prototypes for the externs match (since all arguments have the same type, we
+just have to check that the number of arguments match).  Second, we want to
+allow 'extern'ing a function and then defining a body for it.  This is useful
+when defining mutually recursive functions.</p>
+
+<p>In order to implement this, the code above first checks to see if there is
+a collision on the name of the function.  If so, it deletes the function we just
+created (by calling <tt>eraseFromParent</tt>) and then calling 
+<tt>getFunction</tt> to get the existing function with the specified name.  Note
+that many APIs in LLVM have "erase" forms and "remove" forms.  The "remove" form
+unlinks the object from its parent (e.g. a Function from a Module) and returns
+it.  The "erase" form unlinks the object and then deletes it.</p>
+   
+<div class="doc_code">
+<pre>
+    // If F already has a body, reject this.
+    if (!F-&gt;empty()) {
+      ErrorF("redefinition of function");
+      return 0;
+    }
+    
+    // If F took a different number of args, reject.
+    if (F-&gt;arg_size() != Args.size()) {
+      ErrorF("redefinition of function with different # args");
+      return 0;
+    }
+  }
+</pre>
+</div>
+
+<p>In order to verify the logic above, we first check to see if the pre-existing
+function is "empty".  In this case, empty means that it has no basic blocks in
+it, which means it has no body.  If it has no body, it is a forward 
+declaration.  Since we don't allow anything after a full definition of the
+function, the code rejects this case.  If the previous reference to a function
+was an 'extern', we simply verify that the number of arguments for that
+definition and this one match up.  If not, we emit an error.</p>
+
+<div class="doc_code">
+<pre>
+  // Set names for all arguments.
+  unsigned Idx = 0;
+  for (Function::arg_iterator AI = F-&gt;arg_begin(); Idx != Args.size();
+       ++AI, ++Idx) {
+    AI-&gt;setName(Args[Idx]);
+    
+    // Add arguments to variable symbol table.
+    NamedValues[Args[Idx]] = AI;
+  }
+  return F;
+}
+</pre>
+</div>
+
+<p>The last bit of code for prototypes loops over all of the arguments in the
+function, setting the name of the LLVM Argument objects to match, and registering
+the arguments in the <tt>NamedValues</tt> map for future use by the
+<tt>VariableExprAST</tt> AST node.  Once this is set up, it returns the Function
+object to the caller.  Note that we don't check for conflicting 
+argument names here (e.g. "extern foo(a b a)").  Doing so would be very
+straight-forward with the mechanics we have already used above.</p>
+
+<div class="doc_code">
+<pre>
+Function *FunctionAST::Codegen() {
+  NamedValues.clear();
+  
+  Function *TheFunction = Proto-&gt;Codegen();
+  if (TheFunction == 0)
+    return 0;
+</pre>
+</div>
+
+<p>Code generation for function definitions starts out simply enough: we just
+codegen the prototype (Proto) and verify that it is ok.  We then clear out the
+<tt>NamedValues</tt> map to make sure that there isn't anything in it from the
+last function we compiled.  Code generation of the prototype ensures that there
+is an LLVM Function object that is ready to go for us.</p>
+
+<div class="doc_code">
+<pre>
+  // Create a new basic block to start insertion into.
+  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
+  Builder.SetInsertPoint(BB);
+  
+  if (Value *RetVal = Body-&gt;Codegen()) {
+</pre>
+</div>
+
+<p>Now we get to the point where the <tt>Builder</tt> is set up.  The first
+line creates a new <a href="http://en.wikipedia.org/wiki/Basic_block">basic
+block</a> (named "entry"), which is inserted into <tt>TheFunction</tt>.  The
+second line then tells the builder that new instructions should be inserted into
+the end of the new basic block.  Basic blocks in LLVM are an important part
+of functions that define the <a 
+href="http://en.wikipedia.org/wiki/Control_flow_graph">Control Flow Graph</a>.
+Since we don't have any control flow, our functions will only contain one 
+block at this point.  We'll fix this in <a href="LangImpl5.html">Chapter 5</a> :).</p>
+
+<div class="doc_code">
+<pre>
+  if (Value *RetVal = Body-&gt;Codegen()) {
+    // Finish off the function.
+    Builder.CreateRet(RetVal);
+
+    // Validate the generated code, checking for consistency.
+    verifyFunction(*TheFunction);
+
+    return TheFunction;
+  }
+</pre>
+</div>
+
+<p>Once the insertion point is set up, we call the <tt>CodeGen()</tt> method for
+the root expression of the function.  If no error happens, this emits code to
+compute the expression into the entry block and returns the value that was
+computed.  Assuming no error, we then create an LLVM <a 
+href="../LangRef.html#i_ret">ret instruction</a>, which completes the function.
+Once the function is built, we call <tt>verifyFunction</tt>, which
+is provided by LLVM.  This function does a variety of consistency checks on the
+generated code, to determine if our compiler is doing everything right.  Using
+this is important: it can catch a lot of bugs.  Once the function is finished
+and validated, we return it.</p>
+  
+<div class="doc_code">
+<pre>
+  // Error reading body, remove function.
+  TheFunction-&gt;eraseFromParent();
+  return 0;
+}
+</pre>
+</div>
+
+<p>The only piece left here is handling of the error case.  For simplicity, we
+handle this by merely deleting the function we produced with the 
+<tt>eraseFromParent</tt> method.  This allows the user to redefine a function
+that they incorrectly typed in before: if we didn't delete it, it would live in
+the symbol table, with a body, preventing future redefinition.</p>
+
+<p>This code does have a bug, though.  Since the <tt>PrototypeAST::Codegen</tt>
+can return a previously defined forward declaration, our code can actually delete
+a forward declaration.  There are a number of ways to fix this bug, see what you
+can come up with!  Here is a testcase:</p>
+
+<div class="doc_code">
+<pre>
+extern foo(a b);     # ok, defines foo.
+def foo(a b) c;      # error, 'c' is invalid.
+def bar() foo(1, 2); # error, unknown function "foo"
+</pre>
+</div>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="driver">Driver Changes and 
+Closing Thoughts</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+For now, code generation to LLVM doesn't really get us much, except that we can
+look at the pretty IR calls.  The sample code inserts calls to Codegen into the
+"<tt>HandleDefinition</tt>", "<tt>HandleExtern</tt>" etc functions, and then
+dumps out the LLVM IR.  This gives a nice way to look at the LLVM IR for simple
+functions.  For example:
+</p>
+
+<div class="doc_code">
+<pre>
+ready> <b>4+5</b>;
+Read top-level expression:
+define double @""() {
+entry:
+        ret double 9.000000e+00
+}
+</pre>
+</div>
+
+<p>Note how the parser turns the top-level expression into anonymous functions
+for us.  This will be handy when we add <a href="LangImpl4.html#jit">JIT 
+support</a> in the next chapter.  Also note that the code is very literally
+transcribed, no optimizations are being performed except simple constant
+folding done by IRBuilder.  We will 
+<a href="LangImpl4.html#trivialconstfold">add optimizations</a> explicitly in
+the next chapter.</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def foo(a b) a*a + 2*a*b + b*b;</b>
+Read function definition:
+define double @foo(double %a, double %b) {
+entry:
+        %multmp = fmul double %a, %a
+        %multmp1 = fmul double 2.000000e+00, %a
+        %multmp2 = fmul double %multmp1, %b
+        %addtmp = fadd double %multmp, %multmp2
+        %multmp3 = fmul double %b, %b
+        %addtmp4 = fadd double %addtmp, %multmp3
+        ret double %addtmp4
+}
+</pre>
+</div>
+
+<p>This shows some simple arithmetic. Notice the striking similarity to the
+LLVM builder calls that we use to create the instructions.</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def bar(a) foo(a, 4.0) + bar(31337);</b>
+Read function definition:
+define double @bar(double %a) {
+entry:
+        %calltmp = call double @foo( double %a, double 4.000000e+00 )
+        %calltmp1 = call double @bar( double 3.133700e+04 )
+        %addtmp = fadd double %calltmp, %calltmp1
+        ret double %addtmp
+}
+</pre>
+</div>
+
+<p>This shows some function calls.  Note that this function will take a long
+time to execute if you call it.  In the future we'll add conditional control 
+flow to actually make recursion useful :).</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>extern cos(x);</b>
+Read extern: 
+declare double @cos(double)
+
+ready&gt; <b>cos(1.234);</b>
+Read top-level expression:
+define double @""() {
+entry:
+        %calltmp = call double @cos( double 1.234000e+00 )
+        ret double %calltmp
+}
+</pre>
+</div>
+
+<p>This shows an extern for the libm "cos" function, and a call to it.</p>
+
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>^D</b>
+; ModuleID = 'my cool jit'
+
+define double @""() {
+entry:
+        %addtmp = fadd double 4.000000e+00, 5.000000e+00
+        ret double %addtmp
+}
+
+define double @foo(double %a, double %b) {
+entry:
+        %multmp = fmul double %a, %a
+        %multmp1 = fmul double 2.000000e+00, %a
+        %multmp2 = fmul double %multmp1, %b
+        %addtmp = fadd double %multmp, %multmp2
+        %multmp3 = fmul double %b, %b
+        %addtmp4 = fadd double %addtmp, %multmp3
+        ret double %addtmp4
+}
+
+define double @bar(double %a) {
+entry:
+        %calltmp = call double @foo( double %a, double 4.000000e+00 )
+        %calltmp1 = call double @bar( double 3.133700e+04 )
+        %addtmp = fadd double %calltmp, %calltmp1
+        ret double %addtmp
+}
+
+declare double @cos(double)
+
+define double @""() {
+entry:
+        %calltmp = call double @cos( double 1.234000e+00 )
+        ret double %calltmp
+}
+</pre>
+</div>
+
+<p>When you quit the current demo, it dumps out the IR for the entire module
+generated.  Here you can see the big picture with all the functions referencing
+each other.</p>
+
+<p>This wraps up the third chapter of the Kaleidoscope tutorial.  Up next, we'll
+describe how to <a href="LangImpl4.html">add JIT codegen and optimizer
+support</a> to this so we can actually start running code!</p>
+
+</div>
+
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with the
+LLVM code generator.    Because this uses the LLVM libraries, we need to link
+them in.  To do this, we use the <a 
+href="http://llvm.org/cmds/llvm-config.html">llvm-config</a> tool to inform
+our makefile/command line about which options to use:</p>
+
+<div class="doc_code">
+<pre>
+   # Compile
+   g++ -g -O3 toy.cpp `llvm-config --cppflags --ldflags --libs core` -o toy
+   # Run
+   ./toy
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<div class="doc_code">
+<pre>
+// To build this:
+// See example below.
+
+#include "llvm/DerivedTypes.h"
+#include "llvm/LLVMContext.h"
+#include "llvm/Module.h"
+#include "llvm/Analysis/Verifier.h"
+#include "llvm/Support/IRBuilder.h"
+#include &lt;cstdio&gt;
+#include &lt;string&gt;
+#include &lt;map&gt;
+#include &lt;vector&gt;
+using namespace llvm;
+
+//===----------------------------------------------------------------------===//
+// Lexer
+//===----------------------------------------------------------------------===//
+
+// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
+// of these for known things.
+enum Token {
+  tok_eof = -1,
+
+  // commands
+  tok_def = -2, tok_extern = -3,
+
+  // primary
+  tok_identifier = -4, tok_number = -5
+};
+
+static std::string IdentifierStr;  // Filled in if tok_identifier
+static double NumVal;              // Filled in if tok_number
+
+/// gettok - Return the next token from standard input.
+static int gettok() {
+  static int LastChar = ' ';
+
+  // Skip any whitespace.
+  while (isspace(LastChar))
+    LastChar = getchar();
+
+  if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
+    IdentifierStr = LastChar;
+    while (isalnum((LastChar = getchar())))
+      IdentifierStr += LastChar;
+
+    if (IdentifierStr == "def") return tok_def;
+    if (IdentifierStr == "extern") return tok_extern;
+    return tok_identifier;
+  }
+
+  if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
+    std::string NumStr;
+    do {
+      NumStr += LastChar;
+      LastChar = getchar();
+    } while (isdigit(LastChar) || LastChar == '.');
+
+    NumVal = strtod(NumStr.c_str(), 0);
+    return tok_number;
+  }
+
+  if (LastChar == '#') {
+    // Comment until end of line.
+    do LastChar = getchar();
+    while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp;&amp; LastChar != '\r');
+    
+    if (LastChar != EOF)
+      return gettok();
+  }
+  
+  // Check for end of file.  Don't eat the EOF.
+  if (LastChar == EOF)
+    return tok_eof;
+
+  // Otherwise, just return the character as its ascii value.
+  int ThisChar = LastChar;
+  LastChar = getchar();
+  return ThisChar;
+}
+
+//===----------------------------------------------------------------------===//
+// Abstract Syntax Tree (aka Parse Tree)
+//===----------------------------------------------------------------------===//
+
+/// ExprAST - Base class for all expression nodes.
+class ExprAST {
+public:
+  virtual ~ExprAST() {}
+  virtual Value *Codegen() = 0;
+};
+
+/// NumberExprAST - Expression class for numeric literals like "1.0".
+class NumberExprAST : public ExprAST {
+  double Val;
+public:
+  NumberExprAST(double val) : Val(val) {}
+  virtual Value *Codegen();
+};
+
+/// VariableExprAST - Expression class for referencing a variable, like "a".
+class VariableExprAST : public ExprAST {
+  std::string Name;
+public:
+  VariableExprAST(const std::string &amp;name) : Name(name) {}
+  virtual Value *Codegen();
+};
+
+/// BinaryExprAST - Expression class for a binary operator.
+class BinaryExprAST : public ExprAST {
+  char Op;
+  ExprAST *LHS, *RHS;
+public:
+  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
+    : Op(op), LHS(lhs), RHS(rhs) {}
+  virtual Value *Codegen();
+};
+
+/// CallExprAST - Expression class for function calls.
+class CallExprAST : public ExprAST {
+  std::string Callee;
+  std::vector&lt;ExprAST*&gt; Args;
+public:
+  CallExprAST(const std::string &amp;callee, std::vector&lt;ExprAST*&gt; &amp;args)
+    : Callee(callee), Args(args) {}
+  virtual Value *Codegen();
+};
+
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its name, and its argument names (thus implicitly the number
+/// of arguments the function takes).
+class PrototypeAST {
+  std::string Name;
+  std::vector&lt;std::string&gt; Args;
+public:
+  PrototypeAST(const std::string &amp;name, const std::vector&lt;std::string&gt; &amp;args)
+    : Name(name), Args(args) {}
+  
+  Function *Codegen();
+};
+
+/// FunctionAST - This class represents a function definition itself.
+class FunctionAST {
+  PrototypeAST *Proto;
+  ExprAST *Body;
+public:
+  FunctionAST(PrototypeAST *proto, ExprAST *body)
+    : Proto(proto), Body(body) {}
+  
+  Function *Codegen();
+};
+
+//===----------------------------------------------------------------------===//
+// Parser
+//===----------------------------------------------------------------------===//
+
+/// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
+/// token the parser is looking at.  getNextToken reads another token from the
+/// lexer and updates CurTok with its results.
+static int CurTok;
+static int getNextToken() {
+  return CurTok = gettok();
+}
+
+/// BinopPrecedence - This holds the precedence for each binary operator that is
+/// defined.
+static std::map&lt;char, int&gt; BinopPrecedence;
+
+/// GetTokPrecedence - Get the precedence of the pending binary operator token.
+static int GetTokPrecedence() {
+  if (!isascii(CurTok))
+    return -1;
+  
+  // Make sure it's a declared binop.
+  int TokPrec = BinopPrecedence[CurTok];
+  if (TokPrec &lt;= 0) return -1;
+  return TokPrec;
+}
+
+/// Error* - These are little helper functions for error handling.
+ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
+PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
+FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
+
+static ExprAST *ParseExpression();
+
+/// identifierexpr
+///   ::= identifier
+///   ::= identifier '(' expression* ')'
+static ExprAST *ParseIdentifierExpr() {
+  std::string IdName = IdentifierStr;
+  
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '(') // Simple variable ref.
+    return new VariableExprAST(IdName);
+  
+  // Call.
+  getNextToken();  // eat (
+  std::vector&lt;ExprAST*&gt; Args;
+  if (CurTok != ')') {
+    while (1) {
+      ExprAST *Arg = ParseExpression();
+      if (!Arg) return 0;
+      Args.push_back(Arg);
+
+      if (CurTok == ')') break;
+
+      if (CurTok != ',')
+        return Error("Expected ')' or ',' in argument list");
+      getNextToken();
+    }
+  }
+
+  // Eat the ')'.
+  getNextToken();
+  
+  return new CallExprAST(IdName, Args);
+}
+
+/// numberexpr ::= number
+static ExprAST *ParseNumberExpr() {
+  ExprAST *Result = new NumberExprAST(NumVal);
+  getNextToken(); // consume the number
+  return Result;
+}
+
+/// parenexpr ::= '(' expression ')'
+static ExprAST *ParseParenExpr() {
+  getNextToken();  // eat (.
+  ExprAST *V = ParseExpression();
+  if (!V) return 0;
+  
+  if (CurTok != ')')
+    return Error("expected ')'");
+  getNextToken();  // eat ).
+  return V;
+}
+
+/// primary
+///   ::= identifierexpr
+///   ::= numberexpr
+///   ::= parenexpr
+static ExprAST *ParsePrimary() {
+  switch (CurTok) {
+  default: return Error("unknown token when expecting an expression");
+  case tok_identifier: return ParseIdentifierExpr();
+  case tok_number:     return ParseNumberExpr();
+  case '(':            return ParseParenExpr();
+  }
+}
+
+/// binoprhs
+///   ::= ('+' primary)*
+static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+  // If this is a binop, find its precedence.
+  while (1) {
+    int TokPrec = GetTokPrecedence();
+    
+    // If this is a binop that binds at least as tightly as the current binop,
+    // consume it, otherwise we are done.
+    if (TokPrec &lt; ExprPrec)
+      return LHS;
+    
+    // Okay, we know this is a binop.
+    int BinOp = CurTok;
+    getNextToken();  // eat binop
+    
+    // Parse the primary expression after the binary operator.
+    ExprAST *RHS = ParsePrimary();
+    if (!RHS) return 0;
+    
+    // If BinOp binds less tightly with RHS than the operator after RHS, let
+    // the pending operator take RHS as its LHS.
+    int NextPrec = GetTokPrecedence();
+    if (TokPrec &lt; NextPrec) {
+      RHS = ParseBinOpRHS(TokPrec+1, RHS);
+      if (RHS == 0) return 0;
+    }
+    
+    // Merge LHS/RHS.
+    LHS = new BinaryExprAST(BinOp, LHS, RHS);
+  }
+}
+
+/// expression
+///   ::= primary binoprhs
+///
+static ExprAST *ParseExpression() {
+  ExprAST *LHS = ParsePrimary();
+  if (!LHS) return 0;
+  
+  return ParseBinOpRHS(0, LHS);
+}
+
+/// prototype
+///   ::= id '(' id* ')'
+static PrototypeAST *ParsePrototype() {
+  if (CurTok != tok_identifier)
+    return ErrorP("Expected function name in prototype");
+
+  std::string FnName = IdentifierStr;
+  getNextToken();
+  
+  if (CurTok != '(')
+    return ErrorP("Expected '(' in prototype");
+  
+  std::vector&lt;std::string&gt; ArgNames;
+  while (getNextToken() == tok_identifier)
+    ArgNames.push_back(IdentifierStr);
+  if (CurTok != ')')
+    return ErrorP("Expected ')' in prototype");
+  
+  // success.
+  getNextToken();  // eat ')'.
+  
+  return new PrototypeAST(FnName, ArgNames);
+}
+
+/// definition ::= 'def' prototype expression
+static FunctionAST *ParseDefinition() {
+  getNextToken();  // eat def.
+  PrototypeAST *Proto = ParsePrototype();
+  if (Proto == 0) return 0;
+
+  if (ExprAST *E = ParseExpression())
+    return new FunctionAST(Proto, E);
+  return 0;
+}
+
+/// toplevelexpr ::= expression
+static FunctionAST *ParseTopLevelExpr() {
+  if (ExprAST *E = ParseExpression()) {
+    // Make an anonymous proto.
+    PrototypeAST *Proto = new PrototypeAST("", std::vector&lt;std::string&gt;());
+    return new FunctionAST(Proto, E);
+  }
+  return 0;
+}
+
+/// external ::= 'extern' prototype
+static PrototypeAST *ParseExtern() {
+  getNextToken();  // eat extern.
+  return ParsePrototype();
+}
+
+//===----------------------------------------------------------------------===//
+// Code Generation
+//===----------------------------------------------------------------------===//
+
+static Module *TheModule;
+static IRBuilder&lt;&gt; Builder(getGlobalContext());
+static std::map&lt;std::string, Value*&gt; NamedValues;
+
+Value *ErrorV(const char *Str) { Error(Str); return 0; }
+
+Value *NumberExprAST::Codegen() {
+  return ConstantFP::get(getGlobalContext(), APFloat(Val));
+}
+
+Value *VariableExprAST::Codegen() {
+  // Look this variable up in the function.
+  Value *V = NamedValues[Name];
+  return V ? V : ErrorV("Unknown variable name");
+}
+
+Value *BinaryExprAST::Codegen() {
+  Value *L = LHS-&gt;Codegen();
+  Value *R = RHS-&gt;Codegen();
+  if (L == 0 || R == 0) return 0;
+  
+  switch (Op) {
+  case '+': return Builder.CreateAdd(L, R, "addtmp");
+  case '-': return Builder.CreateSub(L, R, "subtmp");
+  case '*': return Builder.CreateMul(L, R, "multmp");
+  case '&lt;':
+    L = Builder.CreateFCmpULT(L, R, "cmptmp");
+    // Convert bool 0/1 to double 0.0 or 1.0
+    return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
+                                "booltmp");
+  default: return ErrorV("invalid binary operator");
+  }
+}
+
+Value *CallExprAST::Codegen() {
+  // Look up the name in the global module table.
+  Function *CalleeF = TheModule-&gt;getFunction(Callee);
+  if (CalleeF == 0)
+    return ErrorV("Unknown function referenced");
+  
+  // If argument mismatch error.
+  if (CalleeF-&gt;arg_size() != Args.size())
+    return ErrorV("Incorrect # arguments passed");
+
+  std::vector&lt;Value*&gt; ArgsV;
+  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
+    ArgsV.push_back(Args[i]-&gt;Codegen());
+    if (ArgsV.back() == 0) return 0;
+  }
+  
+  return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp");
+}
+
+Function *PrototypeAST::Codegen() {
+  // Make the function type:  double(double,double) etc.
+  std::vector&lt;const Type*&gt; Doubles(Args.size(),
+                                   Type::getDoubleTy(getGlobalContext()));
+  FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
+                                       Doubles, false);
+  
+  Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
+  
+  // If F conflicted, there was already something named 'Name'.  If it has a
+  // body, don't allow redefinition or reextern.
+  if (F-&gt;getName() != Name) {
+    // Delete the one we just made and get the existing one.
+    F-&gt;eraseFromParent();
+    F = TheModule-&gt;getFunction(Name);
+    
+    // If F already has a body, reject this.
+    if (!F-&gt;empty()) {
+      ErrorF("redefinition of function");
+      return 0;
+    }
+    
+    // If F took a different number of args, reject.
+    if (F-&gt;arg_size() != Args.size()) {
+      ErrorF("redefinition of function with different # args");
+      return 0;
+    }
+  }
+  
+  // Set names for all arguments.
+  unsigned Idx = 0;
+  for (Function::arg_iterator AI = F-&gt;arg_begin(); Idx != Args.size();
+       ++AI, ++Idx) {
+    AI-&gt;setName(Args[Idx]);
+    
+    // Add arguments to variable symbol table.
+    NamedValues[Args[Idx]] = AI;
+  }
+  
+  return F;
+}
+
+Function *FunctionAST::Codegen() {
+  NamedValues.clear();
+  
+  Function *TheFunction = Proto-&gt;Codegen();
+  if (TheFunction == 0)
+    return 0;
+  
+  // Create a new basic block to start insertion into.
+  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
+  Builder.SetInsertPoint(BB);
+  
+  if (Value *RetVal = Body-&gt;Codegen()) {
+    // Finish off the function.
+    Builder.CreateRet(RetVal);
+
+    // Validate the generated code, checking for consistency.
+    verifyFunction(*TheFunction);
+
+    return TheFunction;
+  }
+  
+  // Error reading body, remove function.
+  TheFunction-&gt;eraseFromParent();
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Top-Level parsing and JIT Driver
+//===----------------------------------------------------------------------===//
+
+static void HandleDefinition() {
+  if (FunctionAST *F = ParseDefinition()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      fprintf(stderr, "Read function definition:");
+      LF-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleExtern() {
+  if (PrototypeAST *P = ParseExtern()) {
+    if (Function *F = P-&gt;Codegen()) {
+      fprintf(stderr, "Read extern: ");
+      F-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleTopLevelExpression() {
+  // Evaluate a top-level expression into an anonymous function.
+  if (FunctionAST *F = ParseTopLevelExpr()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      fprintf(stderr, "Read top-level expression:");
+      LF-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+/// top ::= definition | external | expression | ';'
+static void MainLoop() {
+  while (1) {
+    fprintf(stderr, "ready&gt; ");
+    switch (CurTok) {
+    case tok_eof:    return;
+    case ';':        getNextToken(); break;  // ignore top-level semicolons.
+    case tok_def:    HandleDefinition(); break;
+    case tok_extern: HandleExtern(); break;
+    default:         HandleTopLevelExpression(); break;
+    }
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// "Library" functions that can be "extern'd" from user code.
+//===----------------------------------------------------------------------===//
+
+/// putchard - putchar that takes a double and returns 0.
+extern "C" 
+double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Main driver code.
+//===----------------------------------------------------------------------===//
+
+int main() {
+  LLVMContext &amp;Context = getGlobalContext();
+
+  // Install standard binary operators.
+  // 1 is lowest precedence.
+  BinopPrecedence['&lt;'] = 10;
+  BinopPrecedence['+'] = 20;
+  BinopPrecedence['-'] = 20;
+  BinopPrecedence['*'] = 40;  // highest.
+
+  // Prime the first token.
+  fprintf(stderr, "ready&gt; ");
+  getNextToken();
+
+  // Make the module, which holds all the code.
+  TheModule = new Module("my cool jit", Context);
+
+  // Run the main "interpreter loop" now.
+  MainLoop();
+
+  // Print out all of the generated code.
+  TheModule-&gt;dump();
+
+  return 0;
+}
+</pre>
+</div>
+<a href="LangImpl4.html">Next: Adding JIT and Optimizer Support</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/LangImpl4.html b/docs/tutorial/LangImpl4.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Adding JIT and Optimizer Support</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Adding JIT and Optimizer Support</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 4
+  <ol>
+    <li><a href="#intro">Chapter 4 Introduction</a></li>
+    <li><a href="#trivialconstfold">Trivial Constant Folding</a></li>
+    <li><a href="#optimizerpasses">LLVM Optimization Passes</a></li>
+    <li><a href="#jit">Adding a JIT Compiler</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="LangImpl5.html">Chapter 5</a>: Extending the Language: Control 
+Flow</li>
+</ul>
+
+<div class="doc_author">
+  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 4 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 4 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  Chapters 1-3 described the implementation of a simple
+language and added support for generating LLVM IR.  This chapter describes
+two new techniques: adding optimizer support to your language, and adding JIT
+compiler support.  These additions will demonstrate how to get nice, efficient code 
+for the Kaleidoscope language.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="trivialconstfold">Trivial Constant
+Folding</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Our demonstration for Chapter 3 is elegant and easy to extend.  Unfortunately,
+it does not produce wonderful code.  The IRBuilder, however, does give us
+obvious optimizations when compiling simple code:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def test(x) 1+2+x;</b>
+Read function definition:
+define double @test(double %x) {
+entry:
+        %addtmp = fadd double 3.000000e+00, %x
+        ret double %addtmp
+}
+</pre>
+</div>
+
+<p>This code is not a literal transcription of the AST built by parsing the 
+input. That would be:
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def test(x) 1+2+x;</b>
+Read function definition:
+define double @test(double %x) {
+entry:
+        %addtmp = fadd double 2.000000e+00, 1.000000e+00
+        %addtmp1 = fadd double %addtmp, %x
+        ret double %addtmp1
+}
+</pre>
+</div>
+
+<p>Constant folding, as seen above, in particular, is a very common and very
+important optimization: so much so that many language implementors implement
+constant folding support in their AST representation.</p>
+
+<p>With LLVM, you don't need this support in the AST.  Since all calls to build 
+LLVM IR go through the LLVM IR builder, the builder itself checked to see if 
+there was a constant folding opportunity when you call it.  If so, it just does 
+the constant fold and return the constant instead of creating an instruction.
+
+<p>Well, that was easy :).  In practice, we recommend always using
+<tt>IRBuilder</tt> when generating code like this.  It has no
+"syntactic overhead" for its use (you don't have to uglify your compiler with
+constant checks everywhere) and it can dramatically reduce the amount of
+LLVM IR that is generated in some cases (particular for languages with a macro
+preprocessor or that use a lot of constants).</p>
+
+<p>On the other hand, the <tt>IRBuilder</tt> is limited by the fact
+that it does all of its analysis inline with the code as it is built.  If you
+take a slightly more complex example:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def test(x) (1+2+x)*(x+(1+2));</b>
+ready> Read function definition:
+define double @test(double %x) {
+entry:
+        %addtmp = fadd double 3.000000e+00, %x
+        %addtmp1 = fadd double %x, 3.000000e+00
+        %multmp = fmul double %addtmp, %addtmp1
+        ret double %multmp
+}
+</pre>
+</div>
+
+<p>In this case, the LHS and RHS of the multiplication are the same value.  We'd
+really like to see this generate "<tt>tmp = x+3; result = tmp*tmp;</tt>" instead
+of computing "<tt>x+3</tt>" twice.</p>
+
+<p>Unfortunately, no amount of local analysis will be able to detect and correct
+this.  This requires two transformations: reassociation of expressions (to 
+make the add's lexically identical) and Common Subexpression Elimination (CSE)
+to  delete the redundant add instruction.  Fortunately, LLVM provides a broad
+range of optimizations that you can use, in the form of "passes".</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="optimizerpasses">LLVM Optimization
+ Passes</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>LLVM provides many optimization passes, which do many different sorts of
+things and have different tradeoffs.  Unlike other systems, LLVM doesn't hold
+to the mistaken notion that one set of optimizations is right for all languages
+and for all situations.  LLVM allows a compiler implementor to make complete
+decisions about what optimizations to use, in which order, and in what
+situation.</p>
+
+<p>As a concrete example, LLVM supports both "whole module" passes, which look
+across as large of body of code as they can (often a whole file, but if run 
+at link time, this can be a substantial portion of the whole program).  It also
+supports and includes "per-function" passes which just operate on a single
+function at a time, without looking at other functions.  For more information
+on passes and how they are run, see the <a href="../WritingAnLLVMPass.html">How
+to Write a Pass</a> document and the <a href="../Passes.html">List of LLVM 
+Passes</a>.</p>
+
+<p>For Kaleidoscope, we are currently generating functions on the fly, one at
+a time, as the user types them in.  We aren't shooting for the ultimate
+optimization experience in this setting, but we also want to catch the easy and
+quick stuff where possible.  As such, we will choose to run a few per-function
+optimizations as the user types the function in.  If we wanted to make a "static
+Kaleidoscope compiler", we would use exactly the code we have now, except that
+we would defer running the optimizer until the entire file has been parsed.</p>
+
+<p>In order to get per-function optimizations going, we need to set up a
+<a href="../WritingAnLLVMPass.html#passmanager">FunctionPassManager</a> to hold and
+organize the LLVM optimizations that we want to run.  Once we have that, we can
+add a set of optimizations to run.  The code looks like this:</p>
+
+<div class="doc_code">
+<pre>
+  FunctionPassManager OurFPM(TheModule);
+
+  // Set up the optimizer pipeline.  Start with registering info about how the
+  // target lays out data structures.
+  OurFPM.add(new TargetData(*TheExecutionEngine->getTargetData()));
+  // Do simple "peephole" optimizations and bit-twiddling optzns.
+  OurFPM.add(createInstructionCombiningPass());
+  // Reassociate expressions.
+  OurFPM.add(createReassociatePass());
+  // Eliminate Common SubExpressions.
+  OurFPM.add(createGVNPass());
+  // Simplify the control flow graph (deleting unreachable blocks, etc).
+  OurFPM.add(createCFGSimplificationPass());
+
+  OurFPM.doInitialization();
+
+  // Set the global so the code gen can use this.
+  TheFPM = &amp;OurFPM;
+
+  // Run the main "interpreter loop" now.
+  MainLoop();
+</pre>
+</div>
+
+<p>This code defines a <tt>FunctionPassManager</tt>, "<tt>OurFPM</tt>".  It
+requires a pointer to the <tt>Module</tt> to construct itself.  Once it is set
+up, we use a series of "add" calls to add a bunch of LLVM passes.  The first
+pass is basically boilerplate, it adds a pass so that later optimizations know
+how the data structures in the program are laid out.  The
+"<tt>TheExecutionEngine</tt>" variable is related to the JIT, which we will get
+to in the next section.</p>
+
+<p>In this case, we choose to add 4 optimization passes.  The passes we chose
+here are a pretty standard set of "cleanup" optimizations that are useful for
+a wide variety of code.  I won't delve into what they do but, believe me,
+they are a good starting place :).</p>
+
+<p>Once the PassManager is set up, we need to make use of it.  We do this by
+running it after our newly created function is constructed (in 
+<tt>FunctionAST::Codegen</tt>), but before it is returned to the client:</p>
+
+<div class="doc_code">
+<pre>
+  if (Value *RetVal = Body->Codegen()) {
+    // Finish off the function.
+    Builder.CreateRet(RetVal);
+
+    // Validate the generated code, checking for consistency.
+    verifyFunction(*TheFunction);
+
+    <b>// Optimize the function.
+    TheFPM-&gt;run(*TheFunction);</b>
+    
+    return TheFunction;
+  }
+</pre>
+</div>
+
+<p>As you can see, this is pretty straightforward.  The 
+<tt>FunctionPassManager</tt> optimizes and updates the LLVM Function* in place,
+improving (hopefully) its body.  With this in place, we can try our test above
+again:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def test(x) (1+2+x)*(x+(1+2));</b>
+ready> Read function definition:
+define double @test(double %x) {
+entry:
+        %addtmp = fadd double %x, 3.000000e+00
+        %multmp = fmul double %addtmp, %addtmp
+        ret double %multmp
+}
+</pre>
+</div>
+
+<p>As expected, we now get our nicely optimized code, saving a floating point
+add instruction from every execution of this function.</p>
+
+<p>LLVM provides a wide variety of optimizations that can be used in certain
+circumstances.  Some <a href="../Passes.html">documentation about the various 
+passes</a> is available, but it isn't very complete.  Another good source of
+ideas can come from looking at the passes that <tt>llvm-gcc</tt> or
+<tt>llvm-ld</tt> run to get started.  The "<tt>opt</tt>" tool allows you to 
+experiment with passes from the command line, so you can see if they do
+anything.</p>
+
+<p>Now that we have reasonable code coming out of our front-end, lets talk about
+executing it!</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="jit">Adding a JIT Compiler</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Code that is available in LLVM IR can have a wide variety of tools 
+applied to it.  For example, you can run optimizations on it (as we did above),
+you can dump it out in textual or binary forms, you can compile the code to an
+assembly file (.s) for some target, or you can JIT compile it.  The nice thing
+about the LLVM IR representation is that it is the "common currency" between
+many different parts of the compiler.
+</p>
+
+<p>In this section, we'll add JIT compiler support to our interpreter.  The
+basic idea that we want for Kaleidoscope is to have the user enter function
+bodies as they do now, but immediately evaluate the top-level expressions they
+type in.  For example, if they type in "1 + 2;", we should evaluate and print
+out 3.  If they define a function, they should be able to call it from the 
+command line.</p>
+
+<p>In order to do this, we first declare and initialize the JIT.  This is done
+by adding a global variable and a call in <tt>main</tt>:</p>
+
+<div class="doc_code">
+<pre>
+<b>static ExecutionEngine *TheExecutionEngine;</b>
+...
+int main() {
+  ..
+  <b>// Create the JIT.  This takes ownership of the module.
+  TheExecutionEngine = EngineBuilder(TheModule).create();</b>
+  ..
+}
+</pre>
+</div>
+
+<p>This creates an abstract "Execution Engine" which can be either a JIT
+compiler or the LLVM interpreter.  LLVM will automatically pick a JIT compiler
+for you if one is available for your platform, otherwise it will fall back to
+the interpreter.</p>
+
+<p>Once the <tt>ExecutionEngine</tt> is created, the JIT is ready to be used.
+There are a variety of APIs that are useful, but the simplest one is the
+"<tt>getPointerToFunction(F)</tt>" method.  This method JIT compiles the
+specified LLVM Function and returns a function pointer to the generated machine
+code.  In our case, this means that we can change the code that parses a
+top-level expression to look like this:</p>
+
+<div class="doc_code">
+<pre>
+static void HandleTopLevelExpression() {
+  // Evaluate a top-level expression into an anonymous function.
+  if (FunctionAST *F = ParseTopLevelExpr()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      LF->dump();  // Dump the function for exposition purposes.
+    
+      <b>// JIT the function, returning a function pointer.
+      void *FPtr = TheExecutionEngine-&gt;getPointerToFunction(LF);
+      
+      // Cast it to the right type (takes no arguments, returns a double) so we
+      // can call it as a native function.
+      double (*FP)() = (double (*)())(intptr_t)FPtr;
+      fprintf(stderr, "Evaluated to %f\n", FP());</b>
+    }
+</pre>
+</div>
+
+<p>Recall that we compile top-level expressions into a self-contained LLVM
+function that takes no arguments and returns the computed double.  Because the 
+LLVM JIT compiler matches the native platform ABI, this means that you can just
+cast the result pointer to a function pointer of that type and call it directly.
+This means, there is no difference between JIT compiled code and native machine
+code that is statically linked into your application.</p>
+
+<p>With just these two changes, lets see how Kaleidoscope works now!</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>4+5;</b>
+define double @""() {
+entry:
+        ret double 9.000000e+00
+}
+
+<em>Evaluated to 9.000000</em>
+</pre>
+</div>
+
+<p>Well this looks like it is basically working.  The dump of the function
+shows the "no argument function that always returns double" that we synthesize
+for each top-level expression that is typed in.  This demonstrates very basic
+functionality, but can we do more?</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def testfunc(x y) x + y*2; </b> 
+Read function definition:
+define double @testfunc(double %x, double %y) {
+entry:
+        %multmp = fmul double %y, 2.000000e+00
+        %addtmp = fadd double %multmp, %x
+        ret double %addtmp
+}
+
+ready&gt; <b>testfunc(4, 10);</b>
+define double @""() {
+entry:
+        %calltmp = call double @testfunc( double 4.000000e+00, double 1.000000e+01 )
+        ret double %calltmp
+}
+
+<em>Evaluated to 24.000000</em>
+</pre>
+</div>
+
+<p>This illustrates that we can now call user code, but there is something a bit
+subtle going on here.  Note that we only invoke the JIT on the anonymous
+functions that <em>call testfunc</em>, but we never invoked it
+on <em>testfunc</em> itself.  What actually happened here is that the JIT
+scanned for all non-JIT'd functions transitively called from the anonymous
+function and compiled all of them before returning
+from <tt>getPointerToFunction()</tt>.</p>
+
+<p>The JIT provides a number of other more advanced interfaces for things like
+freeing allocated machine code, rejit'ing functions to update them, etc.
+However, even with this simple code, we get some surprisingly powerful
+capabilities - check this out (I removed the dump of the anonymous functions,
+you should get the idea by now :) :</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>extern sin(x);</b>
+Read extern: 
+declare double @sin(double)
+
+ready&gt; <b>extern cos(x);</b>
+Read extern: 
+declare double @cos(double)
+
+ready&gt; <b>sin(1.0);</b>
+<em>Evaluated to 0.841471</em>
+
+ready&gt; <b>def foo(x) sin(x)*sin(x) + cos(x)*cos(x);</b>
+Read function definition:
+define double @foo(double %x) {
+entry:
+        %calltmp = call double @sin( double %x )
+        %multmp = fmul double %calltmp, %calltmp
+        %calltmp2 = call double @cos( double %x )
+        %multmp4 = fmul double %calltmp2, %calltmp2
+        %addtmp = fadd double %multmp, %multmp4
+        ret double %addtmp
+}
+
+ready&gt; <b>foo(4.0);</b>
+<em>Evaluated to 1.000000</em>
+</pre>
+</div>
+
+<p>Whoa, how does the JIT know about sin and cos?  The answer is surprisingly
+simple: in this
+example, the JIT started execution of a function and got to a function call.  It
+realized that the function was not yet JIT compiled and invoked the standard set
+of routines to resolve the function.  In this case, there is no body defined
+for the function, so the JIT ended up calling "<tt>dlsym("sin")</tt>" on the
+Kaleidoscope process itself.
+Since "<tt>sin</tt>" is defined within the JIT's address space, it simply
+patches up calls in the module to call the libm version of <tt>sin</tt>
+directly.</p>
+
+<p>The LLVM JIT provides a number of interfaces (look in the 
+<tt>ExecutionEngine.h</tt> file) for controlling how unknown functions get
+resolved.  It allows you to establish explicit mappings between IR objects and
+addresses (useful for LLVM global variables that you want to map to static
+tables, for example), allows you to dynamically decide on the fly based on the
+function name, and even allows you to have the JIT compile functions lazily the
+first time they're called.</p>
+
+<p>One interesting application of this is that we can now extend the language
+by writing arbitrary C++ code to implement operations.  For example, if we add:
+</p>
+
+<div class="doc_code">
+<pre>
+/// putchard - putchar that takes a double and returns 0.
+extern "C" 
+double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+</pre>
+</div>
+
+<p>Now we can produce simple output to the console by using things like:
+"<tt>extern putchard(x); putchard(120);</tt>", which prints a lowercase 'x' on
+the console (120 is the ASCII code for 'x').  Similar code could be used to 
+implement file I/O, console input, and many other capabilities in
+Kaleidoscope.</p>
+
+<p>This completes the JIT and optimizer chapter of the Kaleidoscope tutorial. At
+this point, we can compile a non-Turing-complete programming language, optimize
+and JIT compile it in a user-driven way.  Next up we'll look into <a 
+href="LangImpl5.html">extending the language with control flow constructs</a>,
+tackling some interesting LLVM IR issues along the way.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with the
+LLVM JIT and optimizer.  To build this example, use:
+</p>
+
+<div class="doc_code">
+<pre>
+   # Compile
+   g++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
+   # Run
+   ./toy
+</pre>
+</div>
+
+<p>
+If you are compiling this on Linux, make sure to add the "-rdynamic" option 
+as well.  This makes sure that the external functions are resolved properly 
+at runtime.</p>
+
+<p>Here is the code:</p>
+
+<div class="doc_code">
+<pre>
+#include "llvm/DerivedTypes.h"
+#include "llvm/ExecutionEngine/ExecutionEngine.h"
+#include "llvm/ExecutionEngine/JIT.h"
+#include "llvm/LLVMContext.h"
+#include "llvm/Module.h"
+#include "llvm/PassManager.h"
+#include "llvm/Analysis/Verifier.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Target/TargetSelect.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Support/IRBuilder.h"
+#include &lt;cstdio&gt;
+#include &lt;string&gt;
+#include &lt;map&gt;
+#include &lt;vector&gt;
+using namespace llvm;
+
+//===----------------------------------------------------------------------===//
+// Lexer
+//===----------------------------------------------------------------------===//
+
+// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
+// of these for known things.
+enum Token {
+  tok_eof = -1,
+
+  // commands
+  tok_def = -2, tok_extern = -3,
+
+  // primary
+  tok_identifier = -4, tok_number = -5
+};
+
+static std::string IdentifierStr;  // Filled in if tok_identifier
+static double NumVal;              // Filled in if tok_number
+
+/// gettok - Return the next token from standard input.
+static int gettok() {
+  static int LastChar = ' ';
+
+  // Skip any whitespace.
+  while (isspace(LastChar))
+    LastChar = getchar();
+
+  if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
+    IdentifierStr = LastChar;
+    while (isalnum((LastChar = getchar())))
+      IdentifierStr += LastChar;
+
+    if (IdentifierStr == "def") return tok_def;
+    if (IdentifierStr == "extern") return tok_extern;
+    return tok_identifier;
+  }
+
+  if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
+    std::string NumStr;
+    do {
+      NumStr += LastChar;
+      LastChar = getchar();
+    } while (isdigit(LastChar) || LastChar == '.');
+
+    NumVal = strtod(NumStr.c_str(), 0);
+    return tok_number;
+  }
+
+  if (LastChar == '#') {
+    // Comment until end of line.
+    do LastChar = getchar();
+    while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp;&amp; LastChar != '\r');
+    
+    if (LastChar != EOF)
+      return gettok();
+  }
+  
+  // Check for end of file.  Don't eat the EOF.
+  if (LastChar == EOF)
+    return tok_eof;
+
+  // Otherwise, just return the character as its ascii value.
+  int ThisChar = LastChar;
+  LastChar = getchar();
+  return ThisChar;
+}
+
+//===----------------------------------------------------------------------===//
+// Abstract Syntax Tree (aka Parse Tree)
+//===----------------------------------------------------------------------===//
+
+/// ExprAST - Base class for all expression nodes.
+class ExprAST {
+public:
+  virtual ~ExprAST() {}
+  virtual Value *Codegen() = 0;
+};
+
+/// NumberExprAST - Expression class for numeric literals like "1.0".
+class NumberExprAST : public ExprAST {
+  double Val;
+public:
+  NumberExprAST(double val) : Val(val) {}
+  virtual Value *Codegen();
+};
+
+/// VariableExprAST - Expression class for referencing a variable, like "a".
+class VariableExprAST : public ExprAST {
+  std::string Name;
+public:
+  VariableExprAST(const std::string &amp;name) : Name(name) {}
+  virtual Value *Codegen();
+};
+
+/// BinaryExprAST - Expression class for a binary operator.
+class BinaryExprAST : public ExprAST {
+  char Op;
+  ExprAST *LHS, *RHS;
+public:
+  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
+    : Op(op), LHS(lhs), RHS(rhs) {}
+  virtual Value *Codegen();
+};
+
+/// CallExprAST - Expression class for function calls.
+class CallExprAST : public ExprAST {
+  std::string Callee;
+  std::vector&lt;ExprAST*&gt; Args;
+public:
+  CallExprAST(const std::string &amp;callee, std::vector&lt;ExprAST*&gt; &amp;args)
+    : Callee(callee), Args(args) {}
+  virtual Value *Codegen();
+};
+
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its name, and its argument names (thus implicitly the number
+/// of arguments the function takes).
+class PrototypeAST {
+  std::string Name;
+  std::vector&lt;std::string&gt; Args;
+public:
+  PrototypeAST(const std::string &amp;name, const std::vector&lt;std::string&gt; &amp;args)
+    : Name(name), Args(args) {}
+  
+  Function *Codegen();
+};
+
+/// FunctionAST - This class represents a function definition itself.
+class FunctionAST {
+  PrototypeAST *Proto;
+  ExprAST *Body;
+public:
+  FunctionAST(PrototypeAST *proto, ExprAST *body)
+    : Proto(proto), Body(body) {}
+  
+  Function *Codegen();
+};
+
+//===----------------------------------------------------------------------===//
+// Parser
+//===----------------------------------------------------------------------===//
+
+/// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
+/// token the parser is looking at.  getNextToken reads another token from the
+/// lexer and updates CurTok with its results.
+static int CurTok;
+static int getNextToken() {
+  return CurTok = gettok();
+}
+
+/// BinopPrecedence - This holds the precedence for each binary operator that is
+/// defined.
+static std::map&lt;char, int&gt; BinopPrecedence;
+
+/// GetTokPrecedence - Get the precedence of the pending binary operator token.
+static int GetTokPrecedence() {
+  if (!isascii(CurTok))
+    return -1;
+  
+  // Make sure it's a declared binop.
+  int TokPrec = BinopPrecedence[CurTok];
+  if (TokPrec &lt;= 0) return -1;
+  return TokPrec;
+}
+
+/// Error* - These are little helper functions for error handling.
+ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
+PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
+FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
+
+static ExprAST *ParseExpression();
+
+/// identifierexpr
+///   ::= identifier
+///   ::= identifier '(' expression* ')'
+static ExprAST *ParseIdentifierExpr() {
+  std::string IdName = IdentifierStr;
+  
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '(') // Simple variable ref.
+    return new VariableExprAST(IdName);
+  
+  // Call.
+  getNextToken();  // eat (
+  std::vector&lt;ExprAST*&gt; Args;
+  if (CurTok != ')') {
+    while (1) {
+      ExprAST *Arg = ParseExpression();
+      if (!Arg) return 0;
+      Args.push_back(Arg);
+
+      if (CurTok == ')') break;
+
+      if (CurTok != ',')
+        return Error("Expected ')' or ',' in argument list");
+      getNextToken();
+    }
+  }
+
+  // Eat the ')'.
+  getNextToken();
+  
+  return new CallExprAST(IdName, Args);
+}
+
+/// numberexpr ::= number
+static ExprAST *ParseNumberExpr() {
+  ExprAST *Result = new NumberExprAST(NumVal);
+  getNextToken(); // consume the number
+  return Result;
+}
+
+/// parenexpr ::= '(' expression ')'
+static ExprAST *ParseParenExpr() {
+  getNextToken();  // eat (.
+  ExprAST *V = ParseExpression();
+  if (!V) return 0;
+  
+  if (CurTok != ')')
+    return Error("expected ')'");
+  getNextToken();  // eat ).
+  return V;
+}
+
+/// primary
+///   ::= identifierexpr
+///   ::= numberexpr
+///   ::= parenexpr
+static ExprAST *ParsePrimary() {
+  switch (CurTok) {
+  default: return Error("unknown token when expecting an expression");
+  case tok_identifier: return ParseIdentifierExpr();
+  case tok_number:     return ParseNumberExpr();
+  case '(':            return ParseParenExpr();
+  }
+}
+
+/// binoprhs
+///   ::= ('+' primary)*
+static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+  // If this is a binop, find its precedence.
+  while (1) {
+    int TokPrec = GetTokPrecedence();
+    
+    // If this is a binop that binds at least as tightly as the current binop,
+    // consume it, otherwise we are done.
+    if (TokPrec &lt; ExprPrec)
+      return LHS;
+    
+    // Okay, we know this is a binop.
+    int BinOp = CurTok;
+    getNextToken();  // eat binop
+    
+    // Parse the primary expression after the binary operator.
+    ExprAST *RHS = ParsePrimary();
+    if (!RHS) return 0;
+    
+    // If BinOp binds less tightly with RHS than the operator after RHS, let
+    // the pending operator take RHS as its LHS.
+    int NextPrec = GetTokPrecedence();
+    if (TokPrec &lt; NextPrec) {
+      RHS = ParseBinOpRHS(TokPrec+1, RHS);
+      if (RHS == 0) return 0;
+    }
+    
+    // Merge LHS/RHS.
+    LHS = new BinaryExprAST(BinOp, LHS, RHS);
+  }
+}
+
+/// expression
+///   ::= primary binoprhs
+///
+static ExprAST *ParseExpression() {
+  ExprAST *LHS = ParsePrimary();
+  if (!LHS) return 0;
+  
+  return ParseBinOpRHS(0, LHS);
+}
+
+/// prototype
+///   ::= id '(' id* ')'
+static PrototypeAST *ParsePrototype() {
+  if (CurTok != tok_identifier)
+    return ErrorP("Expected function name in prototype");
+
+  std::string FnName = IdentifierStr;
+  getNextToken();
+  
+  if (CurTok != '(')
+    return ErrorP("Expected '(' in prototype");
+  
+  std::vector&lt;std::string&gt; ArgNames;
+  while (getNextToken() == tok_identifier)
+    ArgNames.push_back(IdentifierStr);
+  if (CurTok != ')')
+    return ErrorP("Expected ')' in prototype");
+  
+  // success.
+  getNextToken();  // eat ')'.
+  
+  return new PrototypeAST(FnName, ArgNames);
+}
+
+/// definition ::= 'def' prototype expression
+static FunctionAST *ParseDefinition() {
+  getNextToken();  // eat def.
+  PrototypeAST *Proto = ParsePrototype();
+  if (Proto == 0) return 0;
+
+  if (ExprAST *E = ParseExpression())
+    return new FunctionAST(Proto, E);
+  return 0;
+}
+
+/// toplevelexpr ::= expression
+static FunctionAST *ParseTopLevelExpr() {
+  if (ExprAST *E = ParseExpression()) {
+    // Make an anonymous proto.
+    PrototypeAST *Proto = new PrototypeAST("", std::vector&lt;std::string&gt;());
+    return new FunctionAST(Proto, E);
+  }
+  return 0;
+}
+
+/// external ::= 'extern' prototype
+static PrototypeAST *ParseExtern() {
+  getNextToken();  // eat extern.
+  return ParsePrototype();
+}
+
+//===----------------------------------------------------------------------===//
+// Code Generation
+//===----------------------------------------------------------------------===//
+
+static Module *TheModule;
+static IRBuilder&lt;&gt; Builder(getGlobalContext());
+static std::map&lt;std::string, Value*&gt; NamedValues;
+static FunctionPassManager *TheFPM;
+
+Value *ErrorV(const char *Str) { Error(Str); return 0; }
+
+Value *NumberExprAST::Codegen() {
+  return ConstantFP::get(getGlobalContext(), APFloat(Val));
+}
+
+Value *VariableExprAST::Codegen() {
+  // Look this variable up in the function.
+  Value *V = NamedValues[Name];
+  return V ? V : ErrorV("Unknown variable name");
+}
+
+Value *BinaryExprAST::Codegen() {
+  Value *L = LHS-&gt;Codegen();
+  Value *R = RHS-&gt;Codegen();
+  if (L == 0 || R == 0) return 0;
+  
+  switch (Op) {
+  case '+': return Builder.CreateAdd(L, R, "addtmp");
+  case '-': return Builder.CreateSub(L, R, "subtmp");
+  case '*': return Builder.CreateMul(L, R, "multmp");
+  case '&lt;':
+    L = Builder.CreateFCmpULT(L, R, "cmptmp");
+    // Convert bool 0/1 to double 0.0 or 1.0
+    return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
+                                "booltmp");
+  default: return ErrorV("invalid binary operator");
+  }
+}
+
+Value *CallExprAST::Codegen() {
+  // Look up the name in the global module table.
+  Function *CalleeF = TheModule-&gt;getFunction(Callee);
+  if (CalleeF == 0)
+    return ErrorV("Unknown function referenced");
+  
+  // If argument mismatch error.
+  if (CalleeF-&gt;arg_size() != Args.size())
+    return ErrorV("Incorrect # arguments passed");
+
+  std::vector&lt;Value*&gt; ArgsV;
+  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
+    ArgsV.push_back(Args[i]-&gt;Codegen());
+    if (ArgsV.back() == 0) return 0;
+  }
+  
+  return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp");
+}
+
+Function *PrototypeAST::Codegen() {
+  // Make the function type:  double(double,double) etc.
+  std::vector&lt;const Type*&gt; Doubles(Args.size(),
+                                   Type::getDoubleTy(getGlobalContext()));
+  FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
+                                       Doubles, false);
+  
+  Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
+  
+  // If F conflicted, there was already something named 'Name'.  If it has a
+  // body, don't allow redefinition or reextern.
+  if (F-&gt;getName() != Name) {
+    // Delete the one we just made and get the existing one.
+    F-&gt;eraseFromParent();
+    F = TheModule-&gt;getFunction(Name);
+    
+    // If F already has a body, reject this.
+    if (!F-&gt;empty()) {
+      ErrorF("redefinition of function");
+      return 0;
+    }
+    
+    // If F took a different number of args, reject.
+    if (F-&gt;arg_size() != Args.size()) {
+      ErrorF("redefinition of function with different # args");
+      return 0;
+    }
+  }
+  
+  // Set names for all arguments.
+  unsigned Idx = 0;
+  for (Function::arg_iterator AI = F-&gt;arg_begin(); Idx != Args.size();
+       ++AI, ++Idx) {
+    AI-&gt;setName(Args[Idx]);
+    
+    // Add arguments to variable symbol table.
+    NamedValues[Args[Idx]] = AI;
+  }
+  
+  return F;
+}
+
+Function *FunctionAST::Codegen() {
+  NamedValues.clear();
+  
+  Function *TheFunction = Proto-&gt;Codegen();
+  if (TheFunction == 0)
+    return 0;
+  
+  // Create a new basic block to start insertion into.
+  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
+  Builder.SetInsertPoint(BB);
+  
+  if (Value *RetVal = Body-&gt;Codegen()) {
+    // Finish off the function.
+    Builder.CreateRet(RetVal);
+
+    // Validate the generated code, checking for consistency.
+    verifyFunction(*TheFunction);
+
+    // Optimize the function.
+    TheFPM-&gt;run(*TheFunction);
+    
+    return TheFunction;
+  }
+  
+  // Error reading body, remove function.
+  TheFunction-&gt;eraseFromParent();
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Top-Level parsing and JIT Driver
+//===----------------------------------------------------------------------===//
+
+static ExecutionEngine *TheExecutionEngine;
+
+static void HandleDefinition() {
+  if (FunctionAST *F = ParseDefinition()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      fprintf(stderr, "Read function definition:");
+      LF-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleExtern() {
+  if (PrototypeAST *P = ParseExtern()) {
+    if (Function *F = P-&gt;Codegen()) {
+      fprintf(stderr, "Read extern: ");
+      F-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleTopLevelExpression() {
+  // Evaluate a top-level expression into an anonymous function.
+  if (FunctionAST *F = ParseTopLevelExpr()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      // JIT the function, returning a function pointer.
+      void *FPtr = TheExecutionEngine-&gt;getPointerToFunction(LF);
+      
+      // Cast it to the right type (takes no arguments, returns a double) so we
+      // can call it as a native function.
+      double (*FP)() = (double (*)())(intptr_t)FPtr;
+      fprintf(stderr, "Evaluated to %f\n", FP());
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+/// top ::= definition | external | expression | ';'
+static void MainLoop() {
+  while (1) {
+    fprintf(stderr, "ready&gt; ");
+    switch (CurTok) {
+    case tok_eof:    return;
+    case ';':        getNextToken(); break;  // ignore top-level semicolons.
+    case tok_def:    HandleDefinition(); break;
+    case tok_extern: HandleExtern(); break;
+    default:         HandleTopLevelExpression(); break;
+    }
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// "Library" functions that can be "extern'd" from user code.
+//===----------------------------------------------------------------------===//
+
+/// putchard - putchar that takes a double and returns 0.
+extern "C" 
+double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Main driver code.
+//===----------------------------------------------------------------------===//
+
+int main() {
+  InitializeNativeTarget();
+  LLVMContext &amp;Context = getGlobalContext();
+
+  // Install standard binary operators.
+  // 1 is lowest precedence.
+  BinopPrecedence['&lt;'] = 10;
+  BinopPrecedence['+'] = 20;
+  BinopPrecedence['-'] = 20;
+  BinopPrecedence['*'] = 40;  // highest.
+
+  // Prime the first token.
+  fprintf(stderr, "ready&gt; ");
+  getNextToken();
+
+  // Make the module, which holds all the code.
+  TheModule = new Module("my cool jit", Context);
+
+  // Create the JIT.  This takes ownership of the module.
+  std::string ErrStr;
+  TheExecutionEngine = EngineBuilder(TheModule).setErrorStr(&ErrStr).create();
+  if (!TheExecutionEngine) {
+    fprintf(stderr, "Could not create ExecutionEngine: %s\n", ErrStr.c_str());
+    exit(1);
+  }
+
+  FunctionPassManager OurFPM(TheModule);
+
+  // Set up the optimizer pipeline.  Start with registering info about how the
+  // target lays out data structures.
+  OurFPM.add(new TargetData(*TheExecutionEngine-&gt;getTargetData()));
+  // Do simple "peephole" optimizations and bit-twiddling optzns.
+  OurFPM.add(createInstructionCombiningPass());
+  // Reassociate expressions.
+  OurFPM.add(createReassociatePass());
+  // Eliminate Common SubExpressions.
+  OurFPM.add(createGVNPass());
+  // Simplify the control flow graph (deleting unreachable blocks, etc).
+  OurFPM.add(createCFGSimplificationPass());
+
+  OurFPM.doInitialization();
+
+  // Set the global so the code gen can use this.
+  TheFPM = &amp;OurFPM;
+
+  // Run the main "interpreter loop" now.
+  MainLoop();
+
+  TheFPM = 0;
+
+  // Print out all of the generated code.
+  TheModule-&gt;dump();
+
+  return 0;
+}
+</pre>
+</div>
+
+<a href="LangImpl5.html">Next: Extending the language: control flow</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Extending the Language: Control Flow</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Extending the Language: Control Flow</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 5
+  <ol>
+    <li><a href="#intro">Chapter 5 Introduction</a></li>
+    <li><a href="#ifthen">If/Then/Else</a>
+    <ol>
+      <li><a href="#iflexer">Lexer Extensions</a></li>
+      <li><a href="#ifast">AST Extensions</a></li>
+      <li><a href="#ifparser">Parser Extensions</a></li>
+      <li><a href="#ifir">LLVM IR</a></li>
+      <li><a href="#ifcodegen">Code Generation</a></li>
+    </ol>
+    </li>
+    <li><a href="#for">'for' Loop Expression</a>
+    <ol>
+      <li><a href="#forlexer">Lexer Extensions</a></li>
+      <li><a href="#forast">AST Extensions</a></li>
+      <li><a href="#forparser">Parser Extensions</a></li>
+      <li><a href="#forir">LLVM IR</a></li>
+      <li><a href="#forcodegen">Code Generation</a></li>
+    </ol>
+    </li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="LangImpl6.html">Chapter 6</a>: Extending the Language: 
+User-defined Operators</li>
+</ul>
+
+<div class="doc_author">
+  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 5 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 5 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  Parts 1-4 described the implementation of the simple
+Kaleidoscope language and included support for generating LLVM IR, followed by
+optimizations and a JIT compiler.  Unfortunately, as presented, Kaleidoscope is
+mostly useless: it has no control flow other than call and return.  This means
+that you can't have conditional branches in the code, significantly limiting its
+power.  In this episode of "build that compiler", we'll extend Kaleidoscope to
+have an if/then/else expression plus a simple 'for' loop.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="ifthen">If/Then/Else</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Extending Kaleidoscope to support if/then/else is quite straightforward.  It
+basically requires adding lexer support for this "new" concept to the lexer,
+parser, AST, and LLVM code emitter.  This example is nice, because it shows how
+easy it is to "grow" a language over time, incrementally extending it as new
+ideas are discovered.</p>
+
+<p>Before we get going on "how" we add this extension, lets talk about "what" we
+want.  The basic idea is that we want to be able to write this sort of thing:
+</p>
+
+<div class="doc_code">
+<pre>
+def fib(x)
+  if x &lt; 3 then
+    1
+  else
+    fib(x-1)+fib(x-2);
+</pre>
+</div>
+
+<p>In Kaleidoscope, every construct is an expression: there are no statements.
+As such, the if/then/else expression needs to return a value like any other.
+Since we're using a mostly functional form, we'll have it evaluate its
+conditional, then return the 'then' or 'else' value based on how the condition
+was resolved.  This is very similar to the C "?:" expression.</p>
+
+<p>The semantics of the if/then/else expression is that it evaluates the
+condition to a boolean equality value: 0.0 is considered to be false and
+everything else is considered to be true.
+If the condition is true, the first subexpression is evaluated and returned, if
+the condition is false, the second subexpression is evaluated and returned.
+Since Kaleidoscope allows side-effects, this behavior is important to nail down.
+</p>
+
+<p>Now that we know what we "want", lets break this down into its constituent
+pieces.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="iflexer">Lexer Extensions for
+If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+
+<div class="doc_text">
+
+<p>The lexer extensions are straightforward.  First we add new enum values
+for the relevant tokens:</p>
+
+<div class="doc_code">
+<pre>
+  // control
+  tok_if = -6, tok_then = -7, tok_else = -8,
+</pre>
+</div>
+
+<p>Once we have that, we recognize the new keywords in the lexer. This is pretty simple
+stuff:</p>
+
+<div class="doc_code">
+<pre>
+    ...
+    if (IdentifierStr == "def") return tok_def;
+    if (IdentifierStr == "extern") return tok_extern;
+    <b>if (IdentifierStr == "if") return tok_if;
+    if (IdentifierStr == "then") return tok_then;
+    if (IdentifierStr == "else") return tok_else;</b>
+    return tok_identifier;
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifast">AST Extensions for
+ If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>To represent the new expression we add a new AST node for it:</p>
+
+<div class="doc_code">
+<pre>
+/// IfExprAST - Expression class for if/then/else.
+class IfExprAST : public ExprAST {
+  ExprAST *Cond, *Then, *Else;
+public:
+  IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else)
+    : Cond(cond), Then(then), Else(_else) {}
+  virtual Value *Codegen();
+};
+</pre>
+</div>
+
+<p>The AST node just has pointers to the various subexpressions.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifparser">Parser Extensions for
+If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>Now that we have the relevant tokens coming from the lexer and we have the
+AST node to build, our parsing logic is relatively straightforward.  First we
+define a new parsing function:</p>
+
+<div class="doc_code">
+<pre>
+/// ifexpr ::= 'if' expression 'then' expression 'else' expression
+static ExprAST *ParseIfExpr() {
+  getNextToken();  // eat the if.
+  
+  // condition.
+  ExprAST *Cond = ParseExpression();
+  if (!Cond) return 0;
+  
+  if (CurTok != tok_then)
+    return Error("expected then");
+  getNextToken();  // eat the then
+  
+  ExprAST *Then = ParseExpression();
+  if (Then == 0) return 0;
+  
+  if (CurTok != tok_else)
+    return Error("expected else");
+  
+  getNextToken();
+  
+  ExprAST *Else = ParseExpression();
+  if (!Else) return 0;
+  
+  return new IfExprAST(Cond, Then, Else);
+}
+</pre>
+</div>
+
+<p>Next we hook it up as a primary expression:</p>
+
+<div class="doc_code">
+<pre>
+static ExprAST *ParsePrimary() {
+  switch (CurTok) {
+  default: return Error("unknown token when expecting an expression");
+  case tok_identifier: return ParseIdentifierExpr();
+  case tok_number:     return ParseNumberExpr();
+  case '(':            return ParseParenExpr();
+  <b>case tok_if:         return ParseIfExpr();</b>
+  }
+}
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifir">LLVM IR for If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>Now that we have it parsing and building the AST, the final piece is adding
+LLVM code generation support.  This is the most interesting part of the
+if/then/else example, because this is where it starts to introduce new concepts.
+All of the code above has been thoroughly described in previous chapters.
+</p>
+
+<p>To motivate the code we want to produce, lets take a look at a simple
+example.  Consider:</p>
+
+<div class="doc_code">
+<pre>
+extern foo();
+extern bar();
+def baz(x) if x then foo() else bar();
+</pre>
+</div>
+
+<p>If you disable optimizations, the code you'll (soon) get from Kaleidoscope
+looks like this:</p>
+
+<div class="doc_code">
+<pre>
+declare double @foo()
+
+declare double @bar()
+
+define double @baz(double %x) {
+entry:
+	%ifcond = fcmp one double %x, 0.000000e+00
+	br i1 %ifcond, label %then, label %else
+
+then:		; preds = %entry
+	%calltmp = call double @foo()
+	br label %ifcont
+
+else:		; preds = %entry
+	%calltmp1 = call double @bar()
+	br label %ifcont
+
+ifcont:		; preds = %else, %then
+	%iftmp = phi double [ %calltmp, %then ], [ %calltmp1, %else ]
+	ret double %iftmp
+}
+</pre>
+</div>
+
+<p>To visualize the control flow graph, you can use a nifty feature of the LLVM
+'<a href="http://llvm.org/cmds/opt.html">opt</a>' tool.  If you put this LLVM IR
+into "t.ll" and run "<tt>llvm-as &lt; t.ll | opt -analyze -view-cfg</tt>", <a
+href="../ProgrammersManual.html#ViewGraph">a window will pop up</a> and you'll
+see this graph:</p>
+
+<div style="text-align: center"><img src="LangImpl5-cfg.png" alt="Example CFG" width="423" 
+height="315"></div>
+
+<p>Another way to get this is to call "<tt>F-&gt;viewCFG()</tt>" or
+"<tt>F-&gt;viewCFGOnly()</tt>" (where F is a "<tt>Function*</tt>") either by
+inserting actual calls into the code and recompiling or by calling these in the
+debugger.  LLVM has many nice features for visualizing various graphs.</p>
+
+<p>Getting back to the generated code, it is fairly simple: the entry block 
+evaluates the conditional expression ("x" in our case here) and compares the
+result to 0.0 with the "<tt><a href="../LangRef.html#i_fcmp">fcmp</a> one</tt>"
+instruction ('one' is "Ordered and Not Equal").  Based on the result of this
+expression, the code jumps to either the "then" or "else" blocks, which contain
+the expressions for the true/false cases.</p>
+
+<p>Once the then/else blocks are finished executing, they both branch back to the
+'ifcont' block to execute the code that happens after the if/then/else.  In this
+case the only thing left to do is to return to the caller of the function.  The
+question then becomes: how does the code know which expression to return?</p>
+
+<p>The answer to this question involves an important SSA operation: the
+<a href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Phi
+operation</a>.  If you're not familiar with SSA, <a 
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">the wikipedia
+article</a> is a good introduction and there are various other introductions to
+it available on your favorite search engine.  The short version is that
+"execution" of the Phi operation requires "remembering" which block control came
+from.  The Phi operation takes on the value corresponding to the input control
+block.  In this case, if control comes in from the "then" block, it gets the
+value of "calltmp".  If control comes from the "else" block, it gets the value
+of "calltmp1".</p>
+
+<p>At this point, you are probably starting to think "Oh no! This means my
+simple and elegant front-end will have to start generating SSA form in order to
+use LLVM!".  Fortunately, this is not the case, and we strongly advise
+<em>not</em> implementing an SSA construction algorithm in your front-end
+unless there is an amazingly good reason to do so.  In practice, there are two
+sorts of values that float around in code written for your average imperative
+programming language that might need Phi nodes:</p>
+
+<ol>
+<li>Code that involves user variables: <tt>x = 1; x = x + 1; </tt></li>
+<li>Values that are implicit in the structure of your AST, such as the Phi node
+in this case.</li>
+</ol>
+
+<p>In <a href="LangImpl7.html">Chapter 7</a> of this tutorial ("mutable 
+variables"), we'll talk about #1
+in depth.  For now, just believe me that you don't need SSA construction to
+handle this case.  For #2, you have the choice of using the techniques that we will 
+describe for #1, or you can insert Phi nodes directly, if convenient.  In this 
+case, it is really really easy to generate the Phi node, so we choose to do it
+directly.</p>
+
+<p>Okay, enough of the motivation and overview, lets generate code!</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifcodegen">Code Generation for 
+If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>In order to generate code for this, we implement the <tt>Codegen</tt> method
+for <tt>IfExprAST</tt>:</p>
+
+<div class="doc_code">
+<pre>
+Value *IfExprAST::Codegen() {
+  Value *CondV = Cond-&gt;Codegen();
+  if (CondV == 0) return 0;
+  
+  // Convert condition to a bool by comparing equal to 0.0.
+  CondV = Builder.CreateFCmpONE(CondV, 
+                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                "ifcond");
+</pre>
+</div>
+
+<p>This code is straightforward and similar to what we saw before.  We emit the
+expression for the condition, then compare that value to zero to get a truth
+value as a 1-bit (bool) value.</p>
+
+<div class="doc_code">
+<pre>
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+  
+  // Create blocks for the then and else cases.  Insert the 'then' block at the
+  // end of the function.
+  BasicBlock *ThenBB = BasicBlock::Create(getGlobalContext(), "then", TheFunction);
+  BasicBlock *ElseBB = BasicBlock::Create(getGlobalContext(), "else");
+  BasicBlock *MergeBB = BasicBlock::Create(getGlobalContext(), "ifcont");
+
+  Builder.CreateCondBr(CondV, ThenBB, ElseBB);
+</pre>
+</div>
+
+<p>This code creates the basic blocks that are related to the if/then/else
+statement, and correspond directly to the blocks in the example above.  The
+first line gets the current Function object that is being built.  It
+gets this by asking the builder for the current BasicBlock, and asking that
+block for its "parent" (the function it is currently embedded into).</p>
+
+<p>Once it has that, it creates three blocks.  Note that it passes "TheFunction"
+into the constructor for the "then" block.  This causes the constructor to
+automatically insert the new block into the end of the specified function.  The
+other two blocks are created, but aren't yet inserted into the function.</p>
+
+<p>Once the blocks are created, we can emit the conditional branch that chooses
+between them.  Note that creating new blocks does not implicitly affect the
+IRBuilder, so it is still inserting into the block that the condition
+went into.  Also note that it is creating a branch to the "then" block and the
+"else" block, even though the "else" block isn't inserted into the function yet.
+This is all ok: it is the standard way that LLVM supports forward 
+references.</p>
+
+<div class="doc_code">
+<pre>
+  // Emit then value.
+  Builder.SetInsertPoint(ThenBB);
+  
+  Value *ThenV = Then-&gt;Codegen();
+  if (ThenV == 0) return 0;
+  
+  Builder.CreateBr(MergeBB);
+  // Codegen of 'Then' can change the current block, update ThenBB for the PHI.
+  ThenBB = Builder.GetInsertBlock();
+</pre>
+</div>
+
+<p>After the conditional branch is inserted, we move the builder to start
+inserting into the "then" block.  Strictly speaking, this call moves the
+insertion point to be at the end of the specified block.  However, since the
+"then" block is empty, it also starts out by inserting at the beginning of the
+block.  :)</p>
+
+<p>Once the insertion point is set, we recursively codegen the "then" expression
+from the AST.  To finish off the "then" block, we create an unconditional branch
+to the merge block.  One interesting (and very important) aspect of the LLVM IR
+is that it <a href="../LangRef.html#functionstructure">requires all basic blocks
+to be "terminated"</a> with a <a href="../LangRef.html#terminators">control flow
+instruction</a> such as return or branch.  This means that all control flow,
+<em>including fall throughs</em> must be made explicit in the LLVM IR.  If you
+violate this rule, the verifier will emit an error.</p>
+
+<p>The final line here is quite subtle, but is very important.  The basic issue
+is that when we create the Phi node in the merge block, we need to set up the
+block/value pairs that indicate how the Phi will work.  Importantly, the Phi
+node expects to have an entry for each predecessor of the block in the CFG.  Why
+then, are we getting the current block when we just set it to ThenBB 5 lines
+above?  The problem is that the "Then" expression may actually itself change the
+block that the Builder is emitting into if, for example, it contains a nested
+"if/then/else" expression.  Because calling Codegen recursively could
+arbitrarily change the notion of the current block, we are required to get an
+up-to-date value for code that will set up the Phi node.</p>
+
+<div class="doc_code">
+<pre>
+  // Emit else block.
+  TheFunction-&gt;getBasicBlockList().push_back(ElseBB);
+  Builder.SetInsertPoint(ElseBB);
+  
+  Value *ElseV = Else-&gt;Codegen();
+  if (ElseV == 0) return 0;
+  
+  Builder.CreateBr(MergeBB);
+  // Codegen of 'Else' can change the current block, update ElseBB for the PHI.
+  ElseBB = Builder.GetInsertBlock();
+</pre>
+</div>
+
+<p>Code generation for the 'else' block is basically identical to codegen for
+the 'then' block.  The only significant difference is the first line, which adds
+the 'else' block to the function.  Recall previously that the 'else' block was
+created, but not added to the function.  Now that the 'then' and 'else' blocks
+are emitted, we can finish up with the merge code:</p>
+
+<div class="doc_code">
+<pre>
+  // Emit merge block.
+  TheFunction->getBasicBlockList().push_back(MergeBB);
+  Builder.SetInsertPoint(MergeBB);
+  PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()),
+                                  "iftmp");
+  
+  PN->addIncoming(ThenV, ThenBB);
+  PN->addIncoming(ElseV, ElseBB);
+  return PN;
+}
+</pre>
+</div>
+
+<p>The first two lines here are now familiar: the first adds the "merge" block
+to the Function object (it was previously floating, like the else block above).
+The second block changes the insertion point so that newly created code will go
+into the "merge" block.  Once that is done, we need to create the PHI node and
+set up the block/value pairs for the PHI.</p>
+
+<p>Finally, the CodeGen function returns the phi node as the value computed by
+the if/then/else expression.  In our example above, this returned value will 
+feed into the code for the top-level function, which will create the return
+instruction.</p>
+
+<p>Overall, we now have the ability to execute conditional code in
+Kaleidoscope.  With this extension, Kaleidoscope is a fairly complete language
+that can calculate a wide variety of numeric functions.  Next up we'll add
+another useful expression that is familiar from non-functional languages...</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="for">'for' Loop Expression</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Now that we know how to add basic control flow constructs to the language,
+we have the tools to add more powerful things.  Lets add something more
+aggressive, a 'for' expression:</p>
+
+<div class="doc_code">
+<pre>
+ extern putchard(char)
+ def printstar(n)
+   for i = 1, i &lt; n, 1.0 in
+     putchard(42);  # ascii 42 = '*'
+     
+ # print 100 '*' characters
+ printstar(100);
+</pre>
+</div>
+
+<p>This expression defines a new variable ("i" in this case) which iterates from
+a starting value, while the condition ("i &lt; n" in this case) is true, 
+incrementing by an optional step value ("1.0" in this case).  If the step value
+is omitted, it defaults to 1.0.  While the loop is true, it executes its 
+body expression.  Because we don't have anything better to return, we'll just
+define the loop as always returning 0.0.  In the future when we have mutable
+variables, it will get more useful.</p>
+
+<p>As before, lets talk about the changes that we need to Kaleidoscope to
+support this.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forlexer">Lexer Extensions for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The lexer extensions are the same sort of thing as for if/then/else:</p>
+
+<div class="doc_code">
+<pre>
+  ... in enum Token ...
+  // control
+  tok_if = -6, tok_then = -7, tok_else = -8,
+<b>  tok_for = -9, tok_in = -10</b>
+
+  ... in gettok ...
+  if (IdentifierStr == "def") return tok_def;
+  if (IdentifierStr == "extern") return tok_extern;
+  if (IdentifierStr == "if") return tok_if;
+  if (IdentifierStr == "then") return tok_then;
+  if (IdentifierStr == "else") return tok_else;
+  <b>if (IdentifierStr == "for") return tok_for;
+  if (IdentifierStr == "in") return tok_in;</b>
+  return tok_identifier;
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forast">AST Extensions for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The AST node is just as simple.  It basically boils down to capturing
+the variable name and the constituent expressions in the node.</p>
+
+<div class="doc_code">
+<pre>
+/// ForExprAST - Expression class for for/in.
+class ForExprAST : public ExprAST {
+  std::string VarName;
+  ExprAST *Start, *End, *Step, *Body;
+public:
+  ForExprAST(const std::string &amp;varname, ExprAST *start, ExprAST *end,
+             ExprAST *step, ExprAST *body)
+    : VarName(varname), Start(start), End(end), Step(step), Body(body) {}
+  virtual Value *Codegen();
+};
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forparser">Parser Extensions for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The parser code is also fairly standard.  The only interesting thing here is
+handling of the optional step value.  The parser code handles it by checking to
+see if the second comma is present.  If not, it sets the step value to null in
+the AST node:</p>
+
+<div class="doc_code">
+<pre>
+/// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
+static ExprAST *ParseForExpr() {
+  getNextToken();  // eat the for.
+
+  if (CurTok != tok_identifier)
+    return Error("expected identifier after for");
+  
+  std::string IdName = IdentifierStr;
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '=')
+    return Error("expected '=' after for");
+  getNextToken();  // eat '='.
+  
+  
+  ExprAST *Start = ParseExpression();
+  if (Start == 0) return 0;
+  if (CurTok != ',')
+    return Error("expected ',' after for start value");
+  getNextToken();
+  
+  ExprAST *End = ParseExpression();
+  if (End == 0) return 0;
+  
+  // The step value is optional.
+  ExprAST *Step = 0;
+  if (CurTok == ',') {
+    getNextToken();
+    Step = ParseExpression();
+    if (Step == 0) return 0;
+  }
+  
+  if (CurTok != tok_in)
+    return Error("expected 'in' after for");
+  getNextToken();  // eat 'in'.
+  
+  ExprAST *Body = ParseExpression();
+  if (Body == 0) return 0;
+
+  return new ForExprAST(IdName, Start, End, Step, Body);
+}
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forir">LLVM IR for 
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>Now we get to the good part: the LLVM IR we want to generate for this thing.
+With the simple example above, we get this LLVM IR (note that this dump is
+generated with optimizations disabled for clarity):
+</p>
+
+<div class="doc_code">
+<pre>
+declare double @putchard(double)
+
+define double @printstar(double %n) {
+entry:
+        ; initial value = 1.0 (inlined into phi)
+	br label %loop
+
+loop:		; preds = %loop, %entry
+	%i = phi double [ 1.000000e+00, %entry ], [ %nextvar, %loop ]
+        ; body
+	%calltmp = call double @putchard( double 4.200000e+01 )
+        ; increment
+	%nextvar = fadd double %i, 1.000000e+00
+
+        ; termination test
+	%cmptmp = fcmp ult double %i, %n
+	%booltmp = uitofp i1 %cmptmp to double
+	%loopcond = fcmp one double %booltmp, 0.000000e+00
+	br i1 %loopcond, label %loop, label %afterloop
+
+afterloop:		; preds = %loop
+        ; loop always returns 0.0
+	ret double 0.000000e+00
+}
+</pre>
+</div>
+
+<p>This loop contains all the same constructs we saw before: a phi node, several
+expressions, and some basic blocks.  Lets see how this fits together.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forcodegen">Code Generation for 
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The first part of Codegen is very simple: we just output the start expression
+for the loop value:</p>
+
+<div class="doc_code">
+<pre>
+Value *ForExprAST::Codegen() {
+  // Emit the start code first, without 'variable' in scope.
+  Value *StartVal = Start-&gt;Codegen();
+  if (StartVal == 0) return 0;
+</pre>
+</div>
+
+<p>With this out of the way, the next step is to set up the LLVM basic block
+for the start of the loop body.  In the case above, the whole loop body is one
+block, but remember that the body code itself could consist of multiple blocks
+(e.g. if it contains an if/then/else or a for/in expression).</p>
+
+<div class="doc_code">
+<pre>
+  // Make the new basic block for the loop header, inserting after current
+  // block.
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+  BasicBlock *PreheaderBB = Builder.GetInsertBlock();
+  BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction);
+  
+  // Insert an explicit fall through from the current block to the LoopBB.
+  Builder.CreateBr(LoopBB);
+</pre>
+</div>
+
+<p>This code is similar to what we saw for if/then/else.  Because we will need
+it to create the Phi node, we remember the block that falls through into the
+loop.  Once we have that, we create the actual block that starts the loop and
+create an unconditional branch for the fall-through between the two blocks.</p>
+  
+<div class="doc_code">
+<pre>
+  // Start insertion in LoopBB.
+  Builder.SetInsertPoint(LoopBB);
+  
+  // Start the PHI node with an entry for Start.
+  PHINode *Variable = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), VarName.c_str());
+  Variable-&gt;addIncoming(StartVal, PreheaderBB);
+</pre>
+</div>
+
+<p>Now that the "preheader" for the loop is set up, we switch to emitting code
+for the loop body.  To begin with, we move the insertion point and create the
+PHI node for the loop induction variable.  Since we already know the incoming
+value for the starting value, we add it to the Phi node.  Note that the Phi will
+eventually get a second value for the backedge, but we can't set it up yet
+(because it doesn't exist!).</p>
+
+<div class="doc_code">
+<pre>
+  // Within the loop, the variable is defined equal to the PHI node.  If it
+  // shadows an existing variable, we have to restore it, so save it now.
+  Value *OldVal = NamedValues[VarName];
+  NamedValues[VarName] = Variable;
+  
+  // Emit the body of the loop.  This, like any other expr, can change the
+  // current BB.  Note that we ignore the value computed by the body, but don't
+  // allow an error.
+  if (Body-&gt;Codegen() == 0)
+    return 0;
+</pre>
+</div>
+
+<p>Now the code starts to get more interesting.  Our 'for' loop introduces a new
+variable to the symbol table.  This means that our symbol table can now contain
+either function arguments or loop variables.  To handle this, before we codegen
+the body of the loop, we add the loop variable as the current value for its
+name.  Note that it is possible that there is a variable of the same name in the
+outer scope.  It would be easy to make this an error (emit an error and return
+null if there is already an entry for VarName) but we choose to allow shadowing
+of variables.  In order to handle this correctly, we remember the Value that
+we are potentially shadowing in <tt>OldVal</tt> (which will be null if there is
+no shadowed variable).</p>
+
+<p>Once the loop variable is set into the symbol table, the code recursively
+codegen's the body.  This allows the body to use the loop variable: any
+references to it will naturally find it in the symbol table.</p>
+
+<div class="doc_code">
+<pre>
+  // Emit the step value.
+  Value *StepVal;
+  if (Step) {
+    StepVal = Step-&gt;Codegen();
+    if (StepVal == 0) return 0;
+  } else {
+    // If not specified, use 1.0.
+    StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0));
+  }
+  
+  Value *NextVar = Builder.CreateAdd(Variable, StepVal, "nextvar");
+</pre>
+</div>
+
+<p>Now that the body is emitted, we compute the next value of the iteration
+variable by adding the step value, or 1.0 if it isn't present. '<tt>NextVar</tt>'
+will be the value of the loop variable on the next iteration of the loop.</p>
+
+<div class="doc_code">
+<pre>
+  // Compute the end condition.
+  Value *EndCond = End-&gt;Codegen();
+  if (EndCond == 0) return EndCond;
+  
+  // Convert condition to a bool by comparing equal to 0.0.
+  EndCond = Builder.CreateFCmpONE(EndCond, 
+                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                  "loopcond");
+</pre>
+</div>
+
+<p>Finally, we evaluate the exit value of the loop, to determine whether the
+loop should exit.  This mirrors the condition evaluation for the if/then/else
+statement.</p>
+      
+<div class="doc_code">
+<pre>
+  // Create the "after loop" block and insert it.
+  BasicBlock *LoopEndBB = Builder.GetInsertBlock();
+  BasicBlock *AfterBB = BasicBlock::Create(getGlobalContext(), "afterloop", TheFunction);
+  
+  // Insert the conditional branch into the end of LoopEndBB.
+  Builder.CreateCondBr(EndCond, LoopBB, AfterBB);
+  
+  // Any new code will be inserted in AfterBB.
+  Builder.SetInsertPoint(AfterBB);
+</pre>
+</div>
+
+<p>With the code for the body of the loop complete, we just need to finish up
+the control flow for it.  This code remembers the end block (for the phi node), then creates the block for the loop exit ("afterloop").  Based on the value of the
+exit condition, it creates a conditional branch that chooses between executing
+the loop again and exiting the loop.  Any future code is emitted in the
+"afterloop" block, so it sets the insertion position to it.</p>
+  
+<div class="doc_code">
+<pre>
+  // Add a new entry to the PHI node for the backedge.
+  Variable-&gt;addIncoming(NextVar, LoopEndBB);
+  
+  // Restore the unshadowed variable.
+  if (OldVal)
+    NamedValues[VarName] = OldVal;
+  else
+    NamedValues.erase(VarName);
+  
+  // for expr always returns 0.0.
+  return Constant::getNullValue(Type::getDoubleTy(getGlobalContext()));
+}
+</pre>
+</div>
+
+<p>The final code handles various cleanups: now that we have the "NextVar"
+value, we can add the incoming value to the loop PHI node.  After that, we
+remove the loop variable from the symbol table, so that it isn't in scope after
+the for loop.  Finally, code generation of the for loop always returns 0.0, so
+that is what we return from <tt>ForExprAST::Codegen</tt>.</p>
+
+<p>With this, we conclude the "adding control flow to Kaleidoscope" chapter of
+the tutorial.  In this chapter we added two control flow constructs, and used them to motivate a couple of aspects of the LLVM IR that are important for front-end implementors
+to know.  In the next chapter of our saga, we will get a bit crazier and add
+<a href="LangImpl6.html">user-defined operators</a> to our poor innocent 
+language.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with the
+if/then/else and for expressions..  To build this example, use:
+</p>
+
+<div class="doc_code">
+<pre>
+   # Compile
+   g++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
+   # Run
+   ./toy
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<div class="doc_code">
+<pre>
+#include "llvm/DerivedTypes.h"
+#include "llvm/ExecutionEngine/ExecutionEngine.h"
+#include "llvm/ExecutionEngine/JIT.h"
+#include "llvm/LLVMContext.h"
+#include "llvm/Module.h"
+#include "llvm/PassManager.h"
+#include "llvm/Analysis/Verifier.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Target/TargetSelect.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Support/IRBuilder.h"
+#include &lt;cstdio&gt;
+#include &lt;string&gt;
+#include &lt;map&gt;
+#include &lt;vector&gt;
+using namespace llvm;
+
+//===----------------------------------------------------------------------===//
+// Lexer
+//===----------------------------------------------------------------------===//
+
+// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
+// of these for known things.
+enum Token {
+  tok_eof = -1,
+
+  // commands
+  tok_def = -2, tok_extern = -3,
+
+  // primary
+  tok_identifier = -4, tok_number = -5,
+  
+  // control
+  tok_if = -6, tok_then = -7, tok_else = -8,
+  tok_for = -9, tok_in = -10
+};
+
+static std::string IdentifierStr;  // Filled in if tok_identifier
+static double NumVal;              // Filled in if tok_number
+
+/// gettok - Return the next token from standard input.
+static int gettok() {
+  static int LastChar = ' ';
+
+  // Skip any whitespace.
+  while (isspace(LastChar))
+    LastChar = getchar();
+
+  if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
+    IdentifierStr = LastChar;
+    while (isalnum((LastChar = getchar())))
+      IdentifierStr += LastChar;
+
+    if (IdentifierStr == "def") return tok_def;
+    if (IdentifierStr == "extern") return tok_extern;
+    if (IdentifierStr == "if") return tok_if;
+    if (IdentifierStr == "then") return tok_then;
+    if (IdentifierStr == "else") return tok_else;
+    if (IdentifierStr == "for") return tok_for;
+    if (IdentifierStr == "in") return tok_in;
+    return tok_identifier;
+  }
+
+  if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
+    std::string NumStr;
+    do {
+      NumStr += LastChar;
+      LastChar = getchar();
+    } while (isdigit(LastChar) || LastChar == '.');
+
+    NumVal = strtod(NumStr.c_str(), 0);
+    return tok_number;
+  }
+
+  if (LastChar == '#') {
+    // Comment until end of line.
+    do LastChar = getchar();
+    while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp;&amp; LastChar != '\r');
+    
+    if (LastChar != EOF)
+      return gettok();
+  }
+  
+  // Check for end of file.  Don't eat the EOF.
+  if (LastChar == EOF)
+    return tok_eof;
+
+  // Otherwise, just return the character as its ascii value.
+  int ThisChar = LastChar;
+  LastChar = getchar();
+  return ThisChar;
+}
+
+//===----------------------------------------------------------------------===//
+// Abstract Syntax Tree (aka Parse Tree)
+//===----------------------------------------------------------------------===//
+
+/// ExprAST - Base class for all expression nodes.
+class ExprAST {
+public:
+  virtual ~ExprAST() {}
+  virtual Value *Codegen() = 0;
+};
+
+/// NumberExprAST - Expression class for numeric literals like "1.0".
+class NumberExprAST : public ExprAST {
+  double Val;
+public:
+  NumberExprAST(double val) : Val(val) {}
+  virtual Value *Codegen();
+};
+
+/// VariableExprAST - Expression class for referencing a variable, like "a".
+class VariableExprAST : public ExprAST {
+  std::string Name;
+public:
+  VariableExprAST(const std::string &amp;name) : Name(name) {}
+  virtual Value *Codegen();
+};
+
+/// BinaryExprAST - Expression class for a binary operator.
+class BinaryExprAST : public ExprAST {
+  char Op;
+  ExprAST *LHS, *RHS;
+public:
+  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
+    : Op(op), LHS(lhs), RHS(rhs) {}
+  virtual Value *Codegen();
+};
+
+/// CallExprAST - Expression class for function calls.
+class CallExprAST : public ExprAST {
+  std::string Callee;
+  std::vector&lt;ExprAST*&gt; Args;
+public:
+  CallExprAST(const std::string &amp;callee, std::vector&lt;ExprAST*&gt; &amp;args)
+    : Callee(callee), Args(args) {}
+  virtual Value *Codegen();
+};
+
+/// IfExprAST - Expression class for if/then/else.
+class IfExprAST : public ExprAST {
+  ExprAST *Cond, *Then, *Else;
+public:
+  IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else)
+  : Cond(cond), Then(then), Else(_else) {}
+  virtual Value *Codegen();
+};
+
+/// ForExprAST - Expression class for for/in.
+class ForExprAST : public ExprAST {
+  std::string VarName;
+  ExprAST *Start, *End, *Step, *Body;
+public:
+  ForExprAST(const std::string &amp;varname, ExprAST *start, ExprAST *end,
+             ExprAST *step, ExprAST *body)
+    : VarName(varname), Start(start), End(end), Step(step), Body(body) {}
+  virtual Value *Codegen();
+};
+
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its name, and its argument names (thus implicitly the number
+/// of arguments the function takes).
+class PrototypeAST {
+  std::string Name;
+  std::vector&lt;std::string&gt; Args;
+public:
+  PrototypeAST(const std::string &amp;name, const std::vector&lt;std::string&gt; &amp;args)
+    : Name(name), Args(args) {}
+  
+  Function *Codegen();
+};
+
+/// FunctionAST - This class represents a function definition itself.
+class FunctionAST {
+  PrototypeAST *Proto;
+  ExprAST *Body;
+public:
+  FunctionAST(PrototypeAST *proto, ExprAST *body)
+    : Proto(proto), Body(body) {}
+  
+  Function *Codegen();
+};
+
+//===----------------------------------------------------------------------===//
+// Parser
+//===----------------------------------------------------------------------===//
+
+/// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
+/// token the parser is looking at.  getNextToken reads another token from the
+/// lexer and updates CurTok with its results.
+static int CurTok;
+static int getNextToken() {
+  return CurTok = gettok();
+}
+
+/// BinopPrecedence - This holds the precedence for each binary operator that is
+/// defined.
+static std::map&lt;char, int&gt; BinopPrecedence;
+
+/// GetTokPrecedence - Get the precedence of the pending binary operator token.
+static int GetTokPrecedence() {
+  if (!isascii(CurTok))
+    return -1;
+  
+  // Make sure it's a declared binop.
+  int TokPrec = BinopPrecedence[CurTok];
+  if (TokPrec &lt;= 0) return -1;
+  return TokPrec;
+}
+
+/// Error* - These are little helper functions for error handling.
+ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
+PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
+FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
+
+static ExprAST *ParseExpression();
+
+/// identifierexpr
+///   ::= identifier
+///   ::= identifier '(' expression* ')'
+static ExprAST *ParseIdentifierExpr() {
+  std::string IdName = IdentifierStr;
+  
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '(') // Simple variable ref.
+    return new VariableExprAST(IdName);
+  
+  // Call.
+  getNextToken();  // eat (
+  std::vector&lt;ExprAST*&gt; Args;
+  if (CurTok != ')') {
+    while (1) {
+      ExprAST *Arg = ParseExpression();
+      if (!Arg) return 0;
+      Args.push_back(Arg);
+
+      if (CurTok == ')') break;
+
+      if (CurTok != ',')
+        return Error("Expected ')' or ',' in argument list");
+      getNextToken();
+    }
+  }
+
+  // Eat the ')'.
+  getNextToken();
+  
+  return new CallExprAST(IdName, Args);
+}
+
+/// numberexpr ::= number
+static ExprAST *ParseNumberExpr() {
+  ExprAST *Result = new NumberExprAST(NumVal);
+  getNextToken(); // consume the number
+  return Result;
+}
+
+/// parenexpr ::= '(' expression ')'
+static ExprAST *ParseParenExpr() {
+  getNextToken();  // eat (.
+  ExprAST *V = ParseExpression();
+  if (!V) return 0;
+  
+  if (CurTok != ')')
+    return Error("expected ')'");
+  getNextToken();  // eat ).
+  return V;
+}
+
+/// ifexpr ::= 'if' expression 'then' expression 'else' expression
+static ExprAST *ParseIfExpr() {
+  getNextToken();  // eat the if.
+  
+  // condition.
+  ExprAST *Cond = ParseExpression();
+  if (!Cond) return 0;
+  
+  if (CurTok != tok_then)
+    return Error("expected then");
+  getNextToken();  // eat the then
+  
+  ExprAST *Then = ParseExpression();
+  if (Then == 0) return 0;
+  
+  if (CurTok != tok_else)
+    return Error("expected else");
+  
+  getNextToken();
+  
+  ExprAST *Else = ParseExpression();
+  if (!Else) return 0;
+  
+  return new IfExprAST(Cond, Then, Else);
+}
+
+/// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
+static ExprAST *ParseForExpr() {
+  getNextToken();  // eat the for.
+
+  if (CurTok != tok_identifier)
+    return Error("expected identifier after for");
+  
+  std::string IdName = IdentifierStr;
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '=')
+    return Error("expected '=' after for");
+  getNextToken();  // eat '='.
+  
+  
+  ExprAST *Start = ParseExpression();
+  if (Start == 0) return 0;
+  if (CurTok != ',')
+    return Error("expected ',' after for start value");
+  getNextToken();
+  
+  ExprAST *End = ParseExpression();
+  if (End == 0) return 0;
+  
+  // The step value is optional.
+  ExprAST *Step = 0;
+  if (CurTok == ',') {
+    getNextToken();
+    Step = ParseExpression();
+    if (Step == 0) return 0;
+  }
+  
+  if (CurTok != tok_in)
+    return Error("expected 'in' after for");
+  getNextToken();  // eat 'in'.
+  
+  ExprAST *Body = ParseExpression();
+  if (Body == 0) return 0;
+
+  return new ForExprAST(IdName, Start, End, Step, Body);
+}
+
+/// primary
+///   ::= identifierexpr
+///   ::= numberexpr
+///   ::= parenexpr
+///   ::= ifexpr
+///   ::= forexpr
+static ExprAST *ParsePrimary() {
+  switch (CurTok) {
+  default: return Error("unknown token when expecting an expression");
+  case tok_identifier: return ParseIdentifierExpr();
+  case tok_number:     return ParseNumberExpr();
+  case '(':            return ParseParenExpr();
+  case tok_if:         return ParseIfExpr();
+  case tok_for:        return ParseForExpr();
+  }
+}
+
+/// binoprhs
+///   ::= ('+' primary)*
+static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+  // If this is a binop, find its precedence.
+  while (1) {
+    int TokPrec = GetTokPrecedence();
+    
+    // If this is a binop that binds at least as tightly as the current binop,
+    // consume it, otherwise we are done.
+    if (TokPrec &lt; ExprPrec)
+      return LHS;
+    
+    // Okay, we know this is a binop.
+    int BinOp = CurTok;
+    getNextToken();  // eat binop
+    
+    // Parse the primary expression after the binary operator.
+    ExprAST *RHS = ParsePrimary();
+    if (!RHS) return 0;
+    
+    // If BinOp binds less tightly with RHS than the operator after RHS, let
+    // the pending operator take RHS as its LHS.
+    int NextPrec = GetTokPrecedence();
+    if (TokPrec &lt; NextPrec) {
+      RHS = ParseBinOpRHS(TokPrec+1, RHS);
+      if (RHS == 0) return 0;
+    }
+    
+    // Merge LHS/RHS.
+    LHS = new BinaryExprAST(BinOp, LHS, RHS);
+  }
+}
+
+/// expression
+///   ::= primary binoprhs
+///
+static ExprAST *ParseExpression() {
+  ExprAST *LHS = ParsePrimary();
+  if (!LHS) return 0;
+  
+  return ParseBinOpRHS(0, LHS);
+}
+
+/// prototype
+///   ::= id '(' id* ')'
+static PrototypeAST *ParsePrototype() {
+  if (CurTok != tok_identifier)
+    return ErrorP("Expected function name in prototype");
+
+  std::string FnName = IdentifierStr;
+  getNextToken();
+  
+  if (CurTok != '(')
+    return ErrorP("Expected '(' in prototype");
+  
+  std::vector&lt;std::string&gt; ArgNames;
+  while (getNextToken() == tok_identifier)
+    ArgNames.push_back(IdentifierStr);
+  if (CurTok != ')')
+    return ErrorP("Expected ')' in prototype");
+  
+  // success.
+  getNextToken();  // eat ')'.
+  
+  return new PrototypeAST(FnName, ArgNames);
+}
+
+/// definition ::= 'def' prototype expression
+static FunctionAST *ParseDefinition() {
+  getNextToken();  // eat def.
+  PrototypeAST *Proto = ParsePrototype();
+  if (Proto == 0) return 0;
+
+  if (ExprAST *E = ParseExpression())
+    return new FunctionAST(Proto, E);
+  return 0;
+}
+
+/// toplevelexpr ::= expression
+static FunctionAST *ParseTopLevelExpr() {
+  if (ExprAST *E = ParseExpression()) {
+    // Make an anonymous proto.
+    PrototypeAST *Proto = new PrototypeAST("", std::vector&lt;std::string&gt;());
+    return new FunctionAST(Proto, E);
+  }
+  return 0;
+}
+
+/// external ::= 'extern' prototype
+static PrototypeAST *ParseExtern() {
+  getNextToken();  // eat extern.
+  return ParsePrototype();
+}
+
+//===----------------------------------------------------------------------===//
+// Code Generation
+//===----------------------------------------------------------------------===//
+
+static Module *TheModule;
+static IRBuilder&lt;&gt; Builder(getGlobalContext());
+static std::map&lt;std::string, Value*&gt; NamedValues;
+static FunctionPassManager *TheFPM;
+
+Value *ErrorV(const char *Str) { Error(Str); return 0; }
+
+Value *NumberExprAST::Codegen() {
+  return ConstantFP::get(getGlobalContext(), APFloat(Val));
+}
+
+Value *VariableExprAST::Codegen() {
+  // Look this variable up in the function.
+  Value *V = NamedValues[Name];
+  return V ? V : ErrorV("Unknown variable name");
+}
+
+Value *BinaryExprAST::Codegen() {
+  Value *L = LHS-&gt;Codegen();
+  Value *R = RHS-&gt;Codegen();
+  if (L == 0 || R == 0) return 0;
+  
+  switch (Op) {
+  case '+': return Builder.CreateAdd(L, R, "addtmp");
+  case '-': return Builder.CreateSub(L, R, "subtmp");
+  case '*': return Builder.CreateMul(L, R, "multmp");
+  case '&lt;':
+    L = Builder.CreateFCmpULT(L, R, "cmptmp");
+    // Convert bool 0/1 to double 0.0 or 1.0
+    return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
+                                "booltmp");
+  default: return ErrorV("invalid binary operator");
+  }
+}
+
+Value *CallExprAST::Codegen() {
+  // Look up the name in the global module table.
+  Function *CalleeF = TheModule-&gt;getFunction(Callee);
+  if (CalleeF == 0)
+    return ErrorV("Unknown function referenced");
+  
+  // If argument mismatch error.
+  if (CalleeF-&gt;arg_size() != Args.size())
+    return ErrorV("Incorrect # arguments passed");
+
+  std::vector&lt;Value*&gt; ArgsV;
+  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
+    ArgsV.push_back(Args[i]-&gt;Codegen());
+    if (ArgsV.back() == 0) return 0;
+  }
+  
+  return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp");
+}
+
+Value *IfExprAST::Codegen() {
+  Value *CondV = Cond-&gt;Codegen();
+  if (CondV == 0) return 0;
+  
+  // Convert condition to a bool by comparing equal to 0.0.
+  CondV = Builder.CreateFCmpONE(CondV, 
+                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                "ifcond");
+  
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+  
+  // Create blocks for the then and else cases.  Insert the 'then' block at the
+  // end of the function.
+  BasicBlock *ThenBB = BasicBlock::Create(getGlobalContext(), "then", TheFunction);
+  BasicBlock *ElseBB = BasicBlock::Create(getGlobalContext(), "else");
+  BasicBlock *MergeBB = BasicBlock::Create(getGlobalContext(), "ifcont");
+  
+  Builder.CreateCondBr(CondV, ThenBB, ElseBB);
+  
+  // Emit then value.
+  Builder.SetInsertPoint(ThenBB);
+  
+  Value *ThenV = Then-&gt;Codegen();
+  if (ThenV == 0) return 0;
+  
+  Builder.CreateBr(MergeBB);
+  // Codegen of 'Then' can change the current block, update ThenBB for the PHI.
+  ThenBB = Builder.GetInsertBlock();
+  
+  // Emit else block.
+  TheFunction-&gt;getBasicBlockList().push_back(ElseBB);
+  Builder.SetInsertPoint(ElseBB);
+  
+  Value *ElseV = Else-&gt;Codegen();
+  if (ElseV == 0) return 0;
+  
+  Builder.CreateBr(MergeBB);
+  // Codegen of 'Else' can change the current block, update ElseBB for the PHI.
+  ElseBB = Builder.GetInsertBlock();
+  
+  // Emit merge block.
+  TheFunction-&gt;getBasicBlockList().push_back(MergeBB);
+  Builder.SetInsertPoint(MergeBB);
+  PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()),
+                                  "iftmp");
+  
+  PN-&gt;addIncoming(ThenV, ThenBB);
+  PN-&gt;addIncoming(ElseV, ElseBB);
+  return PN;
+}
+
+Value *ForExprAST::Codegen() {
+  // Output this as:
+  //   ...
+  //   start = startexpr
+  //   goto loop
+  // loop: 
+  //   variable = phi [start, loopheader], [nextvariable, loopend]
+  //   ...
+  //   bodyexpr
+  //   ...
+  // loopend:
+  //   step = stepexpr
+  //   nextvariable = variable + step
+  //   endcond = endexpr
+  //   br endcond, loop, endloop
+  // outloop:
+  
+  // Emit the start code first, without 'variable' in scope.
+  Value *StartVal = Start-&gt;Codegen();
+  if (StartVal == 0) return 0;
+  
+  // Make the new basic block for the loop header, inserting after current
+  // block.
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+  BasicBlock *PreheaderBB = Builder.GetInsertBlock();
+  BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction);
+  
+  // Insert an explicit fall through from the current block to the LoopBB.
+  Builder.CreateBr(LoopBB);
+
+  // Start insertion in LoopBB.
+  Builder.SetInsertPoint(LoopBB);
+  
+  // Start the PHI node with an entry for Start.
+  PHINode *Variable = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), VarName.c_str());
+  Variable-&gt;addIncoming(StartVal, PreheaderBB);
+  
+  // Within the loop, the variable is defined equal to the PHI node.  If it
+  // shadows an existing variable, we have to restore it, so save it now.
+  Value *OldVal = NamedValues[VarName];
+  NamedValues[VarName] = Variable;
+  
+  // Emit the body of the loop.  This, like any other expr, can change the
+  // current BB.  Note that we ignore the value computed by the body, but don't
+  // allow an error.
+  if (Body-&gt;Codegen() == 0)
+    return 0;
+  
+  // Emit the step value.
+  Value *StepVal;
+  if (Step) {
+    StepVal = Step-&gt;Codegen();
+    if (StepVal == 0) return 0;
+  } else {
+    // If not specified, use 1.0.
+    StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0));
+  }
+  
+  Value *NextVar = Builder.CreateAdd(Variable, StepVal, "nextvar");
+
+  // Compute the end condition.
+  Value *EndCond = End-&gt;Codegen();
+  if (EndCond == 0) return EndCond;
+  
+  // Convert condition to a bool by comparing equal to 0.0.
+  EndCond = Builder.CreateFCmpONE(EndCond, 
+                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                  "loopcond");
+  
+  // Create the "after loop" block and insert it.
+  BasicBlock *LoopEndBB = Builder.GetInsertBlock();
+  BasicBlock *AfterBB = BasicBlock::Create(getGlobalContext(), "afterloop", TheFunction);
+  
+  // Insert the conditional branch into the end of LoopEndBB.
+  Builder.CreateCondBr(EndCond, LoopBB, AfterBB);
+  
+  // Any new code will be inserted in AfterBB.
+  Builder.SetInsertPoint(AfterBB);
+  
+  // Add a new entry to the PHI node for the backedge.
+  Variable-&gt;addIncoming(NextVar, LoopEndBB);
+  
+  // Restore the unshadowed variable.
+  if (OldVal)
+    NamedValues[VarName] = OldVal;
+  else
+    NamedValues.erase(VarName);
+
+  
+  // for expr always returns 0.0.
+  return Constant::getNullValue(Type::getDoubleTy(getGlobalContext()));
+}
+
+Function *PrototypeAST::Codegen() {
+  // Make the function type:  double(double,double) etc.
+  std::vector&lt;const Type*&gt; Doubles(Args.size(),
+                                   Type::getDoubleTy(getGlobalContext()));
+  FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
+                                       Doubles, false);
+  
+  Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
+  
+  // If F conflicted, there was already something named 'Name'.  If it has a
+  // body, don't allow redefinition or reextern.
+  if (F-&gt;getName() != Name) {
+    // Delete the one we just made and get the existing one.
+    F-&gt;eraseFromParent();
+    F = TheModule-&gt;getFunction(Name);
+    
+    // If F already has a body, reject this.
+    if (!F-&gt;empty()) {
+      ErrorF("redefinition of function");
+      return 0;
+    }
+    
+    // If F took a different number of args, reject.
+    if (F-&gt;arg_size() != Args.size()) {
+      ErrorF("redefinition of function with different # args");
+      return 0;
+    }
+  }
+  
+  // Set names for all arguments.
+  unsigned Idx = 0;
+  for (Function::arg_iterator AI = F-&gt;arg_begin(); Idx != Args.size();
+       ++AI, ++Idx) {
+    AI-&gt;setName(Args[Idx]);
+    
+    // Add arguments to variable symbol table.
+    NamedValues[Args[Idx]] = AI;
+  }
+  
+  return F;
+}
+
+Function *FunctionAST::Codegen() {
+  NamedValues.clear();
+  
+  Function *TheFunction = Proto-&gt;Codegen();
+  if (TheFunction == 0)
+    return 0;
+  
+  // Create a new basic block to start insertion into.
+  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
+  Builder.SetInsertPoint(BB);
+  
+  if (Value *RetVal = Body-&gt;Codegen()) {
+    // Finish off the function.
+    Builder.CreateRet(RetVal);
+
+    // Validate the generated code, checking for consistency.
+    verifyFunction(*TheFunction);
+
+    // Optimize the function.
+    TheFPM-&gt;run(*TheFunction);
+    
+    return TheFunction;
+  }
+  
+  // Error reading body, remove function.
+  TheFunction-&gt;eraseFromParent();
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Top-Level parsing and JIT Driver
+//===----------------------------------------------------------------------===//
+
+static ExecutionEngine *TheExecutionEngine;
+
+static void HandleDefinition() {
+  if (FunctionAST *F = ParseDefinition()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      fprintf(stderr, "Read function definition:");
+      LF-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleExtern() {
+  if (PrototypeAST *P = ParseExtern()) {
+    if (Function *F = P-&gt;Codegen()) {
+      fprintf(stderr, "Read extern: ");
+      F-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleTopLevelExpression() {
+  // Evaluate a top-level expression into an anonymous function.
+  if (FunctionAST *F = ParseTopLevelExpr()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      // JIT the function, returning a function pointer.
+      void *FPtr = TheExecutionEngine-&gt;getPointerToFunction(LF);
+      
+      // Cast it to the right type (takes no arguments, returns a double) so we
+      // can call it as a native function.
+      double (*FP)() = (double (*)())(intptr_t)FPtr;
+      fprintf(stderr, "Evaluated to %f\n", FP());
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+/// top ::= definition | external | expression | ';'
+static void MainLoop() {
+  while (1) {
+    fprintf(stderr, "ready&gt; ");
+    switch (CurTok) {
+    case tok_eof:    return;
+    case ';':        getNextToken(); break;  // ignore top-level semicolons.
+    case tok_def:    HandleDefinition(); break;
+    case tok_extern: HandleExtern(); break;
+    default:         HandleTopLevelExpression(); break;
+    }
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// "Library" functions that can be "extern'd" from user code.
+//===----------------------------------------------------------------------===//
+
+/// putchard - putchar that takes a double and returns 0.
+extern "C" 
+double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Main driver code.
+//===----------------------------------------------------------------------===//
+
+int main() {
+  InitializeNativeTarget();
+  LLVMContext &amp;Context = getGlobalContext();
+
+  // Install standard binary operators.
+  // 1 is lowest precedence.
+  BinopPrecedence['&lt;'] = 10;
+  BinopPrecedence['+'] = 20;
+  BinopPrecedence['-'] = 20;
+  BinopPrecedence['*'] = 40;  // highest.
+
+  // Prime the first token.
+  fprintf(stderr, "ready&gt; ");
+  getNextToken();
+
+  // Make the module, which holds all the code.
+  TheModule = new Module("my cool jit", Context);
+
+  // Create the JIT.  This takes ownership of the module.
+  std::string ErrStr;
+  TheExecutionEngine = EngineBuilder(TheModule).setErrorStr(&ErrStr).create();
+  if (!TheExecutionEngine) {
+    fprintf(stderr, "Could not create ExecutionEngine: %s\n", ErrStr.c_str());
+    exit(1);
+  }
+
+  FunctionPassManager OurFPM(TheModule);
+
+  // Set up the optimizer pipeline.  Start with registering info about how the
+  // target lays out data structures.
+  OurFPM.add(new TargetData(*TheExecutionEngine-&gt;getTargetData()));
+  // Do simple "peephole" optimizations and bit-twiddling optzns.
+  OurFPM.add(createInstructionCombiningPass());
+  // Reassociate expressions.
+  OurFPM.add(createReassociatePass());
+  // Eliminate Common SubExpressions.
+  OurFPM.add(createGVNPass());
+  // Simplify the control flow graph (deleting unreachable blocks, etc).
+  OurFPM.add(createCFGSimplificationPass());
+
+  OurFPM.doInitialization();
+
+  // Set the global so the code gen can use this.
+  TheFPM = &amp;OurFPM;
+
+  // Run the main "interpreter loop" now.
+  MainLoop();
+
+  TheFPM = 0;
+
+  // Print out all of the generated code.
+  TheModule-&gt;dump();
+
+  return 0;
+}
+</pre>
+</div>
+
+<a href="LangImpl6.html">Next: Extending the language: user-defined operators</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/LangImpl6.html b/docs/tutorial/LangImpl6.html
new file mode 100644
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Extending the Language: User-defined Operators</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Extending the Language: User-defined Operators</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 6
+  <ol>
+    <li><a href="#intro">Chapter 6 Introduction</a></li>
+    <li><a href="#idea">User-defined Operators: the Idea</a></li>
+    <li><a href="#binary">User-defined Binary Operators</a></li>
+    <li><a href="#unary">User-defined Unary Operators</a></li>
+    <li><a href="#example">Kicking the Tires</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="LangImpl7.html">Chapter 7</a>: Extending the Language: Mutable
+Variables / SSA Construction</li>
+</ul>
+
+<div class="doc_author">
+  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 6 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 6 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  At this point in our tutorial, we now have a fully
+functional language that is fairly minimal, but also useful.  There
+is still one big problem with it, however. Our language doesn't have many 
+useful operators (like division, logical negation, or even any comparisons 
+besides less-than).</p>
+
+<p>This chapter of the tutorial takes a wild digression into adding user-defined
+operators to the simple and beautiful Kaleidoscope language. This digression now gives 
+us a simple and ugly language in some ways, but also a powerful one at the same time.
+One of the great things about creating your own language is that you get to
+decide what is good or bad.  In this tutorial we'll assume that it is okay to
+use this as a way to show some interesting parsing techniques.</p>
+
+<p>At the end of this tutorial, we'll run through an example Kaleidoscope 
+application that <a href="#example">renders the Mandelbrot set</a>.  This gives 
+an example of what you can build with Kaleidoscope and its feature set.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="idea">User-defined Operators: the Idea</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+The "operator overloading" that we will add to Kaleidoscope is more general than
+languages like C++.  In C++, you are only allowed to redefine existing
+operators: you can't programatically change the grammar, introduce new
+operators, change precedence levels, etc.  In this chapter, we will add this
+capability to Kaleidoscope, which will let the user round out the set of
+operators that are supported.</p>
+
+<p>The point of going into user-defined operators in a tutorial like this is to
+show the power and flexibility of using a hand-written parser.  Thus far, the parser
+we have been implementing uses recursive descent for most parts of the grammar and 
+operator precedence parsing for the expressions.  See <a 
+href="LangImpl2.html">Chapter 2</a> for details.  Without using operator
+precedence parsing, it would be very difficult to allow the programmer to
+introduce new operators into the grammar: the grammar is dynamically extensible
+as the JIT runs.</p>
+
+<p>The two specific features we'll add are programmable unary operators (right
+now, Kaleidoscope has no unary operators at all) as well as binary operators.
+An example of this is:</p>
+
+<div class="doc_code">
+<pre>
+# Logical unary not.
+def unary!(v)
+  if v then
+    0
+  else
+    1;
+
+# Define &gt; with the same precedence as &lt;.
+def binary&gt; 10 (LHS RHS)
+  RHS &lt; LHS;
+
+# Binary "logical or", (note that it does not "short circuit")
+def binary| 5 (LHS RHS)
+  if LHS then
+    1
+  else if RHS then
+    1
+  else
+    0;
+
+# Define = with slightly lower precedence than relationals.
+def binary= 9 (LHS RHS)
+  !(LHS &lt; RHS | LHS &gt; RHS);
+</pre>
+</div>
+
+<p>Many languages aspire to being able to implement their standard runtime
+library in the language itself.  In Kaleidoscope, we can implement significant
+parts of the language in the library!</p>
+
+<p>We will break down implementation of these features into two parts:
+implementing support for user-defined binary operators and adding unary
+operators.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="binary">User-defined Binary Operators</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Adding support for user-defined binary operators is pretty simple with our
+current framework.  We'll first add support for the unary/binary keywords:</p>
+
+<div class="doc_code">
+<pre>
+enum Token {
+  ...
+  <b>// operators
+  tok_binary = -11, tok_unary = -12</b>
+};
+...
+static int gettok() {
+...
+    if (IdentifierStr == "for") return tok_for;
+    if (IdentifierStr == "in") return tok_in;
+    <b>if (IdentifierStr == "binary") return tok_binary;
+    if (IdentifierStr == "unary") return tok_unary;</b>
+    return tok_identifier;
+</pre>
+</div>
+
+<p>This just adds lexer support for the unary and binary keywords, like we
+did in <a href="LangImpl5.html#iflexer">previous chapters</a>.  One nice thing
+about our current AST, is that we represent binary operators with full generalisation
+by using their ASCII code as the opcode.  For our extended operators, we'll use this
+same representation, so we don't need any new AST or parser support.</p>
+
+<p>On the other hand, we have to be able to represent the definitions of these
+new operators, in the "def binary| 5" part of the function definition.  In our
+grammar so far, the "name" for the function definition is parsed as the
+"prototype" production and into the <tt>PrototypeAST</tt> AST node.  To
+represent our new user-defined operators as prototypes, we have to extend
+the  <tt>PrototypeAST</tt> AST node like this:</p>
+
+<div class="doc_code">
+<pre>
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its argument names as well as if it is an operator.
+class PrototypeAST {
+  std::string Name;
+  std::vector&lt;std::string&gt; Args;
+  <b>bool isOperator;
+  unsigned Precedence;  // Precedence if a binary op.</b>
+public:
+  PrototypeAST(const std::string &amp;name, const std::vector&lt;std::string&gt; &amp;args,
+               <b>bool isoperator = false, unsigned prec = 0</b>)
+  : Name(name), Args(args), <b>isOperator(isoperator), Precedence(prec)</b> {}
+  
+  <b>bool isUnaryOp() const { return isOperator &amp;&amp; Args.size() == 1; }
+  bool isBinaryOp() const { return isOperator &amp;&amp; Args.size() == 2; }
+  
+  char getOperatorName() const {
+    assert(isUnaryOp() || isBinaryOp());
+    return Name[Name.size()-1];
+  }
+  
+  unsigned getBinaryPrecedence() const { return Precedence; }</b>
+  
+  Function *Codegen();
+};
+</pre>
+</div>
+
+<p>Basically, in addition to knowing a name for the prototype, we now keep track
+of whether it was an operator, and if it was, what precedence level the operator
+is at.  The precedence is only used for binary operators (as you'll see below,
+it just doesn't apply for unary operators).  Now that we have a way to represent
+the prototype for a user-defined operator, we need to parse it:</p>
+
+<div class="doc_code">
+<pre>
+/// prototype
+///   ::= id '(' id* ')'
+<b>///   ::= binary LETTER number? (id, id)</b>
+static PrototypeAST *ParsePrototype() {
+  std::string FnName;
+  
+  <b>unsigned Kind = 0;  // 0 = identifier, 1 = unary, 2 = binary.
+  unsigned BinaryPrecedence = 30;</b>
+  
+  switch (CurTok) {
+  default:
+    return ErrorP("Expected function name in prototype");
+  case tok_identifier:
+    FnName = IdentifierStr;
+    Kind = 0;
+    getNextToken();
+    break;
+  <b>case tok_binary:
+    getNextToken();
+    if (!isascii(CurTok))
+      return ErrorP("Expected binary operator");
+    FnName = "binary";
+    FnName += (char)CurTok;
+    Kind = 2;
+    getNextToken();
+    
+    // Read the precedence if present.
+    if (CurTok == tok_number) {
+      if (NumVal &lt; 1 || NumVal &gt; 100)
+        return ErrorP("Invalid precedecnce: must be 1..100");
+      BinaryPrecedence = (unsigned)NumVal;
+      getNextToken();
+    }
+    break;</b>
+  }
+  
+  if (CurTok != '(')
+    return ErrorP("Expected '(' in prototype");
+  
+  std::vector&lt;std::string&gt; ArgNames;
+  while (getNextToken() == tok_identifier)
+    ArgNames.push_back(IdentifierStr);
+  if (CurTok != ')')
+    return ErrorP("Expected ')' in prototype");
+  
+  // success.
+  getNextToken();  // eat ')'.
+  
+  <b>// Verify right number of names for operator.
+  if (Kind &amp;&amp; ArgNames.size() != Kind)
+    return ErrorP("Invalid number of operands for operator");
+  
+  return new PrototypeAST(FnName, ArgNames, Kind != 0, BinaryPrecedence);</b>
+}
+</pre>
+</div>
+
+<p>This is all fairly straightforward parsing code, and we have already seen
+a lot of similar code in the past.  One interesting part about the code above is 
+the couple lines that set up <tt>FnName</tt> for binary operators.  This builds names 
+like "binary@" for a newly defined "@" operator.  This then takes advantage of the 
+fact that symbol names in the LLVM symbol table are allowed to have any character in
+them, including embedded nul characters.</p>
+
+<p>The next interesting thing to add, is codegen support for these binary operators.
+Given our current structure, this is a simple addition of a default case for our
+existing binary operator node:</p>
+
+<div class="doc_code">
+<pre>
+Value *BinaryExprAST::Codegen() {
+  Value *L = LHS-&gt;Codegen();
+  Value *R = RHS-&gt;Codegen();
+  if (L == 0 || R == 0) return 0;
+  
+  switch (Op) {
+  case '+': return Builder.CreateAdd(L, R, "addtmp");
+  case '-': return Builder.CreateSub(L, R, "subtmp");
+  case '*': return Builder.CreateMul(L, R, "multmp");
+  case '&lt;':
+    L = Builder.CreateFCmpULT(L, R, "cmptmp");
+    // Convert bool 0/1 to double 0.0 or 1.0
+    return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
+                                "booltmp");
+  <b>default: break;</b>
+  }
+  
+  <b>// If it wasn't a builtin binary operator, it must be a user defined one. Emit
+  // a call to it.
+  Function *F = TheModule-&gt;getFunction(std::string("binary")+Op);
+  assert(F &amp;&amp; "binary operator not found!");
+  
+  Value *Ops[] = { L, R };
+  return Builder.CreateCall(F, Ops, Ops+2, "binop");</b>
+}
+
+</pre>
+</div>
+
+<p>As you can see above, the new code is actually really simple.  It just does
+a lookup for the appropriate operator in the symbol table and generates a 
+function call to it.  Since user-defined operators are just built as normal
+functions (because the "prototype" boils down to a function with the right
+name) everything falls into place.</p>
+
+<p>The final piece of code we are missing, is a bit of top-level magic:</p>
+
+<div class="doc_code">
+<pre>
+Function *FunctionAST::Codegen() {
+  NamedValues.clear();
+  
+  Function *TheFunction = Proto->Codegen();
+  if (TheFunction == 0)
+    return 0;
+  
+  <b>// If this is an operator, install it.
+  if (Proto-&gt;isBinaryOp())
+    BinopPrecedence[Proto->getOperatorName()] = Proto->getBinaryPrecedence();</b>
+  
+  // Create a new basic block to start insertion into.
+  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
+  Builder.SetInsertPoint(BB);
+  
+  if (Value *RetVal = Body-&gt;Codegen()) {
+    ...
+</pre>
+</div>
+
+<p>Basically, before codegening a function, if it is a user-defined operator, we
+register it in the precedence table.  This allows the binary operator parsing
+logic we already have in place to handle it.  Since we are working on a fully-general operator precedence parser, this is all we need to do to "extend the grammar".</p>
+
+<p>Now we have useful user-defined binary operators.  This builds a lot
+on the previous framework we built for other operators.  Adding unary operators
+is a bit more challenging, because we don't have any framework for it yet - lets
+see what it takes.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="unary">User-defined Unary Operators</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Since we don't currently support unary operators in the Kaleidoscope
+language, we'll need to add everything to support them.  Above, we added simple
+support for the 'unary' keyword to the lexer.  In addition to that, we need an
+AST node:</p>
+
+<div class="doc_code">
+<pre>
+/// UnaryExprAST - Expression class for a unary operator.
+class UnaryExprAST : public ExprAST {
+  char Opcode;
+  ExprAST *Operand;
+public:
+  UnaryExprAST(char opcode, ExprAST *operand) 
+    : Opcode(opcode), Operand(operand) {}
+  virtual Value *Codegen();
+};
+</pre>
+</div>
+
+<p>This AST node is very simple and obvious by now.  It directly mirrors the
+binary operator AST node, except that it only has one child.  With this, we
+need to add the parsing logic.  Parsing a unary operator is pretty simple: we'll
+add a new function to do it:</p>
+
+<div class="doc_code">
+<pre>
+/// unary
+///   ::= primary
+///   ::= '!' unary
+static ExprAST *ParseUnary() {
+  // If the current token is not an operator, it must be a primary expr.
+  if (!isascii(CurTok) || CurTok == '(' || CurTok == ',')
+    return ParsePrimary();
+  
+  // If this is a unary operator, read it.
+  int Opc = CurTok;
+  getNextToken();
+  if (ExprAST *Operand = ParseUnary())
+    return new UnaryExprAST(Opc, Operand);
+  return 0;
+}
+</pre>
+</div>
+
+<p>The grammar we add is pretty straightforward here.  If we see a unary
+operator when parsing a primary operator, we eat the operator as a prefix and
+parse the remaining piece as another unary operator.  This allows us to handle
+multiple unary operators (e.g. "!!x").  Note that unary operators can't have 
+ambiguous parses like binary operators can, so there is no need for precedence
+information.</p>
+
+<p>The problem with this function, is that we need to call ParseUnary from somewhere.
+To do this, we change previous callers of ParsePrimary to call ParseUnary
+instead:</p>
+
+<div class="doc_code">
+<pre>
+/// binoprhs
+///   ::= ('+' unary)*
+static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+  ...
+    <b>// Parse the unary expression after the binary operator.
+    ExprAST *RHS = ParseUnary();
+    if (!RHS) return 0;</b>
+  ...
+}
+/// expression
+///   ::= unary binoprhs
+///
+static ExprAST *ParseExpression() {
+  <b>ExprAST *LHS = ParseUnary();</b>
+  if (!LHS) return 0;
+  
+  return ParseBinOpRHS(0, LHS);
+}
+</pre>
+</div>
+
+<p>With these two simple changes, we are now able to parse unary operators and build the
+AST for them.  Next up, we need to add parser support for prototypes, to parse
+the unary operator prototype.  We extend the binary operator code above 
+with:</p>
+
+<div class="doc_code">
+<pre>
+/// prototype
+///   ::= id '(' id* ')'
+///   ::= binary LETTER number? (id, id)
+<b>///   ::= unary LETTER (id)</b>
+static PrototypeAST *ParsePrototype() {
+  std::string FnName;
+  
+  unsigned Kind = 0;  // 0 = identifier, 1 = unary, 2 = binary.
+  unsigned BinaryPrecedence = 30;
+  
+  switch (CurTok) {
+  default:
+    return ErrorP("Expected function name in prototype");
+  case tok_identifier:
+    FnName = IdentifierStr;
+    Kind = 0;
+    getNextToken();
+    break;
+  <b>case tok_unary:
+    getNextToken();
+    if (!isascii(CurTok))
+      return ErrorP("Expected unary operator");
+    FnName = "unary";
+    FnName += (char)CurTok;
+    Kind = 1;
+    getNextToken();
+    break;</b>
+  case tok_binary:
+    ...
+</pre>
+</div>
+
+<p>As with binary operators, we name unary operators with a name that includes
+the operator character.  This assists us at code generation time.  Speaking of,
+the final piece we need to add is codegen support for unary operators.  It looks
+like this:</p>
+
+<div class="doc_code">
+<pre>
+Value *UnaryExprAST::Codegen() {
+  Value *OperandV = Operand->Codegen();
+  if (OperandV == 0) return 0;
+  
+  Function *F = TheModule->getFunction(std::string("unary")+Opcode);
+  if (F == 0)
+    return ErrorV("Unknown unary operator");
+  
+  return Builder.CreateCall(F, OperandV, "unop");
+}
+</pre>
+</div>
+
+<p>This code is similar to, but simpler than, the code for binary operators.  It
+is simpler primarily because it doesn't need to handle any predefined operators.
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="example">Kicking the Tires</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>It is somewhat hard to believe, but with a few simple extensions we've
+covered in the last chapters, we have grown a real-ish language.  With this, we 
+can do a lot of interesting things, including I/O, math, and a bunch of other
+things.  For example, we can now add a nice sequencing operator (printd is
+defined to print out the specified value and a newline):</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>extern printd(x);</b>
+Read extern: declare double @printd(double)
+ready&gt; <b>def binary : 1 (x y) 0;  # Low-precedence operator that ignores operands.</b>
+..
+ready&gt; <b>printd(123) : printd(456) : printd(789);</b>
+123.000000
+456.000000
+789.000000
+Evaluated to 0.000000
+</pre>
+</div>
+
+<p>We can also define a bunch of other "primitive" operations, such as:</p>
+
+<div class="doc_code">
+<pre>
+# Logical unary not.
+def unary!(v)
+  if v then
+    0
+  else
+    1;
+    
+# Unary negate.
+def unary-(v)
+  0-v;
+
+# Define &gt; with the same precedence as &gt;.
+def binary&gt; 10 (LHS RHS)
+  RHS &lt; LHS;
+
+# Binary logical or, which does not short circuit. 
+def binary| 5 (LHS RHS)
+  if LHS then
+    1
+  else if RHS then
+    1
+  else
+    0;
+
+# Binary logical and, which does not short circuit. 
+def binary&amp; 6 (LHS RHS)
+  if !LHS then
+    0
+  else
+    !!RHS;
+
+# Define = with slightly lower precedence than relationals.
+def binary = 9 (LHS RHS)
+  !(LHS &lt; RHS | LHS &gt; RHS);
+
+</pre>
+</div>
+
+
+<p>Given the previous if/then/else support, we can also define interesting
+functions for I/O.  For example, the following prints out a character whose
+"density" reflects the value passed in: the lower the value, the denser the
+character:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt;
+<b>
+extern putchard(char)
+def printdensity(d)
+  if d &gt; 8 then
+    putchard(32)  # ' '
+  else if d &gt; 4 then
+    putchard(46)  # '.'
+  else if d &gt; 2 then
+    putchard(43)  # '+'
+  else
+    putchard(42); # '*'</b>
+...
+ready&gt; <b>printdensity(1): printdensity(2): printdensity(3) : 
+          printdensity(4): printdensity(5): printdensity(9): putchard(10);</b>
+*++.. 
+Evaluated to 0.000000
+</pre>
+</div>
+
+<p>Based on these simple primitive operations, we can start to define more
+interesting things.  For example, here's a little function that solves for the
+number of iterations it takes a function in the complex plane to
+converge:</p>
+
+<div class="doc_code">
+<pre>
+# determine whether the specific location diverges.
+# Solve for z = z^2 + c in the complex plane.
+def mandleconverger(real imag iters creal cimag)
+  if iters &gt; 255 | (real*real + imag*imag &gt; 4) then
+    iters
+  else
+    mandleconverger(real*real - imag*imag + creal,
+                    2*real*imag + cimag,
+                    iters+1, creal, cimag);
+
+# return the number of iterations required for the iteration to escape
+def mandleconverge(real imag)
+  mandleconverger(real, imag, 0, real, imag);
+</pre>
+</div>
+
+<p>This "z = z<sup>2</sup> + c" function is a beautiful little creature that is the basis
+for computation of the <a 
+href="http://en.wikipedia.org/wiki/Mandelbrot_set">Mandelbrot Set</a>.  Our
+<tt>mandelconverge</tt> function returns the number of iterations that it takes
+for a complex orbit to escape, saturating to 255.  This is not a very useful
+function by itself, but if you plot its value over a two-dimensional plane,
+you can see the Mandelbrot set.  Given that we are limited to using putchard
+here, our amazing graphical output is limited, but we can whip together
+something using the density plotter above:</p>
+
+<div class="doc_code">
+<pre>
+# compute and plot the mandlebrot set with the specified 2 dimensional range
+# info.
+def mandelhelp(xmin xmax xstep   ymin ymax ystep)
+  for y = ymin, y &lt; ymax, ystep in (
+    (for x = xmin, x &lt; xmax, xstep in
+       printdensity(mandleconverge(x,y)))
+    : putchard(10)
+  )
+ 
+# mandel - This is a convenient helper function for ploting the mandelbrot set
+# from the specified position with the specified Magnification.
+def mandel(realstart imagstart realmag imagmag) 
+  mandelhelp(realstart, realstart+realmag*78, realmag,
+             imagstart, imagstart+imagmag*40, imagmag);
+</pre>
+</div>
+
+<p>Given this, we can try plotting out the mandlebrot set!  Lets try it out:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>mandel(-2.3, -1.3, 0.05, 0.07);</b>
+*******************************+++++++++++*************************************
+*************************+++++++++++++++++++++++*******************************
+**********************+++++++++++++++++++++++++++++****************************
+*******************+++++++++++++++++++++.. ...++++++++*************************
+*****************++++++++++++++++++++++.... ...+++++++++***********************
+***************+++++++++++++++++++++++.....   ...+++++++++*********************
+**************+++++++++++++++++++++++....     ....+++++++++********************
+*************++++++++++++++++++++++......      .....++++++++*******************
+************+++++++++++++++++++++.......       .......+++++++******************
+***********+++++++++++++++++++....                ... .+++++++*****************
+**********+++++++++++++++++.......                     .+++++++****************
+*********++++++++++++++...........                    ...+++++++***************
+********++++++++++++............                      ...++++++++**************
+********++++++++++... ..........                        .++++++++**************
+*******+++++++++.....                                   .+++++++++*************
+*******++++++++......                                  ..+++++++++*************
+*******++++++.......                                   ..+++++++++*************
+*******+++++......                                     ..+++++++++*************
+*******.... ....                                      ...+++++++++*************
+*******.... .                                         ...+++++++++*************
+*******+++++......                                    ...+++++++++*************
+*******++++++.......                                   ..+++++++++*************
+*******++++++++......                                   .+++++++++*************
+*******+++++++++.....                                  ..+++++++++*************
+********++++++++++... ..........                        .++++++++**************
+********++++++++++++............                      ...++++++++**************
+*********++++++++++++++..........                     ...+++++++***************
+**********++++++++++++++++........                     .+++++++****************
+**********++++++++++++++++++++....                ... ..+++++++****************
+***********++++++++++++++++++++++.......       .......++++++++*****************
+************+++++++++++++++++++++++......      ......++++++++******************
+**************+++++++++++++++++++++++....      ....++++++++********************
+***************+++++++++++++++++++++++.....   ...+++++++++*********************
+*****************++++++++++++++++++++++....  ...++++++++***********************
+*******************+++++++++++++++++++++......++++++++*************************
+*********************++++++++++++++++++++++.++++++++***************************
+*************************+++++++++++++++++++++++*******************************
+******************************+++++++++++++************************************
+*******************************************************************************
+*******************************************************************************
+*******************************************************************************
+Evaluated to 0.000000
+ready&gt; <b>mandel(-2, -1, 0.02, 0.04);</b>
+**************************+++++++++++++++++++++++++++++++++++++++++++++++++++++
+***********************++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+*********************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++.
+*******************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++...
+*****************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++.....
+***************++++++++++++++++++++++++++++++++++++++++++++++++++++++++........
+**************++++++++++++++++++++++++++++++++++++++++++++++++++++++...........
+************+++++++++++++++++++++++++++++++++++++++++++++++++++++..............
+***********++++++++++++++++++++++++++++++++++++++++++++++++++........        . 
+**********++++++++++++++++++++++++++++++++++++++++++++++.............          
+********+++++++++++++++++++++++++++++++++++++++++++..................          
+*******+++++++++++++++++++++++++++++++++++++++.......................          
+******+++++++++++++++++++++++++++++++++++...........................           
+*****++++++++++++++++++++++++++++++++............................              
+*****++++++++++++++++++++++++++++...............................               
+****++++++++++++++++++++++++++......   .........................               
+***++++++++++++++++++++++++.........     ......    ...........                 
+***++++++++++++++++++++++............                                          
+**+++++++++++++++++++++..............                                          
+**+++++++++++++++++++................                                          
+*++++++++++++++++++.................                                           
+*++++++++++++++++............ ...                                              
+*++++++++++++++..............                                                  
+*+++....++++................                                                   
+*..........  ...........                                                       
+*                                                                              
+*..........  ...........                                                       
+*+++....++++................                                                   
+*++++++++++++++..............                                                  
+*++++++++++++++++............ ...                                              
+*++++++++++++++++++.................                                           
+**+++++++++++++++++++................                                          
+**+++++++++++++++++++++..............                                          
+***++++++++++++++++++++++............                                          
+***++++++++++++++++++++++++.........     ......    ...........                 
+****++++++++++++++++++++++++++......   .........................               
+*****++++++++++++++++++++++++++++...............................               
+*****++++++++++++++++++++++++++++++++............................              
+******+++++++++++++++++++++++++++++++++++...........................           
+*******+++++++++++++++++++++++++++++++++++++++.......................          
+********+++++++++++++++++++++++++++++++++++++++++++..................          
+Evaluated to 0.000000
+ready&gt; <b>mandel(-0.9, -1.4, 0.02, 0.03);</b>
+*******************************************************************************
+*******************************************************************************
+*******************************************************************************
+**********+++++++++++++++++++++************************************************
+*+++++++++++++++++++++++++++++++++++++++***************************************
++++++++++++++++++++++++++++++++++++++++++++++**********************************
+++++++++++++++++++++++++++++++++++++++++++++++++++*****************************
+++++++++++++++++++++++++++++++++++++++++++++++++++++++*************************
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++**********************
++++++++++++++++++++++++++++++++++.........++++++++++++++++++*******************
++++++++++++++++++++++++++++++++....   ......+++++++++++++++++++****************
++++++++++++++++++++++++++++++.......  ........+++++++++++++++++++**************
+++++++++++++++++++++++++++++........   ........++++++++++++++++++++************
++++++++++++++++++++++++++++.........     ..  ...+++++++++++++++++++++**********
+++++++++++++++++++++++++++...........        ....++++++++++++++++++++++********
+++++++++++++++++++++++++.............       .......++++++++++++++++++++++******
++++++++++++++++++++++++.............        ........+++++++++++++++++++++++****
+++++++++++++++++++++++...........           ..........++++++++++++++++++++++***
+++++++++++++++++++++...........                .........++++++++++++++++++++++*
+++++++++++++++++++............                  ...........++++++++++++++++++++
+++++++++++++++++...............                 .............++++++++++++++++++
+++++++++++++++.................                 ...............++++++++++++++++
+++++++++++++..................                  .................++++++++++++++
++++++++++..................                      .................+++++++++++++
+++++++........        .                               .........  ..++++++++++++
+++............                                         ......    ....++++++++++
+..............                                                    ...++++++++++
+..............                                                    ....+++++++++
+..............                                                    .....++++++++
+.............                                                    ......++++++++
+...........                                                     .......++++++++
+.........                                                       ........+++++++
+.........                                                       ........+++++++
+.........                                                           ....+++++++
+........                                                             ...+++++++
+.......                                                              ...+++++++
+                                                                    ....+++++++
+                                                                   .....+++++++
+                                                                    ....+++++++
+                                                                    ....+++++++
+                                                                    ....+++++++
+Evaluated to 0.000000
+ready&gt; <b>^D</b>
+</pre>
+</div>
+
+<p>At this point, you may be starting to realize that Kaleidoscope is a real
+and powerful language.  It may not be self-similar :), but it can be used to
+plot things that are!</p>
+
+<p>With this, we conclude the "adding user-defined operators" chapter of the
+tutorial.  We have successfully augmented our language, adding the ability to extend the
+language in the library, and we have shown how this can be used to build a simple but
+interesting end-user application in Kaleidoscope.  At this point, Kaleidoscope
+can build a variety of applications that are functional and can call functions
+with side-effects, but it can't actually define and mutate a variable itself.
+</p>
+
+<p>Strikingly, variable mutation is an important feature of some
+languages, and it is not at all obvious how to <a href="LangImpl7.html">add
+support for mutable variables</a> without having to add an "SSA construction"
+phase to your front-end.  In the next chapter, we will describe how you can
+add variable mutation without building SSA in your front-end.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with the
+if/then/else and for expressions..  To build this example, use:
+</p>
+
+<div class="doc_code">
+<pre>
+   # Compile
+   g++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
+   # Run
+   ./toy
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<div class="doc_code">
+<pre>
+#include "llvm/DerivedTypes.h"
+#include "llvm/ExecutionEngine/ExecutionEngine.h"
+#include "llvm/ExecutionEngine/JIT.h"
+#include "llvm/LLVMContext.h"
+#include "llvm/Module.h"
+#include "llvm/PassManager.h"
+#include "llvm/Analysis/Verifier.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Target/TargetSelect.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Support/IRBuilder.h"
+#include &lt;cstdio&gt;
+#include &lt;string&gt;
+#include &lt;map&gt;
+#include &lt;vector&gt;
+using namespace llvm;
+
+//===----------------------------------------------------------------------===//
+// Lexer
+//===----------------------------------------------------------------------===//
+
+// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
+// of these for known things.
+enum Token {
+  tok_eof = -1,
+
+  // commands
+  tok_def = -2, tok_extern = -3,
+
+  // primary
+  tok_identifier = -4, tok_number = -5,
+  
+  // control
+  tok_if = -6, tok_then = -7, tok_else = -8,
+  tok_for = -9, tok_in = -10,
+  
+  // operators
+  tok_binary = -11, tok_unary = -12
+};
+
+static std::string IdentifierStr;  // Filled in if tok_identifier
+static double NumVal;              // Filled in if tok_number
+
+/// gettok - Return the next token from standard input.
+static int gettok() {
+  static int LastChar = ' ';
+
+  // Skip any whitespace.
+  while (isspace(LastChar))
+    LastChar = getchar();
+
+  if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
+    IdentifierStr = LastChar;
+    while (isalnum((LastChar = getchar())))
+      IdentifierStr += LastChar;
+
+    if (IdentifierStr == "def") return tok_def;
+    if (IdentifierStr == "extern") return tok_extern;
+    if (IdentifierStr == "if") return tok_if;
+    if (IdentifierStr == "then") return tok_then;
+    if (IdentifierStr == "else") return tok_else;
+    if (IdentifierStr == "for") return tok_for;
+    if (IdentifierStr == "in") return tok_in;
+    if (IdentifierStr == "binary") return tok_binary;
+    if (IdentifierStr == "unary") return tok_unary;
+    return tok_identifier;
+  }
+
+  if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
+    std::string NumStr;
+    do {
+      NumStr += LastChar;
+      LastChar = getchar();
+    } while (isdigit(LastChar) || LastChar == '.');
+
+    NumVal = strtod(NumStr.c_str(), 0);
+    return tok_number;
+  }
+
+  if (LastChar == '#') {
+    // Comment until end of line.
+    do LastChar = getchar();
+    while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp;&amp; LastChar != '\r');
+    
+    if (LastChar != EOF)
+      return gettok();
+  }
+  
+  // Check for end of file.  Don't eat the EOF.
+  if (LastChar == EOF)
+    return tok_eof;
+
+  // Otherwise, just return the character as its ascii value.
+  int ThisChar = LastChar;
+  LastChar = getchar();
+  return ThisChar;
+}
+
+//===----------------------------------------------------------------------===//
+// Abstract Syntax Tree (aka Parse Tree)
+//===----------------------------------------------------------------------===//
+
+/// ExprAST - Base class for all expression nodes.
+class ExprAST {
+public:
+  virtual ~ExprAST() {}
+  virtual Value *Codegen() = 0;
+};
+
+/// NumberExprAST - Expression class for numeric literals like "1.0".
+class NumberExprAST : public ExprAST {
+  double Val;
+public:
+  NumberExprAST(double val) : Val(val) {}
+  virtual Value *Codegen();
+};
+
+/// VariableExprAST - Expression class for referencing a variable, like "a".
+class VariableExprAST : public ExprAST {
+  std::string Name;
+public:
+  VariableExprAST(const std::string &amp;name) : Name(name) {}
+  virtual Value *Codegen();
+};
+
+/// UnaryExprAST - Expression class for a unary operator.
+class UnaryExprAST : public ExprAST {
+  char Opcode;
+  ExprAST *Operand;
+public:
+  UnaryExprAST(char opcode, ExprAST *operand) 
+    : Opcode(opcode), Operand(operand) {}
+  virtual Value *Codegen();
+};
+
+/// BinaryExprAST - Expression class for a binary operator.
+class BinaryExprAST : public ExprAST {
+  char Op;
+  ExprAST *LHS, *RHS;
+public:
+  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
+    : Op(op), LHS(lhs), RHS(rhs) {}
+  virtual Value *Codegen();
+};
+
+/// CallExprAST - Expression class for function calls.
+class CallExprAST : public ExprAST {
+  std::string Callee;
+  std::vector&lt;ExprAST*&gt; Args;
+public:
+  CallExprAST(const std::string &amp;callee, std::vector&lt;ExprAST*&gt; &amp;args)
+    : Callee(callee), Args(args) {}
+  virtual Value *Codegen();
+};
+
+/// IfExprAST - Expression class for if/then/else.
+class IfExprAST : public ExprAST {
+  ExprAST *Cond, *Then, *Else;
+public:
+  IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else)
+  : Cond(cond), Then(then), Else(_else) {}
+  virtual Value *Codegen();
+};
+
+/// ForExprAST - Expression class for for/in.
+class ForExprAST : public ExprAST {
+  std::string VarName;
+  ExprAST *Start, *End, *Step, *Body;
+public:
+  ForExprAST(const std::string &amp;varname, ExprAST *start, ExprAST *end,
+             ExprAST *step, ExprAST *body)
+    : VarName(varname), Start(start), End(end), Step(step), Body(body) {}
+  virtual Value *Codegen();
+};
+
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its name, and its argument names (thus implicitly the number
+/// of arguments the function takes), as well as if it is an operator.
+class PrototypeAST {
+  std::string Name;
+  std::vector&lt;std::string&gt; Args;
+  bool isOperator;
+  unsigned Precedence;  // Precedence if a binary op.
+public:
+  PrototypeAST(const std::string &amp;name, const std::vector&lt;std::string&gt; &amp;args,
+               bool isoperator = false, unsigned prec = 0)
+  : Name(name), Args(args), isOperator(isoperator), Precedence(prec) {}
+  
+  bool isUnaryOp() const { return isOperator &amp;&amp; Args.size() == 1; }
+  bool isBinaryOp() const { return isOperator &amp;&amp; Args.size() == 2; }
+  
+  char getOperatorName() const {
+    assert(isUnaryOp() || isBinaryOp());
+    return Name[Name.size()-1];
+  }
+  
+  unsigned getBinaryPrecedence() const { return Precedence; }
+  
+  Function *Codegen();
+};
+
+/// FunctionAST - This class represents a function definition itself.
+class FunctionAST {
+  PrototypeAST *Proto;
+  ExprAST *Body;
+public:
+  FunctionAST(PrototypeAST *proto, ExprAST *body)
+    : Proto(proto), Body(body) {}
+  
+  Function *Codegen();
+};
+
+//===----------------------------------------------------------------------===//
+// Parser
+//===----------------------------------------------------------------------===//
+
+/// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
+/// token the parser is looking at.  getNextToken reads another token from the
+/// lexer and updates CurTok with its results.
+static int CurTok;
+static int getNextToken() {
+  return CurTok = gettok();
+}
+
+/// BinopPrecedence - This holds the precedence for each binary operator that is
+/// defined.
+static std::map&lt;char, int&gt; BinopPrecedence;
+
+/// GetTokPrecedence - Get the precedence of the pending binary operator token.
+static int GetTokPrecedence() {
+  if (!isascii(CurTok))
+    return -1;
+  
+  // Make sure it's a declared binop.
+  int TokPrec = BinopPrecedence[CurTok];
+  if (TokPrec &lt;= 0) return -1;
+  return TokPrec;
+}
+
+/// Error* - These are little helper functions for error handling.
+ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
+PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
+FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
+
+static ExprAST *ParseExpression();
+
+/// identifierexpr
+///   ::= identifier
+///   ::= identifier '(' expression* ')'
+static ExprAST *ParseIdentifierExpr() {
+  std::string IdName = IdentifierStr;
+  
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '(') // Simple variable ref.
+    return new VariableExprAST(IdName);
+  
+  // Call.
+  getNextToken();  // eat (
+  std::vector&lt;ExprAST*&gt; Args;
+  if (CurTok != ')') {
+    while (1) {
+      ExprAST *Arg = ParseExpression();
+      if (!Arg) return 0;
+      Args.push_back(Arg);
+
+      if (CurTok == ')') break;
+
+      if (CurTok != ',')
+        return Error("Expected ')' or ',' in argument list");
+      getNextToken();
+    }
+  }
+
+  // Eat the ')'.
+  getNextToken();
+  
+  return new CallExprAST(IdName, Args);
+}
+
+/// numberexpr ::= number
+static ExprAST *ParseNumberExpr() {
+  ExprAST *Result = new NumberExprAST(NumVal);
+  getNextToken(); // consume the number
+  return Result;
+}
+
+/// parenexpr ::= '(' expression ')'
+static ExprAST *ParseParenExpr() {
+  getNextToken();  // eat (.
+  ExprAST *V = ParseExpression();
+  if (!V) return 0;
+  
+  if (CurTok != ')')
+    return Error("expected ')'");
+  getNextToken();  // eat ).
+  return V;
+}
+
+/// ifexpr ::= 'if' expression 'then' expression 'else' expression
+static ExprAST *ParseIfExpr() {
+  getNextToken();  // eat the if.
+  
+  // condition.
+  ExprAST *Cond = ParseExpression();
+  if (!Cond) return 0;
+  
+  if (CurTok != tok_then)
+    return Error("expected then");
+  getNextToken();  // eat the then
+  
+  ExprAST *Then = ParseExpression();
+  if (Then == 0) return 0;
+  
+  if (CurTok != tok_else)
+    return Error("expected else");
+  
+  getNextToken();
+  
+  ExprAST *Else = ParseExpression();
+  if (!Else) return 0;
+  
+  return new IfExprAST(Cond, Then, Else);
+}
+
+/// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
+static ExprAST *ParseForExpr() {
+  getNextToken();  // eat the for.
+
+  if (CurTok != tok_identifier)
+    return Error("expected identifier after for");
+  
+  std::string IdName = IdentifierStr;
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '=')
+    return Error("expected '=' after for");
+  getNextToken();  // eat '='.
+  
+  
+  ExprAST *Start = ParseExpression();
+  if (Start == 0) return 0;
+  if (CurTok != ',')
+    return Error("expected ',' after for start value");
+  getNextToken();
+  
+  ExprAST *End = ParseExpression();
+  if (End == 0) return 0;
+  
+  // The step value is optional.
+  ExprAST *Step = 0;
+  if (CurTok == ',') {
+    getNextToken();
+    Step = ParseExpression();
+    if (Step == 0) return 0;
+  }
+  
+  if (CurTok != tok_in)
+    return Error("expected 'in' after for");
+  getNextToken();  // eat 'in'.
+  
+  ExprAST *Body = ParseExpression();
+  if (Body == 0) return 0;
+
+  return new ForExprAST(IdName, Start, End, Step, Body);
+}
+
+/// primary
+///   ::= identifierexpr
+///   ::= numberexpr
+///   ::= parenexpr
+///   ::= ifexpr
+///   ::= forexpr
+static ExprAST *ParsePrimary() {
+  switch (CurTok) {
+  default: return Error("unknown token when expecting an expression");
+  case tok_identifier: return ParseIdentifierExpr();
+  case tok_number:     return ParseNumberExpr();
+  case '(':            return ParseParenExpr();
+  case tok_if:         return ParseIfExpr();
+  case tok_for:        return ParseForExpr();
+  }
+}
+
+/// unary
+///   ::= primary
+///   ::= '!' unary
+static ExprAST *ParseUnary() {
+  // If the current token is not an operator, it must be a primary expr.
+  if (!isascii(CurTok) || CurTok == '(' || CurTok == ',')
+    return ParsePrimary();
+  
+  // If this is a unary operator, read it.
+  int Opc = CurTok;
+  getNextToken();
+  if (ExprAST *Operand = ParseUnary())
+    return new UnaryExprAST(Opc, Operand);
+  return 0;
+}
+
+/// binoprhs
+///   ::= ('+' unary)*
+static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+  // If this is a binop, find its precedence.
+  while (1) {
+    int TokPrec = GetTokPrecedence();
+    
+    // If this is a binop that binds at least as tightly as the current binop,
+    // consume it, otherwise we are done.
+    if (TokPrec &lt; ExprPrec)
+      return LHS;
+    
+    // Okay, we know this is a binop.
+    int BinOp = CurTok;
+    getNextToken();  // eat binop
+    
+    // Parse the unary expression after the binary operator.
+    ExprAST *RHS = ParseUnary();
+    if (!RHS) return 0;
+    
+    // If BinOp binds less tightly with RHS than the operator after RHS, let
+    // the pending operator take RHS as its LHS.
+    int NextPrec = GetTokPrecedence();
+    if (TokPrec &lt; NextPrec) {
+      RHS = ParseBinOpRHS(TokPrec+1, RHS);
+      if (RHS == 0) return 0;
+    }
+    
+    // Merge LHS/RHS.
+    LHS = new BinaryExprAST(BinOp, LHS, RHS);
+  }
+}
+
+/// expression
+///   ::= unary binoprhs
+///
+static ExprAST *ParseExpression() {
+  ExprAST *LHS = ParseUnary();
+  if (!LHS) return 0;
+  
+  return ParseBinOpRHS(0, LHS);
+}
+
+/// prototype
+///   ::= id '(' id* ')'
+///   ::= binary LETTER number? (id, id)
+///   ::= unary LETTER (id)
+static PrototypeAST *ParsePrototype() {
+  std::string FnName;
+  
+  unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary.
+  unsigned BinaryPrecedence = 30;
+  
+  switch (CurTok) {
+  default:
+    return ErrorP("Expected function name in prototype");
+  case tok_identifier:
+    FnName = IdentifierStr;
+    Kind = 0;
+    getNextToken();
+    break;
+  case tok_unary:
+    getNextToken();
+    if (!isascii(CurTok))
+      return ErrorP("Expected unary operator");
+    FnName = "unary";
+    FnName += (char)CurTok;
+    Kind = 1;
+    getNextToken();
+    break;
+  case tok_binary:
+    getNextToken();
+    if (!isascii(CurTok))
+      return ErrorP("Expected binary operator");
+    FnName = "binary";
+    FnName += (char)CurTok;
+    Kind = 2;
+    getNextToken();
+    
+    // Read the precedence if present.
+    if (CurTok == tok_number) {
+      if (NumVal &lt; 1 || NumVal &gt; 100)
+        return ErrorP("Invalid precedecnce: must be 1..100");
+      BinaryPrecedence = (unsigned)NumVal;
+      getNextToken();
+    }
+    break;
+  }
+  
+  if (CurTok != '(')
+    return ErrorP("Expected '(' in prototype");
+  
+  std::vector&lt;std::string&gt; ArgNames;
+  while (getNextToken() == tok_identifier)
+    ArgNames.push_back(IdentifierStr);
+  if (CurTok != ')')
+    return ErrorP("Expected ')' in prototype");
+  
+  // success.
+  getNextToken();  // eat ')'.
+  
+  // Verify right number of names for operator.
+  if (Kind &amp;&amp; ArgNames.size() != Kind)
+    return ErrorP("Invalid number of operands for operator");
+  
+  return new PrototypeAST(FnName, ArgNames, Kind != 0, BinaryPrecedence);
+}
+
+/// definition ::= 'def' prototype expression
+static FunctionAST *ParseDefinition() {
+  getNextToken();  // eat def.
+  PrototypeAST *Proto = ParsePrototype();
+  if (Proto == 0) return 0;
+
+  if (ExprAST *E = ParseExpression())
+    return new FunctionAST(Proto, E);
+  return 0;
+}
+
+/// toplevelexpr ::= expression
+static FunctionAST *ParseTopLevelExpr() {
+  if (ExprAST *E = ParseExpression()) {
+    // Make an anonymous proto.
+    PrototypeAST *Proto = new PrototypeAST("", std::vector&lt;std::string&gt;());
+    return new FunctionAST(Proto, E);
+  }
+  return 0;
+}
+
+/// external ::= 'extern' prototype
+static PrototypeAST *ParseExtern() {
+  getNextToken();  // eat extern.
+  return ParsePrototype();
+}
+
+//===----------------------------------------------------------------------===//
+// Code Generation
+//===----------------------------------------------------------------------===//
+
+static Module *TheModule;
+static IRBuilder&lt;&gt; Builder(getGlobalContext());
+static std::map&lt;std::string, Value*&gt; NamedValues;
+static FunctionPassManager *TheFPM;
+
+Value *ErrorV(const char *Str) { Error(Str); return 0; }
+
+Value *NumberExprAST::Codegen() {
+  return ConstantFP::get(getGlobalContext(), APFloat(Val));
+}
+
+Value *VariableExprAST::Codegen() {
+  // Look this variable up in the function.
+  Value *V = NamedValues[Name];
+  return V ? V : ErrorV("Unknown variable name");
+}
+
+Value *UnaryExprAST::Codegen() {
+  Value *OperandV = Operand-&gt;Codegen();
+  if (OperandV == 0) return 0;
+  
+  Function *F = TheModule-&gt;getFunction(std::string("unary")+Opcode);
+  if (F == 0)
+    return ErrorV("Unknown unary operator");
+  
+  return Builder.CreateCall(F, OperandV, "unop");
+}
+
+Value *BinaryExprAST::Codegen() {
+  Value *L = LHS-&gt;Codegen();
+  Value *R = RHS-&gt;Codegen();
+  if (L == 0 || R == 0) return 0;
+  
+  switch (Op) {
+  case '+': return Builder.CreateAdd(L, R, "addtmp");
+  case '-': return Builder.CreateSub(L, R, "subtmp");
+  case '*': return Builder.CreateMul(L, R, "multmp");
+  case '&lt;':
+    L = Builder.CreateFCmpULT(L, R, "cmptmp");
+    // Convert bool 0/1 to double 0.0 or 1.0
+    return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
+                                "booltmp");
+  default: break;
+  }
+  
+  // If it wasn't a builtin binary operator, it must be a user defined one. Emit
+  // a call to it.
+  Function *F = TheModule-&gt;getFunction(std::string("binary")+Op);
+  assert(F &amp;&amp; "binary operator not found!");
+  
+  Value *Ops[] = { L, R };
+  return Builder.CreateCall(F, Ops, Ops+2, "binop");
+}
+
+Value *CallExprAST::Codegen() {
+  // Look up the name in the global module table.
+  Function *CalleeF = TheModule-&gt;getFunction(Callee);
+  if (CalleeF == 0)
+    return ErrorV("Unknown function referenced");
+  
+  // If argument mismatch error.
+  if (CalleeF-&gt;arg_size() != Args.size())
+    return ErrorV("Incorrect # arguments passed");
+
+  std::vector&lt;Value*&gt; ArgsV;
+  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
+    ArgsV.push_back(Args[i]-&gt;Codegen());
+    if (ArgsV.back() == 0) return 0;
+  }
+  
+  return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp");
+}
+
+Value *IfExprAST::Codegen() {
+  Value *CondV = Cond-&gt;Codegen();
+  if (CondV == 0) return 0;
+  
+  // Convert condition to a bool by comparing equal to 0.0.
+  CondV = Builder.CreateFCmpONE(CondV, 
+                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                "ifcond");
+  
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+  
+  // Create blocks for the then and else cases.  Insert the 'then' block at the
+  // end of the function.
+  BasicBlock *ThenBB = BasicBlock::Create(getGlobalContext(), "then", TheFunction);
+  BasicBlock *ElseBB = BasicBlock::Create(getGlobalContext(), "else");
+  BasicBlock *MergeBB = BasicBlock::Create(getGlobalContext(), "ifcont");
+  
+  Builder.CreateCondBr(CondV, ThenBB, ElseBB);
+  
+  // Emit then value.
+  Builder.SetInsertPoint(ThenBB);
+  
+  Value *ThenV = Then-&gt;Codegen();
+  if (ThenV == 0) return 0;
+  
+  Builder.CreateBr(MergeBB);
+  // Codegen of 'Then' can change the current block, update ThenBB for the PHI.
+  ThenBB = Builder.GetInsertBlock();
+  
+  // Emit else block.
+  TheFunction-&gt;getBasicBlockList().push_back(ElseBB);
+  Builder.SetInsertPoint(ElseBB);
+  
+  Value *ElseV = Else-&gt;Codegen();
+  if (ElseV == 0) return 0;
+  
+  Builder.CreateBr(MergeBB);
+  // Codegen of 'Else' can change the current block, update ElseBB for the PHI.
+  ElseBB = Builder.GetInsertBlock();
+  
+  // Emit merge block.
+  TheFunction-&gt;getBasicBlockList().push_back(MergeBB);
+  Builder.SetInsertPoint(MergeBB);
+  PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()),
+                                  "iftmp");
+  
+  PN-&gt;addIncoming(ThenV, ThenBB);
+  PN-&gt;addIncoming(ElseV, ElseBB);
+  return PN;
+}
+
+Value *ForExprAST::Codegen() {
+  // Output this as:
+  //   ...
+  //   start = startexpr
+  //   goto loop
+  // loop: 
+  //   variable = phi [start, loopheader], [nextvariable, loopend]
+  //   ...
+  //   bodyexpr
+  //   ...
+  // loopend:
+  //   step = stepexpr
+  //   nextvariable = variable + step
+  //   endcond = endexpr
+  //   br endcond, loop, endloop
+  // outloop:
+  
+  // Emit the start code first, without 'variable' in scope.
+  Value *StartVal = Start-&gt;Codegen();
+  if (StartVal == 0) return 0;
+  
+  // Make the new basic block for the loop header, inserting after current
+  // block.
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+  BasicBlock *PreheaderBB = Builder.GetInsertBlock();
+  BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction);
+  
+  // Insert an explicit fall through from the current block to the LoopBB.
+  Builder.CreateBr(LoopBB);
+
+  // Start insertion in LoopBB.
+  Builder.SetInsertPoint(LoopBB);
+  
+  // Start the PHI node with an entry for Start.
+  PHINode *Variable = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), VarName.c_str());
+  Variable-&gt;addIncoming(StartVal, PreheaderBB);
+  
+  // Within the loop, the variable is defined equal to the PHI node.  If it
+  // shadows an existing variable, we have to restore it, so save it now.
+  Value *OldVal = NamedValues[VarName];
+  NamedValues[VarName] = Variable;
+  
+  // Emit the body of the loop.  This, like any other expr, can change the
+  // current BB.  Note that we ignore the value computed by the body, but don't
+  // allow an error.
+  if (Body-&gt;Codegen() == 0)
+    return 0;
+  
+  // Emit the step value.
+  Value *StepVal;
+  if (Step) {
+    StepVal = Step-&gt;Codegen();
+    if (StepVal == 0) return 0;
+  } else {
+    // If not specified, use 1.0.
+    StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0));
+  }
+  
+  Value *NextVar = Builder.CreateAdd(Variable, StepVal, "nextvar");
+
+  // Compute the end condition.
+  Value *EndCond = End-&gt;Codegen();
+  if (EndCond == 0) return EndCond;
+  
+  // Convert condition to a bool by comparing equal to 0.0.
+  EndCond = Builder.CreateFCmpONE(EndCond, 
+                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                  "loopcond");
+  
+  // Create the "after loop" block and insert it.
+  BasicBlock *LoopEndBB = Builder.GetInsertBlock();
+  BasicBlock *AfterBB = BasicBlock::Create(getGlobalContext(), "afterloop", TheFunction);
+  
+  // Insert the conditional branch into the end of LoopEndBB.
+  Builder.CreateCondBr(EndCond, LoopBB, AfterBB);
+  
+  // Any new code will be inserted in AfterBB.
+  Builder.SetInsertPoint(AfterBB);
+  
+  // Add a new entry to the PHI node for the backedge.
+  Variable-&gt;addIncoming(NextVar, LoopEndBB);
+  
+  // Restore the unshadowed variable.
+  if (OldVal)
+    NamedValues[VarName] = OldVal;
+  else
+    NamedValues.erase(VarName);
+
+  
+  // for expr always returns 0.0.
+  return Constant::getNullValue(Type::getDoubleTy(getGlobalContext()));
+}
+
+Function *PrototypeAST::Codegen() {
+  // Make the function type:  double(double,double) etc.
+  std::vector&lt;const Type*&gt; Doubles(Args.size(),
+                                   Type::getDoubleTy(getGlobalContext()));
+  FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
+                                       Doubles, false);
+  
+  Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
+  
+  // If F conflicted, there was already something named 'Name'.  If it has a
+  // body, don't allow redefinition or reextern.
+  if (F-&gt;getName() != Name) {
+    // Delete the one we just made and get the existing one.
+    F-&gt;eraseFromParent();
+    F = TheModule-&gt;getFunction(Name);
+    
+    // If F already has a body, reject this.
+    if (!F-&gt;empty()) {
+      ErrorF("redefinition of function");
+      return 0;
+    }
+    
+    // If F took a different number of args, reject.
+    if (F-&gt;arg_size() != Args.size()) {
+      ErrorF("redefinition of function with different # args");
+      return 0;
+    }
+  }
+  
+  // Set names for all arguments.
+  unsigned Idx = 0;
+  for (Function::arg_iterator AI = F-&gt;arg_begin(); Idx != Args.size();
+       ++AI, ++Idx) {
+    AI-&gt;setName(Args[Idx]);
+    
+    // Add arguments to variable symbol table.
+    NamedValues[Args[Idx]] = AI;
+  }
+  
+  return F;
+}
+
+Function *FunctionAST::Codegen() {
+  NamedValues.clear();
+  
+  Function *TheFunction = Proto-&gt;Codegen();
+  if (TheFunction == 0)
+    return 0;
+  
+  // If this is an operator, install it.
+  if (Proto-&gt;isBinaryOp())
+    BinopPrecedence[Proto-&gt;getOperatorName()] = Proto-&gt;getBinaryPrecedence();
+  
+  // Create a new basic block to start insertion into.
+  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
+  Builder.SetInsertPoint(BB);
+  
+  if (Value *RetVal = Body-&gt;Codegen()) {
+    // Finish off the function.
+    Builder.CreateRet(RetVal);
+
+    // Validate the generated code, checking for consistency.
+    verifyFunction(*TheFunction);
+
+    // Optimize the function.
+    TheFPM-&gt;run(*TheFunction);
+    
+    return TheFunction;
+  }
+  
+  // Error reading body, remove function.
+  TheFunction-&gt;eraseFromParent();
+
+  if (Proto-&gt;isBinaryOp())
+    BinopPrecedence.erase(Proto-&gt;getOperatorName());
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Top-Level parsing and JIT Driver
+//===----------------------------------------------------------------------===//
+
+static ExecutionEngine *TheExecutionEngine;
+
+static void HandleDefinition() {
+  if (FunctionAST *F = ParseDefinition()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      fprintf(stderr, "Read function definition:");
+      LF-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleExtern() {
+  if (PrototypeAST *P = ParseExtern()) {
+    if (Function *F = P-&gt;Codegen()) {
+      fprintf(stderr, "Read extern: ");
+      F-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleTopLevelExpression() {
+  // Evaluate a top-level expression into an anonymous function.
+  if (FunctionAST *F = ParseTopLevelExpr()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      // JIT the function, returning a function pointer.
+      void *FPtr = TheExecutionEngine-&gt;getPointerToFunction(LF);
+      
+      // Cast it to the right type (takes no arguments, returns a double) so we
+      // can call it as a native function.
+      double (*FP)() = (double (*)())(intptr_t)FPtr;
+      fprintf(stderr, "Evaluated to %f\n", FP());
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+/// top ::= definition | external | expression | ';'
+static void MainLoop() {
+  while (1) {
+    fprintf(stderr, "ready&gt; ");
+    switch (CurTok) {
+    case tok_eof:    return;
+    case ';':        getNextToken(); break;  // ignore top-level semicolons.
+    case tok_def:    HandleDefinition(); break;
+    case tok_extern: HandleExtern(); break;
+    default:         HandleTopLevelExpression(); break;
+    }
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// "Library" functions that can be "extern'd" from user code.
+//===----------------------------------------------------------------------===//
+
+/// putchard - putchar that takes a double and returns 0.
+extern "C" 
+double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+
+/// printd - printf that takes a double prints it as "%f\n", returning 0.
+extern "C" 
+double printd(double X) {
+  printf("%f\n", X);
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Main driver code.
+//===----------------------------------------------------------------------===//
+
+int main() {
+  InitializeNativeTarget();
+  LLVMContext &amp;Context = getGlobalContext();
+
+  // Install standard binary operators.
+  // 1 is lowest precedence.
+  BinopPrecedence['&lt;'] = 10;
+  BinopPrecedence['+'] = 20;
+  BinopPrecedence['-'] = 20;
+  BinopPrecedence['*'] = 40;  // highest.
+
+  // Prime the first token.
+  fprintf(stderr, "ready&gt; ");
+  getNextToken();
+
+  // Make the module, which holds all the code.
+  TheModule = new Module("my cool jit", Context);
+
+  // Create the JIT.  This takes ownership of the module.
+  std::string ErrStr;
+  TheExecutionEngine = EngineBuilder(TheModule).setErrorStr(&ErrStr).create();
+  if (!TheExecutionEngine) {
+    fprintf(stderr, "Could not create ExecutionEngine: %s\n", ErrStr.c_str());
+    exit(1);
+  }
+
+  FunctionPassManager OurFPM(TheModule);
+
+  // Set up the optimizer pipeline.  Start with registering info about how the
+  // target lays out data structures.
+  OurFPM.add(new TargetData(*TheExecutionEngine-&gt;getTargetData()));
+  // Do simple "peephole" optimizations and bit-twiddling optzns.
+  OurFPM.add(createInstructionCombiningPass());
+  // Reassociate expressions.
+  OurFPM.add(createReassociatePass());
+  // Eliminate Common SubExpressions.
+  OurFPM.add(createGVNPass());
+  // Simplify the control flow graph (deleting unreachable blocks, etc).
+  OurFPM.add(createCFGSimplificationPass());
+
+  OurFPM.doInitialization();
+
+  // Set the global so the code gen can use this.
+  TheFPM = &amp;OurFPM;
+
+  // Run the main "interpreter loop" now.
+  MainLoop();
+
+  TheFPM = 0;
+
+  // Print out all of the generated code.
+  TheModule-&gt;dump();
+
+  return 0;
+}
+</pre>
+</div>
+
+<a href="LangImpl7.html">Next: Extending the language: mutable variables / SSA construction</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/LangImpl7.html b/docs/tutorial/LangImpl7.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Extending the Language: Mutable Variables / SSA
+         construction</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Extending the Language: Mutable Variables</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 7
+  <ol>
+    <li><a href="#intro">Chapter 7 Introduction</a></li>
+    <li><a href="#why">Why is this a hard problem?</a></li>
+    <li><a href="#memory">Memory in LLVM</a></li>
+    <li><a href="#kalvars">Mutable Variables in Kaleidoscope</a></li>
+    <li><a href="#adjustments">Adjusting Existing Variables for
+     Mutation</a></li>
+    <li><a href="#assignment">New Assignment Operator</a></li>
+    <li><a href="#localvars">User-defined Local Variables</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="LangImpl8.html">Chapter 8</a>: Conclusion and other useful LLVM
+ tidbits</li>
+</ul>
+
+<div class="doc_author">
+  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 7 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 7 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  In chapters 1 through 6, we've built a very
+respectable, albeit simple, <a 
+href="http://en.wikipedia.org/wiki/Functional_programming">functional
+programming language</a>.  In our journey, we learned some parsing techniques,
+how to build and represent an AST, how to build LLVM IR, and how to optimize
+the resultant code as well as JIT compile it.</p>
+
+<p>While Kaleidoscope is interesting as a functional language, the fact that it
+is functional makes it "too easy" to generate LLVM IR for it.  In particular, a 
+functional language makes it very easy to build LLVM IR directly in <a 
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">SSA form</a>.
+Since LLVM requires that the input code be in SSA form, this is a very nice
+property and it is often unclear to newcomers how to generate code for an
+imperative language with mutable variables.</p>
+
+<p>The short (and happy) summary of this chapter is that there is no need for
+your front-end to build SSA form: LLVM provides highly tuned and well tested
+support for this, though the way it works is a bit unexpected for some.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="why">Why is this a hard problem?</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+To understand why mutable variables cause complexities in SSA construction, 
+consider this extremely simple C example:
+</p>
+
+<div class="doc_code">
+<pre>
+int G, H;
+int test(_Bool Condition) {
+  int X;
+  if (Condition)
+    X = G;
+  else
+    X = H;
+  return X;
+}
+</pre>
+</div>
+
+<p>In this case, we have the variable "X", whose value depends on the path 
+executed in the program.  Because there are two different possible values for X
+before the return instruction, a PHI node is inserted to merge the two values.
+The LLVM IR that we want for this example looks like this:</p>
+
+<div class="doc_code">
+<pre>
+@G = weak global i32 0   ; type of @G is i32*
+@H = weak global i32 0   ; type of @H is i32*
+
+define i32 @test(i1 %Condition) {
+entry:
+	br i1 %Condition, label %cond_true, label %cond_false
+
+cond_true:
+	%X.0 = load i32* @G
+	br label %cond_next
+
+cond_false:
+	%X.1 = load i32* @H
+	br label %cond_next
+
+cond_next:
+	%X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
+	ret i32 %X.2
+}
+</pre>
+</div>
+
+<p>In this example, the loads from the G and H global variables are explicit in
+the LLVM IR, and they live in the then/else branches of the if statement
+(cond_true/cond_false).  In order to merge the incoming values, the X.2 phi node
+in the cond_next block selects the right value to use based on where control 
+flow is coming from: if control flow comes from the cond_false block, X.2 gets
+the value of X.1.  Alternatively, if control flow comes from cond_true, it gets
+the value of X.0.  The intent of this chapter is not to explain the details of
+SSA form.  For more information, see one of the many <a 
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">online 
+references</a>.</p>
+
+<p>The question for this article is "who places the phi nodes when lowering 
+assignments to mutable variables?".  The issue here is that LLVM 
+<em>requires</em> that its IR be in SSA form: there is no "non-ssa" mode for it.
+However, SSA construction requires non-trivial algorithms and data structures,
+so it is inconvenient and wasteful for every front-end to have to reproduce this
+logic.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="memory">Memory in LLVM</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>The 'trick' here is that while LLVM does require all register values to be
+in SSA form, it does not require (or permit) memory objects to be in SSA form.
+In the example above, note that the loads from G and H are direct accesses to
+G and H: they are not renamed or versioned.  This differs from some other
+compiler systems, which do try to version memory objects.  In LLVM, instead of
+encoding dataflow analysis of memory into the LLVM IR, it is handled with <a 
+href="../WritingAnLLVMPass.html">Analysis Passes</a> which are computed on
+demand.</p>
+
+<p>
+With this in mind, the high-level idea is that we want to make a stack variable
+(which lives in memory, because it is on the stack) for each mutable object in
+a function.  To take advantage of this trick, we need to talk about how LLVM
+represents stack variables.
+</p>
+
+<p>In LLVM, all memory accesses are explicit with load/store instructions, and
+it is carefully designed not to have (or need) an "address-of" operator.  Notice
+how the type of the @G/@H global variables is actually "i32*" even though the 
+variable is defined as "i32".  What this means is that @G defines <em>space</em>
+for an i32 in the global data area, but its <em>name</em> actually refers to the
+address for that space.  Stack variables work the same way, except that instead of 
+being declared with global variable definitions, they are declared with the 
+<a href="../LangRef.html#i_alloca">LLVM alloca instruction</a>:</p>
+
+<div class="doc_code">
+<pre>
+define i32 @example() {
+entry:
+	%X = alloca i32           ; type of %X is i32*.
+	...
+	%tmp = load i32* %X       ; load the stack value %X from the stack.
+	%tmp2 = add i32 %tmp, 1   ; increment it
+	store i32 %tmp2, i32* %X  ; store it back
+	...
+</pre>
+</div>
+
+<p>This code shows an example of how you can declare and manipulate a stack
+variable in the LLVM IR.  Stack memory allocated with the alloca instruction is
+fully general: you can pass the address of the stack slot to functions, you can
+store it in other variables, etc.  In our example above, we could rewrite the
+example to use the alloca technique to avoid using a PHI node:</p>
+
+<div class="doc_code">
+<pre>
+@G = weak global i32 0   ; type of @G is i32*
+@H = weak global i32 0   ; type of @H is i32*
+
+define i32 @test(i1 %Condition) {
+entry:
+	%X = alloca i32           ; type of %X is i32*.
+	br i1 %Condition, label %cond_true, label %cond_false
+
+cond_true:
+	%X.0 = load i32* @G
+        store i32 %X.0, i32* %X   ; Update X
+	br label %cond_next
+
+cond_false:
+	%X.1 = load i32* @H
+        store i32 %X.1, i32* %X   ; Update X
+	br label %cond_next
+
+cond_next:
+	%X.2 = load i32* %X       ; Read X
+	ret i32 %X.2
+}
+</pre>
+</div>
+
+<p>With this, we have discovered a way to handle arbitrary mutable variables
+without the need to create Phi nodes at all:</p>
+
+<ol>
+<li>Each mutable variable becomes a stack allocation.</li>
+<li>Each read of the variable becomes a load from the stack.</li>
+<li>Each update of the variable becomes a store to the stack.</li>
+<li>Taking the address of a variable just uses the stack address directly.</li>
+</ol>
+
+<p>While this solution has solved our immediate problem, it introduced another
+one: we have now apparently introduced a lot of stack traffic for very simple
+and common operations, a major performance problem.  Fortunately for us, the
+LLVM optimizer has a highly-tuned optimization pass named "mem2reg" that handles
+this case, promoting allocas like this into SSA registers, inserting Phi nodes
+as appropriate.  If you run this example through the pass, for example, you'll
+get:</p>
+
+<div class="doc_code">
+<pre>
+$ <b>llvm-as &lt; example.ll | opt -mem2reg | llvm-dis</b>
+@G = weak global i32 0
+@H = weak global i32 0
+
+define i32 @test(i1 %Condition) {
+entry:
+	br i1 %Condition, label %cond_true, label %cond_false
+
+cond_true:
+	%X.0 = load i32* @G
+	br label %cond_next
+
+cond_false:
+	%X.1 = load i32* @H
+	br label %cond_next
+
+cond_next:
+	%X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
+	ret i32 %X.01
+}
+</pre>
+</div>
+
+<p>The mem2reg pass implements the standard "iterated dominance frontier"
+algorithm for constructing SSA form and has a number of optimizations that speed
+up (very common) degenerate cases. The mem2reg optimization pass is the answer to dealing 
+with mutable variables, and we highly recommend that you depend on it.  Note that
+mem2reg only works on variables in certain circumstances:</p>
+
+<ol>
+<li>mem2reg is alloca-driven: it looks for allocas and if it can handle them, it
+promotes them.  It does not apply to global variables or heap allocations.</li>
+
+<li>mem2reg only looks for alloca instructions in the entry block of the
+function.  Being in the entry block guarantees that the alloca is only executed
+once, which makes analysis simpler.</li>
+
+<li>mem2reg only promotes allocas whose uses are direct loads and stores.  If
+the address of the stack object is passed to a function, or if any funny pointer
+arithmetic is involved, the alloca will not be promoted.</li>
+
+<li>mem2reg only works on allocas of <a 
+href="../LangRef.html#t_classifications">first class</a> 
+values (such as pointers, scalars and vectors), and only if the array size
+of the allocation is 1 (or missing in the .ll file).  mem2reg is not capable of
+promoting structs or arrays to registers.  Note that the "scalarrepl" pass is
+more powerful and can promote structs, "unions", and arrays in many cases.</li>
+
+</ol>
+
+<p>
+All of these properties are easy to satisfy for most imperative languages, and
+we'll illustrate it below with Kaleidoscope.  The final question you may be
+asking is: should I bother with this nonsense for my front-end?  Wouldn't it be
+better if I just did SSA construction directly, avoiding use of the mem2reg
+optimization pass?  In short, we strongly recommend that you use this technique
+for building SSA form, unless there is an extremely good reason not to.  Using
+this technique is:</p>
+
+<ul>
+<li>Proven and well tested: llvm-gcc and clang both use this technique for local
+mutable variables.  As such, the most common clients of LLVM are using this to
+handle a bulk of their variables.  You can be sure that bugs are found fast and
+fixed early.</li>
+
+<li>Extremely Fast: mem2reg has a number of special cases that make it fast in
+common cases as well as fully general.  For example, it has fast-paths for
+variables that are only used in a single block, variables that only have one
+assignment point, good heuristics to avoid insertion of unneeded phi nodes, etc.
+</li>
+
+<li>Needed for debug info generation: <a href="../SourceLevelDebugging.html">
+Debug information in LLVM</a> relies on having the address of the variable
+exposed so that debug info can be attached to it.  This technique dovetails 
+very naturally with this style of debug info.</li>
+</ul>
+
+<p>If nothing else, this makes it much easier to get your front-end up and 
+running, and is very simple to implement.  Lets extend Kaleidoscope with mutable
+variables now!
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="kalvars">Mutable Variables in 
+Kaleidoscope</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Now that we know the sort of problem we want to tackle, lets see what this
+looks like in the context of our little Kaleidoscope language.  We're going to
+add two features:</p>
+
+<ol>
+<li>The ability to mutate variables with the '=' operator.</li>
+<li>The ability to define new variables.</li>
+</ol>
+
+<p>While the first item is really what this is about, we only have variables
+for incoming arguments as well as for induction variables, and redefining those only
+goes so far :).  Also, the ability to define new variables is a
+useful thing regardless of whether you will be mutating them.  Here's a
+motivating example that shows how we could use these:</p>
+
+<div class="doc_code">
+<pre>
+# Define ':' for sequencing: as a low-precedence operator that ignores operands
+# and just returns the RHS.
+def binary : 1 (x y) y;
+
+# Recursive fib, we could do this before.
+def fib(x)
+  if (x &lt; 3) then
+    1
+  else
+    fib(x-1)+fib(x-2);
+
+# Iterative fib.
+def fibi(x)
+  <b>var a = 1, b = 1, c in</b>
+  (for i = 3, i &lt; x in 
+     <b>c = a + b</b> :
+     <b>a = b</b> :
+     <b>b = c</b>) :
+  b;
+
+# Call it. 
+fibi(10);
+</pre>
+</div>
+
+<p>
+In order to mutate variables, we have to change our existing variables to use
+the "alloca trick".  Once we have that, we'll add our new operator, then extend
+Kaleidoscope to support new variable definitions.
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="adjustments">Adjusting Existing Variables for
+Mutation</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+The symbol table in Kaleidoscope is managed at code generation time by the 
+'<tt>NamedValues</tt>' map.  This map currently keeps track of the LLVM "Value*"
+that holds the double value for the named variable.  In order to support
+mutation, we need to change this slightly, so that it <tt>NamedValues</tt> holds
+the <em>memory location</em> of the variable in question.  Note that this 
+change is a refactoring: it changes the structure of the code, but does not
+(by itself) change the behavior of the compiler.  All of these changes are 
+isolated in the Kaleidoscope code generator.</p>
+
+<p>
+At this point in Kaleidoscope's development, it only supports variables for two
+things: incoming arguments to functions and the induction variable of 'for'
+loops.  For consistency, we'll allow mutation of these variables in addition to
+other user-defined variables.  This means that these will both need memory
+locations.
+</p>
+
+<p>To start our transformation of Kaleidoscope, we'll change the NamedValues
+map so that it maps to AllocaInst* instead of Value*.  Once we do this, the C++ 
+compiler will tell us what parts of the code we need to update:</p>
+
+<div class="doc_code">
+<pre>
+static std::map&lt;std::string, AllocaInst*&gt; NamedValues;
+</pre>
+</div>
+
+<p>Also, since we will need to create these alloca's, we'll use a helper
+function that ensures that the allocas are created in the entry block of the
+function:</p>
+
+<div class="doc_code">
+<pre>
+/// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of
+/// the function.  This is used for mutable variables etc.
+static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
+                                          const std::string &amp;VarName) {
+  IRBuilder&lt;&gt; TmpB(&amp;TheFunction-&gt;getEntryBlock(),
+                 TheFunction-&gt;getEntryBlock().begin());
+  return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0,
+                           VarName.c_str());
+}
+</pre>
+</div>
+
+<p>This funny looking code creates an IRBuilder object that is pointing at
+the first instruction (.begin()) of the entry block.  It then creates an alloca
+with the expected name and returns it.  Because all values in Kaleidoscope are
+doubles, there is no need to pass in a type to use.</p>
+
+<p>With this in place, the first functionality change we want to make is to
+variable references.  In our new scheme, variables live on the stack, so code
+generating a reference to them actually needs to produce a load from the stack
+slot:</p>
+
+<div class="doc_code">
+<pre>
+Value *VariableExprAST::Codegen() {
+  // Look this variable up in the function.
+  Value *V = NamedValues[Name];
+  if (V == 0) return ErrorV("Unknown variable name");
+
+  <b>// Load the value.
+  return Builder.CreateLoad(V, Name.c_str());</b>
+}
+</pre>
+</div>
+
+<p>As you can see, this is pretty straightforward.  Now we need to update the
+things that define the variables to set up the alloca.  We'll start with 
+<tt>ForExprAST::Codegen</tt> (see the <a href="#code">full code listing</a> for
+the unabridged code):</p>
+
+<div class="doc_code">
+<pre>
+  Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+  <b>// Create an alloca for the variable in the entry block.
+  AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);</b>
+  
+    // Emit the start code first, without 'variable' in scope.
+  Value *StartVal = Start-&gt;Codegen();
+  if (StartVal == 0) return 0;
+  
+  <b>// Store the value into the alloca.
+  Builder.CreateStore(StartVal, Alloca);</b>
+  ...
+
+  // Compute the end condition.
+  Value *EndCond = End-&gt;Codegen();
+  if (EndCond == 0) return EndCond;
+  
+  <b>// Reload, increment, and restore the alloca.  This handles the case where
+  // the body of the loop mutates the variable.
+  Value *CurVar = Builder.CreateLoad(Alloca);
+  Value *NextVar = Builder.CreateAdd(CurVar, StepVal, "nextvar");
+  Builder.CreateStore(NextVar, Alloca);</b>
+  ...
+</pre>
+</div>
+
+<p>This code is virtually identical to the code <a 
+href="LangImpl5.html#forcodegen">before we allowed mutable variables</a>.  The
+big difference is that we no longer have to construct a PHI node, and we use
+load/store to access the variable as needed.</p>
+
+<p>To support mutable argument variables, we need to also make allocas for them.
+The code for this is also pretty simple:</p>
+
+<div class="doc_code">
+<pre>
+/// CreateArgumentAllocas - Create an alloca for each argument and register the
+/// argument in the symbol table so that references to it will succeed.
+void PrototypeAST::CreateArgumentAllocas(Function *F) {
+  Function::arg_iterator AI = F-&gt;arg_begin();
+  for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) {
+    // Create an alloca for this variable.
+    AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]);
+
+    // Store the initial value into the alloca.
+    Builder.CreateStore(AI, Alloca);
+
+    // Add arguments to variable symbol table.
+    NamedValues[Args[Idx]] = Alloca;
+  }
+}
+</pre>
+</div>
+
+<p>For each argument, we make an alloca, store the input value to the function
+into the alloca, and register the alloca as the memory location for the
+argument.  This method gets invoked by <tt>FunctionAST::Codegen</tt> right after
+it sets up the entry block for the function.</p>
+
+<p>The final missing piece is adding the mem2reg pass, which allows us to get
+good codegen once again:</p>
+
+<div class="doc_code">
+<pre>
+    // Set up the optimizer pipeline.  Start with registering info about how the
+    // target lays out data structures.
+    OurFPM.add(new TargetData(*TheExecutionEngine-&gt;getTargetData()));
+    <b>// Promote allocas to registers.
+    OurFPM.add(createPromoteMemoryToRegisterPass());</b>
+    // Do simple "peephole" optimizations and bit-twiddling optzns.
+    OurFPM.add(createInstructionCombiningPass());
+    // Reassociate expressions.
+    OurFPM.add(createReassociatePass());
+</pre>
+</div>
+
+<p>It is interesting to see what the code looks like before and after the
+mem2reg optimization runs.  For example, this is the before/after code for our
+recursive fib function.  Before the optimization:</p>
+
+<div class="doc_code">
+<pre>
+define double @fib(double %x) {
+entry:
+	<b>%x1 = alloca double
+	store double %x, double* %x1
+	%x2 = load double* %x1</b>
+	%cmptmp = fcmp ult double %x2, 3.000000e+00
+	%booltmp = uitofp i1 %cmptmp to double
+	%ifcond = fcmp one double %booltmp, 0.000000e+00
+	br i1 %ifcond, label %then, label %else
+
+then:		; preds = %entry
+	br label %ifcont
+
+else:		; preds = %entry
+	<b>%x3 = load double* %x1</b>
+	%subtmp = fsub double %x3, 1.000000e+00
+	%calltmp = call double @fib( double %subtmp )
+	<b>%x4 = load double* %x1</b>
+	%subtmp5 = fsub double %x4, 2.000000e+00
+	%calltmp6 = call double @fib( double %subtmp5 )
+	%addtmp = fadd double %calltmp, %calltmp6
+	br label %ifcont
+
+ifcont:		; preds = %else, %then
+	%iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
+	ret double %iftmp
+}
+</pre>
+</div>
+
+<p>Here there is only one variable (x, the input argument) but you can still
+see the extremely simple-minded code generation strategy we are using.  In the
+entry block, an alloca is created, and the initial input value is stored into
+it.  Each reference to the variable does a reload from the stack.  Also, note
+that we didn't modify the if/then/else expression, so it still inserts a PHI
+node.  While we could make an alloca for it, it is actually easier to create a 
+PHI node for it, so we still just make the PHI.</p>
+
+<p>Here is the code after the mem2reg pass runs:</p>
+
+<div class="doc_code">
+<pre>
+define double @fib(double %x) {
+entry:
+	%cmptmp = fcmp ult double <b>%x</b>, 3.000000e+00
+	%booltmp = uitofp i1 %cmptmp to double
+	%ifcond = fcmp one double %booltmp, 0.000000e+00
+	br i1 %ifcond, label %then, label %else
+
+then:
+	br label %ifcont
+
+else:
+	%subtmp = fsub double <b>%x</b>, 1.000000e+00
+	%calltmp = call double @fib( double %subtmp )
+	%subtmp5 = fsub double <b>%x</b>, 2.000000e+00
+	%calltmp6 = call double @fib( double %subtmp5 )
+	%addtmp = fadd double %calltmp, %calltmp6
+	br label %ifcont
+
+ifcont:		; preds = %else, %then
+	%iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
+	ret double %iftmp
+}
+</pre>
+</div>
+
+<p>This is a trivial case for mem2reg, since there are no redefinitions of the
+variable.  The point of showing this is to calm your tension about inserting
+such blatent inefficiencies :).</p>
+
+<p>After the rest of the optimizers run, we get:</p>
+
+<div class="doc_code">
+<pre>
+define double @fib(double %x) {
+entry:
+	%cmptmp = fcmp ult double %x, 3.000000e+00
+	%booltmp = uitofp i1 %cmptmp to double
+	%ifcond = fcmp ueq double %booltmp, 0.000000e+00
+	br i1 %ifcond, label %else, label %ifcont
+
+else:
+	%subtmp = fsub double %x, 1.000000e+00
+	%calltmp = call double @fib( double %subtmp )
+	%subtmp5 = fsub double %x, 2.000000e+00
+	%calltmp6 = call double @fib( double %subtmp5 )
+	%addtmp = fadd double %calltmp, %calltmp6
+	ret double %addtmp
+
+ifcont:
+	ret double 1.000000e+00
+}
+</pre>
+</div>
+
+<p>Here we see that the simplifycfg pass decided to clone the return instruction
+into the end of the 'else' block.  This allowed it to eliminate some branches
+and the PHI node.</p>
+
+<p>Now that all symbol table references are updated to use stack variables, 
+we'll add the assignment operator.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="assignment">New Assignment Operator</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>With our current framework, adding a new assignment operator is really
+simple.  We will parse it just like any other binary operator, but handle it
+internally (instead of allowing the user to define it).  The first step is to
+set a precedence:</p>
+
+<div class="doc_code">
+<pre>
+ int main() {
+   // Install standard binary operators.
+   // 1 is lowest precedence.
+   <b>BinopPrecedence['='] = 2;</b>
+   BinopPrecedence['&lt;'] = 10;
+   BinopPrecedence['+'] = 20;
+   BinopPrecedence['-'] = 20;
+</pre>
+</div>
+
+<p>Now that the parser knows the precedence of the binary operator, it takes
+care of all the parsing and AST generation.  We just need to implement codegen
+for the assignment operator.  This looks like:</p> 
+
+<div class="doc_code">
+<pre>
+Value *BinaryExprAST::Codegen() {
+  // Special case '=' because we don't want to emit the LHS as an expression.
+  if (Op == '=') {
+    // Assignment requires the LHS to be an identifier.
+    VariableExprAST *LHSE = dynamic_cast&lt;VariableExprAST*&gt;(LHS);
+    if (!LHSE)
+      return ErrorV("destination of '=' must be a variable");
+</pre>
+</div>
+
+<p>Unlike the rest of the binary operators, our assignment operator doesn't
+follow the "emit LHS, emit RHS, do computation" model.  As such, it is handled
+as a special case before the other binary operators are handled.  The other 
+strange thing is that it requires the LHS to be a variable.  It is invalid to
+have "(x+1) = expr" - only things like "x = expr" are allowed.
+</p>
+
+<div class="doc_code">
+<pre>
+    // Codegen the RHS.
+    Value *Val = RHS-&gt;Codegen();
+    if (Val == 0) return 0;
+
+    // Look up the name.
+    Value *Variable = NamedValues[LHSE-&gt;getName()];
+    if (Variable == 0) return ErrorV("Unknown variable name");
+
+    Builder.CreateStore(Val, Variable);
+    return Val;
+  }
+  ...  
+</pre>
+</div>
+
+<p>Once we have the variable, codegen'ing the assignment is straightforward:
+we emit the RHS of the assignment, create a store, and return the computed
+value.  Returning a value allows for chained assignments like "X = (Y = Z)".</p>
+
+<p>Now that we have an assignment operator, we can mutate loop variables and
+arguments.  For example, we can now run code like this:</p>
+
+<div class="doc_code">
+<pre>
+# Function to print a double.
+extern printd(x);
+
+# Define ':' for sequencing: as a low-precedence operator that ignores operands
+# and just returns the RHS.
+def binary : 1 (x y) y;
+
+def test(x)
+  printd(x) :
+  x = 4 :
+  printd(x);
+
+test(123);
+</pre>
+</div>
+
+<p>When run, this example prints "123" and then "4", showing that we did
+actually mutate the value!  Okay, we have now officially implemented our goal:
+getting this to work requires SSA construction in the general case.  However,
+to be really useful, we want the ability to define our own local variables, lets
+add this next! 
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="localvars">User-defined Local 
+Variables</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Adding var/in is just like any other other extensions we made to 
+Kaleidoscope: we extend the lexer, the parser, the AST and the code generator.
+The first step for adding our new 'var/in' construct is to extend the lexer.
+As before, this is pretty trivial, the code looks like this:</p>
+
+<div class="doc_code">
+<pre>
+enum Token {
+  ...
+  <b>// var definition
+  tok_var = -13</b>
+...
+}
+...
+static int gettok() {
+...
+    if (IdentifierStr == "in") return tok_in;
+    if (IdentifierStr == "binary") return tok_binary;
+    if (IdentifierStr == "unary") return tok_unary;
+    <b>if (IdentifierStr == "var") return tok_var;</b>
+    return tok_identifier;
+...
+</pre>
+</div>
+
+<p>The next step is to define the AST node that we will construct.  For var/in,
+it looks like this:</p>
+
+<div class="doc_code">
+<pre>
+/// VarExprAST - Expression class for var/in
+class VarExprAST : public ExprAST {
+  std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; VarNames;
+  ExprAST *Body;
+public:
+  VarExprAST(const std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; &amp;varnames,
+             ExprAST *body)
+  : VarNames(varnames), Body(body) {}
+  
+  virtual Value *Codegen();
+};
+</pre>
+</div>
+
+<p>var/in allows a list of names to be defined all at once, and each name can
+optionally have an initializer value.  As such, we capture this information in
+the VarNames vector.  Also, var/in has a body, this body is allowed to access
+the variables defined by the var/in.</p>
+
+<p>With this in place, we can define the parser pieces.  The first thing we do is add
+it as a primary expression:</p>
+
+<div class="doc_code">
+<pre>
+/// primary
+///   ::= identifierexpr
+///   ::= numberexpr
+///   ::= parenexpr
+///   ::= ifexpr
+///   ::= forexpr
+<b>///   ::= varexpr</b>
+static ExprAST *ParsePrimary() {
+  switch (CurTok) {
+  default: return Error("unknown token when expecting an expression");
+  case tok_identifier: return ParseIdentifierExpr();
+  case tok_number:     return ParseNumberExpr();
+  case '(':            return ParseParenExpr();
+  case tok_if:         return ParseIfExpr();
+  case tok_for:        return ParseForExpr();
+  <b>case tok_var:        return ParseVarExpr();</b>
+  }
+}
+</pre>
+</div>
+
+<p>Next we define ParseVarExpr:</p>
+
+<div class="doc_code">
+<pre>
+/// varexpr ::= 'var' identifier ('=' expression)? 
+//                    (',' identifier ('=' expression)?)* 'in' expression
+static ExprAST *ParseVarExpr() {
+  getNextToken();  // eat the var.
+
+  std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; VarNames;
+
+  // At least one variable name is required.
+  if (CurTok != tok_identifier)
+    return Error("expected identifier after var");
+</pre>
+</div>
+
+<p>The first part of this code parses the list of identifier/expr pairs into the
+local <tt>VarNames</tt> vector.  
+
+<div class="doc_code">
+<pre>
+  while (1) {
+    std::string Name = IdentifierStr;
+    getNextToken();  // eat identifier.
+
+    // Read the optional initializer.
+    ExprAST *Init = 0;
+    if (CurTok == '=') {
+      getNextToken(); // eat the '='.
+      
+      Init = ParseExpression();
+      if (Init == 0) return 0;
+    }
+    
+    VarNames.push_back(std::make_pair(Name, Init));
+    
+    // End of var list, exit loop.
+    if (CurTok != ',') break;
+    getNextToken(); // eat the ','.
+    
+    if (CurTok != tok_identifier)
+      return Error("expected identifier list after var");
+  }
+</pre>
+</div>
+
+<p>Once all the variables are parsed, we then parse the body and create the
+AST node:</p>
+
+<div class="doc_code">
+<pre>
+  // At this point, we have to have 'in'.
+  if (CurTok != tok_in)
+    return Error("expected 'in' keyword after 'var'");
+  getNextToken();  // eat 'in'.
+  
+  ExprAST *Body = ParseExpression();
+  if (Body == 0) return 0;
+  
+  return new VarExprAST(VarNames, Body);
+}
+</pre>
+</div>
+
+<p>Now that we can parse and represent the code, we need to support emission of
+LLVM IR for it.  This code starts out with:</p>
+
+<div class="doc_code">
+<pre>
+Value *VarExprAST::Codegen() {
+  std::vector&lt;AllocaInst *&gt; OldBindings;
+  
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+
+  // Register all variables and emit their initializer.
+  for (unsigned i = 0, e = VarNames.size(); i != e; ++i) {
+    const std::string &amp;VarName = VarNames[i].first;
+    ExprAST *Init = VarNames[i].second;
+</pre>
+</div>
+
+<p>Basically it loops over all the variables, installing them one at a time.
+For each variable we put into the symbol table, we remember the previous value
+that we replace in OldBindings.</p>
+
+<div class="doc_code">
+<pre>
+    // Emit the initializer before adding the variable to scope, this prevents
+    // the initializer from referencing the variable itself, and permits stuff
+    // like this:
+    //  var a = 1 in
+    //    var a = a in ...   # refers to outer 'a'.
+    Value *InitVal;
+    if (Init) {
+      InitVal = Init-&gt;Codegen();
+      if (InitVal == 0) return 0;
+    } else { // If not specified, use 0.0.
+      InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0));
+    }
+    
+    AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+    Builder.CreateStore(InitVal, Alloca);
+
+    // Remember the old variable binding so that we can restore the binding when
+    // we unrecurse.
+    OldBindings.push_back(NamedValues[VarName]);
+    
+    // Remember this binding.
+    NamedValues[VarName] = Alloca;
+  }
+</pre>
+</div>
+
+<p>There are more comments here than code.  The basic idea is that we emit the
+initializer, create the alloca, then update the symbol table to point to it.
+Once all the variables are installed in the symbol table, we evaluate the body
+of the var/in expression:</p>
+
+<div class="doc_code">
+<pre>
+  // Codegen the body, now that all vars are in scope.
+  Value *BodyVal = Body-&gt;Codegen();
+  if (BodyVal == 0) return 0;
+</pre>
+</div>
+
+<p>Finally, before returning, we restore the previous variable bindings:</p>
+
+<div class="doc_code">
+<pre>
+  // Pop all our variables from scope.
+  for (unsigned i = 0, e = VarNames.size(); i != e; ++i)
+    NamedValues[VarNames[i].first] = OldBindings[i];
+
+  // Return the body computation.
+  return BodyVal;
+}
+</pre>
+</div>
+
+<p>The end result of all of this is that we get properly scoped variable 
+definitions, and we even (trivially) allow mutation of them :).</p>
+
+<p>With this, we completed what we set out to do.  Our nice iterative fib
+example from the intro compiles and runs just fine.  The mem2reg pass optimizes
+all of our stack variables into SSA registers, inserting PHI nodes where needed,
+and our front-end remains simple: no "iterated dominance frontier" computation
+anywhere in sight.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with mutable
+variables and var/in support.  To build this example, use:
+</p>
+
+<div class="doc_code">
+<pre>
+   # Compile
+   g++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
+   # Run
+   ./toy
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<div class="doc_code">
+<pre>
+#include "llvm/DerivedTypes.h"
+#include "llvm/ExecutionEngine/ExecutionEngine.h"
+#include "llvm/ExecutionEngine/JIT.h"
+#include "llvm/LLVMContext.h"
+#include "llvm/Module.h"
+#include "llvm/PassManager.h"
+#include "llvm/Analysis/Verifier.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Target/TargetSelect.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Support/IRBuilder.h"
+#include &lt;cstdio&gt;
+#include &lt;string&gt;
+#include &lt;map&gt;
+#include &lt;vector&gt;
+using namespace llvm;
+
+//===----------------------------------------------------------------------===//
+// Lexer
+//===----------------------------------------------------------------------===//
+
+// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
+// of these for known things.
+enum Token {
+  tok_eof = -1,
+
+  // commands
+  tok_def = -2, tok_extern = -3,
+
+  // primary
+  tok_identifier = -4, tok_number = -5,
+  
+  // control
+  tok_if = -6, tok_then = -7, tok_else = -8,
+  tok_for = -9, tok_in = -10,
+  
+  // operators
+  tok_binary = -11, tok_unary = -12,
+  
+  // var definition
+  tok_var = -13
+};
+
+static std::string IdentifierStr;  // Filled in if tok_identifier
+static double NumVal;              // Filled in if tok_number
+
+/// gettok - Return the next token from standard input.
+static int gettok() {
+  static int LastChar = ' ';
+
+  // Skip any whitespace.
+  while (isspace(LastChar))
+    LastChar = getchar();
+
+  if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
+    IdentifierStr = LastChar;
+    while (isalnum((LastChar = getchar())))
+      IdentifierStr += LastChar;
+
+    if (IdentifierStr == "def") return tok_def;
+    if (IdentifierStr == "extern") return tok_extern;
+    if (IdentifierStr == "if") return tok_if;
+    if (IdentifierStr == "then") return tok_then;
+    if (IdentifierStr == "else") return tok_else;
+    if (IdentifierStr == "for") return tok_for;
+    if (IdentifierStr == "in") return tok_in;
+    if (IdentifierStr == "binary") return tok_binary;
+    if (IdentifierStr == "unary") return tok_unary;
+    if (IdentifierStr == "var") return tok_var;
+    return tok_identifier;
+  }
+
+  if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
+    std::string NumStr;
+    do {
+      NumStr += LastChar;
+      LastChar = getchar();
+    } while (isdigit(LastChar) || LastChar == '.');
+
+    NumVal = strtod(NumStr.c_str(), 0);
+    return tok_number;
+  }
+
+  if (LastChar == '#') {
+    // Comment until end of line.
+    do LastChar = getchar();
+    while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp;&amp; LastChar != '\r');
+    
+    if (LastChar != EOF)
+      return gettok();
+  }
+  
+  // Check for end of file.  Don't eat the EOF.
+  if (LastChar == EOF)
+    return tok_eof;
+
+  // Otherwise, just return the character as its ascii value.
+  int ThisChar = LastChar;
+  LastChar = getchar();
+  return ThisChar;
+}
+
+//===----------------------------------------------------------------------===//
+// Abstract Syntax Tree (aka Parse Tree)
+//===----------------------------------------------------------------------===//
+
+/// ExprAST - Base class for all expression nodes.
+class ExprAST {
+public:
+  virtual ~ExprAST() {}
+  virtual Value *Codegen() = 0;
+};
+
+/// NumberExprAST - Expression class for numeric literals like "1.0".
+class NumberExprAST : public ExprAST {
+  double Val;
+public:
+  NumberExprAST(double val) : Val(val) {}
+  virtual Value *Codegen();
+};
+
+/// VariableExprAST - Expression class for referencing a variable, like "a".
+class VariableExprAST : public ExprAST {
+  std::string Name;
+public:
+  VariableExprAST(const std::string &amp;name) : Name(name) {}
+  const std::string &amp;getName() const { return Name; }
+  virtual Value *Codegen();
+};
+
+/// UnaryExprAST - Expression class for a unary operator.
+class UnaryExprAST : public ExprAST {
+  char Opcode;
+  ExprAST *Operand;
+public:
+  UnaryExprAST(char opcode, ExprAST *operand) 
+    : Opcode(opcode), Operand(operand) {}
+  virtual Value *Codegen();
+};
+
+/// BinaryExprAST - Expression class for a binary operator.
+class BinaryExprAST : public ExprAST {
+  char Op;
+  ExprAST *LHS, *RHS;
+public:
+  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
+    : Op(op), LHS(lhs), RHS(rhs) {}
+  virtual Value *Codegen();
+};
+
+/// CallExprAST - Expression class for function calls.
+class CallExprAST : public ExprAST {
+  std::string Callee;
+  std::vector&lt;ExprAST*&gt; Args;
+public:
+  CallExprAST(const std::string &amp;callee, std::vector&lt;ExprAST*&gt; &amp;args)
+    : Callee(callee), Args(args) {}
+  virtual Value *Codegen();
+};
+
+/// IfExprAST - Expression class for if/then/else.
+class IfExprAST : public ExprAST {
+  ExprAST *Cond, *Then, *Else;
+public:
+  IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else)
+  : Cond(cond), Then(then), Else(_else) {}
+  virtual Value *Codegen();
+};
+
+/// ForExprAST - Expression class for for/in.
+class ForExprAST : public ExprAST {
+  std::string VarName;
+  ExprAST *Start, *End, *Step, *Body;
+public:
+  ForExprAST(const std::string &amp;varname, ExprAST *start, ExprAST *end,
+             ExprAST *step, ExprAST *body)
+    : VarName(varname), Start(start), End(end), Step(step), Body(body) {}
+  virtual Value *Codegen();
+};
+
+/// VarExprAST - Expression class for var/in
+class VarExprAST : public ExprAST {
+  std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; VarNames;
+  ExprAST *Body;
+public:
+  VarExprAST(const std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; &amp;varnames,
+             ExprAST *body)
+  : VarNames(varnames), Body(body) {}
+  
+  virtual Value *Codegen();
+};
+
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its name, and its argument names (thus implicitly the number
+/// of arguments the function takes), as well as if it is an operator.
+class PrototypeAST {
+  std::string Name;
+  std::vector&lt;std::string&gt; Args;
+  bool isOperator;
+  unsigned Precedence;  // Precedence if a binary op.
+public:
+  PrototypeAST(const std::string &amp;name, const std::vector&lt;std::string&gt; &amp;args,
+               bool isoperator = false, unsigned prec = 0)
+  : Name(name), Args(args), isOperator(isoperator), Precedence(prec) {}
+  
+  bool isUnaryOp() const { return isOperator &amp;&amp; Args.size() == 1; }
+  bool isBinaryOp() const { return isOperator &amp;&amp; Args.size() == 2; }
+  
+  char getOperatorName() const {
+    assert(isUnaryOp() || isBinaryOp());
+    return Name[Name.size()-1];
+  }
+  
+  unsigned getBinaryPrecedence() const { return Precedence; }
+  
+  Function *Codegen();
+  
+  void CreateArgumentAllocas(Function *F);
+};
+
+/// FunctionAST - This class represents a function definition itself.
+class FunctionAST {
+  PrototypeAST *Proto;
+  ExprAST *Body;
+public:
+  FunctionAST(PrototypeAST *proto, ExprAST *body)
+    : Proto(proto), Body(body) {}
+  
+  Function *Codegen();
+};
+
+//===----------------------------------------------------------------------===//
+// Parser
+//===----------------------------------------------------------------------===//
+
+/// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
+/// token the parser is looking at.  getNextToken reads another token from the
+/// lexer and updates CurTok with its results.
+static int CurTok;
+static int getNextToken() {
+  return CurTok = gettok();
+}
+
+/// BinopPrecedence - This holds the precedence for each binary operator that is
+/// defined.
+static std::map&lt;char, int&gt; BinopPrecedence;
+
+/// GetTokPrecedence - Get the precedence of the pending binary operator token.
+static int GetTokPrecedence() {
+  if (!isascii(CurTok))
+    return -1;
+  
+  // Make sure it's a declared binop.
+  int TokPrec = BinopPrecedence[CurTok];
+  if (TokPrec &lt;= 0) return -1;
+  return TokPrec;
+}
+
+/// Error* - These are little helper functions for error handling.
+ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
+PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
+FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
+
+static ExprAST *ParseExpression();
+
+/// identifierexpr
+///   ::= identifier
+///   ::= identifier '(' expression* ')'
+static ExprAST *ParseIdentifierExpr() {
+  std::string IdName = IdentifierStr;
+  
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '(') // Simple variable ref.
+    return new VariableExprAST(IdName);
+  
+  // Call.
+  getNextToken();  // eat (
+  std::vector&lt;ExprAST*&gt; Args;
+  if (CurTok != ')') {
+    while (1) {
+      ExprAST *Arg = ParseExpression();
+      if (!Arg) return 0;
+      Args.push_back(Arg);
+
+      if (CurTok == ')') break;
+
+      if (CurTok != ',')
+        return Error("Expected ')' or ',' in argument list");
+      getNextToken();
+    }
+  }
+
+  // Eat the ')'.
+  getNextToken();
+  
+  return new CallExprAST(IdName, Args);
+}
+
+/// numberexpr ::= number
+static ExprAST *ParseNumberExpr() {
+  ExprAST *Result = new NumberExprAST(NumVal);
+  getNextToken(); // consume the number
+  return Result;
+}
+
+/// parenexpr ::= '(' expression ')'
+static ExprAST *ParseParenExpr() {
+  getNextToken();  // eat (.
+  ExprAST *V = ParseExpression();
+  if (!V) return 0;
+  
+  if (CurTok != ')')
+    return Error("expected ')'");
+  getNextToken();  // eat ).
+  return V;
+}
+
+/// ifexpr ::= 'if' expression 'then' expression 'else' expression
+static ExprAST *ParseIfExpr() {
+  getNextToken();  // eat the if.
+  
+  // condition.
+  ExprAST *Cond = ParseExpression();
+  if (!Cond) return 0;
+  
+  if (CurTok != tok_then)
+    return Error("expected then");
+  getNextToken();  // eat the then
+  
+  ExprAST *Then = ParseExpression();
+  if (Then == 0) return 0;
+  
+  if (CurTok != tok_else)
+    return Error("expected else");
+  
+  getNextToken();
+  
+  ExprAST *Else = ParseExpression();
+  if (!Else) return 0;
+  
+  return new IfExprAST(Cond, Then, Else);
+}
+
+/// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
+static ExprAST *ParseForExpr() {
+  getNextToken();  // eat the for.
+
+  if (CurTok != tok_identifier)
+    return Error("expected identifier after for");
+  
+  std::string IdName = IdentifierStr;
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '=')
+    return Error("expected '=' after for");
+  getNextToken();  // eat '='.
+  
+  
+  ExprAST *Start = ParseExpression();
+  if (Start == 0) return 0;
+  if (CurTok != ',')
+    return Error("expected ',' after for start value");
+  getNextToken();
+  
+  ExprAST *End = ParseExpression();
+  if (End == 0) return 0;
+  
+  // The step value is optional.
+  ExprAST *Step = 0;
+  if (CurTok == ',') {
+    getNextToken();
+    Step = ParseExpression();
+    if (Step == 0) return 0;
+  }
+  
+  if (CurTok != tok_in)
+    return Error("expected 'in' after for");
+  getNextToken();  // eat 'in'.
+  
+  ExprAST *Body = ParseExpression();
+  if (Body == 0) return 0;
+
+  return new ForExprAST(IdName, Start, End, Step, Body);
+}
+
+/// varexpr ::= 'var' identifier ('=' expression)? 
+//                    (',' identifier ('=' expression)?)* 'in' expression
+static ExprAST *ParseVarExpr() {
+  getNextToken();  // eat the var.
+
+  std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; VarNames;
+
+  // At least one variable name is required.
+  if (CurTok != tok_identifier)
+    return Error("expected identifier after var");
+  
+  while (1) {
+    std::string Name = IdentifierStr;
+    getNextToken();  // eat identifier.
+
+    // Read the optional initializer.
+    ExprAST *Init = 0;
+    if (CurTok == '=') {
+      getNextToken(); // eat the '='.
+      
+      Init = ParseExpression();
+      if (Init == 0) return 0;
+    }
+    
+    VarNames.push_back(std::make_pair(Name, Init));
+    
+    // End of var list, exit loop.
+    if (CurTok != ',') break;
+    getNextToken(); // eat the ','.
+    
+    if (CurTok != tok_identifier)
+      return Error("expected identifier list after var");
+  }
+  
+  // At this point, we have to have 'in'.
+  if (CurTok != tok_in)
+    return Error("expected 'in' keyword after 'var'");
+  getNextToken();  // eat 'in'.
+  
+  ExprAST *Body = ParseExpression();
+  if (Body == 0) return 0;
+  
+  return new VarExprAST(VarNames, Body);
+}
+
+/// primary
+///   ::= identifierexpr
+///   ::= numberexpr
+///   ::= parenexpr
+///   ::= ifexpr
+///   ::= forexpr
+///   ::= varexpr
+static ExprAST *ParsePrimary() {
+  switch (CurTok) {
+  default: return Error("unknown token when expecting an expression");
+  case tok_identifier: return ParseIdentifierExpr();
+  case tok_number:     return ParseNumberExpr();
+  case '(':            return ParseParenExpr();
+  case tok_if:         return ParseIfExpr();
+  case tok_for:        return ParseForExpr();
+  case tok_var:        return ParseVarExpr();
+  }
+}
+
+/// unary
+///   ::= primary
+///   ::= '!' unary
+static ExprAST *ParseUnary() {
+  // If the current token is not an operator, it must be a primary expr.
+  if (!isascii(CurTok) || CurTok == '(' || CurTok == ',')
+    return ParsePrimary();
+  
+  // If this is a unary operator, read it.
+  int Opc = CurTok;
+  getNextToken();
+  if (ExprAST *Operand = ParseUnary())
+    return new UnaryExprAST(Opc, Operand);
+  return 0;
+}
+
+/// binoprhs
+///   ::= ('+' unary)*
+static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+  // If this is a binop, find its precedence.
+  while (1) {
+    int TokPrec = GetTokPrecedence();
+    
+    // If this is a binop that binds at least as tightly as the current binop,
+    // consume it, otherwise we are done.
+    if (TokPrec &lt; ExprPrec)
+      return LHS;
+    
+    // Okay, we know this is a binop.
+    int BinOp = CurTok;
+    getNextToken();  // eat binop
+    
+    // Parse the unary expression after the binary operator.
+    ExprAST *RHS = ParseUnary();
+    if (!RHS) return 0;
+    
+    // If BinOp binds less tightly with RHS than the operator after RHS, let
+    // the pending operator take RHS as its LHS.
+    int NextPrec = GetTokPrecedence();
+    if (TokPrec &lt; NextPrec) {
+      RHS = ParseBinOpRHS(TokPrec+1, RHS);
+      if (RHS == 0) return 0;
+    }
+    
+    // Merge LHS/RHS.
+    LHS = new BinaryExprAST(BinOp, LHS, RHS);
+  }
+}
+
+/// expression
+///   ::= unary binoprhs
+///
+static ExprAST *ParseExpression() {
+  ExprAST *LHS = ParseUnary();
+  if (!LHS) return 0;
+  
+  return ParseBinOpRHS(0, LHS);
+}
+
+/// prototype
+///   ::= id '(' id* ')'
+///   ::= binary LETTER number? (id, id)
+///   ::= unary LETTER (id)
+static PrototypeAST *ParsePrototype() {
+  std::string FnName;
+  
+  unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary.
+  unsigned BinaryPrecedence = 30;
+  
+  switch (CurTok) {
+  default:
+    return ErrorP("Expected function name in prototype");
+  case tok_identifier:
+    FnName = IdentifierStr;
+    Kind = 0;
+    getNextToken();
+    break;
+  case tok_unary:
+    getNextToken();
+    if (!isascii(CurTok))
+      return ErrorP("Expected unary operator");
+    FnName = "unary";
+    FnName += (char)CurTok;
+    Kind = 1;
+    getNextToken();
+    break;
+  case tok_binary:
+    getNextToken();
+    if (!isascii(CurTok))
+      return ErrorP("Expected binary operator");
+    FnName = "binary";
+    FnName += (char)CurTok;
+    Kind = 2;
+    getNextToken();
+    
+    // Read the precedence if present.
+    if (CurTok == tok_number) {
+      if (NumVal &lt; 1 || NumVal &gt; 100)
+        return ErrorP("Invalid precedecnce: must be 1..100");
+      BinaryPrecedence = (unsigned)NumVal;
+      getNextToken();
+    }
+    break;
+  }
+  
+  if (CurTok != '(')
+    return ErrorP("Expected '(' in prototype");
+  
+  std::vector&lt;std::string&gt; ArgNames;
+  while (getNextToken() == tok_identifier)
+    ArgNames.push_back(IdentifierStr);
+  if (CurTok != ')')
+    return ErrorP("Expected ')' in prototype");
+  
+  // success.
+  getNextToken();  // eat ')'.
+  
+  // Verify right number of names for operator.
+  if (Kind &amp;&amp; ArgNames.size() != Kind)
+    return ErrorP("Invalid number of operands for operator");
+  
+  return new PrototypeAST(FnName, ArgNames, Kind != 0, BinaryPrecedence);
+}
+
+/// definition ::= 'def' prototype expression
+static FunctionAST *ParseDefinition() {
+  getNextToken();  // eat def.
+  PrototypeAST *Proto = ParsePrototype();
+  if (Proto == 0) return 0;
+
+  if (ExprAST *E = ParseExpression())
+    return new FunctionAST(Proto, E);
+  return 0;
+}
+
+/// toplevelexpr ::= expression
+static FunctionAST *ParseTopLevelExpr() {
+  if (ExprAST *E = ParseExpression()) {
+    // Make an anonymous proto.
+    PrototypeAST *Proto = new PrototypeAST("", std::vector&lt;std::string&gt;());
+    return new FunctionAST(Proto, E);
+  }
+  return 0;
+}
+
+/// external ::= 'extern' prototype
+static PrototypeAST *ParseExtern() {
+  getNextToken();  // eat extern.
+  return ParsePrototype();
+}
+
+//===----------------------------------------------------------------------===//
+// Code Generation
+//===----------------------------------------------------------------------===//
+
+static Module *TheModule;
+static IRBuilder&lt;&gt; Builder(getGlobalContext());
+static std::map&lt;std::string, AllocaInst*&gt; NamedValues;
+static FunctionPassManager *TheFPM;
+
+Value *ErrorV(const char *Str) { Error(Str); return 0; }
+
+/// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of
+/// the function.  This is used for mutable variables etc.
+static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
+                                          const std::string &amp;VarName) {
+  IRBuilder&lt;&gt; TmpB(&amp;TheFunction-&gt;getEntryBlock(),
+                 TheFunction-&gt;getEntryBlock().begin());
+  return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0,
+                           VarName.c_str());
+}
+
+Value *NumberExprAST::Codegen() {
+  return ConstantFP::get(getGlobalContext(), APFloat(Val));
+}
+
+Value *VariableExprAST::Codegen() {
+  // Look this variable up in the function.
+  Value *V = NamedValues[Name];
+  if (V == 0) return ErrorV("Unknown variable name");
+
+  // Load the value.
+  return Builder.CreateLoad(V, Name.c_str());
+}
+
+Value *UnaryExprAST::Codegen() {
+  Value *OperandV = Operand-&gt;Codegen();
+  if (OperandV == 0) return 0;
+  
+  Function *F = TheModule-&gt;getFunction(std::string("unary")+Opcode);
+  if (F == 0)
+    return ErrorV("Unknown unary operator");
+  
+  return Builder.CreateCall(F, OperandV, "unop");
+}
+
+Value *BinaryExprAST::Codegen() {
+  // Special case '=' because we don't want to emit the LHS as an expression.
+  if (Op == '=') {
+    // Assignment requires the LHS to be an identifier.
+    VariableExprAST *LHSE = dynamic_cast&lt;VariableExprAST*&gt;(LHS);
+    if (!LHSE)
+      return ErrorV("destination of '=' must be a variable");
+    // Codegen the RHS.
+    Value *Val = RHS-&gt;Codegen();
+    if (Val == 0) return 0;
+
+    // Look up the name.
+    Value *Variable = NamedValues[LHSE-&gt;getName()];
+    if (Variable == 0) return ErrorV("Unknown variable name");
+
+    Builder.CreateStore(Val, Variable);
+    return Val;
+  }
+  
+  Value *L = LHS-&gt;Codegen();
+  Value *R = RHS-&gt;Codegen();
+  if (L == 0 || R == 0) return 0;
+  
+  switch (Op) {
+  case '+': return Builder.CreateAdd(L, R, "addtmp");
+  case '-': return Builder.CreateSub(L, R, "subtmp");
+  case '*': return Builder.CreateMul(L, R, "multmp");
+  case '&lt;':
+    L = Builder.CreateFCmpULT(L, R, "cmptmp");
+    // Convert bool 0/1 to double 0.0 or 1.0
+    return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
+                                "booltmp");
+  default: break;
+  }
+  
+  // If it wasn't a builtin binary operator, it must be a user defined one. Emit
+  // a call to it.
+  Function *F = TheModule-&gt;getFunction(std::string("binary")+Op);
+  assert(F &amp;&amp; "binary operator not found!");
+  
+  Value *Ops[] = { L, R };
+  return Builder.CreateCall(F, Ops, Ops+2, "binop");
+}
+
+Value *CallExprAST::Codegen() {
+  // Look up the name in the global module table.
+  Function *CalleeF = TheModule-&gt;getFunction(Callee);
+  if (CalleeF == 0)
+    return ErrorV("Unknown function referenced");
+  
+  // If argument mismatch error.
+  if (CalleeF-&gt;arg_size() != Args.size())
+    return ErrorV("Incorrect # arguments passed");
+
+  std::vector&lt;Value*&gt; ArgsV;
+  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
+    ArgsV.push_back(Args[i]-&gt;Codegen());
+    if (ArgsV.back() == 0) return 0;
+  }
+  
+  return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp");
+}
+
+Value *IfExprAST::Codegen() {
+  Value *CondV = Cond-&gt;Codegen();
+  if (CondV == 0) return 0;
+  
+  // Convert condition to a bool by comparing equal to 0.0.
+  CondV = Builder.CreateFCmpONE(CondV, 
+                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                "ifcond");
+  
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+  
+  // Create blocks for the then and else cases.  Insert the 'then' block at the
+  // end of the function.
+  BasicBlock *ThenBB = BasicBlock::Create(getGlobalContext(), "then", TheFunction);
+  BasicBlock *ElseBB = BasicBlock::Create(getGlobalContext(), "else");
+  BasicBlock *MergeBB = BasicBlock::Create(getGlobalContext(), "ifcont");
+  
+  Builder.CreateCondBr(CondV, ThenBB, ElseBB);
+  
+  // Emit then value.
+  Builder.SetInsertPoint(ThenBB);
+  
+  Value *ThenV = Then-&gt;Codegen();
+  if (ThenV == 0) return 0;
+  
+  Builder.CreateBr(MergeBB);
+  // Codegen of 'Then' can change the current block, update ThenBB for the PHI.
+  ThenBB = Builder.GetInsertBlock();
+  
+  // Emit else block.
+  TheFunction-&gt;getBasicBlockList().push_back(ElseBB);
+  Builder.SetInsertPoint(ElseBB);
+  
+  Value *ElseV = Else-&gt;Codegen();
+  if (ElseV == 0) return 0;
+  
+  Builder.CreateBr(MergeBB);
+  // Codegen of 'Else' can change the current block, update ElseBB for the PHI.
+  ElseBB = Builder.GetInsertBlock();
+  
+  // Emit merge block.
+  TheFunction-&gt;getBasicBlockList().push_back(MergeBB);
+  Builder.SetInsertPoint(MergeBB);
+  PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()),
+                                  "iftmp");
+  
+  PN-&gt;addIncoming(ThenV, ThenBB);
+  PN-&gt;addIncoming(ElseV, ElseBB);
+  return PN;
+}
+
+Value *ForExprAST::Codegen() {
+  // Output this as:
+  //   var = alloca double
+  //   ...
+  //   start = startexpr
+  //   store start -&gt; var
+  //   goto loop
+  // loop: 
+  //   ...
+  //   bodyexpr
+  //   ...
+  // loopend:
+  //   step = stepexpr
+  //   endcond = endexpr
+  //
+  //   curvar = load var
+  //   nextvar = curvar + step
+  //   store nextvar -&gt; var
+  //   br endcond, loop, endloop
+  // outloop:
+  
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+
+  // Create an alloca for the variable in the entry block.
+  AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+  
+  // Emit the start code first, without 'variable' in scope.
+  Value *StartVal = Start-&gt;Codegen();
+  if (StartVal == 0) return 0;
+  
+  // Store the value into the alloca.
+  Builder.CreateStore(StartVal, Alloca);
+  
+  // Make the new basic block for the loop header, inserting after current
+  // block.
+  BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction);
+  
+  // Insert an explicit fall through from the current block to the LoopBB.
+  Builder.CreateBr(LoopBB);
+
+  // Start insertion in LoopBB.
+  Builder.SetInsertPoint(LoopBB);
+  
+  // Within the loop, the variable is defined equal to the PHI node.  If it
+  // shadows an existing variable, we have to restore it, so save it now.
+  AllocaInst *OldVal = NamedValues[VarName];
+  NamedValues[VarName] = Alloca;
+  
+  // Emit the body of the loop.  This, like any other expr, can change the
+  // current BB.  Note that we ignore the value computed by the body, but don't
+  // allow an error.
+  if (Body-&gt;Codegen() == 0)
+    return 0;
+  
+  // Emit the step value.
+  Value *StepVal;
+  if (Step) {
+    StepVal = Step-&gt;Codegen();
+    if (StepVal == 0) return 0;
+  } else {
+    // If not specified, use 1.0.
+    StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0));
+  }
+  
+  // Compute the end condition.
+  Value *EndCond = End-&gt;Codegen();
+  if (EndCond == 0) return EndCond;
+  
+  // Reload, increment, and restore the alloca.  This handles the case where
+  // the body of the loop mutates the variable.
+  Value *CurVar = Builder.CreateLoad(Alloca, VarName.c_str());
+  Value *NextVar = Builder.CreateAdd(CurVar, StepVal, "nextvar");
+  Builder.CreateStore(NextVar, Alloca);
+  
+  // Convert condition to a bool by comparing equal to 0.0.
+  EndCond = Builder.CreateFCmpONE(EndCond, 
+                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                  "loopcond");
+  
+  // Create the "after loop" block and insert it.
+  BasicBlock *AfterBB = BasicBlock::Create(getGlobalContext(), "afterloop", TheFunction);
+  
+  // Insert the conditional branch into the end of LoopEndBB.
+  Builder.CreateCondBr(EndCond, LoopBB, AfterBB);
+  
+  // Any new code will be inserted in AfterBB.
+  Builder.SetInsertPoint(AfterBB);
+  
+  // Restore the unshadowed variable.
+  if (OldVal)
+    NamedValues[VarName] = OldVal;
+  else
+    NamedValues.erase(VarName);
+
+  
+  // for expr always returns 0.0.
+  return Constant::getNullValue(Type::getDoubleTy(getGlobalContext()));
+}
+
+Value *VarExprAST::Codegen() {
+  std::vector&lt;AllocaInst *&gt; OldBindings;
+  
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+
+  // Register all variables and emit their initializer.
+  for (unsigned i = 0, e = VarNames.size(); i != e; ++i) {
+    const std::string &amp;VarName = VarNames[i].first;
+    ExprAST *Init = VarNames[i].second;
+    
+    // Emit the initializer before adding the variable to scope, this prevents
+    // the initializer from referencing the variable itself, and permits stuff
+    // like this:
+    //  var a = 1 in
+    //    var a = a in ...   # refers to outer 'a'.
+    Value *InitVal;
+    if (Init) {
+      InitVal = Init-&gt;Codegen();
+      if (InitVal == 0) return 0;
+    } else { // If not specified, use 0.0.
+      InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0));
+    }
+    
+    AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+    Builder.CreateStore(InitVal, Alloca);
+
+    // Remember the old variable binding so that we can restore the binding when
+    // we unrecurse.
+    OldBindings.push_back(NamedValues[VarName]);
+    
+    // Remember this binding.
+    NamedValues[VarName] = Alloca;
+  }
+  
+  // Codegen the body, now that all vars are in scope.
+  Value *BodyVal = Body-&gt;Codegen();
+  if (BodyVal == 0) return 0;
+  
+  // Pop all our variables from scope.
+  for (unsigned i = 0, e = VarNames.size(); i != e; ++i)
+    NamedValues[VarNames[i].first] = OldBindings[i];
+
+  // Return the body computation.
+  return BodyVal;
+}
+
+Function *PrototypeAST::Codegen() {
+  // Make the function type:  double(double,double) etc.
+  std::vector&lt;const Type*&gt; Doubles(Args.size(),
+                                   Type::getDoubleTy(getGlobalContext()));
+  FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
+                                       Doubles, false);
+  
+  Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
+  
+  // If F conflicted, there was already something named 'Name'.  If it has a
+  // body, don't allow redefinition or reextern.
+  if (F-&gt;getName() != Name) {
+    // Delete the one we just made and get the existing one.
+    F-&gt;eraseFromParent();
+    F = TheModule-&gt;getFunction(Name);
+    
+    // If F already has a body, reject this.
+    if (!F-&gt;empty()) {
+      ErrorF("redefinition of function");
+      return 0;
+    }
+    
+    // If F took a different number of args, reject.
+    if (F-&gt;arg_size() != Args.size()) {
+      ErrorF("redefinition of function with different # args");
+      return 0;
+    }
+  }
+  
+  // Set names for all arguments.
+  unsigned Idx = 0;
+  for (Function::arg_iterator AI = F-&gt;arg_begin(); Idx != Args.size();
+       ++AI, ++Idx)
+    AI-&gt;setName(Args[Idx]);
+    
+  return F;
+}
+
+/// CreateArgumentAllocas - Create an alloca for each argument and register the
+/// argument in the symbol table so that references to it will succeed.
+void PrototypeAST::CreateArgumentAllocas(Function *F) {
+  Function::arg_iterator AI = F-&gt;arg_begin();
+  for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) {
+    // Create an alloca for this variable.
+    AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]);
+
+    // Store the initial value into the alloca.
+    Builder.CreateStore(AI, Alloca);
+
+    // Add arguments to variable symbol table.
+    NamedValues[Args[Idx]] = Alloca;
+  }
+}
+
+Function *FunctionAST::Codegen() {
+  NamedValues.clear();
+  
+  Function *TheFunction = Proto-&gt;Codegen();
+  if (TheFunction == 0)
+    return 0;
+  
+  // If this is an operator, install it.
+  if (Proto-&gt;isBinaryOp())
+    BinopPrecedence[Proto-&gt;getOperatorName()] = Proto-&gt;getBinaryPrecedence();
+  
+  // Create a new basic block to start insertion into.
+  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
+  Builder.SetInsertPoint(BB);
+  
+  // Add all arguments to the symbol table and create their allocas.
+  Proto-&gt;CreateArgumentAllocas(TheFunction);
+
+  if (Value *RetVal = Body-&gt;Codegen()) {
+    // Finish off the function.
+    Builder.CreateRet(RetVal);
+
+    // Validate the generated code, checking for consistency.
+    verifyFunction(*TheFunction);
+
+    // Optimize the function.
+    TheFPM-&gt;run(*TheFunction);
+    
+    return TheFunction;
+  }
+  
+  // Error reading body, remove function.
+  TheFunction-&gt;eraseFromParent();
+
+  if (Proto-&gt;isBinaryOp())
+    BinopPrecedence.erase(Proto-&gt;getOperatorName());
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Top-Level parsing and JIT Driver
+//===----------------------------------------------------------------------===//
+
+static ExecutionEngine *TheExecutionEngine;
+
+static void HandleDefinition() {
+  if (FunctionAST *F = ParseDefinition()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      fprintf(stderr, "Read function definition:");
+      LF-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleExtern() {
+  if (PrototypeAST *P = ParseExtern()) {
+    if (Function *F = P-&gt;Codegen()) {
+      fprintf(stderr, "Read extern: ");
+      F-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleTopLevelExpression() {
+  // Evaluate a top-level expression into an anonymous function.
+  if (FunctionAST *F = ParseTopLevelExpr()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      // JIT the function, returning a function pointer.
+      void *FPtr = TheExecutionEngine-&gt;getPointerToFunction(LF);
+      
+      // Cast it to the right type (takes no arguments, returns a double) so we
+      // can call it as a native function.
+      double (*FP)() = (double (*)())(intptr_t)FPtr;
+      fprintf(stderr, "Evaluated to %f\n", FP());
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+/// top ::= definition | external | expression | ';'
+static void MainLoop() {
+  while (1) {
+    fprintf(stderr, "ready&gt; ");
+    switch (CurTok) {
+    case tok_eof:    return;
+    case ';':        getNextToken(); break;  // ignore top-level semicolons.
+    case tok_def:    HandleDefinition(); break;
+    case tok_extern: HandleExtern(); break;
+    default:         HandleTopLevelExpression(); break;
+    }
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// "Library" functions that can be "extern'd" from user code.
+//===----------------------------------------------------------------------===//
+
+/// putchard - putchar that takes a double and returns 0.
+extern "C" 
+double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+
+/// printd - printf that takes a double prints it as "%f\n", returning 0.
+extern "C" 
+double printd(double X) {
+  printf("%f\n", X);
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Main driver code.
+//===----------------------------------------------------------------------===//
+
+int main() {
+  InitializeNativeTarget();
+  LLVMContext &amp;Context = getGlobalContext();
+
+  // Install standard binary operators.
+  // 1 is lowest precedence.
+  BinopPrecedence['='] = 2;
+  BinopPrecedence['&lt;'] = 10;
+  BinopPrecedence['+'] = 20;
+  BinopPrecedence['-'] = 20;
+  BinopPrecedence['*'] = 40;  // highest.
+
+  // Prime the first token.
+  fprintf(stderr, "ready&gt; ");
+  getNextToken();
+
+  // Make the module, which holds all the code.
+  TheModule = new Module("my cool jit", Context);
+
+  // Create the JIT.  This takes ownership of the module.
+  std::string ErrStr;
+  TheExecutionEngine = EngineBuilder(TheModule).setErrorStr(&amp;ErrStr).create();
+  if (!TheExecutionEngine) {
+    fprintf(stderr, "Could not create ExecutionEngine: %s\n", ErrStr.c_str());
+    exit(1);
+  }
+
+  FunctionPassManager OurFPM(TheModule);
+
+  // Set up the optimizer pipeline.  Start with registering info about how the
+  // target lays out data structures.
+  OurFPM.add(new TargetData(*TheExecutionEngine-&gt;getTargetData()));
+  // Promote allocas to registers.
+  OurFPM.add(createPromoteMemoryToRegisterPass());
+  // Do simple "peephole" optimizations and bit-twiddling optzns.
+  OurFPM.add(createInstructionCombiningPass());
+  // Reassociate expressions.
+  OurFPM.add(createReassociatePass());
+  // Eliminate Common SubExpressions.
+  OurFPM.add(createGVNPass());
+  // Simplify the control flow graph (deleting unreachable blocks, etc).
+  OurFPM.add(createCFGSimplificationPass());
+
+  OurFPM.doInitialization();
+
+  // Set the global so the code gen can use this.
+  TheFPM = &amp;OurFPM;
+
+  // Run the main "interpreter loop" now.
+  MainLoop();
+
+  TheFPM = 0;
+
+  // Print out all of the generated code.
+  TheModule-&gt;dump();
+
+  return 0;
+}
+</pre>
+</div>
+
+<a href="LangImpl8.html">Next: Conclusion and other useful LLVM tidbits</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/LangImpl8.html b/docs/tutorial/LangImpl8.html
new file mode 100644
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--- /dev/null
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Conclusion and other useful LLVM tidbits</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Conclusion and other useful LLVM
+ tidbits</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 8
+  <ol>
+    <li><a href="#conclusion">Tutorial Conclusion</a></li>
+    <li><a href="#llvmirproperties">Properties of LLVM IR</a>
+    <ul>
+      <li><a href="#targetindep">Target Independence</a></li>
+      <li><a href="#safety">Safety Guarantees</a></li>
+      <li><a href="#langspecific">Language-Specific Optimizations</a></li>
+    </ul>
+    </li>
+    <li><a href="#tipsandtricks">Tips and Tricks</a>
+    <ul>
+      <li><a href="#offsetofsizeof">Implementing portable 
+                                    offsetof/sizeof</a></li>
+      <li><a href="#gcstack">Garbage Collected Stack Frames</a></li>
+    </ul>
+    </li>
+  </ol>
+</li>
+</ul>
+
+
+<div class="doc_author">
+  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="conclusion">Tutorial Conclusion</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to the the final chapter of the "<a href="index.html">Implementing a
+language with LLVM</a>" tutorial.  In the course of this tutorial, we have grown
+our little Kaleidoscope language from being a useless toy, to being a
+semi-interesting (but probably still useless) toy. :)</p>
+
+<p>It is interesting to see how far we've come, and how little code it has
+taken.  We built the entire lexer, parser, AST, code generator, and an 
+interactive run-loop (with a JIT!) by-hand in under 700 lines of
+(non-comment/non-blank) code.</p>
+
+<p>Our little language supports a couple of interesting features: it supports
+user defined binary and unary operators, it uses JIT compilation for immediate
+evaluation, and it supports a few control flow constructs with SSA construction.
+</p>
+
+<p>Part of the idea of this tutorial was to show you how easy and fun it can be
+to define, build, and play with languages.  Building a compiler need not be a
+scary or mystical process!  Now that you've seen some of the basics, I strongly
+encourage you to take the code and hack on it.  For example, try adding:</p>
+
+<ul>
+<li><b>global variables</b> - While global variables have questional value in
+modern software engineering, they are often useful when putting together quick
+little hacks like the Kaleidoscope compiler itself.  Fortunately, our current
+setup makes it very easy to add global variables: just have value lookup check
+to see if an unresolved variable is in the global variable symbol table before
+rejecting it.  To create a new global variable, make an instance of the LLVM
+<tt>GlobalVariable</tt> class.</li>
+
+<li><b>typed variables</b> - Kaleidoscope currently only supports variables of
+type double.  This gives the language a very nice elegance, because only
+supporting one type means that you never have to specify types.  Different
+languages have different ways of handling this.  The easiest way is to require
+the user to specify types for every variable definition, and record the type
+of the variable in the symbol table along with its Value*.</li>
+
+<li><b>arrays, structs, vectors, etc</b> - Once you add types, you can start
+extending the type system in all sorts of interesting ways.  Simple arrays are
+very easy and are quite useful for many different applications.  Adding them is
+mostly an exercise in learning how the LLVM <a 
+href="../LangRef.html#i_getelementptr">getelementptr</a> instruction works: it
+is so nifty/unconventional, it <a 
+href="../GetElementPtr.html">has its own FAQ</a>!  If you add support
+for recursive types (e.g. linked lists), make sure to read the <a 
+href="../ProgrammersManual.html#TypeResolve">section in the LLVM
+Programmer's Manual</a> that describes how to construct them.</li>
+
+<li><b>standard runtime</b> - Our current language allows the user to access
+arbitrary external functions, and we use it for things like "printd" and
+"putchard".  As you extend the language to add higher-level constructs, often
+these constructs make the most sense if they are lowered to calls into a
+language-supplied runtime.  For example, if you add hash tables to the language,
+it would probably make sense to add the routines to a runtime, instead of 
+inlining them all the way.</li>
+
+<li><b>memory management</b> - Currently we can only access the stack in
+Kaleidoscope.  It would also be useful to be able to allocate heap memory,
+either with calls to the standard libc malloc/free interface or with a garbage
+collector.  If you would like to use garbage collection, note that LLVM fully
+supports <a href="../GarbageCollection.html">Accurate Garbage Collection</a>
+including algorithms that move objects and need to scan/update the stack.</li>
+
+<li><b>debugger support</b> - LLVM supports generation of <a 
+href="../SourceLevelDebugging.html">DWARF Debug info</a> which is understood by
+common debuggers like GDB.  Adding support for debug info is fairly 
+straightforward.  The best way to understand it is to compile some C/C++ code
+with "<tt>llvm-gcc -g -O0</tt>" and taking a look at what it produces.</li>
+
+<li><b>exception handling support</b> - LLVM supports generation of <a 
+href="../ExceptionHandling.html">zero cost exceptions</a> which interoperate
+with code compiled in other languages.  You could also generate code by
+implicitly making every function return an error value and checking it.  You 
+could also make explicit use of setjmp/longjmp.  There are many different ways
+to go here.</li>
+
+<li><b>object orientation, generics, database access, complex numbers,
+geometric programming, ...</b> - Really, there is
+no end of crazy features that you can add to the language.</li>
+
+<li><b>unusual domains</b> - We've been talking about applying LLVM to a domain
+that many people are interested in: building a compiler for a specific language.
+However, there are many other domains that can use compiler technology that are
+not typically considered.  For example, LLVM has been used to implement OpenGL
+graphics acceleration, translate C++ code to ActionScript, and many other
+cute and clever things.  Maybe you will be the first to JIT compile a regular
+expression interpreter into native code with LLVM?</li>
+
+</ul>
+
+<p>
+Have fun - try doing something crazy and unusual.  Building a language like
+everyone else always has, is much less fun than trying something a little crazy
+or off the wall and seeing how it turns out.  If you get stuck or want to talk
+about it, feel free to email the <a 
+href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing 
+list</a>: it has lots of people who are interested in languages and are often
+willing to help out.
+</p>
+
+<p>Before we end this tutorial, I want to talk about some "tips and tricks" for generating
+LLVM IR.  These are some of the more subtle things that may not be obvious, but
+are very useful if you want to take advantage of LLVM's capabilities.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="llvmirproperties">Properties of the LLVM 
+IR</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>We have a couple common questions about code in the LLVM IR form - lets just
+get these out of the way right now, shall we?</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="targetindep">Target 
+Independence</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>Kaleidoscope is an example of a "portable language": any program written in
+Kaleidoscope will work the same way on any target that it runs on.  Many other
+languages have this property, e.g. lisp, java, haskell, javascript, python, etc
+(note that while these languages are portable, not all their libraries are).</p>
+
+<p>One nice aspect of LLVM is that it is often capable of preserving target
+independence in the IR: you can take the LLVM IR for a Kaleidoscope-compiled 
+program and run it on any target that LLVM supports, even emitting C code and
+compiling that on targets that LLVM doesn't support natively.  You can trivially
+tell that the Kaleidoscope compiler generates target-independent code because it
+never queries for any target-specific information when generating code.</p>
+
+<p>The fact that LLVM provides a compact, target-independent, representation for
+code gets a lot of people excited.  Unfortunately, these people are usually
+thinking about C or a language from the C family when they are asking questions
+about language portability.  I say "unfortunately", because there is really no
+way to make (fully general) C code portable, other than shipping the source code
+around (and of course, C source code is not actually portable in general
+either - ever port a really old application from 32- to 64-bits?).</p>
+
+<p>The problem with C (again, in its full generality) is that it is heavily
+laden with target specific assumptions.  As one simple example, the preprocessor
+often destructively removes target-independence from the code when it processes
+the input text:</p>
+
+<div class="doc_code">
+<pre>
+#ifdef __i386__
+  int X = 1;
+#else
+  int X = 42;
+#endif
+</pre>
+</div>
+
+<p>While it is possible to engineer more and more complex solutions to problems
+like this, it cannot be solved in full generality in a way that is better than shipping
+the actual source code.</p>
+
+<p>That said, there are interesting subsets of C that can be made portable.  If
+you are willing to fix primitive types to a fixed size (say int = 32-bits, 
+and long = 64-bits), don't care about ABI compatibility with existing binaries,
+and are willing to give up some other minor features, you can have portable
+code.  This can make sense for specialized domains such as an
+in-kernel language.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="safety">Safety Guarantees</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>Many of the languages above are also "safe" languages: it is impossible for
+a program written in Java to corrupt its address space and crash the process
+(assuming the JVM has no bugs).
+Safety is an interesting property that requires a combination of language
+design, runtime support, and often operating system support.</p>
+
+<p>It is certainly possible to implement a safe language in LLVM, but LLVM IR
+does not itself guarantee safety.  The LLVM IR allows unsafe pointer casts,
+use after free bugs, buffer over-runs, and a variety of other problems.  Safety
+needs to be implemented as a layer on top of LLVM and, conveniently, several
+groups have investigated this.  Ask on the <a 
+href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing 
+list</a> if you are interested in more details.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="langspecific">Language-Specific 
+Optimizations</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>One thing about LLVM that turns off many people is that it does not solve all
+the world's problems in one system (sorry 'world hunger', someone else will have
+to solve you some other day).  One specific complaint is that people perceive
+LLVM as being incapable of performing high-level language-specific optimization:
+LLVM "loses too much information".</p>
+
+<p>Unfortunately, this is really not the place to give you a full and unified
+version of "Chris Lattner's theory of compiler design".  Instead, I'll make a
+few observations:</p>
+
+<p>First, you're right that LLVM does lose information.  For example, as of this
+writing, there is no way to distinguish in the LLVM IR whether an SSA-value came
+from a C "int" or a C "long" on an ILP32 machine (other than debug info).  Both
+get compiled down to an 'i32' value and the information about what it came from
+is lost.  The more general issue here, is that the LLVM type system uses
+"structural equivalence" instead of "name equivalence".  Another place this
+surprises people is if you have two types in a high-level language that have the
+same structure (e.g. two different structs that have a single int field): these
+types will compile down into a single LLVM type and it will be impossible to
+tell what it came from.</p>
+
+<p>Second, while LLVM does lose information, LLVM is not a fixed target: we 
+continue to enhance and improve it in many different ways.  In addition to
+adding new features (LLVM did not always support exceptions or debug info), we
+also extend the IR to capture important information for optimization (e.g.
+whether an argument is sign or zero extended, information about pointers
+aliasing, etc).  Many of the enhancements are user-driven: people want LLVM to
+include some specific feature, so they go ahead and extend it.</p>
+
+<p>Third, it is <em>possible and easy</em> to add language-specific
+optimizations, and you have a number of choices in how to do it.  As one trivial
+example, it is easy to add language-specific optimization passes that
+"know" things about code compiled for a language.  In the case of the C family,
+there is an optimization pass that "knows" about the standard C library
+functions.  If you call "exit(0)" in main(), it knows that it is safe to
+optimize that into "return 0;" because C specifies what the 'exit'
+function does.</p>
+
+<p>In addition to simple library knowledge, it is possible to embed a variety of
+other language-specific information into the LLVM IR.  If you have a specific
+need and run into a wall, please bring the topic up on the llvmdev list.  At the
+very worst, you can always treat LLVM as if it were a "dumb code generator" and
+implement the high-level optimizations you desire in your front-end, on the
+language-specific AST.
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="tipsandtricks">Tips and Tricks</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>There is a variety of useful tips and tricks that you come to know after
+working on/with LLVM that aren't obvious at first glance.  Instead of letting
+everyone rediscover them, this section talks about some of these issues.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="offsetofsizeof">Implementing portable
+offsetof/sizeof</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>One interesting thing that comes up, if you are trying to keep the code 
+generated by your compiler "target independent", is that you often need to know
+the size of some LLVM type or the offset of some field in an llvm structure.
+For example, you might need to pass the size of a type into a function that
+allocates memory.</p>
+
+<p>Unfortunately, this can vary widely across targets: for example the width of
+a pointer is trivially target-specific.  However, there is a <a 
+href="http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt">clever
+way to use the getelementptr instruction</a> that allows you to compute this
+in a portable way.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="gcstack">Garbage Collected 
+Stack Frames</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>Some languages want to explicitly manage their stack frames, often so that
+they are garbage collected or to allow easy implementation of closures.  There
+are often better ways to implement these features than explicit stack frames,
+but <a 
+href="http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt">LLVM
+does support them,</a> if you want.  It requires your front-end to convert the
+code into <a 
+href="http://en.wikipedia.org/wiki/Continuation-passing_style">Continuation
+Passing Style</a> and the use of tail calls (which LLVM also supports).</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/Makefile b/docs/tutorial/Makefile
new file mode 100644
index 0000000000..9082ad4d85
--- /dev/null
+++ b/docs/tutorial/Makefile
@@ -0,0 +1,28 @@
+##===- docs/tutorial/Makefile ------------------------------*- Makefile -*-===##
+#
+#                     The LLVM Compiler Infrastructure
+#
+# This file is distributed under the University of Illinois Open Source
+# License. See LICENSE.TXT for details.
+#
+##===----------------------------------------------------------------------===##
+
+LEVEL := ../..
+include $(LEVEL)/Makefile.common
+
+HTML       := $(wildcard $(PROJ_SRC_DIR)/*.html)
+EXTRA_DIST := $(HTML) index.html
+HTML_DIR   := $(DESTDIR)$(PROJ_docsdir)/html/tutorial
+
+install-local:: $(HTML)
+	$(Echo) Installing HTML Tutorial Documentation
+	$(Verb) $(MKDIR) $(HTML_DIR)
+	$(Verb) $(DataInstall) $(HTML) $(HTML_DIR)
+	$(Verb) $(DataInstall) $(PROJ_SRC_DIR)/index.html $(HTML_DIR)
+
+uninstall-local::
+	$(Echo) Uninstalling Tutorial Documentation
+	$(Verb) $(RM) -rf $(HTML_DIR)
+
+printvars::
+	$(Echo) "HTML           : " '$(HTML)'
diff --git a/docs/tutorial/OCamlLangImpl1.html b/docs/tutorial/OCamlLangImpl1.html
new file mode 100644
index 0000000000..98c1124cc1
--- /dev/null
+++ b/docs/tutorial/OCamlLangImpl1.html
@@ -0,0 +1,365 @@
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Tutorial Introduction and the Lexer</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <meta name="author" content="Erick Tryzelaar">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Tutorial Introduction and the Lexer</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 1
+  <ol>
+    <li><a href="#intro">Tutorial Introduction</a></li>
+    <li><a href="#language">The Basic Language</a></li>
+    <li><a href="#lexer">The Lexer</a></li>
+  </ol>
+</li>
+<li><a href="OCamlLangImpl2.html">Chapter 2</a>: Implementing a Parser and
+AST</li>
+</ul>
+
+<div class="doc_author">
+	<p>
+		Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
+		and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
+	</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Tutorial Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to the "Implementing a language with LLVM" tutorial.  This tutorial
+runs through the implementation of a simple language, showing how fun and
+easy it can be.  This tutorial will get you up and started as well as help to
+build a framework you can extend to other languages.  The code in this tutorial
+can also be used as a playground to hack on other LLVM specific things.
+</p>
+
+<p>
+The goal of this tutorial is to progressively unveil our language, describing
+how it is built up over time.  This will let us cover a fairly broad range of
+language design and LLVM-specific usage issues, showing and explaining the code
+for it all along the way, without overwhelming you with tons of details up
+front.</p>
+
+<p>It is useful to point out ahead of time that this tutorial is really about
+teaching compiler techniques and LLVM specifically, <em>not</em> about teaching
+modern and sane software engineering principles.  In practice, this means that
+we'll take a number of shortcuts to simplify the exposition.  For example, the
+code leaks memory, uses global variables all over the place, doesn't use nice
+design patterns like <a
+href="http://en.wikipedia.org/wiki/Visitor_pattern">visitors</a>, etc... but it
+is very simple.  If you dig in and use the code as a basis for future projects,
+fixing these deficiencies shouldn't be hard.</p>
+
+<p>I've tried to put this tutorial together in a way that makes chapters easy to
+skip over if you are already familiar with or are uninterested in the various
+pieces.  The structure of the tutorial is:
+</p>
+
+<ul>
+<li><b><a href="#language">Chapter #1</a>: Introduction to the Kaleidoscope
+language, and the definition of its Lexer</b> - This shows where we are going
+and the basic functionality that we want it to do.  In order to make this
+tutorial maximally understandable and hackable, we choose to implement
+everything in Objective Caml instead of using lexer and parser generators.
+LLVM obviously works just fine with such tools, feel free to use one if you
+prefer.</li>
+<li><b><a href="OCamlLangImpl2.html">Chapter #2</a>: Implementing a Parser and
+AST</b> - With the lexer in place, we can talk about parsing techniques and
+basic AST construction.  This tutorial describes recursive descent parsing and
+operator precedence parsing.  Nothing in Chapters 1 or 2 is LLVM-specific,
+the code doesn't even link in LLVM at this point. :)</li>
+<li><b><a href="OCamlLangImpl3.html">Chapter #3</a>: Code generation to LLVM
+IR</b> - With the AST ready, we can show off how easy generation of LLVM IR
+really is.</li>
+<li><b><a href="OCamlLangImpl4.html">Chapter #4</a>: Adding JIT and Optimizer
+Support</b> - Because a lot of people are interested in using LLVM as a JIT,
+we'll dive right into it and show you the 3 lines it takes to add JIT support.
+LLVM is also useful in many other ways, but this is one simple and "sexy" way
+to shows off its power. :)</li>
+<li><b><a href="OCamlLangImpl5.html">Chapter #5</a>: Extending the Language:
+Control Flow</b> - With the language up and running, we show how to extend it
+with control flow operations (if/then/else and a 'for' loop).  This gives us a
+chance to talk about simple SSA construction and control flow.</li>
+<li><b><a href="OCamlLangImpl6.html">Chapter #6</a>: Extending the Language:
+User-defined Operators</b> - This is a silly but fun chapter that talks about
+extending the language to let the user program define their own arbitrary
+unary and binary operators (with assignable precedence!).  This lets us build a
+significant piece of the "language" as library routines.</li>
+<li><b><a href="OCamlLangImpl7.html">Chapter #7</a>: Extending the Language:
+Mutable Variables</b> - This chapter talks about adding user-defined local
+variables along with an assignment operator.  The interesting part about this
+is how easy and trivial it is to construct SSA form in LLVM: no, LLVM does
+<em>not</em> require your front-end to construct SSA form!</li>
+<li><b><a href="OCamlLangImpl8.html">Chapter #8</a>: Conclusion and other
+useful LLVM tidbits</b> - This chapter wraps up the series by talking about
+potential ways to extend the language, but also includes a bunch of pointers to
+info about "special topics" like adding garbage collection support, exceptions,
+debugging, support for "spaghetti stacks", and a bunch of other tips and
+tricks.</li>
+
+</ul>
+
+<p>By the end of the tutorial, we'll have written a bit less than 700 lines of
+non-comment, non-blank, lines of code.  With this small amount of code, we'll
+have built up a very reasonable compiler for a non-trivial language including
+a hand-written lexer, parser, AST, as well as code generation support with a JIT
+compiler.  While other systems may have interesting "hello world" tutorials,
+I think the breadth of this tutorial is a great testament to the strengths of
+LLVM and why you should consider it if you're interested in language or compiler
+design.</p>
+
+<p>A note about this tutorial: we expect you to extend the language and play
+with it on your own.  Take the code and go crazy hacking away at it, compilers
+don't need to be scary creatures - it can be a lot of fun to play with
+languages!</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="language">The Basic Language</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>This tutorial will be illustrated with a toy language that we'll call
+"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>" (derived
+from "meaning beautiful, form, and view").
+Kaleidoscope is a procedural language that allows you to define functions, use
+conditionals, math, etc.  Over the course of the tutorial, we'll extend
+Kaleidoscope to support the if/then/else construct, a for loop, user defined
+operators, JIT compilation with a simple command line interface, etc.</p>
+
+<p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
+64-bit floating point type (aka 'float' in O'Caml parlance).  As such, all
+values are implicitly double precision and the language doesn't require type
+declarations.  This gives the language a very nice and simple syntax.  For
+example, the following simple example computes <a
+href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>
+
+<div class="doc_code">
+<pre>
+# Compute the x'th fibonacci number.
+def fib(x)
+  if x &lt; 3 then
+    1
+  else
+    fib(x-1)+fib(x-2)
+
+# This expression will compute the 40th number.
+fib(40)
+</pre>
+</div>
+
+<p>We also allow Kaleidoscope to call into standard library functions (the LLVM
+JIT makes this completely trivial).  This means that you can use the 'extern'
+keyword to define a function before you use it (this is also useful for mutually
+recursive functions).  For example:</p>
+
+<div class="doc_code">
+<pre>
+extern sin(arg);
+extern cos(arg);
+extern atan2(arg1 arg2);
+
+atan2(sin(.4), cos(42))
+</pre>
+</div>
+
+<p>A more interesting example is included in Chapter 6 where we write a little
+Kaleidoscope application that <a href="OCamlLangImpl6.html#example">displays
+a Mandelbrot Set</a> at various levels of magnification.</p>
+
+<p>Lets dive into the implementation of this language!</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="lexer">The Lexer</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>When it comes to implementing a language, the first thing needed is
+the ability to process a text file and recognize what it says.  The traditional
+way to do this is to use a "<a
+href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
+to break the input up into "tokens".  Each token returned by the lexer includes
+a token code and potentially some metadata (e.g. the numeric value of a number).
+First, we define the possibilities:
+</p>
+
+<div class="doc_code">
+<pre>
+(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
+ * these others for known things. *)
+type token =
+  (* commands *)
+  | Def | Extern
+
+  (* primary *)
+  | Ident of string | Number of float
+
+  (* unknown *)
+  | Kwd of char
+</pre>
+</div>
+
+<p>Each token returned by our lexer will be one of the token variant values.
+An unknown character like '+' will be returned as <tt>Token.Kwd '+'</tt>.  If
+the curr token is an identifier, the value will be <tt>Token.Ident s</tt>.  If
+the current token is a numeric literal (like 1.0), the value will be
+<tt>Token.Number 1.0</tt>.
+</p>
+
+<p>The actual implementation of the lexer is a collection of functions driven
+by a function named <tt>Lexer.lex</tt>.  The <tt>Lexer.lex</tt> function is
+called to return the next token from standard input.  We will use
+<a href="http://caml.inria.fr/pub/docs/manual-camlp4/index.html">Camlp4</a>
+to simplify the tokenization of the standard input.  Its definition starts
+as:</p>
+
+<div class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer
+ *===----------------------------------------------------------------------===*)
+
+let rec lex = parser
+  (* Skip any whitespace. *)
+  | [&lt; ' (' ' | '\n' | '\r' | '\t'); stream &gt;] -&gt; lex stream
+</pre>
+</div>
+
+<p>
+<tt>Lexer.lex</tt> works by recursing over a <tt>char Stream.t</tt> to read
+characters one at a time from the standard input.  It eats them as it recognizes
+them and stores them in in a <tt>Token.token</tt> variant.  The first thing that
+it has to do is ignore whitespace between tokens.  This is accomplished with the
+recursive call above.</p>
+
+<p>The next thing <tt>Lexer.lex</tt> needs to do is recognize identifiers and
+specific keywords like "def".  Kaleidoscope does this with a pattern match
+and a helper function.<p>
+
+<div class="doc_code">
+<pre>
+  (* identifier: [a-zA-Z][a-zA-Z0-9] *)
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+
+...
+
+and lex_ident buffer = parser
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+</pre>
+</div>
+
+<p>Numeric values are similar:</p>
+
+<div class="doc_code">
+<pre>
+  (* number: [0-9.]+ *)
+  | [&lt; ' ('0' .. '9' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+
+...
+
+and lex_number buffer = parser
+  | [&lt; ' ('0' .. '9' | '.' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      [&lt; 'Token.Number (float_of_string (Buffer.contents buffer)); stream &gt;]
+</pre>
+</div>
+
+<p>This is all pretty straight-forward code for processing input.  When reading
+a numeric value from input, we use the ocaml <tt>float_of_string</tt> function
+to convert it to a numeric value that we store in <tt>Token.Number</tt>.  Note
+that this isn't doing sufficient error checking: it will raise <tt>Failure</tt>
+if the string "1.23.45.67".  Feel free to extend it :).  Next we handle
+comments:
+</p>
+
+<div class="doc_code">
+<pre>
+  (* Comment until end of line. *)
+  | [&lt; ' ('#'); stream &gt;] -&gt;
+      lex_comment stream
+
+...
+
+and lex_comment = parser
+  | [&lt; ' ('\n'); stream=lex &gt;] -&gt; stream
+  | [&lt; 'c; e=lex_comment &gt;] -&gt; e
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+</pre>
+</div>
+
+<p>We handle comments by skipping to the end of the line and then return the
+next token.  Finally, if the input doesn't match one of the above cases, it is
+either an operator character like '+' or the end of the file.  These are handled
+with this code:</p>
+
+<div class="doc_code">
+<pre>
+  (* Otherwise, just return the character as its ascii value. *)
+  | [&lt; 'c; stream &gt;] -&gt;
+      [&lt; 'Token.Kwd c; lex stream &gt;]
+
+  (* end of stream. *)
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+</pre>
+</div>
+
+<p>With this, we have the complete lexer for the basic Kaleidoscope language
+(the <a href="OCamlLangImpl2.html#code">full code listing</a> for the Lexer is
+available in the <a href="OCamlLangImpl2.html">next chapter</a> of the
+tutorial).  Next we'll <a href="OCamlLangImpl2.html">build a simple parser that
+uses this to build an Abstract Syntax Tree</a>.  When we have that, we'll
+include a driver so that you can use the lexer and parser together.
+</p>
+
+<a href="OCamlLangImpl2.html">Next: Implementing a Parser and AST</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/OCamlLangImpl2.html b/docs/tutorial/OCamlLangImpl2.html
new file mode 100644
index 0000000000..666510979f
--- /dev/null
+++ b/docs/tutorial/OCamlLangImpl2.html
@@ -0,0 +1,1045 @@
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Implementing a Parser and AST</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <meta name="author" content="Erick Tryzelaar">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Implementing a Parser and AST</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 2
+  <ol>
+    <li><a href="#intro">Chapter 2 Introduction</a></li>
+    <li><a href="#ast">The Abstract Syntax Tree (AST)</a></li>
+    <li><a href="#parserbasics">Parser Basics</a></li>
+    <li><a href="#parserprimexprs">Basic Expression Parsing</a></li>
+    <li><a href="#parserbinops">Binary Expression Parsing</a></li>
+    <li><a href="#parsertop">Parsing the Rest</a></li>
+    <li><a href="#driver">The Driver</a></li>
+    <li><a href="#conclusions">Conclusions</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="OCamlLangImpl3.html">Chapter 3</a>: Code generation to LLVM IR</li>
+</ul>
+
+<div class="doc_author">
+	<p>
+		Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
+		and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
+	</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 2 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 2 of the "<a href="index.html">Implementing a language
+with LLVM in Objective Caml</a>" tutorial.  This chapter shows you how to use
+the lexer, built in <a href="OCamlLangImpl1.html">Chapter 1</a>, to build a
+full <a href="http://en.wikipedia.org/wiki/Parsing">parser</a> for our
+Kaleidoscope language.  Once we have a parser, we'll define and build an <a
+href="http://en.wikipedia.org/wiki/Abstract_syntax_tree">Abstract Syntax
+Tree</a> (AST).</p>
+
+<p>The parser we will build uses a combination of <a
+href="http://en.wikipedia.org/wiki/Recursive_descent_parser">Recursive Descent
+Parsing</a> and <a href=
+"http://en.wikipedia.org/wiki/Operator-precedence_parser">Operator-Precedence
+Parsing</a> to parse the Kaleidoscope language (the latter for
+binary expressions and the former for everything else).  Before we get to
+parsing though, lets talk about the output of the parser: the Abstract Syntax
+Tree.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="ast">The Abstract Syntax Tree (AST)</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>The AST for a program captures its behavior in such a way that it is easy for
+later stages of the compiler (e.g. code generation) to interpret.  We basically
+want one object for each construct in the language, and the AST should closely
+model the language.  In Kaleidoscope, we have expressions, a prototype, and a
+function object.  We'll start with expressions first:</p>
+
+<div class="doc_code">
+<pre>
+(* expr - Base type for all expression nodes. *)
+type expr =
+  (* variant for numeric literals like "1.0". *)
+  | Number of float
+</pre>
+</div>
+
+<p>The code above shows the definition of the base ExprAST class and one
+subclass which we use for numeric literals.  The important thing to note about
+this code is that the Number variant captures the numeric value of the
+literal as an instance variable. This allows later phases of the compiler to
+know what the stored numeric value is.</p>
+
+<p>Right now we only create the AST,  so there are no useful functions on
+them.  It would be very easy to add a function to pretty print the code,
+for example.  Here are the other expression AST node definitions that we'll use
+in the basic form of the Kaleidoscope language:
+</p>
+
+<div class="doc_code">
+<pre>
+  (* variant for referencing a variable, like "a". *)
+  | Variable of string
+
+  (* variant for a binary operator. *)
+  | Binary of char * expr * expr
+
+  (* variant for function calls. *)
+  | Call of string * expr array
+</pre>
+</div>
+
+<p>This is all (intentionally) rather straight-forward: variables capture the
+variable name, binary operators capture their opcode (e.g. '+'), and calls
+capture a function name as well as a list of any argument expressions.  One thing
+that is nice about our AST is that it captures the language features without
+talking about the syntax of the language.  Note that there is no discussion about
+precedence of binary operators, lexical structure, etc.</p>
+
+<p>For our basic language, these are all of the expression nodes we'll define.
+Because it doesn't have conditional control flow, it isn't Turing-complete;
+we'll fix that in a later installment.  The two things we need next are a way
+to talk about the interface to a function, and a way to talk about functions
+themselves:</p>
+
+<div class="doc_code">
+<pre>
+(* proto - This type represents the "prototype" for a function, which captures
+ * its name, and its argument names (thus implicitly the number of arguments the
+ * function takes). *)
+type proto = Prototype of string * string array
+
+(* func - This type represents a function definition itself. *)
+type func = Function of proto * expr
+</pre>
+</div>
+
+<p>In Kaleidoscope, functions are typed with just a count of their arguments.
+Since all values are double precision floating point, the type of each argument
+doesn't need to be stored anywhere.  In a more aggressive and realistic
+language, the "expr" variants would probably have a type field.</p>
+
+<p>With this scaffolding, we can now talk about parsing expressions and function
+bodies in Kaleidoscope.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="parserbasics">Parser Basics</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Now that we have an AST to build, we need to define the parser code to build
+it.  The idea here is that we want to parse something like "x+y" (which is
+returned as three tokens by the lexer) into an AST that could be generated with
+calls like this:</p>
+
+<div class="doc_code">
+<pre>
+  let x = Variable "x" in
+  let y = Variable "y" in
+  let result = Binary ('+', x, y) in
+  ...
+</pre>
+</div>
+
+<p>
+The error handling routines make use of the builtin <tt>Stream.Failure</tt> and
+<tt>Stream.Error</tt>s.  <tt>Stream.Failure</tt> is raised when the parser is
+unable to find any matching token in the first position of a pattern.
+<tt>Stream.Error</tt> is raised when the first token matches, but the rest do
+not.  The error recovery in our parser will not be the best and is not
+particular user-friendly, but it will be enough for our tutorial.  These
+exceptions make it easier to handle errors in routines that have various return
+types.</p>
+
+<p>With these basic types and exceptions, we can implement the first
+piece of our grammar: numeric literals.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="parserprimexprs">Basic Expression
+ Parsing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>We start with numeric literals, because they are the simplest to process.
+For each production in our grammar, we'll define a function which parses that
+production.  We call this class of expressions "primary" expressions, for
+reasons that will become more clear <a href="OCamlLangImpl6.html#unary">
+later in the tutorial</a>.  In order to parse an arbitrary primary expression,
+we need to determine what sort of expression it is.  For numeric literals, we
+have:</p>
+
+<div class="doc_code">
+<pre>
+(* primary
+ *   ::= identifier
+ *   ::= numberexpr
+ *   ::= parenexpr *)
+parse_primary = parser
+  (* numberexpr ::= number *)
+  | [&lt; 'Token.Number n &gt;] -&gt; Ast.Number n
+</pre>
+</div>
+
+<p>This routine is very simple: it expects to be called when the current token
+is a <tt>Token.Number</tt> token.  It takes the current number value, creates
+a <tt>Ast.Number</tt> node, advances the lexer to the next token, and finally
+returns.</p>
+
+<p>There are some interesting aspects to this.  The most important one is that
+this routine eats all of the tokens that correspond to the production and
+returns the lexer buffer with the next token (which is not part of the grammar
+production) ready to go.  This is a fairly standard way to go for recursive
+descent parsers.  For a better example, the parenthesis operator is defined like
+this:</p>
+
+<div class="doc_code">
+<pre>
+  (* parenexpr ::= '(' expression ')' *)
+  | [&lt; 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" &gt;] -&gt; e
+</pre>
+</div>
+
+<p>This function illustrates a number of interesting things about the
+parser:</p>
+
+<p>
+1) It shows how we use the <tt>Stream.Error</tt> exception.  When called, this
+function expects that the current token is a '(' token, but after parsing the
+subexpression, it is possible that there is no ')' waiting.  For example, if
+the user types in "(4 x" instead of "(4)", the parser should emit an error.
+Because errors can occur, the parser needs a way to indicate that they
+happened. In our parser, we use the camlp4 shortcut syntax <tt>token ?? "parse
+error"</tt>, where if the token before the <tt>??</tt> does not match, then
+<tt>Stream.Error "parse error"</tt> will be raised.</p>
+
+<p>2) Another interesting aspect of this function is that it uses recursion by
+calling <tt>Parser.parse_primary</tt> (we will soon see that
+<tt>Parser.parse_primary</tt> can call <tt>Parser.parse_primary</tt>).  This is
+powerful because it allows us to handle recursive grammars, and keeps each
+production very simple.  Note that parentheses do not cause construction of AST
+nodes themselves.  While we could do it this way, the most important role of
+parentheses are to guide the parser and provide grouping.  Once the parser
+constructs the AST, parentheses are not needed.</p>
+
+<p>The next simple production is for handling variable references and function
+calls:</p>
+
+<div class="doc_code">
+<pre>
+  (* identifierexpr
+   *   ::= identifier
+   *   ::= identifier '(' argumentexpr ')' *)
+  | [&lt; 'Token.Ident id; stream &gt;] -&gt;
+      let rec parse_args accumulator = parser
+        | [&lt; e=parse_expr; stream &gt;] -&gt;
+            begin parser
+              | [&lt; 'Token.Kwd ','; e=parse_args (e :: accumulator) &gt;] -&gt; e
+              | [&lt; &gt;] -&gt; e :: accumulator
+            end stream
+        | [&lt; &gt;] -&gt; accumulator
+      in
+      let rec parse_ident id = parser
+        (* Call. *)
+        | [&lt; 'Token.Kwd '(';
+             args=parse_args [];
+             'Token.Kwd ')' ?? "expected ')'"&gt;] -&gt;
+            Ast.Call (id, Array.of_list (List.rev args))
+
+        (* Simple variable ref. *)
+        | [&lt; &gt;] -&gt; Ast.Variable id
+      in
+      parse_ident id stream
+</pre>
+</div>
+
+<p>This routine follows the same style as the other routines.  (It expects to be
+called if the current token is a <tt>Token.Ident</tt> token).  It also has
+recursion and error handling.  One interesting aspect of this is that it uses
+<em>look-ahead</em> to determine if the current identifier is a stand alone
+variable reference or if it is a function call expression.  It handles this by
+checking to see if the token after the identifier is a '(' token, constructing
+either a <tt>Ast.Variable</tt> or <tt>Ast.Call</tt> node as appropriate.
+</p>
+
+<p>We finish up by raising an exception if we received a token we didn't
+expect:</p>
+
+<div class="doc_code">
+<pre>
+  | [&lt; &gt;] -&gt; raise (Stream.Error "unknown token when expecting an expression.")
+</pre>
+</div>
+
+<p>Now that basic expressions are handled, we need to handle binary expressions.
+They are a bit more complex.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="parserbinops">Binary Expression
+ Parsing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Binary expressions are significantly harder to parse because they are often
+ambiguous.  For example, when given the string "x+y*z", the parser can choose
+to parse it as either "(x+y)*z" or "x+(y*z)".  With common definitions from
+mathematics, we expect the later parse, because "*" (multiplication) has
+higher <em>precedence</em> than "+" (addition).</p>
+
+<p>There are many ways to handle this, but an elegant and efficient way is to
+use <a href=
+"http://en.wikipedia.org/wiki/Operator-precedence_parser">Operator-Precedence
+Parsing</a>.  This parsing technique uses the precedence of binary operators to
+guide recursion.  To start with, we need a table of precedences:</p>
+
+<div class="doc_code">
+<pre>
+(* binop_precedence - This holds the precedence for each binary operator that is
+ * defined *)
+let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
+
+(* precedence - Get the precedence of the pending binary operator token. *)
+let precedence c = try Hashtbl.find binop_precedence c with Not_found -&gt; -1
+
+...
+
+let main () =
+  (* Install standard binary operators.
+   * 1 is the lowest precedence. *)
+  Hashtbl.add Parser.binop_precedence '&lt;' 10;
+  Hashtbl.add Parser.binop_precedence '+' 20;
+  Hashtbl.add Parser.binop_precedence '-' 20;
+  Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
+  ...
+</pre>
+</div>
+
+<p>For the basic form of Kaleidoscope, we will only support 4 binary operators
+(this can obviously be extended by you, our brave and intrepid reader).  The
+<tt>Parser.precedence</tt> function returns the precedence for the current
+token, or -1 if the token is not a binary operator.  Having a <tt>Hashtbl.t</tt>
+makes it easy to add new operators and makes it clear that the algorithm doesn't
+depend on the specific operators involved, but it would be easy enough to
+eliminate the <tt>Hashtbl.t</tt> and do the comparisons in the
+<tt>Parser.precedence</tt> function.  (Or just use a fixed-size array).</p>
+
+<p>With the helper above defined, we can now start parsing binary expressions.
+The basic idea of operator precedence parsing is to break down an expression
+with potentially ambiguous binary operators into pieces.  Consider ,for example,
+the expression "a+b+(c+d)*e*f+g".  Operator precedence parsing considers this
+as a stream of primary expressions separated by binary operators.  As such,
+it will first parse the leading primary expression "a", then it will see the
+pairs [+, b] [+, (c+d)] [*, e] [*, f] and [+, g].  Note that because parentheses
+are primary expressions, the binary expression parser doesn't need to worry
+about nested subexpressions like (c+d) at all.
+</p>
+
+<p>
+To start, an expression is a primary expression potentially followed by a
+sequence of [binop,primaryexpr] pairs:</p>
+
+<div class="doc_code">
+<pre>
+(* expression
+ *   ::= primary binoprhs *)
+and parse_expr = parser
+  | [&lt; lhs=parse_primary; stream &gt;] -&gt; parse_bin_rhs 0 lhs stream
+</pre>
+</div>
+
+<p><tt>Parser.parse_bin_rhs</tt> is the function that parses the sequence of
+pairs for us.  It takes a precedence and a pointer to an expression for the part
+that has been parsed so far.   Note that "x" is a perfectly valid expression: As
+such, "binoprhs" is allowed to be empty, in which case it returns the expression
+that is passed into it. In our example above, the code passes the expression for
+"a" into <tt>Parser.parse_bin_rhs</tt> and the current token is "+".</p>
+
+<p>The precedence value passed into <tt>Parser.parse_bin_rhs</tt> indicates the
+<em>minimal operator precedence</em> that the function is allowed to eat.  For
+example, if the current pair stream is [+, x] and <tt>Parser.parse_bin_rhs</tt>
+is passed in a precedence of 40, it will not consume any tokens (because the
+precedence of '+' is only 20).  With this in mind, <tt>Parser.parse_bin_rhs</tt>
+starts with:</p>
+
+<div class="doc_code">
+<pre>
+(* binoprhs
+ *   ::= ('+' primary)* *)
+and parse_bin_rhs expr_prec lhs stream =
+  match Stream.peek stream with
+  (* If this is a binop, find its precedence. *)
+  | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -&gt;
+      let token_prec = precedence c in
+
+      (* If this is a binop that binds at least as tightly as the current binop,
+       * consume it, otherwise we are done. *)
+      if token_prec &lt; expr_prec then lhs else begin
+</pre>
+</div>
+
+<p>This code gets the precedence of the current token and checks to see if if is
+too low.  Because we defined invalid tokens to have a precedence of -1, this
+check implicitly knows that the pair-stream ends when the token stream runs out
+of binary operators.  If this check succeeds, we know that the token is a binary
+operator and that it will be included in this expression:</p>
+
+<div class="doc_code">
+<pre>
+        (* Eat the binop. *)
+        Stream.junk stream;
+
+        (* Okay, we know this is a binop. *)
+        let rhs =
+          match Stream.peek stream with
+          | Some (Token.Kwd c2) -&gt;
+</pre>
+</div>
+
+<p>As such, this code eats (and remembers) the binary operator and then parses
+the primary expression that follows.  This builds up the whole pair, the first of
+which is [+, b] for the running example.</p>
+
+<p>Now that we parsed the left-hand side of an expression and one pair of the
+RHS sequence, we have to decide which way the expression associates.  In
+particular, we could have "(a+b) binop unparsed"  or "a + (b binop unparsed)".
+To determine this, we look ahead at "binop" to determine its precedence and
+compare it to BinOp's precedence (which is '+' in this case):</p>
+
+<div class="doc_code">
+<pre>
+              (* If BinOp binds less tightly with rhs than the operator after
+               * rhs, let the pending operator take rhs as its lhs. *)
+              let next_prec = precedence c2 in
+              if token_prec &lt; next_prec
+</pre>
+</div>
+
+<p>If the precedence of the binop to the right of "RHS" is lower or equal to the
+precedence of our current operator, then we know that the parentheses associate
+as "(a+b) binop ...".  In our example, the current operator is "+" and the next
+operator is "+", we know that they have the same precedence.  In this case we'll
+create the AST node for "a+b", and then continue parsing:</p>
+
+<div class="doc_code">
+<pre>
+          ... if body omitted ...
+        in
+
+        (* Merge lhs/rhs. *)
+        let lhs = Ast.Binary (c, lhs, rhs) in
+        parse_bin_rhs expr_prec lhs stream
+      end
+</pre>
+</div>
+
+<p>In our example above, this will turn "a+b+" into "(a+b)" and execute the next
+iteration of the loop, with "+" as the current token.  The code above will eat,
+remember, and parse "(c+d)" as the primary expression, which makes the
+current pair equal to [+, (c+d)].  It will then evaluate the 'if' conditional above with
+"*" as the binop to the right of the primary.  In this case, the precedence of "*" is
+higher than the precedence of "+" so the if condition will be entered.</p>
+
+<p>The critical question left here is "how can the if condition parse the right
+hand side in full"?  In particular, to build the AST correctly for our example,
+it needs to get all of "(c+d)*e*f" as the RHS expression variable.  The code to
+do this is surprisingly simple (code from the above two blocks duplicated for
+context):</p>
+
+<div class="doc_code">
+<pre>
+          match Stream.peek stream with
+          | Some (Token.Kwd c2) -&gt;
+              (* If BinOp binds less tightly with rhs than the operator after
+               * rhs, let the pending operator take rhs as its lhs. *)
+              if token_prec &lt; precedence c2
+              then <b>parse_bin_rhs (token_prec + 1) rhs stream</b>
+              else rhs
+          | _ -&gt; rhs
+        in
+
+        (* Merge lhs/rhs. *)
+        let lhs = Ast.Binary (c, lhs, rhs) in
+        parse_bin_rhs expr_prec lhs stream
+      end
+</pre>
+</div>
+
+<p>At this point, we know that the binary operator to the RHS of our primary
+has higher precedence than the binop we are currently parsing.  As such, we know
+that any sequence of pairs whose operators are all higher precedence than "+"
+should be parsed together and returned as "RHS".  To do this, we recursively
+invoke the <tt>Parser.parse_bin_rhs</tt> function specifying "token_prec+1" as
+the minimum precedence required for it to continue.  In our example above, this
+will cause it to return the AST node for "(c+d)*e*f" as RHS, which is then set
+as the RHS of the '+' expression.</p>
+
+<p>Finally, on the next iteration of the while loop, the "+g" piece is parsed
+and added to the AST.  With this little bit of code (14 non-trivial lines), we
+correctly handle fully general binary expression parsing in a very elegant way.
+This was a whirlwind tour of this code, and it is somewhat subtle.  I recommend
+running through it with a few tough examples to see how it works.
+</p>
+
+<p>This wraps up handling of expressions.  At this point, we can point the
+parser at an arbitrary token stream and build an expression from it, stopping
+at the first token that is not part of the expression.  Next up we need to
+handle function definitions, etc.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="parsertop">Parsing the Rest</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+The next thing missing is handling of function prototypes.  In Kaleidoscope,
+these are used both for 'extern' function declarations as well as function body
+definitions.  The code to do this is straight-forward and not very interesting
+(once you've survived expressions):
+</p>
+
+<div class="doc_code">
+<pre>
+(* prototype
+ *   ::= id '(' id* ')' *)
+let parse_prototype =
+  let rec parse_args accumulator = parser
+    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
+    | [&lt; &gt;] -&gt; accumulator
+  in
+
+  parser
+  | [&lt; 'Token.Ident id;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+       args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      (* success. *)
+      Ast.Prototype (id, Array.of_list (List.rev args))
+
+  | [&lt; &gt;] -&gt;
+      raise (Stream.Error "expected function name in prototype")
+</pre>
+</div>
+
+<p>Given this, a function definition is very simple, just a prototype plus
+an expression to implement the body:</p>
+
+<div class="doc_code">
+<pre>
+(* definition ::= 'def' prototype expression *)
+let parse_definition = parser
+  | [&lt; 'Token.Def; p=parse_prototype; e=parse_expr &gt;] -&gt;
+      Ast.Function (p, e)
+</pre>
+</div>
+
+<p>In addition, we support 'extern' to declare functions like 'sin' and 'cos' as
+well as to support forward declaration of user functions.  These 'extern's are just
+prototypes with no body:</p>
+
+<div class="doc_code">
+<pre>
+(*  external ::= 'extern' prototype *)
+let parse_extern = parser
+  | [&lt; 'Token.Extern; e=parse_prototype &gt;] -&gt; e
+</pre>
+</div>
+
+<p>Finally, we'll also let the user type in arbitrary top-level expressions and
+evaluate them on the fly.  We will handle this by defining anonymous nullary
+(zero argument) functions for them:</p>
+
+<div class="doc_code">
+<pre>
+(* toplevelexpr ::= expression *)
+let parse_toplevel = parser
+  | [&lt; e=parse_expr &gt;] -&gt;
+      (* Make an anonymous proto. *)
+      Ast.Function (Ast.Prototype ("", [||]), e)
+</pre>
+</div>
+
+<p>Now that we have all the pieces, let's build a little driver that will let us
+actually <em>execute</em> this code we've built!</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="driver">The Driver</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>The driver for this simply invokes all of the parsing pieces with a top-level
+dispatch loop.  There isn't much interesting here, so I'll just include the
+top-level loop.  See <a href="#code">below</a> for full code in the "Top-Level
+Parsing" section.</p>
+
+<div class="doc_code">
+<pre>
+(* top ::= definition | external | expression | ';' *)
+let rec main_loop stream =
+  match Stream.peek stream with
+  | None -&gt; ()
+
+  (* ignore top-level semicolons. *)
+  | Some (Token.Kwd ';') -&gt;
+      Stream.junk stream;
+      main_loop stream
+
+  | Some token -&gt;
+      begin
+        try match token with
+        | Token.Def -&gt;
+            ignore(Parser.parse_definition stream);
+            print_endline "parsed a function definition.";
+        | Token.Extern -&gt;
+            ignore(Parser.parse_extern stream);
+            print_endline "parsed an extern.";
+        | _ -&gt;
+            (* Evaluate a top-level expression into an anonymous function. *)
+            ignore(Parser.parse_toplevel stream);
+            print_endline "parsed a top-level expr";
+        with Stream.Error s -&gt;
+          (* Skip token for error recovery. *)
+          Stream.junk stream;
+          print_endline s;
+      end;
+      print_string "ready&gt; "; flush stdout;
+      main_loop stream
+</pre>
+</div>
+
+<p>The most interesting part of this is that we ignore top-level semicolons.
+Why is this, you ask?  The basic reason is that if you type "4 + 5" at the
+command line, the parser doesn't know whether that is the end of what you will type
+or not.  For example, on the next line you could type "def foo..." in which case
+4+5 is the end of a top-level expression.  Alternatively you could type "* 6",
+which would continue the expression.  Having top-level semicolons allows you to
+type "4+5;", and the parser will know you are done.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="conclusions">Conclusions</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>With just under 300 lines of commented code (240 lines of non-comment,
+non-blank code), we fully defined our minimal language, including a lexer,
+parser, and AST builder.  With this done, the executable will validate
+Kaleidoscope code and tell us if it is grammatically invalid.  For
+example, here is a sample interaction:</p>
+
+<div class="doc_code">
+<pre>
+$ <b>./toy.byte</b>
+ready&gt; <b>def foo(x y) x+foo(y, 4.0);</b>
+Parsed a function definition.
+ready&gt; <b>def foo(x y) x+y y;</b>
+Parsed a function definition.
+Parsed a top-level expr
+ready&gt; <b>def foo(x y) x+y );</b>
+Parsed a function definition.
+Error: unknown token when expecting an expression
+ready&gt; <b>extern sin(a);</b>
+ready&gt; Parsed an extern
+ready&gt; <b>^D</b>
+$
+</pre>
+</div>
+
+<p>There is a lot of room for extension here.  You can define new AST nodes,
+extend the language in many ways, etc.  In the <a href="OCamlLangImpl3.html">
+next installment</a>, we will describe how to generate LLVM Intermediate
+Representation (IR) from the AST.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for this and the previous chapter.
+Note that it is fully self-contained: you don't need LLVM or any external
+libraries at all for this.  (Besides the ocaml standard libraries, of
+course.)  To build this, just compile with:</p>
+
+<div class="doc_code">
+<pre>
+# Compile
+ocamlbuild toy.byte
+# Run
+./toy.byte
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<dl>
+<dt>_tags:</dt>
+<dd class="doc_code">
+<pre>
+&lt;{lexer,parser}.ml&gt;: use_camlp4, pp(camlp4of)
+</pre>
+</dd>
+
+<dt>token.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer Tokens
+ *===----------------------------------------------------------------------===*)
+
+(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
+ * these others for known things. *)
+type token =
+  (* commands *)
+  | Def | Extern
+
+  (* primary *)
+  | Ident of string | Number of float
+
+  (* unknown *)
+  | Kwd of char
+</pre>
+</dd>
+
+<dt>lexer.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer
+ *===----------------------------------------------------------------------===*)
+
+let rec lex = parser
+  (* Skip any whitespace. *)
+  | [&lt; ' (' ' | '\n' | '\r' | '\t'); stream &gt;] -&gt; lex stream
+
+  (* identifier: [a-zA-Z][a-zA-Z0-9] *)
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+
+  (* number: [0-9.]+ *)
+  | [&lt; ' ('0' .. '9' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+
+  (* Comment until end of line. *)
+  | [&lt; ' ('#'); stream &gt;] -&gt;
+      lex_comment stream
+
+  (* Otherwise, just return the character as its ascii value. *)
+  | [&lt; 'c; stream &gt;] -&gt;
+      [&lt; 'Token.Kwd c; lex stream &gt;]
+
+  (* end of stream. *)
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+
+and lex_number buffer = parser
+  | [&lt; ' ('0' .. '9' | '.' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      [&lt; 'Token.Number (float_of_string (Buffer.contents buffer)); stream &gt;]
+
+and lex_ident buffer = parser
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+
+and lex_comment = parser
+  | [&lt; ' ('\n'); stream=lex &gt;] -&gt; stream
+  | [&lt; 'c; e=lex_comment &gt;] -&gt; e
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+</pre>
+</dd>
+
+<dt>ast.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Abstract Syntax Tree (aka Parse Tree)
+ *===----------------------------------------------------------------------===*)
+
+(* expr - Base type for all expression nodes. *)
+type expr =
+  (* variant for numeric literals like "1.0". *)
+  | Number of float
+
+  (* variant for referencing a variable, like "a". *)
+  | Variable of string
+
+  (* variant for a binary operator. *)
+  | Binary of char * expr * expr
+
+  (* variant for function calls. *)
+  | Call of string * expr array
+
+(* proto - This type represents the "prototype" for a function, which captures
+ * its name, and its argument names (thus implicitly the number of arguments the
+ * function takes). *)
+type proto = Prototype of string * string array
+
+(* func - This type represents a function definition itself. *)
+type func = Function of proto * expr
+</pre>
+</dd>
+
+<dt>parser.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===---------------------------------------------------------------------===
+ * Parser
+ *===---------------------------------------------------------------------===*)
+
+(* binop_precedence - This holds the precedence for each binary operator that is
+ * defined *)
+let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
+
+(* precedence - Get the precedence of the pending binary operator token. *)
+let precedence c = try Hashtbl.find binop_precedence c with Not_found -&gt; -1
+
+(* primary
+ *   ::= identifier
+ *   ::= numberexpr
+ *   ::= parenexpr *)
+let rec parse_primary = parser
+  (* numberexpr ::= number *)
+  | [&lt; 'Token.Number n &gt;] -&gt; Ast.Number n
+
+  (* parenexpr ::= '(' expression ')' *)
+  | [&lt; 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" &gt;] -&gt; e
+
+  (* identifierexpr
+   *   ::= identifier
+   *   ::= identifier '(' argumentexpr ')' *)
+  | [&lt; 'Token.Ident id; stream &gt;] -&gt;
+      let rec parse_args accumulator = parser
+        | [&lt; e=parse_expr; stream &gt;] -&gt;
+            begin parser
+              | [&lt; 'Token.Kwd ','; e=parse_args (e :: accumulator) &gt;] -&gt; e
+              | [&lt; &gt;] -&gt; e :: accumulator
+            end stream
+        | [&lt; &gt;] -&gt; accumulator
+      in
+      let rec parse_ident id = parser
+        (* Call. *)
+        | [&lt; 'Token.Kwd '(';
+             args=parse_args [];
+             'Token.Kwd ')' ?? "expected ')'"&gt;] -&gt;
+            Ast.Call (id, Array.of_list (List.rev args))
+
+        (* Simple variable ref. *)
+        | [&lt; &gt;] -&gt; Ast.Variable id
+      in
+      parse_ident id stream
+
+  | [&lt; &gt;] -&gt; raise (Stream.Error "unknown token when expecting an expression.")
+
+(* binoprhs
+ *   ::= ('+' primary)* *)
+and parse_bin_rhs expr_prec lhs stream =
+  match Stream.peek stream with
+  (* If this is a binop, find its precedence. *)
+  | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -&gt;
+      let token_prec = precedence c in
+
+      (* If this is a binop that binds at least as tightly as the current binop,
+       * consume it, otherwise we are done. *)
+      if token_prec &lt; expr_prec then lhs else begin
+        (* Eat the binop. *)
+        Stream.junk stream;
+
+        (* Parse the primary expression after the binary operator. *)
+        let rhs = parse_primary stream in
+
+        (* Okay, we know this is a binop. *)
+        let rhs =
+          match Stream.peek stream with
+          | Some (Token.Kwd c2) -&gt;
+              (* If BinOp binds less tightly with rhs than the operator after
+               * rhs, let the pending operator take rhs as its lhs. *)
+              let next_prec = precedence c2 in
+              if token_prec &lt; next_prec
+              then parse_bin_rhs (token_prec + 1) rhs stream
+              else rhs
+          | _ -&gt; rhs
+        in
+
+        (* Merge lhs/rhs. *)
+        let lhs = Ast.Binary (c, lhs, rhs) in
+        parse_bin_rhs expr_prec lhs stream
+      end
+  | _ -&gt; lhs
+
+(* expression
+ *   ::= primary binoprhs *)
+and parse_expr = parser
+  | [&lt; lhs=parse_primary; stream &gt;] -&gt; parse_bin_rhs 0 lhs stream
+
+(* prototype
+ *   ::= id '(' id* ')' *)
+let parse_prototype =
+  let rec parse_args accumulator = parser
+    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
+    | [&lt; &gt;] -&gt; accumulator
+  in
+
+  parser
+  | [&lt; 'Token.Ident id;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+       args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      (* success. *)
+      Ast.Prototype (id, Array.of_list (List.rev args))
+
+  | [&lt; &gt;] -&gt;
+      raise (Stream.Error "expected function name in prototype")
+
+(* definition ::= 'def' prototype expression *)
+let parse_definition = parser
+  | [&lt; 'Token.Def; p=parse_prototype; e=parse_expr &gt;] -&gt;
+      Ast.Function (p, e)
+
+(* toplevelexpr ::= expression *)
+let parse_toplevel = parser
+  | [&lt; e=parse_expr &gt;] -&gt;
+      (* Make an anonymous proto. *)
+      Ast.Function (Ast.Prototype ("", [||]), e)
+
+(*  external ::= 'extern' prototype *)
+let parse_extern = parser
+  | [&lt; 'Token.Extern; e=parse_prototype &gt;] -&gt; e
+</pre>
+</dd>
+
+<dt>toplevel.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Top-Level parsing and JIT Driver
+ *===----------------------------------------------------------------------===*)
+
+(* top ::= definition | external | expression | ';' *)
+let rec main_loop stream =
+  match Stream.peek stream with
+  | None -&gt; ()
+
+  (* ignore top-level semicolons. *)
+  | Some (Token.Kwd ';') -&gt;
+      Stream.junk stream;
+      main_loop stream
+
+  | Some token -&gt;
+      begin
+        try match token with
+        | Token.Def -&gt;
+            ignore(Parser.parse_definition stream);
+            print_endline "parsed a function definition.";
+        | Token.Extern -&gt;
+            ignore(Parser.parse_extern stream);
+            print_endline "parsed an extern.";
+        | _ -&gt;
+            (* Evaluate a top-level expression into an anonymous function. *)
+            ignore(Parser.parse_toplevel stream);
+            print_endline "parsed a top-level expr";
+        with Stream.Error s -&gt;
+          (* Skip token for error recovery. *)
+          Stream.junk stream;
+          print_endline s;
+      end;
+      print_string "ready&gt; "; flush stdout;
+      main_loop stream
+</pre>
+</dd>
+
+<dt>toy.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Main driver code.
+ *===----------------------------------------------------------------------===*)
+
+let main () =
+  (* Install standard binary operators.
+   * 1 is the lowest precedence. *)
+  Hashtbl.add Parser.binop_precedence '&lt;' 10;
+  Hashtbl.add Parser.binop_precedence '+' 20;
+  Hashtbl.add Parser.binop_precedence '-' 20;
+  Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
+
+  (* Prime the first token. *)
+  print_string "ready&gt; "; flush stdout;
+  let stream = Lexer.lex (Stream.of_channel stdin) in
+
+  (* Run the main "interpreter loop" now. *)
+  Toplevel.main_loop stream;
+;;
+
+main ()
+</pre>
+</dd>
+</dl>
+
+<a href="OCamlLangImpl3.html">Next: Implementing Code Generation to LLVM IR</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a>
+  <a href="mailto:erickt@users.sourceforge.net">Erick Tryzelaar</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/OCamlLangImpl3.html b/docs/tutorial/OCamlLangImpl3.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Implementing code generation to LLVM IR</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <meta name="author" content="Erick Tryzelaar">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Code generation to LLVM IR</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 3
+  <ol>
+    <li><a href="#intro">Chapter 3 Introduction</a></li>
+    <li><a href="#basics">Code Generation Setup</a></li>
+    <li><a href="#exprs">Expression Code Generation</a></li>
+    <li><a href="#funcs">Function Code Generation</a></li>
+    <li><a href="#driver">Driver Changes and Closing Thoughts</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="OCamlLangImpl4.html">Chapter 4</a>: Adding JIT and Optimizer
+Support</li>
+</ul>
+
+<div class="doc_author">
+	<p>
+		Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
+		and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
+	</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 3 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 3 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  This chapter shows you how to transform the <a
+href="OCamlLangImpl2.html">Abstract Syntax Tree</a>, built in Chapter 2, into
+LLVM IR.  This will teach you a little bit about how LLVM does things, as well
+as demonstrate how easy it is to use.  It's much more work to build a lexer and
+parser than it is to generate LLVM IR code. :)
+</p>
+
+<p><b>Please note</b>: the code in this chapter and later require LLVM 2.3 or
+LLVM SVN to work.  LLVM 2.2 and before will not work with it.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="basics">Code Generation Setup</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+In order to generate LLVM IR, we want some simple setup to get started.  First
+we define virtual code generation (codegen) methods in each AST class:</p>
+
+<div class="doc_code">
+<pre>
+let rec codegen_expr = function
+  | Ast.Number n -&gt; ...
+  | Ast.Variable name -&gt; ...
+</pre>
+</div>
+
+<p>The <tt>Codegen.codegen_expr</tt> function says to emit IR for that AST node
+along with all the things it depends on, and they all return an LLVM Value
+object.  "Value" is the class used to represent a "<a
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Static Single
+Assignment (SSA)</a> register" or "SSA value" in LLVM.  The most distinct aspect
+of SSA values is that their value is computed as the related instruction
+executes, and it does not get a new value until (and if) the instruction
+re-executes.  In other words, there is no way to "change" an SSA value.  For
+more information, please read up on <a
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Static Single
+Assignment</a> - the concepts are really quite natural once you grok them.</p>
+
+<p>The
+second thing we want is an "Error" exception like we used for the parser, which
+will be used to report errors found during code generation (for example, use of
+an undeclared parameter):</p>
+
+<div class="doc_code">
+<pre>
+exception Error of string
+
+let the_module = create_module (global_context ()) "my cool jit"
+let builder = builder (global_context ())
+let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
+let double_type = double_type context
+</pre>
+</div>
+
+<p>The static variables will be used during code generation.
+<tt>Codgen.the_module</tt> is the LLVM construct that contains all of the
+functions and global variables in a chunk of code.  In many ways, it is the
+top-level structure that the LLVM IR uses to contain code.</p>
+
+<p>The <tt>Codegen.builder</tt> object is a helper object that makes it easy to
+generate LLVM instructions.  Instances of the <a
+href="http://llvm.org/doxygen/IRBuilder_8h-source.html"><tt>IRBuilder</tt></a>
+class keep track of the current place to insert instructions and has methods to
+create new instructions.</p>
+
+<p>The <tt>Codegen.named_values</tt> map keeps track of which values are defined
+in the current scope and what their LLVM representation is.  (In other words, it
+is a symbol table for the code).  In this form of Kaleidoscope, the only things
+that can be referenced are function parameters.  As such, function parameters
+will be in this map when generating code for their function body.</p>
+
+<p>
+With these basics in place, we can start talking about how to generate code for
+each expression.  Note that this assumes that the <tt>Codgen.builder</tt> has
+been set up to generate code <em>into</em> something.  For now, we'll assume
+that this has already been done, and we'll just use it to emit code.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="exprs">Expression Code Generation</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Generating LLVM code for expression nodes is very straightforward: less
+than 30 lines of commented code for all four of our expression nodes.  First
+we'll do numeric literals:</p>
+
+<div class="doc_code">
+<pre>
+  | Ast.Number n -&gt; const_float double_type n
+</pre>
+</div>
+
+<p>In the LLVM IR, numeric constants are represented with the
+<tt>ConstantFP</tt> class, which holds the numeric value in an <tt>APFloat</tt>
+internally (<tt>APFloat</tt> has the capability of holding floating point
+constants of <em>A</em>rbitrary <em>P</em>recision).  This code basically just
+creates and returns a <tt>ConstantFP</tt>.  Note that in the LLVM IR
+that constants are all uniqued together and shared.  For this reason, the API
+uses "the foo::get(..)" idiom instead of "new foo(..)" or "foo::Create(..)".</p>
+
+<div class="doc_code">
+<pre>
+  | Ast.Variable name -&gt;
+      (try Hashtbl.find named_values name with
+        | Not_found -&gt; raise (Error "unknown variable name"))
+</pre>
+</div>
+
+<p>References to variables are also quite simple using LLVM.  In the simple
+version of Kaleidoscope, we assume that the variable has already been emitted
+somewhere and its value is available.  In practice, the only values that can be
+in the <tt>Codegen.named_values</tt> map are function arguments.  This code
+simply checks to see that the specified name is in the map (if not, an unknown
+variable is being referenced) and returns the value for it.  In future chapters,
+we'll add support for <a href="LangImpl5.html#for">loop induction variables</a>
+in the symbol table, and for <a href="LangImpl7.html#localvars">local
+variables</a>.</p>
+
+<div class="doc_code">
+<pre>
+  | Ast.Binary (op, lhs, rhs) -&gt;
+      let lhs_val = codegen_expr lhs in
+      let rhs_val = codegen_expr rhs in
+      begin
+        match op with
+        | '+' -&gt; build_add lhs_val rhs_val "addtmp" builder
+        | '-' -&gt; build_sub lhs_val rhs_val "subtmp" builder
+        | '*' -&gt; build_mul lhs_val rhs_val "multmp" builder
+        | '&lt;' -&gt;
+            (* Convert bool 0/1 to double 0.0 or 1.0 *)
+            let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
+            build_uitofp i double_type "booltmp" builder
+        | _ -&gt; raise (Error "invalid binary operator")
+      end
+</pre>
+</div>
+
+<p>Binary operators start to get more interesting.  The basic idea here is that
+we recursively emit code for the left-hand side of the expression, then the
+right-hand side, then we compute the result of the binary expression.  In this
+code, we do a simple switch on the opcode to create the right LLVM instruction.
+</p>
+
+<p>In the example above, the LLVM builder class is starting to show its value.
+IRBuilder knows where to insert the newly created instruction, all you have to
+do is specify what instruction to create (e.g. with <tt>Llvm.create_add</tt>),
+which operands to use (<tt>lhs</tt> and <tt>rhs</tt> here) and optionally
+provide a name for the generated instruction.</p>
+
+<p>One nice thing about LLVM is that the name is just a hint.  For instance, if
+the code above emits multiple "addtmp" variables, LLVM will automatically
+provide each one with an increasing, unique numeric suffix.  Local value names
+for instructions are purely optional, but it makes it much easier to read the
+IR dumps.</p>
+
+<p><a href="../LangRef.html#instref">LLVM instructions</a> are constrained by
+strict rules: for example, the Left and Right operators of
+an <a href="../LangRef.html#i_add">add instruction</a> must have the same
+type, and the result type of the add must match the operand types.  Because
+all values in Kaleidoscope are doubles, this makes for very simple code for add,
+sub and mul.</p>
+
+<p>On the other hand, LLVM specifies that the <a
+href="../LangRef.html#i_fcmp">fcmp instruction</a> always returns an 'i1' value
+(a one bit integer).  The problem with this is that Kaleidoscope wants the value to be a 0.0 or 1.0 value.  In order to get these semantics, we combine the fcmp instruction with
+a <a href="../LangRef.html#i_uitofp">uitofp instruction</a>.  This instruction
+converts its input integer into a floating point value by treating the input
+as an unsigned value.  In contrast, if we used the <a
+href="../LangRef.html#i_sitofp">sitofp instruction</a>, the Kaleidoscope '&lt;'
+operator would return 0.0 and -1.0, depending on the input value.</p>
+
+<div class="doc_code">
+<pre>
+  | Ast.Call (callee, args) -&gt;
+      (* Look up the name in the module table. *)
+      let callee =
+        match lookup_function callee the_module with
+        | Some callee -&gt; callee
+        | None -&gt; raise (Error "unknown function referenced")
+      in
+      let params = params callee in
+
+      (* If argument mismatch error. *)
+      if Array.length params == Array.length args then () else
+        raise (Error "incorrect # arguments passed");
+      let args = Array.map codegen_expr args in
+      build_call callee args "calltmp" builder
+</pre>
+</div>
+
+<p>Code generation for function calls is quite straightforward with LLVM.  The
+code above initially does a function name lookup in the LLVM Module's symbol
+table.  Recall that the LLVM Module is the container that holds all of the
+functions we are JIT'ing.  By giving each function the same name as what the
+user specifies, we can use the LLVM symbol table to resolve function names for
+us.</p>
+
+<p>Once we have the function to call, we recursively codegen each argument that
+is to be passed in, and create an LLVM <a href="../LangRef.html#i_call">call
+instruction</a>.  Note that LLVM uses the native C calling conventions by
+default, allowing these calls to also call into standard library functions like
+"sin" and "cos", with no additional effort.</p>
+
+<p>This wraps up our handling of the four basic expressions that we have so far
+in Kaleidoscope.  Feel free to go in and add some more.  For example, by
+browsing the <a href="../LangRef.html">LLVM language reference</a> you'll find
+several other interesting instructions that are really easy to plug into our
+basic framework.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="funcs">Function Code Generation</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Code generation for prototypes and functions must handle a number of
+details, which make their code less beautiful than expression code
+generation, but allows us to illustrate some important points.  First, lets
+talk about code generation for prototypes: they are used both for function
+bodies and external function declarations.  The code starts with:</p>
+
+<div class="doc_code">
+<pre>
+let codegen_proto = function
+  | Ast.Prototype (name, args) -&gt;
+      (* Make the function type: double(double,double) etc. *)
+      let doubles = Array.make (Array.length args) double_type in
+      let ft = function_type double_type doubles in
+      let f =
+        match lookup_function name the_module with
+</pre>
+</div>
+
+<p>This code packs a lot of power into a few lines.  Note first that this
+function returns a "Function*" instead of a "Value*" (although at the moment
+they both are modeled by <tt>llvalue</tt> in ocaml).  Because a "prototype"
+really talks about the external interface for a function (not the value computed
+by an expression), it makes sense for it to return the LLVM Function it
+corresponds to when codegen'd.</p>
+
+<p>The call to <tt>Llvm.function_type</tt> creates the <tt>Llvm.llvalue</tt>
+that should be used for a given Prototype.  Since all function arguments in
+Kaleidoscope are of type double, the first line creates a vector of "N" LLVM
+double types.  It then uses the <tt>Llvm.function_type</tt> method to create a
+function type that takes "N" doubles as arguments, returns one double as a
+result, and that is not vararg (that uses the function
+<tt>Llvm.var_arg_function_type</tt>).  Note that Types in LLVM are uniqued just
+like <tt>Constant</tt>s are, so you don't "new" a type, you "get" it.</p>
+
+<p>The final line above checks if the function has already been defined in
+<tt>Codegen.the_module</tt>. If not, we will create it.</p>
+
+<div class="doc_code">
+<pre>
+        | None -&gt; declare_function name ft the_module
+</pre>
+</div>
+
+<p>This indicates the type and name to use, as well as which module to insert
+into.  By default we assume a function has
+<tt>Llvm.Linkage.ExternalLinkage</tt>.  "<a href="LangRef.html#linkage">external
+linkage</a>" means that the function may be defined outside the current module
+and/or that it is callable by functions outside the module.  The "<tt>name</tt>"
+passed in is the name the user specified: this name is registered in
+"<tt>Codegen.the_module</tt>"s symbol table, which is used by the function call
+code above.</p>
+
+<p>In Kaleidoscope, I choose to allow redefinitions of functions in two cases:
+first, we want to allow 'extern'ing a function more than once, as long as the
+prototypes for the externs match (since all arguments have the same type, we
+just have to check that the number of arguments match).  Second, we want to
+allow 'extern'ing a function and then defining a body for it.  This is useful
+when defining mutually recursive functions.</p>
+
+<div class="doc_code">
+<pre>
+        (* If 'f' conflicted, there was already something named 'name'. If it
+         * has a body, don't allow redefinition or reextern. *)
+        | Some f -&gt;
+            (* If 'f' already has a body, reject this. *)
+            if Array.length (basic_blocks f) == 0 then () else
+              raise (Error "redefinition of function");
+
+            (* If 'f' took a different number of arguments, reject. *)
+            if Array.length (params f) == Array.length args then () else
+              raise (Error "redefinition of function with different # args");
+            f
+      in
+</pre>
+</div>
+
+<p>In order to verify the logic above, we first check to see if the pre-existing
+function is "empty".  In this case, empty means that it has no basic blocks in
+it, which means it has no body.  If it has no body, it is a forward
+declaration.  Since we don't allow anything after a full definition of the
+function, the code rejects this case.  If the previous reference to a function
+was an 'extern', we simply verify that the number of arguments for that
+definition and this one match up.  If not, we emit an error.</p>
+
+<div class="doc_code">
+<pre>
+      (* Set names for all arguments. *)
+      Array.iteri (fun i a -&gt;
+        let n = args.(i) in
+        set_value_name n a;
+        Hashtbl.add named_values n a;
+      ) (params f);
+      f
+</pre>
+</div>
+
+<p>The last bit of code for prototypes loops over all of the arguments in the
+function, setting the name of the LLVM Argument objects to match, and registering
+the arguments in the <tt>Codegen.named_values</tt> map for future use by the
+<tt>Ast.Variable</tt> variant.  Once this is set up, it returns the Function
+object to the caller.  Note that we don't check for conflicting
+argument names here (e.g. "extern foo(a b a)").  Doing so would be very
+straight-forward with the mechanics we have already used above.</p>
+
+<div class="doc_code">
+<pre>
+let codegen_func = function
+  | Ast.Function (proto, body) -&gt;
+      Hashtbl.clear named_values;
+      let the_function = codegen_proto proto in
+</pre>
+</div>
+
+<p>Code generation for function definitions starts out simply enough: we just
+codegen the prototype (Proto) and verify that it is ok.  We then clear out the
+<tt>Codegen.named_values</tt> map to make sure that there isn't anything in it
+from the last function we compiled.  Code generation of the prototype ensures
+that there is an LLVM Function object that is ready to go for us.</p>
+
+<div class="doc_code">
+<pre>
+      (* Create a new basic block to start insertion into. *)
+      let bb = append_block context "entry" the_function in
+      position_at_end bb builder;
+
+      try
+        let ret_val = codegen_expr body in
+</pre>
+</div>
+
+<p>Now we get to the point where the <tt>Codegen.builder</tt> is set up.  The
+first line creates a new
+<a href="http://en.wikipedia.org/wiki/Basic_block">basic block</a> (named
+"entry"), which is inserted into <tt>the_function</tt>.  The second line then
+tells the builder that new instructions should be inserted into the end of the
+new basic block.  Basic blocks in LLVM are an important part of functions that
+define the <a
+href="http://en.wikipedia.org/wiki/Control_flow_graph">Control Flow Graph</a>.
+Since we don't have any control flow, our functions will only contain one
+block at this point.  We'll fix this in <a href="OCamlLangImpl5.html">Chapter
+5</a> :).</p>
+
+<div class="doc_code">
+<pre>
+        let ret_val = codegen_expr body in
+
+        (* Finish off the function. *)
+        let _ = build_ret ret_val builder in
+
+        (* Validate the generated code, checking for consistency. *)
+        Llvm_analysis.assert_valid_function the_function;
+
+        the_function
+</pre>
+</div>
+
+<p>Once the insertion point is set up, we call the <tt>Codegen.codegen_func</tt>
+method for the root expression of the function.  If no error happens, this emits
+code to compute the expression into the entry block and returns the value that
+was computed.  Assuming no error, we then create an LLVM <a
+href="../LangRef.html#i_ret">ret instruction</a>, which completes the function.
+Once the function is built, we call
+<tt>Llvm_analysis.assert_valid_function</tt>, which is provided by LLVM.  This
+function does a variety of consistency checks on the generated code, to
+determine if our compiler is doing everything right.  Using this is important:
+it can catch a lot of bugs.  Once the function is finished and validated, we
+return it.</p>
+
+<div class="doc_code">
+<pre>
+      with e -&gt;
+        delete_function the_function;
+        raise e
+</pre>
+</div>
+
+<p>The only piece left here is handling of the error case.  For simplicity, we
+handle this by merely deleting the function we produced with the
+<tt>Llvm.delete_function</tt> method.  This allows the user to redefine a
+function that they incorrectly typed in before: if we didn't delete it, it
+would live in the symbol table, with a body, preventing future redefinition.</p>
+
+<p>This code does have a bug, though.  Since the <tt>Codegen.codegen_proto</tt>
+can return a previously defined forward declaration, our code can actually delete
+a forward declaration.  There are a number of ways to fix this bug, see what you
+can come up with!  Here is a testcase:</p>
+
+<div class="doc_code">
+<pre>
+extern foo(a b);     # ok, defines foo.
+def foo(a b) c;      # error, 'c' is invalid.
+def bar() foo(1, 2); # error, unknown function "foo"
+</pre>
+</div>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="driver">Driver Changes and
+Closing Thoughts</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+For now, code generation to LLVM doesn't really get us much, except that we can
+look at the pretty IR calls.  The sample code inserts calls to Codegen into the
+"<tt>Toplevel.main_loop</tt>", and then dumps out the LLVM IR.  This gives a
+nice way to look at the LLVM IR for simple functions.  For example:
+</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>4+5</b>;
+Read top-level expression:
+define double @""() {
+entry:
+        %addtmp = fadd double 4.000000e+00, 5.000000e+00
+        ret double %addtmp
+}
+</pre>
+</div>
+
+<p>Note how the parser turns the top-level expression into anonymous functions
+for us.  This will be handy when we add <a href="OCamlLangImpl4.html#jit">JIT
+support</a> in the next chapter.  Also note that the code is very literally
+transcribed, no optimizations are being performed.  We will
+<a href="OCamlLangImpl4.html#trivialconstfold">add optimizations</a> explicitly
+in the next chapter.</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def foo(a b) a*a + 2*a*b + b*b;</b>
+Read function definition:
+define double @foo(double %a, double %b) {
+entry:
+        %multmp = fmul double %a, %a
+        %multmp1 = fmul double 2.000000e+00, %a
+        %multmp2 = fmul double %multmp1, %b
+        %addtmp = fadd double %multmp, %multmp2
+        %multmp3 = fmul double %b, %b
+        %addtmp4 = fadd double %addtmp, %multmp3
+        ret double %addtmp4
+}
+</pre>
+</div>
+
+<p>This shows some simple arithmetic. Notice the striking similarity to the
+LLVM builder calls that we use to create the instructions.</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def bar(a) foo(a, 4.0) + bar(31337);</b>
+Read function definition:
+define double @bar(double %a) {
+entry:
+        %calltmp = call double @foo( double %a, double 4.000000e+00 )
+        %calltmp1 = call double @bar( double 3.133700e+04 )
+        %addtmp = fadd double %calltmp, %calltmp1
+        ret double %addtmp
+}
+</pre>
+</div>
+
+<p>This shows some function calls.  Note that this function will take a long
+time to execute if you call it.  In the future we'll add conditional control
+flow to actually make recursion useful :).</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>extern cos(x);</b>
+Read extern:
+declare double @cos(double)
+
+ready&gt; <b>cos(1.234);</b>
+Read top-level expression:
+define double @""() {
+entry:
+        %calltmp = call double @cos( double 1.234000e+00 )
+        ret double %calltmp
+}
+</pre>
+</div>
+
+<p>This shows an extern for the libm "cos" function, and a call to it.</p>
+
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>^D</b>
+; ModuleID = 'my cool jit'
+
+define double @""() {
+entry:
+        %addtmp = fadd double 4.000000e+00, 5.000000e+00
+        ret double %addtmp
+}
+
+define double @foo(double %a, double %b) {
+entry:
+        %multmp = fmul double %a, %a
+        %multmp1 = fmul double 2.000000e+00, %a
+        %multmp2 = fmul double %multmp1, %b
+        %addtmp = fadd double %multmp, %multmp2
+        %multmp3 = fmul double %b, %b
+        %addtmp4 = fadd double %addtmp, %multmp3
+        ret double %addtmp4
+}
+
+define double @bar(double %a) {
+entry:
+        %calltmp = call double @foo( double %a, double 4.000000e+00 )
+        %calltmp1 = call double @bar( double 3.133700e+04 )
+        %addtmp = fadd double %calltmp, %calltmp1
+        ret double %addtmp
+}
+
+declare double @cos(double)
+
+define double @""() {
+entry:
+        %calltmp = call double @cos( double 1.234000e+00 )
+        ret double %calltmp
+}
+</pre>
+</div>
+
+<p>When you quit the current demo, it dumps out the IR for the entire module
+generated.  Here you can see the big picture with all the functions referencing
+each other.</p>
+
+<p>This wraps up the third chapter of the Kaleidoscope tutorial.  Up next, we'll
+describe how to <a href="OCamlLangImpl4.html">add JIT codegen and optimizer
+support</a> to this so we can actually start running code!</p>
+
+</div>
+
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with the
+LLVM code generator.    Because this uses the LLVM libraries, we need to link
+them in.  To do this, we use the <a
+href="http://llvm.org/cmds/llvm-config.html">llvm-config</a> tool to inform
+our makefile/command line about which options to use:</p>
+
+<div class="doc_code">
+<pre>
+# Compile
+ocamlbuild toy.byte
+# Run
+./toy.byte
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<dl>
+<dt>_tags:</dt>
+<dd class="doc_code">
+<pre>
+&lt;{lexer,parser}.ml&gt;: use_camlp4, pp(camlp4of)
+&lt;*.{byte,native}&gt;: g++, use_llvm, use_llvm_analysis
+</pre>
+</dd>
+
+<dt>myocamlbuild.ml:</dt>
+<dd class="doc_code">
+<pre>
+open Ocamlbuild_plugin;;
+
+ocaml_lib ~extern:true "llvm";;
+ocaml_lib ~extern:true "llvm_analysis";;
+
+flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"]);;
+</pre>
+</dd>
+
+<dt>token.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer Tokens
+ *===----------------------------------------------------------------------===*)
+
+(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
+ * these others for known things. *)
+type token =
+  (* commands *)
+  | Def | Extern
+
+  (* primary *)
+  | Ident of string | Number of float
+
+  (* unknown *)
+  | Kwd of char
+</pre>
+</dd>
+
+<dt>lexer.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer
+ *===----------------------------------------------------------------------===*)
+
+let rec lex = parser
+  (* Skip any whitespace. *)
+  | [&lt; ' (' ' | '\n' | '\r' | '\t'); stream &gt;] -&gt; lex stream
+
+  (* identifier: [a-zA-Z][a-zA-Z0-9] *)
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+
+  (* number: [0-9.]+ *)
+  | [&lt; ' ('0' .. '9' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+
+  (* Comment until end of line. *)
+  | [&lt; ' ('#'); stream &gt;] -&gt;
+      lex_comment stream
+
+  (* Otherwise, just return the character as its ascii value. *)
+  | [&lt; 'c; stream &gt;] -&gt;
+      [&lt; 'Token.Kwd c; lex stream &gt;]
+
+  (* end of stream. *)
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+
+and lex_number buffer = parser
+  | [&lt; ' ('0' .. '9' | '.' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      [&lt; 'Token.Number (float_of_string (Buffer.contents buffer)); stream &gt;]
+
+and lex_ident buffer = parser
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+
+and lex_comment = parser
+  | [&lt; ' ('\n'); stream=lex &gt;] -&gt; stream
+  | [&lt; 'c; e=lex_comment &gt;] -&gt; e
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+</pre>
+</dd>
+
+<dt>ast.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Abstract Syntax Tree (aka Parse Tree)
+ *===----------------------------------------------------------------------===*)
+
+(* expr - Base type for all expression nodes. *)
+type expr =
+  (* variant for numeric literals like "1.0". *)
+  | Number of float
+
+  (* variant for referencing a variable, like "a". *)
+  | Variable of string
+
+  (* variant for a binary operator. *)
+  | Binary of char * expr * expr
+
+  (* variant for function calls. *)
+  | Call of string * expr array
+
+(* proto - This type represents the "prototype" for a function, which captures
+ * its name, and its argument names (thus implicitly the number of arguments the
+ * function takes). *)
+type proto = Prototype of string * string array
+
+(* func - This type represents a function definition itself. *)
+type func = Function of proto * expr
+</pre>
+</dd>
+
+<dt>parser.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===---------------------------------------------------------------------===
+ * Parser
+ *===---------------------------------------------------------------------===*)
+
+(* binop_precedence - This holds the precedence for each binary operator that is
+ * defined *)
+let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
+
+(* precedence - Get the precedence of the pending binary operator token. *)
+let precedence c = try Hashtbl.find binop_precedence c with Not_found -&gt; -1
+
+(* primary
+ *   ::= identifier
+ *   ::= numberexpr
+ *   ::= parenexpr *)
+let rec parse_primary = parser
+  (* numberexpr ::= number *)
+  | [&lt; 'Token.Number n &gt;] -&gt; Ast.Number n
+
+  (* parenexpr ::= '(' expression ')' *)
+  | [&lt; 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" &gt;] -&gt; e
+
+  (* identifierexpr
+   *   ::= identifier
+   *   ::= identifier '(' argumentexpr ')' *)
+  | [&lt; 'Token.Ident id; stream &gt;] -&gt;
+      let rec parse_args accumulator = parser
+        | [&lt; e=parse_expr; stream &gt;] -&gt;
+            begin parser
+              | [&lt; 'Token.Kwd ','; e=parse_args (e :: accumulator) &gt;] -&gt; e
+              | [&lt; &gt;] -&gt; e :: accumulator
+            end stream
+        | [&lt; &gt;] -&gt; accumulator
+      in
+      let rec parse_ident id = parser
+        (* Call. *)
+        | [&lt; 'Token.Kwd '(';
+             args=parse_args [];
+             'Token.Kwd ')' ?? "expected ')'"&gt;] -&gt;
+            Ast.Call (id, Array.of_list (List.rev args))
+
+        (* Simple variable ref. *)
+        | [&lt; &gt;] -&gt; Ast.Variable id
+      in
+      parse_ident id stream
+
+  | [&lt; &gt;] -&gt; raise (Stream.Error "unknown token when expecting an expression.")
+
+(* binoprhs
+ *   ::= ('+' primary)* *)
+and parse_bin_rhs expr_prec lhs stream =
+  match Stream.peek stream with
+  (* If this is a binop, find its precedence. *)
+  | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -&gt;
+      let token_prec = precedence c in
+
+      (* If this is a binop that binds at least as tightly as the current binop,
+       * consume it, otherwise we are done. *)
+      if token_prec &lt; expr_prec then lhs else begin
+        (* Eat the binop. *)
+        Stream.junk stream;
+
+        (* Parse the primary expression after the binary operator. *)
+        let rhs = parse_primary stream in
+
+        (* Okay, we know this is a binop. *)
+        let rhs =
+          match Stream.peek stream with
+          | Some (Token.Kwd c2) -&gt;
+              (* If BinOp binds less tightly with rhs than the operator after
+               * rhs, let the pending operator take rhs as its lhs. *)
+              let next_prec = precedence c2 in
+              if token_prec &lt; next_prec
+              then parse_bin_rhs (token_prec + 1) rhs stream
+              else rhs
+          | _ -&gt; rhs
+        in
+
+        (* Merge lhs/rhs. *)
+        let lhs = Ast.Binary (c, lhs, rhs) in
+        parse_bin_rhs expr_prec lhs stream
+      end
+  | _ -&gt; lhs
+
+(* expression
+ *   ::= primary binoprhs *)
+and parse_expr = parser
+  | [&lt; lhs=parse_primary; stream &gt;] -&gt; parse_bin_rhs 0 lhs stream
+
+(* prototype
+ *   ::= id '(' id* ')' *)
+let parse_prototype =
+  let rec parse_args accumulator = parser
+    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
+    | [&lt; &gt;] -&gt; accumulator
+  in
+
+  parser
+  | [&lt; 'Token.Ident id;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+       args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      (* success. *)
+      Ast.Prototype (id, Array.of_list (List.rev args))
+
+  | [&lt; &gt;] -&gt;
+      raise (Stream.Error "expected function name in prototype")
+
+(* definition ::= 'def' prototype expression *)
+let parse_definition = parser
+  | [&lt; 'Token.Def; p=parse_prototype; e=parse_expr &gt;] -&gt;
+      Ast.Function (p, e)
+
+(* toplevelexpr ::= expression *)
+let parse_toplevel = parser
+  | [&lt; e=parse_expr &gt;] -&gt;
+      (* Make an anonymous proto. *)
+      Ast.Function (Ast.Prototype ("", [||]), e)
+
+(*  external ::= 'extern' prototype *)
+let parse_extern = parser
+  | [&lt; 'Token.Extern; e=parse_prototype &gt;] -&gt; e
+</pre>
+</dd>
+
+<dt>codegen.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Code Generation
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+
+exception Error of string
+
+let context = global_context ()
+let the_module = create_module context "my cool jit"
+let builder = builder context
+let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
+let double_type = double_type context
+
+let rec codegen_expr = function
+  | Ast.Number n -&gt; const_float double_type n
+  | Ast.Variable name -&gt;
+      (try Hashtbl.find named_values name with
+        | Not_found -&gt; raise (Error "unknown variable name"))
+  | Ast.Binary (op, lhs, rhs) -&gt;
+      let lhs_val = codegen_expr lhs in
+      let rhs_val = codegen_expr rhs in
+      begin
+        match op with
+        | '+' -&gt; build_add lhs_val rhs_val "addtmp" builder
+        | '-' -&gt; build_sub lhs_val rhs_val "subtmp" builder
+        | '*' -&gt; build_mul lhs_val rhs_val "multmp" builder
+        | '&lt;' -&gt;
+            (* Convert bool 0/1 to double 0.0 or 1.0 *)
+            let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
+            build_uitofp i double_type "booltmp" builder
+        | _ -&gt; raise (Error "invalid binary operator")
+      end
+  | Ast.Call (callee, args) -&gt;
+      (* Look up the name in the module table. *)
+      let callee =
+        match lookup_function callee the_module with
+        | Some callee -&gt; callee
+        | None -&gt; raise (Error "unknown function referenced")
+      in
+      let params = params callee in
+
+      (* If argument mismatch error. *)
+      if Array.length params == Array.length args then () else
+        raise (Error "incorrect # arguments passed");
+      let args = Array.map codegen_expr args in
+      build_call callee args "calltmp" builder
+
+let codegen_proto = function
+  | Ast.Prototype (name, args) -&gt;
+      (* Make the function type: double(double,double) etc. *)
+      let doubles = Array.make (Array.length args) double_type in
+      let ft = function_type double_type doubles in
+      let f =
+        match lookup_function name the_module with
+        | None -&gt; declare_function name ft the_module
+
+        (* If 'f' conflicted, there was already something named 'name'. If it
+         * has a body, don't allow redefinition or reextern. *)
+        | Some f -&gt;
+            (* If 'f' already has a body, reject this. *)
+            if block_begin f &lt;&gt; At_end f then
+              raise (Error "redefinition of function");
+
+            (* If 'f' took a different number of arguments, reject. *)
+            if element_type (type_of f) &lt;&gt; ft then
+              raise (Error "redefinition of function with different # args");
+            f
+      in
+
+      (* Set names for all arguments. *)
+      Array.iteri (fun i a -&gt;
+        let n = args.(i) in
+        set_value_name n a;
+        Hashtbl.add named_values n a;
+      ) (params f);
+      f
+
+let codegen_func = function
+  | Ast.Function (proto, body) -&gt;
+      Hashtbl.clear named_values;
+      let the_function = codegen_proto proto in
+
+      (* Create a new basic block to start insertion into. *)
+      let bb = append_block context "entry" the_function in
+      position_at_end bb builder;
+
+      try
+        let ret_val = codegen_expr body in
+
+        (* Finish off the function. *)
+        let _ = build_ret ret_val builder in
+
+        (* Validate the generated code, checking for consistency. *)
+        Llvm_analysis.assert_valid_function the_function;
+
+        the_function
+      with e -&gt;
+        delete_function the_function;
+        raise e
+</pre>
+</dd>
+
+<dt>toplevel.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Top-Level parsing and JIT Driver
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+
+(* top ::= definition | external | expression | ';' *)
+let rec main_loop stream =
+  match Stream.peek stream with
+  | None -&gt; ()
+
+  (* ignore top-level semicolons. *)
+  | Some (Token.Kwd ';') -&gt;
+      Stream.junk stream;
+      main_loop stream
+
+  | Some token -&gt;
+      begin
+        try match token with
+        | Token.Def -&gt;
+            let e = Parser.parse_definition stream in
+            print_endline "parsed a function definition.";
+            dump_value (Codegen.codegen_func e);
+        | Token.Extern -&gt;
+            let e = Parser.parse_extern stream in
+            print_endline "parsed an extern.";
+            dump_value (Codegen.codegen_proto e);
+        | _ -&gt;
+            (* Evaluate a top-level expression into an anonymous function. *)
+            let e = Parser.parse_toplevel stream in
+            print_endline "parsed a top-level expr";
+            dump_value (Codegen.codegen_func e);
+        with Stream.Error s | Codegen.Error s -&gt;
+          (* Skip token for error recovery. *)
+          Stream.junk stream;
+          print_endline s;
+      end;
+      print_string "ready&gt; "; flush stdout;
+      main_loop stream
+</pre>
+</dd>
+
+<dt>toy.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Main driver code.
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+
+let main () =
+  (* Install standard binary operators.
+   * 1 is the lowest precedence. *)
+  Hashtbl.add Parser.binop_precedence '&lt;' 10;
+  Hashtbl.add Parser.binop_precedence '+' 20;
+  Hashtbl.add Parser.binop_precedence '-' 20;
+  Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
+
+  (* Prime the first token. *)
+  print_string "ready&gt; "; flush stdout;
+  let stream = Lexer.lex (Stream.of_channel stdin) in
+
+  (* Run the main "interpreter loop" now. *)
+  Toplevel.main_loop stream;
+
+  (* Print out all the generated code. *)
+  dump_module Codegen.the_module
+;;
+
+main ()
+</pre>
+</dd>
+</dl>
+
+<a href="OCamlLangImpl4.html">Next: Adding JIT and Optimizer Support</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/OCamlLangImpl4.html b/docs/tutorial/OCamlLangImpl4.html
new file mode 100644
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+++ b/docs/tutorial/OCamlLangImpl4.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Adding JIT and Optimizer Support</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <meta name="author" content="Erick Tryzelaar">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Adding JIT and Optimizer Support</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 4
+  <ol>
+    <li><a href="#intro">Chapter 4 Introduction</a></li>
+    <li><a href="#trivialconstfold">Trivial Constant Folding</a></li>
+    <li><a href="#optimizerpasses">LLVM Optimization Passes</a></li>
+    <li><a href="#jit">Adding a JIT Compiler</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="OCamlLangImpl5.html">Chapter 5</a>: Extending the Language: Control
+Flow</li>
+</ul>
+
+<div class="doc_author">
+	<p>
+		Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
+		and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
+	</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 4 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 4 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  Chapters 1-3 described the implementation of a simple
+language and added support for generating LLVM IR.  This chapter describes
+two new techniques: adding optimizer support to your language, and adding JIT
+compiler support.  These additions will demonstrate how to get nice, efficient code
+for the Kaleidoscope language.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="trivialconstfold">Trivial Constant
+Folding</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p><b>Note:</b> the default <tt>IRBuilder</tt> now always includes the constant 
+folding optimisations below.<p>
+
+<p>
+Our demonstration for Chapter 3 is elegant and easy to extend.  Unfortunately,
+it does not produce wonderful code.  For example, when compiling simple code,
+we don't get obvious optimizations:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def test(x) 1+2+x;</b>
+Read function definition:
+define double @test(double %x) {
+entry:
+        %addtmp = fadd double 1.000000e+00, 2.000000e+00
+        %addtmp1 = fadd double %addtmp, %x
+        ret double %addtmp1
+}
+</pre>
+</div>
+
+<p>This code is a very, very literal transcription of the AST built by parsing
+the input. As such, this transcription lacks optimizations like constant folding
+(we'd like to get "<tt>add x, 3.0</tt>" in the example above) as well as other
+more important optimizations.  Constant folding, in particular, is a very common
+and very important optimization: so much so that many language implementors
+implement constant folding support in their AST representation.</p>
+
+<p>With LLVM, you don't need this support in the AST.  Since all calls to build
+LLVM IR go through the LLVM builder, it would be nice if the builder itself
+checked to see if there was a constant folding opportunity when you call it.
+If so, it could just do the constant fold and return the constant instead of
+creating an instruction.  This is exactly what the <tt>LLVMFoldingBuilder</tt>
+class does.
+
+<p>All we did was switch from <tt>LLVMBuilder</tt> to
+<tt>LLVMFoldingBuilder</tt>.  Though we change no other code, we now have all of our
+instructions implicitly constant folded without us having to do anything
+about it.  For example, the input above now compiles to:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def test(x) 1+2+x;</b>
+Read function definition:
+define double @test(double %x) {
+entry:
+        %addtmp = fadd double 3.000000e+00, %x
+        ret double %addtmp
+}
+</pre>
+</div>
+
+<p>Well, that was easy :).  In practice, we recommend always using
+<tt>LLVMFoldingBuilder</tt> when generating code like this.  It has no
+"syntactic overhead" for its use (you don't have to uglify your compiler with
+constant checks everywhere) and it can dramatically reduce the amount of
+LLVM IR that is generated in some cases (particular for languages with a macro
+preprocessor or that use a lot of constants).</p>
+
+<p>On the other hand, the <tt>LLVMFoldingBuilder</tt> is limited by the fact
+that it does all of its analysis inline with the code as it is built.  If you
+take a slightly more complex example:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def test(x) (1+2+x)*(x+(1+2));</b>
+ready&gt; Read function definition:
+define double @test(double %x) {
+entry:
+        %addtmp = fadd double 3.000000e+00, %x
+        %addtmp1 = fadd double %x, 3.000000e+00
+        %multmp = fmul double %addtmp, %addtmp1
+        ret double %multmp
+}
+</pre>
+</div>
+
+<p>In this case, the LHS and RHS of the multiplication are the same value.  We'd
+really like to see this generate "<tt>tmp = x+3; result = tmp*tmp;</tt>" instead
+of computing "<tt>x*3</tt>" twice.</p>
+
+<p>Unfortunately, no amount of local analysis will be able to detect and correct
+this.  This requires two transformations: reassociation of expressions (to
+make the add's lexically identical) and Common Subexpression Elimination (CSE)
+to  delete the redundant add instruction.  Fortunately, LLVM provides a broad
+range of optimizations that you can use, in the form of "passes".</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="optimizerpasses">LLVM Optimization
+ Passes</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>LLVM provides many optimization passes, which do many different sorts of
+things and have different tradeoffs.  Unlike other systems, LLVM doesn't hold
+to the mistaken notion that one set of optimizations is right for all languages
+and for all situations.  LLVM allows a compiler implementor to make complete
+decisions about what optimizations to use, in which order, and in what
+situation.</p>
+
+<p>As a concrete example, LLVM supports both "whole module" passes, which look
+across as large of body of code as they can (often a whole file, but if run
+at link time, this can be a substantial portion of the whole program).  It also
+supports and includes "per-function" passes which just operate on a single
+function at a time, without looking at other functions.  For more information
+on passes and how they are run, see the <a href="../WritingAnLLVMPass.html">How
+to Write a Pass</a> document and the <a href="../Passes.html">List of LLVM
+Passes</a>.</p>
+
+<p>For Kaleidoscope, we are currently generating functions on the fly, one at
+a time, as the user types them in.  We aren't shooting for the ultimate
+optimization experience in this setting, but we also want to catch the easy and
+quick stuff where possible.  As such, we will choose to run a few per-function
+optimizations as the user types the function in.  If we wanted to make a "static
+Kaleidoscope compiler", we would use exactly the code we have now, except that
+we would defer running the optimizer until the entire file has been parsed.</p>
+
+<p>In order to get per-function optimizations going, we need to set up a
+<a href="../WritingAnLLVMPass.html#passmanager">Llvm.PassManager</a> to hold and
+organize the LLVM optimizations that we want to run.  Once we have that, we can
+add a set of optimizations to run.  The code looks like this:</p>
+
+<div class="doc_code">
+<pre>
+  (* Create the JIT. *)
+  let the_execution_engine = ExecutionEngine.create Codegen.the_module in
+  let the_fpm = PassManager.create_function Codegen.the_module in
+
+  (* Set up the optimizer pipeline.  Start with registering info about how the
+   * target lays out data structures. *)
+  TargetData.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
+
+  (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
+  add_instruction_combining the_fpm;
+
+  (* reassociate expressions. *)
+  add_reassociation the_fpm;
+
+  (* Eliminate Common SubExpressions. *)
+  add_gvn the_fpm;
+
+  (* Simplify the control flow graph (deleting unreachable blocks, etc). *)
+  add_cfg_simplification the_fpm;
+
+  ignore (PassManager.initialize the_fpm);
+
+  (* Run the main "interpreter loop" now. *)
+  Toplevel.main_loop the_fpm the_execution_engine stream;
+</pre>
+</div>
+
+<p>The meat of the matter here, is the definition of "<tt>the_fpm</tt>".  It
+requires a pointer to the <tt>the_module</tt> to construct itself.  Once it is
+set up, we use a series of "add" calls to add a bunch of LLVM passes.  The
+first pass is basically boilerplate, it adds a pass so that later optimizations
+know how the data structures in the program are laid out.  The
+"<tt>the_execution_engine</tt>" variable is related to the JIT, which we will
+get to in the next section.</p>
+
+<p>In this case, we choose to add 4 optimization passes.  The passes we chose
+here are a pretty standard set of "cleanup" optimizations that are useful for
+a wide variety of code.  I won't delve into what they do but, believe me,
+they are a good starting place :).</p>
+
+<p>Once the <tt>Llvm.PassManager.</tt> is set up, we need to make use of it.
+We do this by running it after our newly created function is constructed (in
+<tt>Codegen.codegen_func</tt>), but before it is returned to the client:</p>
+
+<div class="doc_code">
+<pre>
+let codegen_func the_fpm = function
+      ...
+      try
+        let ret_val = codegen_expr body in
+
+        (* Finish off the function. *)
+        let _ = build_ret ret_val builder in
+
+        (* Validate the generated code, checking for consistency. *)
+        Llvm_analysis.assert_valid_function the_function;
+
+        (* Optimize the function. *)
+        let _ = PassManager.run_function the_function the_fpm in
+
+        the_function
+</pre>
+</div>
+
+<p>As you can see, this is pretty straightforward.  The <tt>the_fpm</tt>
+optimizes and updates the LLVM Function* in place, improving (hopefully) its
+body.  With this in place, we can try our test above again:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def test(x) (1+2+x)*(x+(1+2));</b>
+ready&gt; Read function definition:
+define double @test(double %x) {
+entry:
+        %addtmp = fadd double %x, 3.000000e+00
+        %multmp = fmul double %addtmp, %addtmp
+        ret double %multmp
+}
+</pre>
+</div>
+
+<p>As expected, we now get our nicely optimized code, saving a floating point
+add instruction from every execution of this function.</p>
+
+<p>LLVM provides a wide variety of optimizations that can be used in certain
+circumstances.  Some <a href="../Passes.html">documentation about the various
+passes</a> is available, but it isn't very complete.  Another good source of
+ideas can come from looking at the passes that <tt>llvm-gcc</tt> or
+<tt>llvm-ld</tt> run to get started.  The "<tt>opt</tt>" tool allows you to
+experiment with passes from the command line, so you can see if they do
+anything.</p>
+
+<p>Now that we have reasonable code coming out of our front-end, lets talk about
+executing it!</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="jit">Adding a JIT Compiler</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Code that is available in LLVM IR can have a wide variety of tools
+applied to it.  For example, you can run optimizations on it (as we did above),
+you can dump it out in textual or binary forms, you can compile the code to an
+assembly file (.s) for some target, or you can JIT compile it.  The nice thing
+about the LLVM IR representation is that it is the "common currency" between
+many different parts of the compiler.
+</p>
+
+<p>In this section, we'll add JIT compiler support to our interpreter.  The
+basic idea that we want for Kaleidoscope is to have the user enter function
+bodies as they do now, but immediately evaluate the top-level expressions they
+type in.  For example, if they type in "1 + 2;", we should evaluate and print
+out 3.  If they define a function, they should be able to call it from the
+command line.</p>
+
+<p>In order to do this, we first declare and initialize the JIT.  This is done
+by adding a global variable and a call in <tt>main</tt>:</p>
+
+<div class="doc_code">
+<pre>
+...
+let main () =
+  ...
+  <b>(* Create the JIT. *)
+  let the_execution_engine = ExecutionEngine.create Codegen.the_module in</b>
+  ...
+</pre>
+</div>
+
+<p>This creates an abstract "Execution Engine" which can be either a JIT
+compiler or the LLVM interpreter.  LLVM will automatically pick a JIT compiler
+for you if one is available for your platform, otherwise it will fall back to
+the interpreter.</p>
+
+<p>Once the <tt>Llvm_executionengine.ExecutionEngine.t</tt> is created, the JIT
+is ready to be used.  There are a variety of APIs that are useful, but the
+simplest one is the "<tt>Llvm_executionengine.ExecutionEngine.run_function</tt>"
+function.  This method JIT compiles the specified LLVM Function and returns a
+function pointer to the generated machine code.  In our case, this means that we
+can change the code that parses a top-level expression to look like this:</p>
+
+<div class="doc_code">
+<pre>
+            (* Evaluate a top-level expression into an anonymous function. *)
+            let e = Parser.parse_toplevel stream in
+            print_endline "parsed a top-level expr";
+            let the_function = Codegen.codegen_func the_fpm e in
+            dump_value the_function;
+
+            (* JIT the function, returning a function pointer. *)
+            let result = ExecutionEngine.run_function the_function [||]
+              the_execution_engine in
+
+            print_string "Evaluated to ";
+            print_float (GenericValue.as_float Codegen.double_type result);
+            print_newline ();
+</pre>
+</div>
+
+<p>Recall that we compile top-level expressions into a self-contained LLVM
+function that takes no arguments and returns the computed double.  Because the
+LLVM JIT compiler matches the native platform ABI, this means that you can just
+cast the result pointer to a function pointer of that type and call it directly.
+This means, there is no difference between JIT compiled code and native machine
+code that is statically linked into your application.</p>
+
+<p>With just these two changes, lets see how Kaleidoscope works now!</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>4+5;</b>
+define double @""() {
+entry:
+        ret double 9.000000e+00
+}
+
+<em>Evaluated to 9.000000</em>
+</pre>
+</div>
+
+<p>Well this looks like it is basically working.  The dump of the function
+shows the "no argument function that always returns double" that we synthesize
+for each top level expression that is typed in.  This demonstrates very basic
+functionality, but can we do more?</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>def testfunc(x y) x + y*2; </b>
+Read function definition:
+define double @testfunc(double %x, double %y) {
+entry:
+        %multmp = fmul double %y, 2.000000e+00
+        %addtmp = fadd double %multmp, %x
+        ret double %addtmp
+}
+
+ready&gt; <b>testfunc(4, 10);</b>
+define double @""() {
+entry:
+        %calltmp = call double @testfunc( double 4.000000e+00, double 1.000000e+01 )
+        ret double %calltmp
+}
+
+<em>Evaluated to 24.000000</em>
+</pre>
+</div>
+
+<p>This illustrates that we can now call user code, but there is something a bit
+subtle going on here.  Note that we only invoke the JIT on the anonymous
+functions that <em>call testfunc</em>, but we never invoked it
+on <em>testfunc</em> itself.  What actually happened here is that the JIT
+scanned for all non-JIT'd functions transitively called from the anonymous
+function and compiled all of them before returning
+from <tt>run_function</tt>.</p>
+
+<p>The JIT provides a number of other more advanced interfaces for things like
+freeing allocated machine code, rejit'ing functions to update them, etc.
+However, even with this simple code, we get some surprisingly powerful
+capabilities - check this out (I removed the dump of the anonymous functions,
+you should get the idea by now :) :</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>extern sin(x);</b>
+Read extern:
+declare double @sin(double)
+
+ready&gt; <b>extern cos(x);</b>
+Read extern:
+declare double @cos(double)
+
+ready&gt; <b>sin(1.0);</b>
+<em>Evaluated to 0.841471</em>
+
+ready&gt; <b>def foo(x) sin(x)*sin(x) + cos(x)*cos(x);</b>
+Read function definition:
+define double @foo(double %x) {
+entry:
+        %calltmp = call double @sin( double %x )
+        %multmp = fmul double %calltmp, %calltmp
+        %calltmp2 = call double @cos( double %x )
+        %multmp4 = fmul double %calltmp2, %calltmp2
+        %addtmp = fadd double %multmp, %multmp4
+        ret double %addtmp
+}
+
+ready&gt; <b>foo(4.0);</b>
+<em>Evaluated to 1.000000</em>
+</pre>
+</div>
+
+<p>Whoa, how does the JIT know about sin and cos?  The answer is surprisingly
+simple: in this example, the JIT started execution of a function and got to a
+function call.  It realized that the function was not yet JIT compiled and
+invoked the standard set of routines to resolve the function.  In this case,
+there is no body defined for the function, so the JIT ended up calling
+"<tt>dlsym("sin")</tt>" on the Kaleidoscope process itself.  Since
+"<tt>sin</tt>" is defined within the JIT's address space, it simply patches up
+calls in the module to call the libm version of <tt>sin</tt> directly.</p>
+
+<p>The LLVM JIT provides a number of interfaces (look in the
+<tt>llvm_executionengine.mli</tt> file) for controlling how unknown functions
+get resolved.  It allows you to establish explicit mappings between IR objects
+and addresses (useful for LLVM global variables that you want to map to static
+tables, for example), allows you to dynamically decide on the fly based on the
+function name, and even allows you to have the JIT compile functions lazily the
+first time they're called.</p>
+
+<p>One interesting application of this is that we can now extend the language
+by writing arbitrary C code to implement operations.  For example, if we add:
+</p>
+
+<div class="doc_code">
+<pre>
+/* putchard - putchar that takes a double and returns 0. */
+extern "C"
+double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+</pre>
+</div>
+
+<p>Now we can produce simple output to the console by using things like:
+"<tt>extern putchard(x); putchard(120);</tt>", which prints a lowercase 'x' on
+the console (120 is the ASCII code for 'x').  Similar code could be used to
+implement file I/O, console input, and many other capabilities in
+Kaleidoscope.</p>
+
+<p>This completes the JIT and optimizer chapter of the Kaleidoscope tutorial. At
+this point, we can compile a non-Turing-complete programming language, optimize
+and JIT compile it in a user-driven way.  Next up we'll look into <a
+href="OCamlLangImpl5.html">extending the language with control flow
+constructs</a>, tackling some interesting LLVM IR issues along the way.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with the
+LLVM JIT and optimizer.  To build this example, use:
+</p>
+
+<div class="doc_code">
+<pre>
+# Compile
+ocamlbuild toy.byte
+# Run
+./toy.byte
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<dl>
+<dt>_tags:</dt>
+<dd class="doc_code">
+<pre>
+&lt;{lexer,parser}.ml&gt;: use_camlp4, pp(camlp4of)
+&lt;*.{byte,native}&gt;: g++, use_llvm, use_llvm_analysis
+&lt;*.{byte,native}&gt;: use_llvm_executionengine, use_llvm_target
+&lt;*.{byte,native}&gt;: use_llvm_scalar_opts, use_bindings
+</pre>
+</dd>
+
+<dt>myocamlbuild.ml:</dt>
+<dd class="doc_code">
+<pre>
+open Ocamlbuild_plugin;;
+
+ocaml_lib ~extern:true "llvm";;
+ocaml_lib ~extern:true "llvm_analysis";;
+ocaml_lib ~extern:true "llvm_executionengine";;
+ocaml_lib ~extern:true "llvm_target";;
+ocaml_lib ~extern:true "llvm_scalar_opts";;
+
+flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"]);;
+dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
+</pre>
+</dd>
+
+<dt>token.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer Tokens
+ *===----------------------------------------------------------------------===*)
+
+(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
+ * these others for known things. *)
+type token =
+  (* commands *)
+  | Def | Extern
+
+  (* primary *)
+  | Ident of string | Number of float
+
+  (* unknown *)
+  | Kwd of char
+</pre>
+</dd>
+
+<dt>lexer.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer
+ *===----------------------------------------------------------------------===*)
+
+let rec lex = parser
+  (* Skip any whitespace. *)
+  | [&lt; ' (' ' | '\n' | '\r' | '\t'); stream &gt;] -&gt; lex stream
+
+  (* identifier: [a-zA-Z][a-zA-Z0-9] *)
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+
+  (* number: [0-9.]+ *)
+  | [&lt; ' ('0' .. '9' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+
+  (* Comment until end of line. *)
+  | [&lt; ' ('#'); stream &gt;] -&gt;
+      lex_comment stream
+
+  (* Otherwise, just return the character as its ascii value. *)
+  | [&lt; 'c; stream &gt;] -&gt;
+      [&lt; 'Token.Kwd c; lex stream &gt;]
+
+  (* end of stream. *)
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+
+and lex_number buffer = parser
+  | [&lt; ' ('0' .. '9' | '.' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      [&lt; 'Token.Number (float_of_string (Buffer.contents buffer)); stream &gt;]
+
+and lex_ident buffer = parser
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+
+and lex_comment = parser
+  | [&lt; ' ('\n'); stream=lex &gt;] -&gt; stream
+  | [&lt; 'c; e=lex_comment &gt;] -&gt; e
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+</pre>
+</dd>
+
+<dt>ast.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Abstract Syntax Tree (aka Parse Tree)
+ *===----------------------------------------------------------------------===*)
+
+(* expr - Base type for all expression nodes. *)
+type expr =
+  (* variant for numeric literals like "1.0". *)
+  | Number of float
+
+  (* variant for referencing a variable, like "a". *)
+  | Variable of string
+
+  (* variant for a binary operator. *)
+  | Binary of char * expr * expr
+
+  (* variant for function calls. *)
+  | Call of string * expr array
+
+(* proto - This type represents the "prototype" for a function, which captures
+ * its name, and its argument names (thus implicitly the number of arguments the
+ * function takes). *)
+type proto = Prototype of string * string array
+
+(* func - This type represents a function definition itself. *)
+type func = Function of proto * expr
+</pre>
+</dd>
+
+<dt>parser.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===---------------------------------------------------------------------===
+ * Parser
+ *===---------------------------------------------------------------------===*)
+
+(* binop_precedence - This holds the precedence for each binary operator that is
+ * defined *)
+let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
+
+(* precedence - Get the precedence of the pending binary operator token. *)
+let precedence c = try Hashtbl.find binop_precedence c with Not_found -&gt; -1
+
+(* primary
+ *   ::= identifier
+ *   ::= numberexpr
+ *   ::= parenexpr *)
+let rec parse_primary = parser
+  (* numberexpr ::= number *)
+  | [&lt; 'Token.Number n &gt;] -&gt; Ast.Number n
+
+  (* parenexpr ::= '(' expression ')' *)
+  | [&lt; 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" &gt;] -&gt; e
+
+  (* identifierexpr
+   *   ::= identifier
+   *   ::= identifier '(' argumentexpr ')' *)
+  | [&lt; 'Token.Ident id; stream &gt;] -&gt;
+      let rec parse_args accumulator = parser
+        | [&lt; e=parse_expr; stream &gt;] -&gt;
+            begin parser
+              | [&lt; 'Token.Kwd ','; e=parse_args (e :: accumulator) &gt;] -&gt; e
+              | [&lt; &gt;] -&gt; e :: accumulator
+            end stream
+        | [&lt; &gt;] -&gt; accumulator
+      in
+      let rec parse_ident id = parser
+        (* Call. *)
+        | [&lt; 'Token.Kwd '(';
+             args=parse_args [];
+             'Token.Kwd ')' ?? "expected ')'"&gt;] -&gt;
+            Ast.Call (id, Array.of_list (List.rev args))
+
+        (* Simple variable ref. *)
+        | [&lt; &gt;] -&gt; Ast.Variable id
+      in
+      parse_ident id stream
+
+  | [&lt; &gt;] -&gt; raise (Stream.Error "unknown token when expecting an expression.")
+
+(* binoprhs
+ *   ::= ('+' primary)* *)
+and parse_bin_rhs expr_prec lhs stream =
+  match Stream.peek stream with
+  (* If this is a binop, find its precedence. *)
+  | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -&gt;
+      let token_prec = precedence c in
+
+      (* If this is a binop that binds at least as tightly as the current binop,
+       * consume it, otherwise we are done. *)
+      if token_prec &lt; expr_prec then lhs else begin
+        (* Eat the binop. *)
+        Stream.junk stream;
+
+        (* Parse the primary expression after the binary operator. *)
+        let rhs = parse_primary stream in
+
+        (* Okay, we know this is a binop. *)
+        let rhs =
+          match Stream.peek stream with
+          | Some (Token.Kwd c2) -&gt;
+              (* If BinOp binds less tightly with rhs than the operator after
+               * rhs, let the pending operator take rhs as its lhs. *)
+              let next_prec = precedence c2 in
+              if token_prec &lt; next_prec
+              then parse_bin_rhs (token_prec + 1) rhs stream
+              else rhs
+          | _ -&gt; rhs
+        in
+
+        (* Merge lhs/rhs. *)
+        let lhs = Ast.Binary (c, lhs, rhs) in
+        parse_bin_rhs expr_prec lhs stream
+      end
+  | _ -&gt; lhs
+
+(* expression
+ *   ::= primary binoprhs *)
+and parse_expr = parser
+  | [&lt; lhs=parse_primary; stream &gt;] -&gt; parse_bin_rhs 0 lhs stream
+
+(* prototype
+ *   ::= id '(' id* ')' *)
+let parse_prototype =
+  let rec parse_args accumulator = parser
+    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
+    | [&lt; &gt;] -&gt; accumulator
+  in
+
+  parser
+  | [&lt; 'Token.Ident id;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+       args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      (* success. *)
+      Ast.Prototype (id, Array.of_list (List.rev args))
+
+  | [&lt; &gt;] -&gt;
+      raise (Stream.Error "expected function name in prototype")
+
+(* definition ::= 'def' prototype expression *)
+let parse_definition = parser
+  | [&lt; 'Token.Def; p=parse_prototype; e=parse_expr &gt;] -&gt;
+      Ast.Function (p, e)
+
+(* toplevelexpr ::= expression *)
+let parse_toplevel = parser
+  | [&lt; e=parse_expr &gt;] -&gt;
+      (* Make an anonymous proto. *)
+      Ast.Function (Ast.Prototype ("", [||]), e)
+
+(*  external ::= 'extern' prototype *)
+let parse_extern = parser
+  | [&lt; 'Token.Extern; e=parse_prototype &gt;] -&gt; e
+</pre>
+</dd>
+
+<dt>codegen.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Code Generation
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+
+exception Error of string
+
+let context = global_context ()
+let the_module = create_module context "my cool jit"
+let builder = builder context
+let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
+let double_type = double_type context
+
+let rec codegen_expr = function
+  | Ast.Number n -&gt; const_float double_type n
+  | Ast.Variable name -&gt;
+      (try Hashtbl.find named_values name with
+        | Not_found -&gt; raise (Error "unknown variable name"))
+  | Ast.Binary (op, lhs, rhs) -&gt;
+      let lhs_val = codegen_expr lhs in
+      let rhs_val = codegen_expr rhs in
+      begin
+        match op with
+        | '+' -&gt; build_add lhs_val rhs_val "addtmp" builder
+        | '-' -&gt; build_sub lhs_val rhs_val "subtmp" builder
+        | '*' -&gt; build_mul lhs_val rhs_val "multmp" builder
+        | '&lt;' -&gt;
+            (* Convert bool 0/1 to double 0.0 or 1.0 *)
+            let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
+            build_uitofp i double_type "booltmp" builder
+        | _ -&gt; raise (Error "invalid binary operator")
+      end
+  | Ast.Call (callee, args) -&gt;
+      (* Look up the name in the module table. *)
+      let callee =
+        match lookup_function callee the_module with
+        | Some callee -&gt; callee
+        | None -&gt; raise (Error "unknown function referenced")
+      in
+      let params = params callee in
+
+      (* If argument mismatch error. *)
+      if Array.length params == Array.length args then () else
+        raise (Error "incorrect # arguments passed");
+      let args = Array.map codegen_expr args in
+      build_call callee args "calltmp" builder
+
+let codegen_proto = function
+  | Ast.Prototype (name, args) -&gt;
+      (* Make the function type: double(double,double) etc. *)
+      let doubles = Array.make (Array.length args) double_type in
+      let ft = function_type double_type doubles in
+      let f =
+        match lookup_function name the_module with
+        | None -&gt; declare_function name ft the_module
+
+        (* If 'f' conflicted, there was already something named 'name'. If it
+         * has a body, don't allow redefinition or reextern. *)
+        | Some f -&gt;
+            (* If 'f' already has a body, reject this. *)
+            if block_begin f &lt;&gt; At_end f then
+              raise (Error "redefinition of function");
+
+            (* If 'f' took a different number of arguments, reject. *)
+            if element_type (type_of f) &lt;&gt; ft then
+              raise (Error "redefinition of function with different # args");
+            f
+      in
+
+      (* Set names for all arguments. *)
+      Array.iteri (fun i a -&gt;
+        let n = args.(i) in
+        set_value_name n a;
+        Hashtbl.add named_values n a;
+      ) (params f);
+      f
+
+let codegen_func the_fpm = function
+  | Ast.Function (proto, body) -&gt;
+      Hashtbl.clear named_values;
+      let the_function = codegen_proto proto in
+
+      (* Create a new basic block to start insertion into. *)
+      let bb = append_block context "entry" the_function in
+      position_at_end bb builder;
+
+      try
+        let ret_val = codegen_expr body in
+
+        (* Finish off the function. *)
+        let _ = build_ret ret_val builder in
+
+        (* Validate the generated code, checking for consistency. *)
+        Llvm_analysis.assert_valid_function the_function;
+
+        (* Optimize the function. *)
+        let _ = PassManager.run_function the_function the_fpm in
+
+        the_function
+      with e -&gt;
+        delete_function the_function;
+        raise e
+</pre>
+</dd>
+
+<dt>toplevel.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Top-Level parsing and JIT Driver
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+open Llvm_executionengine
+
+(* top ::= definition | external | expression | ';' *)
+let rec main_loop the_fpm the_execution_engine stream =
+  match Stream.peek stream with
+  | None -&gt; ()
+
+  (* ignore top-level semicolons. *)
+  | Some (Token.Kwd ';') -&gt;
+      Stream.junk stream;
+      main_loop the_fpm the_execution_engine stream
+
+  | Some token -&gt;
+      begin
+        try match token with
+        | Token.Def -&gt;
+            let e = Parser.parse_definition stream in
+            print_endline "parsed a function definition.";
+            dump_value (Codegen.codegen_func the_fpm e);
+        | Token.Extern -&gt;
+            let e = Parser.parse_extern stream in
+            print_endline "parsed an extern.";
+            dump_value (Codegen.codegen_proto e);
+        | _ -&gt;
+            (* Evaluate a top-level expression into an anonymous function. *)
+            let e = Parser.parse_toplevel stream in
+            print_endline "parsed a top-level expr";
+            let the_function = Codegen.codegen_func the_fpm e in
+            dump_value the_function;
+
+            (* JIT the function, returning a function pointer. *)
+            let result = ExecutionEngine.run_function the_function [||]
+              the_execution_engine in
+
+            print_string "Evaluated to ";
+            print_float (GenericValue.as_float Codegen.double_type result);
+            print_newline ();
+        with Stream.Error s | Codegen.Error s -&gt;
+          (* Skip token for error recovery. *)
+          Stream.junk stream;
+          print_endline s;
+      end;
+      print_string "ready&gt; "; flush stdout;
+      main_loop the_fpm the_execution_engine stream
+</pre>
+</dd>
+
+<dt>toy.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Main driver code.
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+open Llvm_executionengine
+open Llvm_target
+open Llvm_scalar_opts
+
+let main () =
+  ignore (initialize_native_target ());
+
+  (* Install standard binary operators.
+   * 1 is the lowest precedence. *)
+  Hashtbl.add Parser.binop_precedence '&lt;' 10;
+  Hashtbl.add Parser.binop_precedence '+' 20;
+  Hashtbl.add Parser.binop_precedence '-' 20;
+  Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
+
+  (* Prime the first token. *)
+  print_string "ready&gt; "; flush stdout;
+  let stream = Lexer.lex (Stream.of_channel stdin) in
+
+  (* Create the JIT. *)
+  let the_execution_engine = ExecutionEngine.create Codegen.the_module in
+  let the_fpm = PassManager.create_function Codegen.the_module in
+
+  (* Set up the optimizer pipeline.  Start with registering info about how the
+   * target lays out data structures. *)
+  TargetData.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
+
+  (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
+  add_instruction_combination the_fpm;
+
+  (* reassociate expressions. *)
+  add_reassociation the_fpm;
+
+  (* Eliminate Common SubExpressions. *)
+  add_gvn the_fpm;
+
+  (* Simplify the control flow graph (deleting unreachable blocks, etc). *)
+  add_cfg_simplification the_fpm;
+
+  ignore (PassManager.initialize the_fpm);
+
+  (* Run the main "interpreter loop" now. *)
+  Toplevel.main_loop the_fpm the_execution_engine stream;
+
+  (* Print out all the generated code. *)
+  dump_module Codegen.the_module
+;;
+
+main ()
+</pre>
+</dd>
+
+<dt>bindings.c</dt>
+<dd class="doc_code">
+<pre>
+#include &lt;stdio.h&gt;
+
+/* putchard - putchar that takes a double and returns 0. */
+extern double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+</pre>
+</dd>
+</dl>
+
+<a href="OCamlLangImpl5.html">Next: Extending the language: control flow</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/OCamlLangImpl5.html b/docs/tutorial/OCamlLangImpl5.html
new file mode 100644
index 0000000000..131d5b252e
--- /dev/null
+++ b/docs/tutorial/OCamlLangImpl5.html
@@ -0,0 +1,1569 @@
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Extending the Language: Control Flow</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <meta name="author" content="Erick Tryzelaar">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Extending the Language: Control Flow</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 5
+  <ol>
+    <li><a href="#intro">Chapter 5 Introduction</a></li>
+    <li><a href="#ifthen">If/Then/Else</a>
+    <ol>
+      <li><a href="#iflexer">Lexer Extensions</a></li>
+      <li><a href="#ifast">AST Extensions</a></li>
+      <li><a href="#ifparser">Parser Extensions</a></li>
+      <li><a href="#ifir">LLVM IR</a></li>
+      <li><a href="#ifcodegen">Code Generation</a></li>
+    </ol>
+    </li>
+    <li><a href="#for">'for' Loop Expression</a>
+    <ol>
+      <li><a href="#forlexer">Lexer Extensions</a></li>
+      <li><a href="#forast">AST Extensions</a></li>
+      <li><a href="#forparser">Parser Extensions</a></li>
+      <li><a href="#forir">LLVM IR</a></li>
+      <li><a href="#forcodegen">Code Generation</a></li>
+    </ol>
+    </li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="OCamlLangImpl6.html">Chapter 6</a>: Extending the Language:
+User-defined Operators</li>
+</ul>
+
+<div class="doc_author">
+	<p>
+		Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
+		and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
+	</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 5 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 5 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  Parts 1-4 described the implementation of the simple
+Kaleidoscope language and included support for generating LLVM IR, followed by
+optimizations and a JIT compiler.  Unfortunately, as presented, Kaleidoscope is
+mostly useless: it has no control flow other than call and return.  This means
+that you can't have conditional branches in the code, significantly limiting its
+power.  In this episode of "build that compiler", we'll extend Kaleidoscope to
+have an if/then/else expression plus a simple 'for' loop.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="ifthen">If/Then/Else</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Extending Kaleidoscope to support if/then/else is quite straightforward.  It
+basically requires adding lexer support for this "new" concept to the lexer,
+parser, AST, and LLVM code emitter.  This example is nice, because it shows how
+easy it is to "grow" a language over time, incrementally extending it as new
+ideas are discovered.</p>
+
+<p>Before we get going on "how" we add this extension, lets talk about "what" we
+want.  The basic idea is that we want to be able to write this sort of thing:
+</p>
+
+<div class="doc_code">
+<pre>
+def fib(x)
+  if x &lt; 3 then
+    1
+  else
+    fib(x-1)+fib(x-2);
+</pre>
+</div>
+
+<p>In Kaleidoscope, every construct is an expression: there are no statements.
+As such, the if/then/else expression needs to return a value like any other.
+Since we're using a mostly functional form, we'll have it evaluate its
+conditional, then return the 'then' or 'else' value based on how the condition
+was resolved.  This is very similar to the C "?:" expression.</p>
+
+<p>The semantics of the if/then/else expression is that it evaluates the
+condition to a boolean equality value: 0.0 is considered to be false and
+everything else is considered to be true.
+If the condition is true, the first subexpression is evaluated and returned, if
+the condition is false, the second subexpression is evaluated and returned.
+Since Kaleidoscope allows side-effects, this behavior is important to nail down.
+</p>
+
+<p>Now that we know what we "want", lets break this down into its constituent
+pieces.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="iflexer">Lexer Extensions for
+If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+
+<div class="doc_text">
+
+<p>The lexer extensions are straightforward.  First we add new variants
+for the relevant tokens:</p>
+
+<div class="doc_code">
+<pre>
+  (* control *)
+  | If | Then | Else | For | In
+</pre>
+</div>
+
+<p>Once we have that, we recognize the new keywords in the lexer. This is pretty simple
+stuff:</p>
+
+<div class="doc_code">
+<pre>
+      ...
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | "if" -&gt; [&lt; 'Token.If; stream &gt;]
+      | "then" -&gt; [&lt; 'Token.Then; stream &gt;]
+      | "else" -&gt; [&lt; 'Token.Else; stream &gt;]
+      | "for" -&gt; [&lt; 'Token.For; stream &gt;]
+      | "in" -&gt; [&lt; 'Token.In; stream &gt;]
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifast">AST Extensions for
+ If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>To represent the new expression we add a new AST variant for it:</p>
+
+<div class="doc_code">
+<pre>
+type expr =
+  ...
+  (* variant for if/then/else. *)
+  | If of expr * expr * expr
+</pre>
+</div>
+
+<p>The AST variant just has pointers to the various subexpressions.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifparser">Parser Extensions for
+If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>Now that we have the relevant tokens coming from the lexer and we have the
+AST node to build, our parsing logic is relatively straightforward.  First we
+define a new parsing function:</p>
+
+<div class="doc_code">
+<pre>
+let rec parse_primary = parser
+  ...
+  (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
+  | [&lt; 'Token.If; c=parse_expr;
+       'Token.Then ?? "expected 'then'"; t=parse_expr;
+       'Token.Else ?? "expected 'else'"; e=parse_expr &gt;] -&gt;
+      Ast.If (c, t, e)
+</pre>
+</div>
+
+<p>Next we hook it up as a primary expression:</p>
+
+<div class="doc_code">
+<pre>
+let rec parse_primary = parser
+  ...
+  (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
+  | [&lt; 'Token.If; c=parse_expr;
+       'Token.Then ?? "expected 'then'"; t=parse_expr;
+       'Token.Else ?? "expected 'else'"; e=parse_expr &gt;] -&gt;
+      Ast.If (c, t, e)
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifir">LLVM IR for If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>Now that we have it parsing and building the AST, the final piece is adding
+LLVM code generation support.  This is the most interesting part of the
+if/then/else example, because this is where it starts to introduce new concepts.
+All of the code above has been thoroughly described in previous chapters.
+</p>
+
+<p>To motivate the code we want to produce, lets take a look at a simple
+example.  Consider:</p>
+
+<div class="doc_code">
+<pre>
+extern foo();
+extern bar();
+def baz(x) if x then foo() else bar();
+</pre>
+</div>
+
+<p>If you disable optimizations, the code you'll (soon) get from Kaleidoscope
+looks like this:</p>
+
+<div class="doc_code">
+<pre>
+declare double @foo()
+
+declare double @bar()
+
+define double @baz(double %x) {
+entry:
+  %ifcond = fcmp one double %x, 0.000000e+00
+  br i1 %ifcond, label %then, label %else
+
+then:    ; preds = %entry
+  %calltmp = call double @foo()
+  br label %ifcont
+
+else:    ; preds = %entry
+  %calltmp1 = call double @bar()
+  br label %ifcont
+
+ifcont:    ; preds = %else, %then
+  %iftmp = phi double [ %calltmp, %then ], [ %calltmp1, %else ]
+  ret double %iftmp
+}
+</pre>
+</div>
+
+<p>To visualize the control flow graph, you can use a nifty feature of the LLVM
+'<a href="http://llvm.org/cmds/opt.html">opt</a>' tool.  If you put this LLVM IR
+into "t.ll" and run "<tt>llvm-as &lt; t.ll | opt -analyze -view-cfg</tt>", <a
+href="../ProgrammersManual.html#ViewGraph">a window will pop up</a> and you'll
+see this graph:</p>
+
+<div style="text-align: center"><img src="LangImpl5-cfg.png" alt="Example CFG" width="423"
+height="315"></div>
+
+<p>Another way to get this is to call "<tt>Llvm_analysis.view_function_cfg
+f</tt>" or "<tt>Llvm_analysis.view_function_cfg_only f</tt>" (where <tt>f</tt>
+is a "<tt>Function</tt>") either by inserting actual calls into the code and
+recompiling or by calling these in the debugger.  LLVM has many nice features
+for visualizing various graphs.</p>
+
+<p>Getting back to the generated code, it is fairly simple: the entry block
+evaluates the conditional expression ("x" in our case here) and compares the
+result to 0.0 with the "<tt><a href="../LangRef.html#i_fcmp">fcmp</a> one</tt>"
+instruction ('one' is "Ordered and Not Equal").  Based on the result of this
+expression, the code jumps to either the "then" or "else" blocks, which contain
+the expressions for the true/false cases.</p>
+
+<p>Once the then/else blocks are finished executing, they both branch back to the
+'ifcont' block to execute the code that happens after the if/then/else.  In this
+case the only thing left to do is to return to the caller of the function.  The
+question then becomes: how does the code know which expression to return?</p>
+
+<p>The answer to this question involves an important SSA operation: the
+<a href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Phi
+operation</a>.  If you're not familiar with SSA, <a
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">the wikipedia
+article</a> is a good introduction and there are various other introductions to
+it available on your favorite search engine.  The short version is that
+"execution" of the Phi operation requires "remembering" which block control came
+from.  The Phi operation takes on the value corresponding to the input control
+block.  In this case, if control comes in from the "then" block, it gets the
+value of "calltmp".  If control comes from the "else" block, it gets the value
+of "calltmp1".</p>
+
+<p>At this point, you are probably starting to think "Oh no! This means my
+simple and elegant front-end will have to start generating SSA form in order to
+use LLVM!".  Fortunately, this is not the case, and we strongly advise
+<em>not</em> implementing an SSA construction algorithm in your front-end
+unless there is an amazingly good reason to do so.  In practice, there are two
+sorts of values that float around in code written for your average imperative
+programming language that might need Phi nodes:</p>
+
+<ol>
+<li>Code that involves user variables: <tt>x = 1; x = x + 1; </tt></li>
+<li>Values that are implicit in the structure of your AST, such as the Phi node
+in this case.</li>
+</ol>
+
+<p>In <a href="OCamlLangImpl7.html">Chapter 7</a> of this tutorial ("mutable
+variables"), we'll talk about #1
+in depth.  For now, just believe me that you don't need SSA construction to
+handle this case.  For #2, you have the choice of using the techniques that we will
+describe for #1, or you can insert Phi nodes directly, if convenient.  In this
+case, it is really really easy to generate the Phi node, so we choose to do it
+directly.</p>
+
+<p>Okay, enough of the motivation and overview, lets generate code!</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifcodegen">Code Generation for
+If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>In order to generate code for this, we implement the <tt>Codegen</tt> method
+for <tt>IfExprAST</tt>:</p>
+
+<div class="doc_code">
+<pre>
+let rec codegen_expr = function
+  ...
+  | Ast.If (cond, then_, else_) -&gt;
+      let cond = codegen_expr cond in
+
+      (* Convert condition to a bool by comparing equal to 0.0 *)
+      let zero = const_float double_type 0.0 in
+      let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
+</pre>
+</div>
+
+<p>This code is straightforward and similar to what we saw before.  We emit the
+expression for the condition, then compare that value to zero to get a truth
+value as a 1-bit (bool) value.</p>
+
+<div class="doc_code">
+<pre>
+      (* Grab the first block so that we might later add the conditional branch
+       * to it at the end of the function. *)
+      let start_bb = insertion_block builder in
+      let the_function = block_parent start_bb in
+
+      let then_bb = append_block context "then" the_function in
+      position_at_end then_bb builder;
+</pre>
+</div>
+
+<p>
+As opposed to the <a href="LangImpl5.html">C++ tutorial</a>, we have to build
+our basic blocks bottom up since we can't have dangling BasicBlocks.  We start
+off by saving a pointer to the first block (which might not be the entry
+block), which we'll need to build a conditional branch later.  We do this by
+asking the <tt>builder</tt> for the current BasicBlock.  The fourth line
+gets the current Function object that is being built.  It gets this by the
+<tt>start_bb</tt> for its "parent" (the function it is currently embedded
+into).</p>
+
+<p>Once it has that, it creates one block.  It is automatically appended into
+the function's list of blocks.</p>
+
+<div class="doc_code">
+<pre>
+      (* Emit 'then' value. *)
+      position_at_end then_bb builder;
+      let then_val = codegen_expr then_ in
+
+      (* Codegen of 'then' can change the current block, update then_bb for the
+       * phi. We create a new name because one is used for the phi node, and the
+       * other is used for the conditional branch. *)
+      let new_then_bb = insertion_block builder in
+</pre>
+</div>
+
+<p>We move the builder to start inserting into the "then" block.  Strictly
+speaking, this call moves the insertion point to be at the end of the specified
+block.  However, since the "then" block is empty, it also starts out by
+inserting at the beginning of the block.  :)</p>
+
+<p>Once the insertion point is set, we recursively codegen the "then" expression
+from the AST.</p>
+
+<p>The final line here is quite subtle, but is very important.  The basic issue
+is that when we create the Phi node in the merge block, we need to set up the
+block/value pairs that indicate how the Phi will work.  Importantly, the Phi
+node expects to have an entry for each predecessor of the block in the CFG.  Why
+then, are we getting the current block when we just set it to ThenBB 5 lines
+above?  The problem is that the "Then" expression may actually itself change the
+block that the Builder is emitting into if, for example, it contains a nested
+"if/then/else" expression.  Because calling Codegen recursively could
+arbitrarily change the notion of the current block, we are required to get an
+up-to-date value for code that will set up the Phi node.</p>
+
+<div class="doc_code">
+<pre>
+      (* Emit 'else' value. *)
+      let else_bb = append_block context "else" the_function in
+      position_at_end else_bb builder;
+      let else_val = codegen_expr else_ in
+
+      (* Codegen of 'else' can change the current block, update else_bb for the
+       * phi. *)
+      let new_else_bb = insertion_block builder in
+</pre>
+</div>
+
+<p>Code generation for the 'else' block is basically identical to codegen for
+the 'then' block.</p>
+
+<div class="doc_code">
+<pre>
+      (* Emit merge block. *)
+      let merge_bb = append_block context "ifcont" the_function in
+      position_at_end merge_bb builder;
+      let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
+      let phi = build_phi incoming "iftmp" builder in
+</pre>
+</div>
+
+<p>The first two lines here are now familiar: the first adds the "merge" block
+to the Function object.  The second block changes the insertion point so that
+newly created code will go into the "merge" block.  Once that is done, we need
+to create the PHI node and set up the block/value pairs for the PHI.</p>
+
+<div class="doc_code">
+<pre>
+      (* Return to the start block to add the conditional branch. *)
+      position_at_end start_bb builder;
+      ignore (build_cond_br cond_val then_bb else_bb builder);
+</pre>
+</div>
+
+<p>Once the blocks are created, we can emit the conditional branch that chooses
+between them.  Note that creating new blocks does not implicitly affect the
+IRBuilder, so it is still inserting into the block that the condition
+went into.  This is why we needed to save the "start" block.</p>
+
+<div class="doc_code">
+<pre>
+      (* Set a unconditional branch at the end of the 'then' block and the
+       * 'else' block to the 'merge' block. *)
+      position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
+      position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
+
+      (* Finally, set the builder to the end of the merge block. *)
+      position_at_end merge_bb builder;
+
+      phi
+</pre>
+</div>
+
+<p>To finish off the blocks, we create an unconditional branch
+to the merge block.  One interesting (and very important) aspect of the LLVM IR
+is that it <a href="../LangRef.html#functionstructure">requires all basic blocks
+to be "terminated"</a> with a <a href="../LangRef.html#terminators">control flow
+instruction</a> such as return or branch.  This means that all control flow,
+<em>including fall throughs</em> must be made explicit in the LLVM IR.  If you
+violate this rule, the verifier will emit an error.
+
+<p>Finally, the CodeGen function returns the phi node as the value computed by
+the if/then/else expression.  In our example above, this returned value will
+feed into the code for the top-level function, which will create the return
+instruction.</p>
+
+<p>Overall, we now have the ability to execute conditional code in
+Kaleidoscope.  With this extension, Kaleidoscope is a fairly complete language
+that can calculate a wide variety of numeric functions.  Next up we'll add
+another useful expression that is familiar from non-functional languages...</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="for">'for' Loop Expression</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Now that we know how to add basic control flow constructs to the language,
+we have the tools to add more powerful things.  Lets add something more
+aggressive, a 'for' expression:</p>
+
+<div class="doc_code">
+<pre>
+ extern putchard(char);
+ def printstar(n)
+   for i = 1, i &lt; n, 1.0 in
+     putchard(42);  # ascii 42 = '*'
+
+ # print 100 '*' characters
+ printstar(100);
+</pre>
+</div>
+
+<p>This expression defines a new variable ("i" in this case) which iterates from
+a starting value, while the condition ("i &lt; n" in this case) is true,
+incrementing by an optional step value ("1.0" in this case).  If the step value
+is omitted, it defaults to 1.0.  While the loop is true, it executes its
+body expression.  Because we don't have anything better to return, we'll just
+define the loop as always returning 0.0.  In the future when we have mutable
+variables, it will get more useful.</p>
+
+<p>As before, lets talk about the changes that we need to Kaleidoscope to
+support this.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forlexer">Lexer Extensions for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The lexer extensions are the same sort of thing as for if/then/else:</p>
+
+<div class="doc_code">
+<pre>
+  ... in Token.token ...
+  (* control *)
+  | If | Then | Else
+  <b>| For | In</b>
+
+  ... in Lexer.lex_ident...
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | "if" -&gt; [&lt; 'Token.If; stream &gt;]
+      | "then" -&gt; [&lt; 'Token.Then; stream &gt;]
+      | "else" -&gt; [&lt; 'Token.Else; stream &gt;]
+      <b>| "for" -&gt; [&lt; 'Token.For; stream &gt;]
+      | "in" -&gt; [&lt; 'Token.In; stream &gt;]</b>
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forast">AST Extensions for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The AST variant is just as simple.  It basically boils down to capturing
+the variable name and the constituent expressions in the node.</p>
+
+<div class="doc_code">
+<pre>
+type expr =
+  ...
+  (* variant for for/in. *)
+  | For of string * expr * expr * expr option * expr
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forparser">Parser Extensions for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The parser code is also fairly standard.  The only interesting thing here is
+handling of the optional step value.  The parser code handles it by checking to
+see if the second comma is present.  If not, it sets the step value to null in
+the AST node:</p>
+
+<div class="doc_code">
+<pre>
+let rec parse_primary = parser
+  ...
+  (* forexpr
+        ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
+  | [&lt; 'Token.For;
+       'Token.Ident id ?? "expected identifier after for";
+       'Token.Kwd '=' ?? "expected '=' after for";
+       stream &gt;] -&gt;
+      begin parser
+        | [&lt;
+             start=parse_expr;
+             'Token.Kwd ',' ?? "expected ',' after for";
+             end_=parse_expr;
+             stream &gt;] -&gt;
+            let step =
+              begin parser
+              | [&lt; 'Token.Kwd ','; step=parse_expr &gt;] -&gt; Some step
+              | [&lt; &gt;] -&gt; None
+              end stream
+            in
+            begin parser
+            | [&lt; 'Token.In; body=parse_expr &gt;] -&gt;
+                Ast.For (id, start, end_, step, body)
+            | [&lt; &gt;] -&gt;
+                raise (Stream.Error "expected 'in' after for")
+            end stream
+        | [&lt; &gt;] -&gt;
+            raise (Stream.Error "expected '=' after for")
+      end stream
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forir">LLVM IR for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>Now we get to the good part: the LLVM IR we want to generate for this thing.
+With the simple example above, we get this LLVM IR (note that this dump is
+generated with optimizations disabled for clarity):
+</p>
+
+<div class="doc_code">
+<pre>
+declare double @putchard(double)
+
+define double @printstar(double %n) {
+entry:
+        ; initial value = 1.0 (inlined into phi)
+  br label %loop
+
+loop:    ; preds = %loop, %entry
+  %i = phi double [ 1.000000e+00, %entry ], [ %nextvar, %loop ]
+        ; body
+  %calltmp = call double @putchard( double 4.200000e+01 )
+        ; increment
+  %nextvar = fadd double %i, 1.000000e+00
+
+        ; termination test
+  %cmptmp = fcmp ult double %i, %n
+  %booltmp = uitofp i1 %cmptmp to double
+  %loopcond = fcmp one double %booltmp, 0.000000e+00
+  br i1 %loopcond, label %loop, label %afterloop
+
+afterloop:    ; preds = %loop
+        ; loop always returns 0.0
+  ret double 0.000000e+00
+}
+</pre>
+</div>
+
+<p>This loop contains all the same constructs we saw before: a phi node, several
+expressions, and some basic blocks.  Lets see how this fits together.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forcodegen">Code Generation for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The first part of Codegen is very simple: we just output the start expression
+for the loop value:</p>
+
+<div class="doc_code">
+<pre>
+let rec codegen_expr = function
+  ...
+  | Ast.For (var_name, start, end_, step, body) -&gt;
+      (* Emit the start code first, without 'variable' in scope. *)
+      let start_val = codegen_expr start in
+</pre>
+</div>
+
+<p>With this out of the way, the next step is to set up the LLVM basic block
+for the start of the loop body.  In the case above, the whole loop body is one
+block, but remember that the body code itself could consist of multiple blocks
+(e.g. if it contains an if/then/else or a for/in expression).</p>
+
+<div class="doc_code">
+<pre>
+      (* Make the new basic block for the loop header, inserting after current
+       * block. *)
+      let preheader_bb = insertion_block builder in
+      let the_function = block_parent preheader_bb in
+      let loop_bb = append_block context "loop" the_function in
+
+      (* Insert an explicit fall through from the current block to the
+       * loop_bb. *)
+      ignore (build_br loop_bb builder);
+</pre>
+</div>
+
+<p>This code is similar to what we saw for if/then/else.  Because we will need
+it to create the Phi node, we remember the block that falls through into the
+loop.  Once we have that, we create the actual block that starts the loop and
+create an unconditional branch for the fall-through between the two blocks.</p>
+
+<div class="doc_code">
+<pre>
+      (* Start insertion in loop_bb. *)
+      position_at_end loop_bb builder;
+
+      (* Start the PHI node with an entry for start. *)
+      let variable = build_phi [(start_val, preheader_bb)] var_name builder in
+</pre>
+</div>
+
+<p>Now that the "preheader" for the loop is set up, we switch to emitting code
+for the loop body.  To begin with, we move the insertion point and create the
+PHI node for the loop induction variable.  Since we already know the incoming
+value for the starting value, we add it to the Phi node.  Note that the Phi will
+eventually get a second value for the backedge, but we can't set it up yet
+(because it doesn't exist!).</p>
+
+<div class="doc_code">
+<pre>
+      (* Within the loop, the variable is defined equal to the PHI node. If it
+       * shadows an existing variable, we have to restore it, so save it
+       * now. *)
+      let old_val =
+        try Some (Hashtbl.find named_values var_name) with Not_found -&gt; None
+      in
+      Hashtbl.add named_values var_name variable;
+
+      (* Emit the body of the loop.  This, like any other expr, can change the
+       * current BB.  Note that we ignore the value computed by the body, but
+       * don't allow an error *)
+      ignore (codegen_expr body);
+</pre>
+</div>
+
+<p>Now the code starts to get more interesting.  Our 'for' loop introduces a new
+variable to the symbol table.  This means that our symbol table can now contain
+either function arguments or loop variables.  To handle this, before we codegen
+the body of the loop, we add the loop variable as the current value for its
+name.  Note that it is possible that there is a variable of the same name in the
+outer scope.  It would be easy to make this an error (emit an error and return
+null if there is already an entry for VarName) but we choose to allow shadowing
+of variables.  In order to handle this correctly, we remember the Value that
+we are potentially shadowing in <tt>old_val</tt> (which will be None if there is
+no shadowed variable).</p>
+
+<p>Once the loop variable is set into the symbol table, the code recursively
+codegen's the body.  This allows the body to use the loop variable: any
+references to it will naturally find it in the symbol table.</p>
+
+<div class="doc_code">
+<pre>
+      (* Emit the step value. *)
+      let step_val =
+        match step with
+        | Some step -&gt; codegen_expr step
+        (* If not specified, use 1.0. *)
+        | None -&gt; const_float double_type 1.0
+      in
+
+      let next_var = build_add variable step_val "nextvar" builder in
+</pre>
+</div>
+
+<p>Now that the body is emitted, we compute the next value of the iteration
+variable by adding the step value, or 1.0 if it isn't present.
+'<tt>next_var</tt>' will be the value of the loop variable on the next iteration
+of the loop.</p>
+
+<div class="doc_code">
+<pre>
+      (* Compute the end condition. *)
+      let end_cond = codegen_expr end_ in
+
+      (* Convert condition to a bool by comparing equal to 0.0. *)
+      let zero = const_float double_type 0.0 in
+      let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
+</pre>
+</div>
+
+<p>Finally, we evaluate the exit value of the loop, to determine whether the
+loop should exit.  This mirrors the condition evaluation for the if/then/else
+statement.</p>
+
+<div class="doc_code">
+<pre>
+      (* Create the "after loop" block and insert it. *)
+      let loop_end_bb = insertion_block builder in
+      let after_bb = append_block context "afterloop" the_function in
+
+      (* Insert the conditional branch into the end of loop_end_bb. *)
+      ignore (build_cond_br end_cond loop_bb after_bb builder);
+
+      (* Any new code will be inserted in after_bb. *)
+      position_at_end after_bb builder;
+</pre>
+</div>
+
+<p>With the code for the body of the loop complete, we just need to finish up
+the control flow for it.  This code remembers the end block (for the phi node), then creates the block for the loop exit ("afterloop").  Based on the value of the
+exit condition, it creates a conditional branch that chooses between executing
+the loop again and exiting the loop.  Any future code is emitted in the
+"afterloop" block, so it sets the insertion position to it.</p>
+
+<div class="doc_code">
+<pre>
+      (* Add a new entry to the PHI node for the backedge. *)
+      add_incoming (next_var, loop_end_bb) variable;
+
+      (* Restore the unshadowed variable. *)
+      begin match old_val with
+      | Some old_val -&gt; Hashtbl.add named_values var_name old_val
+      | None -&gt; ()
+      end;
+
+      (* for expr always returns 0.0. *)
+      const_null double_type
+</pre>
+</div>
+
+<p>The final code handles various cleanups: now that we have the
+"<tt>next_var</tt>" value, we can add the incoming value to the loop PHI node.
+After that, we remove the loop variable from the symbol table, so that it isn't
+in scope after the for loop.  Finally, code generation of the for loop always
+returns 0.0, so that is what we return from <tt>Codegen.codegen_expr</tt>.</p>
+
+<p>With this, we conclude the "adding control flow to Kaleidoscope" chapter of
+the tutorial.  In this chapter we added two control flow constructs, and used
+them to motivate a couple of aspects of the LLVM IR that are important for
+front-end implementors to know.  In the next chapter of our saga, we will get
+a bit crazier and add <a href="OCamlLangImpl6.html">user-defined operators</a>
+to our poor innocent language.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with the
+if/then/else and for expressions..  To build this example, use:
+</p>
+
+<div class="doc_code">
+<pre>
+# Compile
+ocamlbuild toy.byte
+# Run
+./toy.byte
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<dl>
+<dt>_tags:</dt>
+<dd class="doc_code">
+<pre>
+&lt;{lexer,parser}.ml&gt;: use_camlp4, pp(camlp4of)
+&lt;*.{byte,native}&gt;: g++, use_llvm, use_llvm_analysis
+&lt;*.{byte,native}&gt;: use_llvm_executionengine, use_llvm_target
+&lt;*.{byte,native}&gt;: use_llvm_scalar_opts, use_bindings
+</pre>
+</dd>
+
+<dt>myocamlbuild.ml:</dt>
+<dd class="doc_code">
+<pre>
+open Ocamlbuild_plugin;;
+
+ocaml_lib ~extern:true "llvm";;
+ocaml_lib ~extern:true "llvm_analysis";;
+ocaml_lib ~extern:true "llvm_executionengine";;
+ocaml_lib ~extern:true "llvm_target";;
+ocaml_lib ~extern:true "llvm_scalar_opts";;
+
+flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"]);;
+dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
+</pre>
+</dd>
+
+<dt>token.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer Tokens
+ *===----------------------------------------------------------------------===*)
+
+(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
+ * these others for known things. *)
+type token =
+  (* commands *)
+  | Def | Extern
+
+  (* primary *)
+  | Ident of string | Number of float
+
+  (* unknown *)
+  | Kwd of char
+
+  (* control *)
+  | If | Then | Else
+  | For | In
+</pre>
+</dd>
+
+<dt>lexer.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer
+ *===----------------------------------------------------------------------===*)
+
+let rec lex = parser
+  (* Skip any whitespace. *)
+  | [&lt; ' (' ' | '\n' | '\r' | '\t'); stream &gt;] -&gt; lex stream
+
+  (* identifier: [a-zA-Z][a-zA-Z0-9] *)
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+
+  (* number: [0-9.]+ *)
+  | [&lt; ' ('0' .. '9' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+
+  (* Comment until end of line. *)
+  | [&lt; ' ('#'); stream &gt;] -&gt;
+      lex_comment stream
+
+  (* Otherwise, just return the character as its ascii value. *)
+  | [&lt; 'c; stream &gt;] -&gt;
+      [&lt; 'Token.Kwd c; lex stream &gt;]
+
+  (* end of stream. *)
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+
+and lex_number buffer = parser
+  | [&lt; ' ('0' .. '9' | '.' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      [&lt; 'Token.Number (float_of_string (Buffer.contents buffer)); stream &gt;]
+
+and lex_ident buffer = parser
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | "if" -&gt; [&lt; 'Token.If; stream &gt;]
+      | "then" -&gt; [&lt; 'Token.Then; stream &gt;]
+      | "else" -&gt; [&lt; 'Token.Else; stream &gt;]
+      | "for" -&gt; [&lt; 'Token.For; stream &gt;]
+      | "in" -&gt; [&lt; 'Token.In; stream &gt;]
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+
+and lex_comment = parser
+  | [&lt; ' ('\n'); stream=lex &gt;] -&gt; stream
+  | [&lt; 'c; e=lex_comment &gt;] -&gt; e
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+</pre>
+</dd>
+
+<dt>ast.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Abstract Syntax Tree (aka Parse Tree)
+ *===----------------------------------------------------------------------===*)
+
+(* expr - Base type for all expression nodes. *)
+type expr =
+  (* variant for numeric literals like "1.0". *)
+  | Number of float
+
+  (* variant for referencing a variable, like "a". *)
+  | Variable of string
+
+  (* variant for a binary operator. *)
+  | Binary of char * expr * expr
+
+  (* variant for function calls. *)
+  | Call of string * expr array
+
+  (* variant for if/then/else. *)
+  | If of expr * expr * expr
+
+  (* variant for for/in. *)
+  | For of string * expr * expr * expr option * expr
+
+(* proto - This type represents the "prototype" for a function, which captures
+ * its name, and its argument names (thus implicitly the number of arguments the
+ * function takes). *)
+type proto = Prototype of string * string array
+
+(* func - This type represents a function definition itself. *)
+type func = Function of proto * expr
+</pre>
+</dd>
+
+<dt>parser.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===---------------------------------------------------------------------===
+ * Parser
+ *===---------------------------------------------------------------------===*)
+
+(* binop_precedence - This holds the precedence for each binary operator that is
+ * defined *)
+let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
+
+(* precedence - Get the precedence of the pending binary operator token. *)
+let precedence c = try Hashtbl.find binop_precedence c with Not_found -&gt; -1
+
+(* primary
+ *   ::= identifier
+ *   ::= numberexpr
+ *   ::= parenexpr
+ *   ::= ifexpr
+ *   ::= forexpr *)
+let rec parse_primary = parser
+  (* numberexpr ::= number *)
+  | [&lt; 'Token.Number n &gt;] -&gt; Ast.Number n
+
+  (* parenexpr ::= '(' expression ')' *)
+  | [&lt; 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" &gt;] -&gt; e
+
+  (* identifierexpr
+   *   ::= identifier
+   *   ::= identifier '(' argumentexpr ')' *)
+  | [&lt; 'Token.Ident id; stream &gt;] -&gt;
+      let rec parse_args accumulator = parser
+        | [&lt; e=parse_expr; stream &gt;] -&gt;
+            begin parser
+              | [&lt; 'Token.Kwd ','; e=parse_args (e :: accumulator) &gt;] -&gt; e
+              | [&lt; &gt;] -&gt; e :: accumulator
+            end stream
+        | [&lt; &gt;] -&gt; accumulator
+      in
+      let rec parse_ident id = parser
+        (* Call. *)
+        | [&lt; 'Token.Kwd '(';
+             args=parse_args [];
+             'Token.Kwd ')' ?? "expected ')'"&gt;] -&gt;
+            Ast.Call (id, Array.of_list (List.rev args))
+
+        (* Simple variable ref. *)
+        | [&lt; &gt;] -&gt; Ast.Variable id
+      in
+      parse_ident id stream
+
+  (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
+  | [&lt; 'Token.If; c=parse_expr;
+       'Token.Then ?? "expected 'then'"; t=parse_expr;
+       'Token.Else ?? "expected 'else'"; e=parse_expr &gt;] -&gt;
+      Ast.If (c, t, e)
+
+  (* forexpr
+        ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
+  | [&lt; 'Token.For;
+       'Token.Ident id ?? "expected identifier after for";
+       'Token.Kwd '=' ?? "expected '=' after for";
+       stream &gt;] -&gt;
+      begin parser
+        | [&lt;
+             start=parse_expr;
+             'Token.Kwd ',' ?? "expected ',' after for";
+             end_=parse_expr;
+             stream &gt;] -&gt;
+            let step =
+              begin parser
+              | [&lt; 'Token.Kwd ','; step=parse_expr &gt;] -&gt; Some step
+              | [&lt; &gt;] -&gt; None
+              end stream
+            in
+            begin parser
+            | [&lt; 'Token.In; body=parse_expr &gt;] -&gt;
+                Ast.For (id, start, end_, step, body)
+            | [&lt; &gt;] -&gt;
+                raise (Stream.Error "expected 'in' after for")
+            end stream
+        | [&lt; &gt;] -&gt;
+            raise (Stream.Error "expected '=' after for")
+      end stream
+
+  | [&lt; &gt;] -&gt; raise (Stream.Error "unknown token when expecting an expression.")
+
+(* binoprhs
+ *   ::= ('+' primary)* *)
+and parse_bin_rhs expr_prec lhs stream =
+  match Stream.peek stream with
+  (* If this is a binop, find its precedence. *)
+  | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -&gt;
+      let token_prec = precedence c in
+
+      (* If this is a binop that binds at least as tightly as the current binop,
+       * consume it, otherwise we are done. *)
+      if token_prec &lt; expr_prec then lhs else begin
+        (* Eat the binop. *)
+        Stream.junk stream;
+
+        (* Parse the primary expression after the binary operator. *)
+        let rhs = parse_primary stream in
+
+        (* Okay, we know this is a binop. *)
+        let rhs =
+          match Stream.peek stream with
+          | Some (Token.Kwd c2) -&gt;
+              (* If BinOp binds less tightly with rhs than the operator after
+               * rhs, let the pending operator take rhs as its lhs. *)
+              let next_prec = precedence c2 in
+              if token_prec &lt; next_prec
+              then parse_bin_rhs (token_prec + 1) rhs stream
+              else rhs
+          | _ -&gt; rhs
+        in
+
+        (* Merge lhs/rhs. *)
+        let lhs = Ast.Binary (c, lhs, rhs) in
+        parse_bin_rhs expr_prec lhs stream
+      end
+  | _ -&gt; lhs
+
+(* expression
+ *   ::= primary binoprhs *)
+and parse_expr = parser
+  | [&lt; lhs=parse_primary; stream &gt;] -&gt; parse_bin_rhs 0 lhs stream
+
+(* prototype
+ *   ::= id '(' id* ')' *)
+let parse_prototype =
+  let rec parse_args accumulator = parser
+    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
+    | [&lt; &gt;] -&gt; accumulator
+  in
+
+  parser
+  | [&lt; 'Token.Ident id;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+       args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      (* success. *)
+      Ast.Prototype (id, Array.of_list (List.rev args))
+
+  | [&lt; &gt;] -&gt;
+      raise (Stream.Error "expected function name in prototype")
+
+(* definition ::= 'def' prototype expression *)
+let parse_definition = parser
+  | [&lt; 'Token.Def; p=parse_prototype; e=parse_expr &gt;] -&gt;
+      Ast.Function (p, e)
+
+(* toplevelexpr ::= expression *)
+let parse_toplevel = parser
+  | [&lt; e=parse_expr &gt;] -&gt;
+      (* Make an anonymous proto. *)
+      Ast.Function (Ast.Prototype ("", [||]), e)
+
+(*  external ::= 'extern' prototype *)
+let parse_extern = parser
+  | [&lt; 'Token.Extern; e=parse_prototype &gt;] -&gt; e
+</pre>
+</dd>
+
+<dt>codegen.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Code Generation
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+
+exception Error of string
+
+let context = global_context ()
+let the_module = create_module context "my cool jit"
+let builder = builder context
+let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
+let double_type = double_type context
+
+let rec codegen_expr = function
+  | Ast.Number n -&gt; const_float double_type n
+  | Ast.Variable name -&gt;
+      (try Hashtbl.find named_values name with
+        | Not_found -&gt; raise (Error "unknown variable name"))
+  | Ast.Binary (op, lhs, rhs) -&gt;
+      let lhs_val = codegen_expr lhs in
+      let rhs_val = codegen_expr rhs in
+      begin
+        match op with
+        | '+' -&gt; build_add lhs_val rhs_val "addtmp" builder
+        | '-' -&gt; build_sub lhs_val rhs_val "subtmp" builder
+        | '*' -&gt; build_mul lhs_val rhs_val "multmp" builder
+        | '&lt;' -&gt;
+            (* Convert bool 0/1 to double 0.0 or 1.0 *)
+            let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
+            build_uitofp i double_type "booltmp" builder
+        | _ -&gt; raise (Error "invalid binary operator")
+      end
+  | Ast.Call (callee, args) -&gt;
+      (* Look up the name in the module table. *)
+      let callee =
+        match lookup_function callee the_module with
+        | Some callee -&gt; callee
+        | None -&gt; raise (Error "unknown function referenced")
+      in
+      let params = params callee in
+
+      (* If argument mismatch error. *)
+      if Array.length params == Array.length args then () else
+        raise (Error "incorrect # arguments passed");
+      let args = Array.map codegen_expr args in
+      build_call callee args "calltmp" builder
+  | Ast.If (cond, then_, else_) -&gt;
+      let cond = codegen_expr cond in
+
+      (* Convert condition to a bool by comparing equal to 0.0 *)
+      let zero = const_float double_type 0.0 in
+      let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
+
+      (* Grab the first block so that we might later add the conditional branch
+       * to it at the end of the function. *)
+      let start_bb = insertion_block builder in
+      let the_function = block_parent start_bb in
+
+      let then_bb = append_block context "then" the_function in
+
+      (* Emit 'then' value. *)
+      position_at_end then_bb builder;
+      let then_val = codegen_expr then_ in
+
+      (* Codegen of 'then' can change the current block, update then_bb for the
+       * phi. We create a new name because one is used for the phi node, and the
+       * other is used for the conditional branch. *)
+      let new_then_bb = insertion_block builder in
+
+      (* Emit 'else' value. *)
+      let else_bb = append_block context "else" the_function in
+      position_at_end else_bb builder;
+      let else_val = codegen_expr else_ in
+
+      (* Codegen of 'else' can change the current block, update else_bb for the
+       * phi. *)
+      let new_else_bb = insertion_block builder in
+
+      (* Emit merge block. *)
+      let merge_bb = append_block context "ifcont" the_function in
+      position_at_end merge_bb builder;
+      let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
+      let phi = build_phi incoming "iftmp" builder in
+
+      (* Return to the start block to add the conditional branch. *)
+      position_at_end start_bb builder;
+      ignore (build_cond_br cond_val then_bb else_bb builder);
+
+      (* Set a unconditional branch at the end of the 'then' block and the
+       * 'else' block to the 'merge' block. *)
+      position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
+      position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
+
+      (* Finally, set the builder to the end of the merge block. *)
+      position_at_end merge_bb builder;
+
+      phi
+  | Ast.For (var_name, start, end_, step, body) -&gt;
+      (* Emit the start code first, without 'variable' in scope. *)
+      let start_val = codegen_expr start in
+
+      (* Make the new basic block for the loop header, inserting after current
+       * block. *)
+      let preheader_bb = insertion_block builder in
+      let the_function = block_parent preheader_bb in
+      let loop_bb = append_block context "loop" the_function in
+
+      (* Insert an explicit fall through from the current block to the
+       * loop_bb. *)
+      ignore (build_br loop_bb builder);
+
+      (* Start insertion in loop_bb. *)
+      position_at_end loop_bb builder;
+
+      (* Start the PHI node with an entry for start. *)
+      let variable = build_phi [(start_val, preheader_bb)] var_name builder in
+
+      (* Within the loop, the variable is defined equal to the PHI node. If it
+       * shadows an existing variable, we have to restore it, so save it
+       * now. *)
+      let old_val =
+        try Some (Hashtbl.find named_values var_name) with Not_found -&gt; None
+      in
+      Hashtbl.add named_values var_name variable;
+
+      (* Emit the body of the loop.  This, like any other expr, can change the
+       * current BB.  Note that we ignore the value computed by the body, but
+       * don't allow an error *)
+      ignore (codegen_expr body);
+
+      (* Emit the step value. *)
+      let step_val =
+        match step with
+        | Some step -&gt; codegen_expr step
+        (* If not specified, use 1.0. *)
+        | None -&gt; const_float double_type 1.0
+      in
+
+      let next_var = build_add variable step_val "nextvar" builder in
+
+      (* Compute the end condition. *)
+      let end_cond = codegen_expr end_ in
+
+      (* Convert condition to a bool by comparing equal to 0.0. *)
+      let zero = const_float double_type 0.0 in
+      let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
+
+      (* Create the "after loop" block and insert it. *)
+      let loop_end_bb = insertion_block builder in
+      let after_bb = append_block context "afterloop" the_function in
+
+      (* Insert the conditional branch into the end of loop_end_bb. *)
+      ignore (build_cond_br end_cond loop_bb after_bb builder);
+
+      (* Any new code will be inserted in after_bb. *)
+      position_at_end after_bb builder;
+
+      (* Add a new entry to the PHI node for the backedge. *)
+      add_incoming (next_var, loop_end_bb) variable;
+
+      (* Restore the unshadowed variable. *)
+      begin match old_val with
+      | Some old_val -&gt; Hashtbl.add named_values var_name old_val
+      | None -&gt; ()
+      end;
+
+      (* for expr always returns 0.0. *)
+      const_null double_type
+
+let codegen_proto = function
+  | Ast.Prototype (name, args) -&gt;
+      (* Make the function type: double(double,double) etc. *)
+      let doubles = Array.make (Array.length args) double_type in
+      let ft = function_type double_type doubles in
+      let f =
+        match lookup_function name the_module with
+        | None -&gt; declare_function name ft the_module
+
+        (* If 'f' conflicted, there was already something named 'name'. If it
+         * has a body, don't allow redefinition or reextern. *)
+        | Some f -&gt;
+            (* If 'f' already has a body, reject this. *)
+            if block_begin f &lt;&gt; At_end f then
+              raise (Error "redefinition of function");
+
+            (* If 'f' took a different number of arguments, reject. *)
+            if element_type (type_of f) &lt;&gt; ft then
+              raise (Error "redefinition of function with different # args");
+            f
+      in
+
+      (* Set names for all arguments. *)
+      Array.iteri (fun i a -&gt;
+        let n = args.(i) in
+        set_value_name n a;
+        Hashtbl.add named_values n a;
+      ) (params f);
+      f
+
+let codegen_func the_fpm = function
+  | Ast.Function (proto, body) -&gt;
+      Hashtbl.clear named_values;
+      let the_function = codegen_proto proto in
+
+      (* Create a new basic block to start insertion into. *)
+      let bb = append_block context "entry" the_function in
+      position_at_end bb builder;
+
+      try
+        let ret_val = codegen_expr body in
+
+        (* Finish off the function. *)
+        let _ = build_ret ret_val builder in
+
+        (* Validate the generated code, checking for consistency. *)
+        Llvm_analysis.assert_valid_function the_function;
+
+        (* Optimize the function. *)
+        let _ = PassManager.run_function the_function the_fpm in
+
+        the_function
+      with e -&gt;
+        delete_function the_function;
+        raise e
+</pre>
+</dd>
+
+<dt>toplevel.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Top-Level parsing and JIT Driver
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+open Llvm_executionengine
+
+(* top ::= definition | external | expression | ';' *)
+let rec main_loop the_fpm the_execution_engine stream =
+  match Stream.peek stream with
+  | None -&gt; ()
+
+  (* ignore top-level semicolons. *)
+  | Some (Token.Kwd ';') -&gt;
+      Stream.junk stream;
+      main_loop the_fpm the_execution_engine stream
+
+  | Some token -&gt;
+      begin
+        try match token with
+        | Token.Def -&gt;
+            let e = Parser.parse_definition stream in
+            print_endline "parsed a function definition.";
+            dump_value (Codegen.codegen_func the_fpm e);
+        | Token.Extern -&gt;
+            let e = Parser.parse_extern stream in
+            print_endline "parsed an extern.";
+            dump_value (Codegen.codegen_proto e);
+        | _ -&gt;
+            (* Evaluate a top-level expression into an anonymous function. *)
+            let e = Parser.parse_toplevel stream in
+            print_endline "parsed a top-level expr";
+            let the_function = Codegen.codegen_func the_fpm e in
+            dump_value the_function;
+
+            (* JIT the function, returning a function pointer. *)
+            let result = ExecutionEngine.run_function the_function [||]
+              the_execution_engine in
+
+            print_string "Evaluated to ";
+            print_float (GenericValue.as_float Codegen.double_type result);
+            print_newline ();
+        with Stream.Error s | Codegen.Error s -&gt;
+          (* Skip token for error recovery. *)
+          Stream.junk stream;
+          print_endline s;
+      end;
+      print_string "ready&gt; "; flush stdout;
+      main_loop the_fpm the_execution_engine stream
+</pre>
+</dd>
+
+<dt>toy.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Main driver code.
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+open Llvm_executionengine
+open Llvm_target
+open Llvm_scalar_opts
+
+let main () =
+  ignore (initialize_native_target ());
+
+  (* Install standard binary operators.
+   * 1 is the lowest precedence. *)
+  Hashtbl.add Parser.binop_precedence '&lt;' 10;
+  Hashtbl.add Parser.binop_precedence '+' 20;
+  Hashtbl.add Parser.binop_precedence '-' 20;
+  Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
+
+  (* Prime the first token. *)
+  print_string "ready&gt; "; flush stdout;
+  let stream = Lexer.lex (Stream.of_channel stdin) in
+
+  (* Create the JIT. *)
+  let the_execution_engine = ExecutionEngine.create Codegen.the_module in
+  let the_fpm = PassManager.create_function Codegen.the_module in
+
+  (* Set up the optimizer pipeline.  Start with registering info about how the
+   * target lays out data structures. *)
+  TargetData.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
+
+  (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
+  add_instruction_combination the_fpm;
+
+  (* reassociate expressions. *)
+  add_reassociation the_fpm;
+
+  (* Eliminate Common SubExpressions. *)
+  add_gvn the_fpm;
+
+  (* Simplify the control flow graph (deleting unreachable blocks, etc). *)
+  add_cfg_simplification the_fpm;
+
+  ignore (PassManager.initialize the_fpm);
+
+  (* Run the main "interpreter loop" now. *)
+  Toplevel.main_loop the_fpm the_execution_engine stream;
+
+  (* Print out all the generated code. *)
+  dump_module Codegen.the_module
+;;
+
+main ()
+</pre>
+</dd>
+
+<dt>bindings.c</dt>
+<dd class="doc_code">
+<pre>
+#include &lt;stdio.h&gt;
+
+/* putchard - putchar that takes a double and returns 0. */
+extern double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+</pre>
+</dd>
+</dl>
+
+<a href="OCamlLangImpl6.html">Next: Extending the language: user-defined
+operators</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/OCamlLangImpl6.html b/docs/tutorial/OCamlLangImpl6.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Extending the Language: User-defined Operators</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <meta name="author" content="Erick Tryzelaar">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Extending the Language: User-defined Operators</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 6
+  <ol>
+    <li><a href="#intro">Chapter 6 Introduction</a></li>
+    <li><a href="#idea">User-defined Operators: the Idea</a></li>
+    <li><a href="#binary">User-defined Binary Operators</a></li>
+    <li><a href="#unary">User-defined Unary Operators</a></li>
+    <li><a href="#example">Kicking the Tires</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="OCamlLangImpl7.html">Chapter 7</a>: Extending the Language: Mutable
+Variables / SSA Construction</li>
+</ul>
+
+<div class="doc_author">
+	<p>
+		Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
+		and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
+	</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 6 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 6 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  At this point in our tutorial, we now have a fully
+functional language that is fairly minimal, but also useful.  There
+is still one big problem with it, however. Our language doesn't have many
+useful operators (like division, logical negation, or even any comparisons
+besides less-than).</p>
+
+<p>This chapter of the tutorial takes a wild digression into adding user-defined
+operators to the simple and beautiful Kaleidoscope language. This digression now
+gives us a simple and ugly language in some ways, but also a powerful one at the
+same time.  One of the great things about creating your own language is that you
+get to decide what is good or bad.  In this tutorial we'll assume that it is
+okay to use this as a way to show some interesting parsing techniques.</p>
+
+<p>At the end of this tutorial, we'll run through an example Kaleidoscope
+application that <a href="#example">renders the Mandelbrot set</a>.  This gives
+an example of what you can build with Kaleidoscope and its feature set.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="idea">User-defined Operators: the Idea</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+The "operator overloading" that we will add to Kaleidoscope is more general than
+languages like C++.  In C++, you are only allowed to redefine existing
+operators: you can't programatically change the grammar, introduce new
+operators, change precedence levels, etc.  In this chapter, we will add this
+capability to Kaleidoscope, which will let the user round out the set of
+operators that are supported.</p>
+
+<p>The point of going into user-defined operators in a tutorial like this is to
+show the power and flexibility of using a hand-written parser.  Thus far, the parser
+we have been implementing uses recursive descent for most parts of the grammar and
+operator precedence parsing for the expressions.  See <a
+href="OCamlLangImpl2.html">Chapter 2</a> for details.  Without using operator
+precedence parsing, it would be very difficult to allow the programmer to
+introduce new operators into the grammar: the grammar is dynamically extensible
+as the JIT runs.</p>
+
+<p>The two specific features we'll add are programmable unary operators (right
+now, Kaleidoscope has no unary operators at all) as well as binary operators.
+An example of this is:</p>
+
+<div class="doc_code">
+<pre>
+# Logical unary not.
+def unary!(v)
+  if v then
+    0
+  else
+    1;
+
+# Define &gt; with the same precedence as &lt;.
+def binary&gt; 10 (LHS RHS)
+  RHS &lt; LHS;
+
+# Binary "logical or", (note that it does not "short circuit")
+def binary| 5 (LHS RHS)
+  if LHS then
+    1
+  else if RHS then
+    1
+  else
+    0;
+
+# Define = with slightly lower precedence than relationals.
+def binary= 9 (LHS RHS)
+  !(LHS &lt; RHS | LHS &gt; RHS);
+</pre>
+</div>
+
+<p>Many languages aspire to being able to implement their standard runtime
+library in the language itself.  In Kaleidoscope, we can implement significant
+parts of the language in the library!</p>
+
+<p>We will break down implementation of these features into two parts:
+implementing support for user-defined binary operators and adding unary
+operators.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="binary">User-defined Binary Operators</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Adding support for user-defined binary operators is pretty simple with our
+current framework.  We'll first add support for the unary/binary keywords:</p>
+
+<div class="doc_code">
+<pre>
+type token =
+  ...
+  <b>(* operators *)
+  | Binary | Unary</b>
+
+...
+
+and lex_ident buffer = parser
+  ...
+      | "for" -&gt; [&lt; 'Token.For; stream &gt;]
+      | "in" -&gt; [&lt; 'Token.In; stream &gt;]
+      <b>| "binary" -&gt; [&lt; 'Token.Binary; stream &gt;]
+      | "unary" -&gt; [&lt; 'Token.Unary; stream &gt;]</b>
+</pre>
+</div>
+
+<p>This just adds lexer support for the unary and binary keywords, like we
+did in <a href="OCamlLangImpl5.html#iflexer">previous chapters</a>.  One nice
+thing about our current AST, is that we represent binary operators with full
+generalisation by using their ASCII code as the opcode.  For our extended
+operators, we'll use this same representation, so we don't need any new AST or
+parser support.</p>
+
+<p>On the other hand, we have to be able to represent the definitions of these
+new operators, in the "def binary| 5" part of the function definition.  In our
+grammar so far, the "name" for the function definition is parsed as the
+"prototype" production and into the <tt>Ast.Prototype</tt> AST node.  To
+represent our new user-defined operators as prototypes, we have to extend
+the  <tt>Ast.Prototype</tt> AST node like this:</p>
+
+<div class="doc_code">
+<pre>
+(* proto - This type represents the "prototype" for a function, which captures
+ * its name, and its argument names (thus implicitly the number of arguments the
+ * function takes). *)
+type proto =
+  | Prototype of string * string array
+  <b>| BinOpPrototype of string * string array * int</b>
+</pre>
+</div>
+
+<p>Basically, in addition to knowing a name for the prototype, we now keep track
+of whether it was an operator, and if it was, what precedence level the operator
+is at.  The precedence is only used for binary operators (as you'll see below,
+it just doesn't apply for unary operators).  Now that we have a way to represent
+the prototype for a user-defined operator, we need to parse it:</p>
+
+<div class="doc_code">
+<pre>
+(* prototype
+ *   ::= id '(' id* ')'
+ <b>*   ::= binary LETTER number? (id, id)
+ *   ::= unary LETTER number? (id) *)</b>
+let parse_prototype =
+  let rec parse_args accumulator = parser
+    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
+    | [&lt; &gt;] -&gt; accumulator
+  in
+  let parse_operator = parser
+    | [&lt; 'Token.Unary &gt;] -&gt; "unary", 1
+    | [&lt; 'Token.Binary &gt;] -&gt; "binary", 2
+  in
+  let parse_binary_precedence = parser
+    | [&lt; 'Token.Number n &gt;] -&gt; int_of_float n
+    | [&lt; &gt;] -&gt; 30
+  in
+  parser
+  | [&lt; 'Token.Ident id;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+       args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      (* success. *)
+      Ast.Prototype (id, Array.of_list (List.rev args))
+  <b>| [&lt; (prefix, kind)=parse_operator;
+       'Token.Kwd op ?? "expected an operator";
+       (* Read the precedence if present. *)
+       binary_precedence=parse_binary_precedence;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+        args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      let name = prefix ^ (String.make 1 op) in
+      let args = Array.of_list (List.rev args) in
+
+      (* Verify right number of arguments for operator. *)
+      if Array.length args != kind
+      then raise (Stream.Error "invalid number of operands for operator")
+      else
+        if kind == 1 then
+          Ast.Prototype (name, args)
+        else
+          Ast.BinOpPrototype (name, args, binary_precedence)</b>
+  | [&lt; &gt;] -&gt;
+      raise (Stream.Error "expected function name in prototype")
+</pre>
+</div>
+
+<p>This is all fairly straightforward parsing code, and we have already seen
+a lot of similar code in the past.  One interesting part about the code above is
+the couple lines that set up <tt>name</tt> for binary operators.  This builds
+names like "binary@" for a newly defined "@" operator.  This then takes
+advantage of the fact that symbol names in the LLVM symbol table are allowed to
+have any character in them, including embedded nul characters.</p>
+
+<p>The next interesting thing to add, is codegen support for these binary
+operators.  Given our current structure, this is a simple addition of a default
+case for our existing binary operator node:</p>
+
+<div class="doc_code">
+<pre>
+let codegen_expr = function
+  ...
+  | Ast.Binary (op, lhs, rhs) -&gt;
+      let lhs_val = codegen_expr lhs in
+      let rhs_val = codegen_expr rhs in
+      begin
+        match op with
+        | '+' -&gt; build_add lhs_val rhs_val "addtmp" builder
+        | '-' -&gt; build_sub lhs_val rhs_val "subtmp" builder
+        | '*' -&gt; build_mul lhs_val rhs_val "multmp" builder
+        | '&lt;' -&gt;
+            (* Convert bool 0/1 to double 0.0 or 1.0 *)
+            let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
+            build_uitofp i double_type "booltmp" builder
+        <b>| _ -&gt;
+            (* If it wasn't a builtin binary operator, it must be a user defined
+             * one. Emit a call to it. *)
+            let callee = "binary" ^ (String.make 1 op) in
+            let callee =
+              match lookup_function callee the_module with
+              | Some callee -&gt; callee
+              | None -&gt; raise (Error "binary operator not found!")
+            in
+            build_call callee [|lhs_val; rhs_val|] "binop" builder</b>
+      end
+</pre>
+</div>
+
+<p>As you can see above, the new code is actually really simple.  It just does
+a lookup for the appropriate operator in the symbol table and generates a
+function call to it.  Since user-defined operators are just built as normal
+functions (because the "prototype" boils down to a function with the right
+name) everything falls into place.</p>
+
+<p>The final piece of code we are missing, is a bit of top level magic:</p>
+
+<div class="doc_code">
+<pre>
+let codegen_func the_fpm = function
+  | Ast.Function (proto, body) -&gt;
+      Hashtbl.clear named_values;
+      let the_function = codegen_proto proto in
+
+      <b>(* If this is an operator, install it. *)
+      begin match proto with
+      | Ast.BinOpPrototype (name, args, prec) -&gt;
+          let op = name.[String.length name - 1] in
+          Hashtbl.add Parser.binop_precedence op prec;
+      | _ -&gt; ()
+      end;</b>
+
+      (* Create a new basic block to start insertion into. *)
+      let bb = append_block context "entry" the_function in
+      position_at_end bb builder;
+      ...
+</pre>
+</div>
+
+<p>Basically, before codegening a function, if it is a user-defined operator, we
+register it in the precedence table.  This allows the binary operator parsing
+logic we already have in place to handle it.  Since we are working on a
+fully-general operator precedence parser, this is all we need to do to "extend
+the grammar".</p>
+
+<p>Now we have useful user-defined binary operators.  This builds a lot
+on the previous framework we built for other operators.  Adding unary operators
+is a bit more challenging, because we don't have any framework for it yet - lets
+see what it takes.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="unary">User-defined Unary Operators</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Since we don't currently support unary operators in the Kaleidoscope
+language, we'll need to add everything to support them.  Above, we added simple
+support for the 'unary' keyword to the lexer.  In addition to that, we need an
+AST node:</p>
+
+<div class="doc_code">
+<pre>
+type expr =
+  ...
+  (* variant for a unary operator. *)
+  | Unary of char * expr
+  ...
+</pre>
+</div>
+
+<p>This AST node is very simple and obvious by now.  It directly mirrors the
+binary operator AST node, except that it only has one child.  With this, we
+need to add the parsing logic.  Parsing a unary operator is pretty simple: we'll
+add a new function to do it:</p>
+
+<div class="doc_code">
+<pre>
+(* unary
+ *   ::= primary
+ *   ::= '!' unary *)
+and parse_unary = parser
+  (* If this is a unary operator, read it. *)
+  | [&lt; 'Token.Kwd op when op != '(' &amp;&amp; op != ')'; operand=parse_expr &gt;] -&gt;
+      Ast.Unary (op, operand)
+
+  (* If the current token is not an operator, it must be a primary expr. *)
+  | [&lt; stream &gt;] -&gt; parse_primary stream
+</pre>
+</div>
+
+<p>The grammar we add is pretty straightforward here.  If we see a unary
+operator when parsing a primary operator, we eat the operator as a prefix and
+parse the remaining piece as another unary operator.  This allows us to handle
+multiple unary operators (e.g. "!!x").  Note that unary operators can't have
+ambiguous parses like binary operators can, so there is no need for precedence
+information.</p>
+
+<p>The problem with this function, is that we need to call ParseUnary from
+somewhere.  To do this, we change previous callers of ParsePrimary to call
+<tt>parse_unary</tt> instead:</p>
+
+<div class="doc_code">
+<pre>
+(* binoprhs
+ *   ::= ('+' primary)* *)
+and parse_bin_rhs expr_prec lhs stream =
+        ...
+        <b>(* Parse the unary expression after the binary operator. *)
+        let rhs = parse_unary stream in</b>
+        ...
+
+...
+
+(* expression
+ *   ::= primary binoprhs *)
+and parse_expr = parser
+  | [&lt; lhs=<b>parse_unary</b>; stream &gt;] -&gt; parse_bin_rhs 0 lhs stream
+</pre>
+</div>
+
+<p>With these two simple changes, we are now able to parse unary operators and build the
+AST for them.  Next up, we need to add parser support for prototypes, to parse
+the unary operator prototype.  We extend the binary operator code above
+with:</p>
+
+<div class="doc_code">
+<pre>
+(* prototype
+ *   ::= id '(' id* ')'
+ *   ::= binary LETTER number? (id, id)
+ <b>*   ::= unary LETTER number? (id)</b> *)
+let parse_prototype =
+  let rec parse_args accumulator = parser
+    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
+    | [&lt; &gt;] -&gt; accumulator
+  in
+  <b>let parse_operator = parser
+    | [&lt; 'Token.Unary &gt;] -&gt; "unary", 1
+    | [&lt; 'Token.Binary &gt;] -&gt; "binary", 2
+  in</b>
+  let parse_binary_precedence = parser
+    | [&lt; 'Token.Number n &gt;] -&gt; int_of_float n
+    | [&lt; &gt;] -&gt; 30
+  in
+  parser
+  | [&lt; 'Token.Ident id;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+       args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      (* success. *)
+      Ast.Prototype (id, Array.of_list (List.rev args))
+  <b>| [&lt; (prefix, kind)=parse_operator;
+       'Token.Kwd op ?? "expected an operator";
+       (* Read the precedence if present. *)
+       binary_precedence=parse_binary_precedence;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+        args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      let name = prefix ^ (String.make 1 op) in
+      let args = Array.of_list (List.rev args) in
+
+      (* Verify right number of arguments for operator. *)
+      if Array.length args != kind
+      then raise (Stream.Error "invalid number of operands for operator")
+      else
+        if kind == 1 then
+          Ast.Prototype (name, args)
+        else
+          Ast.BinOpPrototype (name, args, binary_precedence)</b>
+  | [&lt; &gt;] -&gt;
+      raise (Stream.Error "expected function name in prototype")
+</pre>
+</div>
+
+<p>As with binary operators, we name unary operators with a name that includes
+the operator character.  This assists us at code generation time.  Speaking of,
+the final piece we need to add is codegen support for unary operators.  It looks
+like this:</p>
+
+<div class="doc_code">
+<pre>
+let rec codegen_expr = function
+  ...
+  | Ast.Unary (op, operand) -&gt;
+      let operand = codegen_expr operand in
+      let callee = "unary" ^ (String.make 1 op) in
+      let callee =
+        match lookup_function callee the_module with
+        | Some callee -&gt; callee
+        | None -&gt; raise (Error "unknown unary operator")
+      in
+      build_call callee [|operand|] "unop" builder
+</pre>
+</div>
+
+<p>This code is similar to, but simpler than, the code for binary operators.  It
+is simpler primarily because it doesn't need to handle any predefined operators.
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="example">Kicking the Tires</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>It is somewhat hard to believe, but with a few simple extensions we've
+covered in the last chapters, we have grown a real-ish language.  With this, we
+can do a lot of interesting things, including I/O, math, and a bunch of other
+things.  For example, we can now add a nice sequencing operator (printd is
+defined to print out the specified value and a newline):</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>extern printd(x);</b>
+Read extern: declare double @printd(double)
+ready&gt; <b>def binary : 1 (x y) 0;  # Low-precedence operator that ignores operands.</b>
+..
+ready&gt; <b>printd(123) : printd(456) : printd(789);</b>
+123.000000
+456.000000
+789.000000
+Evaluated to 0.000000
+</pre>
+</div>
+
+<p>We can also define a bunch of other "primitive" operations, such as:</p>
+
+<div class="doc_code">
+<pre>
+# Logical unary not.
+def unary!(v)
+  if v then
+    0
+  else
+    1;
+
+# Unary negate.
+def unary-(v)
+  0-v;
+
+# Define &gt; with the same precedence as &gt;.
+def binary&gt; 10 (LHS RHS)
+  RHS &lt; LHS;
+
+# Binary logical or, which does not short circuit.
+def binary| 5 (LHS RHS)
+  if LHS then
+    1
+  else if RHS then
+    1
+  else
+    0;
+
+# Binary logical and, which does not short circuit.
+def binary&amp; 6 (LHS RHS)
+  if !LHS then
+    0
+  else
+    !!RHS;
+
+# Define = with slightly lower precedence than relationals.
+def binary = 9 (LHS RHS)
+  !(LHS &lt; RHS | LHS &gt; RHS);
+
+</pre>
+</div>
+
+
+<p>Given the previous if/then/else support, we can also define interesting
+functions for I/O.  For example, the following prints out a character whose
+"density" reflects the value passed in: the lower the value, the denser the
+character:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt;
+<b>
+extern putchard(char)
+def printdensity(d)
+  if d &gt; 8 then
+    putchard(32)  # ' '
+  else if d &gt; 4 then
+    putchard(46)  # '.'
+  else if d &gt; 2 then
+    putchard(43)  # '+'
+  else
+    putchard(42); # '*'</b>
+...
+ready&gt; <b>printdensity(1): printdensity(2): printdensity(3) :
+          printdensity(4): printdensity(5): printdensity(9): putchard(10);</b>
+*++..
+Evaluated to 0.000000
+</pre>
+</div>
+
+<p>Based on these simple primitive operations, we can start to define more
+interesting things.  For example, here's a little function that solves for the
+number of iterations it takes a function in the complex plane to
+converge:</p>
+
+<div class="doc_code">
+<pre>
+# determine whether the specific location diverges.
+# Solve for z = z^2 + c in the complex plane.
+def mandleconverger(real imag iters creal cimag)
+  if iters &gt; 255 | (real*real + imag*imag &gt; 4) then
+    iters
+  else
+    mandleconverger(real*real - imag*imag + creal,
+                    2*real*imag + cimag,
+                    iters+1, creal, cimag);
+
+# return the number of iterations required for the iteration to escape
+def mandleconverge(real imag)
+  mandleconverger(real, imag, 0, real, imag);
+</pre>
+</div>
+
+<p>This "z = z<sup>2</sup> + c" function is a beautiful little creature that is the basis
+for computation of the <a
+href="http://en.wikipedia.org/wiki/Mandelbrot_set">Mandelbrot Set</a>.  Our
+<tt>mandelconverge</tt> function returns the number of iterations that it takes
+for a complex orbit to escape, saturating to 255.  This is not a very useful
+function by itself, but if you plot its value over a two-dimensional plane,
+you can see the Mandelbrot set.  Given that we are limited to using putchard
+here, our amazing graphical output is limited, but we can whip together
+something using the density plotter above:</p>
+
+<div class="doc_code">
+<pre>
+# compute and plot the mandlebrot set with the specified 2 dimensional range
+# info.
+def mandelhelp(xmin xmax xstep   ymin ymax ystep)
+  for y = ymin, y &lt; ymax, ystep in (
+    (for x = xmin, x &lt; xmax, xstep in
+       printdensity(mandleconverge(x,y)))
+    : putchard(10)
+  )
+
+# mandel - This is a convenient helper function for ploting the mandelbrot set
+# from the specified position with the specified Magnification.
+def mandel(realstart imagstart realmag imagmag)
+  mandelhelp(realstart, realstart+realmag*78, realmag,
+             imagstart, imagstart+imagmag*40, imagmag);
+</pre>
+</div>
+
+<p>Given this, we can try plotting out the mandlebrot set!  Lets try it out:</p>
+
+<div class="doc_code">
+<pre>
+ready&gt; <b>mandel(-2.3, -1.3, 0.05, 0.07);</b>
+*******************************+++++++++++*************************************
+*************************+++++++++++++++++++++++*******************************
+**********************+++++++++++++++++++++++++++++****************************
+*******************+++++++++++++++++++++.. ...++++++++*************************
+*****************++++++++++++++++++++++.... ...+++++++++***********************
+***************+++++++++++++++++++++++.....   ...+++++++++*********************
+**************+++++++++++++++++++++++....     ....+++++++++********************
+*************++++++++++++++++++++++......      .....++++++++*******************
+************+++++++++++++++++++++.......       .......+++++++******************
+***********+++++++++++++++++++....                ... .+++++++*****************
+**********+++++++++++++++++.......                     .+++++++****************
+*********++++++++++++++...........                    ...+++++++***************
+********++++++++++++............                      ...++++++++**************
+********++++++++++... ..........                        .++++++++**************
+*******+++++++++.....                                   .+++++++++*************
+*******++++++++......                                  ..+++++++++*************
+*******++++++.......                                   ..+++++++++*************
+*******+++++......                                     ..+++++++++*************
+*******.... ....                                      ...+++++++++*************
+*******.... .                                         ...+++++++++*************
+*******+++++......                                    ...+++++++++*************
+*******++++++.......                                   ..+++++++++*************
+*******++++++++......                                   .+++++++++*************
+*******+++++++++.....                                  ..+++++++++*************
+********++++++++++... ..........                        .++++++++**************
+********++++++++++++............                      ...++++++++**************
+*********++++++++++++++..........                     ...+++++++***************
+**********++++++++++++++++........                     .+++++++****************
+**********++++++++++++++++++++....                ... ..+++++++****************
+***********++++++++++++++++++++++.......       .......++++++++*****************
+************+++++++++++++++++++++++......      ......++++++++******************
+**************+++++++++++++++++++++++....      ....++++++++********************
+***************+++++++++++++++++++++++.....   ...+++++++++*********************
+*****************++++++++++++++++++++++....  ...++++++++***********************
+*******************+++++++++++++++++++++......++++++++*************************
+*********************++++++++++++++++++++++.++++++++***************************
+*************************+++++++++++++++++++++++*******************************
+******************************+++++++++++++************************************
+*******************************************************************************
+*******************************************************************************
+*******************************************************************************
+Evaluated to 0.000000
+ready&gt; <b>mandel(-2, -1, 0.02, 0.04);</b>
+**************************+++++++++++++++++++++++++++++++++++++++++++++++++++++
+***********************++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+*********************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++.
+*******************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++...
+*****************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++.....
+***************++++++++++++++++++++++++++++++++++++++++++++++++++++++++........
+**************++++++++++++++++++++++++++++++++++++++++++++++++++++++...........
+************+++++++++++++++++++++++++++++++++++++++++++++++++++++..............
+***********++++++++++++++++++++++++++++++++++++++++++++++++++........        .
+**********++++++++++++++++++++++++++++++++++++++++++++++.............
+********+++++++++++++++++++++++++++++++++++++++++++..................
+*******+++++++++++++++++++++++++++++++++++++++.......................
+******+++++++++++++++++++++++++++++++++++...........................
+*****++++++++++++++++++++++++++++++++............................
+*****++++++++++++++++++++++++++++...............................
+****++++++++++++++++++++++++++......   .........................
+***++++++++++++++++++++++++.........     ......    ...........
+***++++++++++++++++++++++............
+**+++++++++++++++++++++..............
+**+++++++++++++++++++................
+*++++++++++++++++++.................
+*++++++++++++++++............ ...
+*++++++++++++++..............
+*+++....++++................
+*..........  ...........
+*
+*..........  ...........
+*+++....++++................
+*++++++++++++++..............
+*++++++++++++++++............ ...
+*++++++++++++++++++.................
+**+++++++++++++++++++................
+**+++++++++++++++++++++..............
+***++++++++++++++++++++++............
+***++++++++++++++++++++++++.........     ......    ...........
+****++++++++++++++++++++++++++......   .........................
+*****++++++++++++++++++++++++++++...............................
+*****++++++++++++++++++++++++++++++++............................
+******+++++++++++++++++++++++++++++++++++...........................
+*******+++++++++++++++++++++++++++++++++++++++.......................
+********+++++++++++++++++++++++++++++++++++++++++++..................
+Evaluated to 0.000000
+ready&gt; <b>mandel(-0.9, -1.4, 0.02, 0.03);</b>
+*******************************************************************************
+*******************************************************************************
+*******************************************************************************
+**********+++++++++++++++++++++************************************************
+*+++++++++++++++++++++++++++++++++++++++***************************************
++++++++++++++++++++++++++++++++++++++++++++++**********************************
+++++++++++++++++++++++++++++++++++++++++++++++++++*****************************
+++++++++++++++++++++++++++++++++++++++++++++++++++++++*************************
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++**********************
++++++++++++++++++++++++++++++++++.........++++++++++++++++++*******************
++++++++++++++++++++++++++++++++....   ......+++++++++++++++++++****************
++++++++++++++++++++++++++++++.......  ........+++++++++++++++++++**************
+++++++++++++++++++++++++++++........   ........++++++++++++++++++++************
++++++++++++++++++++++++++++.........     ..  ...+++++++++++++++++++++**********
+++++++++++++++++++++++++++...........        ....++++++++++++++++++++++********
+++++++++++++++++++++++++.............       .......++++++++++++++++++++++******
++++++++++++++++++++++++.............        ........+++++++++++++++++++++++****
+++++++++++++++++++++++...........           ..........++++++++++++++++++++++***
+++++++++++++++++++++...........                .........++++++++++++++++++++++*
+++++++++++++++++++............                  ...........++++++++++++++++++++
+++++++++++++++++...............                 .............++++++++++++++++++
+++++++++++++++.................                 ...............++++++++++++++++
+++++++++++++..................                  .................++++++++++++++
++++++++++..................                      .................+++++++++++++
+++++++........        .                               .........  ..++++++++++++
+++............                                         ......    ....++++++++++
+..............                                                    ...++++++++++
+..............                                                    ....+++++++++
+..............                                                    .....++++++++
+.............                                                    ......++++++++
+...........                                                     .......++++++++
+.........                                                       ........+++++++
+.........                                                       ........+++++++
+.........                                                           ....+++++++
+........                                                             ...+++++++
+.......                                                              ...+++++++
+                                                                    ....+++++++
+                                                                   .....+++++++
+                                                                    ....+++++++
+                                                                    ....+++++++
+                                                                    ....+++++++
+Evaluated to 0.000000
+ready&gt; <b>^D</b>
+</pre>
+</div>
+
+<p>At this point, you may be starting to realize that Kaleidoscope is a real
+and powerful language.  It may not be self-similar :), but it can be used to
+plot things that are!</p>
+
+<p>With this, we conclude the "adding user-defined operators" chapter of the
+tutorial.  We have successfully augmented our language, adding the ability to
+extend the language in the library, and we have shown how this can be used to
+build a simple but interesting end-user application in Kaleidoscope.  At this
+point, Kaleidoscope can build a variety of applications that are functional and
+can call functions with side-effects, but it can't actually define and mutate a
+variable itself.</p>
+
+<p>Strikingly, variable mutation is an important feature of some
+languages, and it is not at all obvious how to <a href="OCamlLangImpl7.html">add
+support for mutable variables</a> without having to add an "SSA construction"
+phase to your front-end.  In the next chapter, we will describe how you can
+add variable mutation without building SSA in your front-end.</p>
+
+</div>
+
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with the
+if/then/else and for expressions..  To build this example, use:
+</p>
+
+<div class="doc_code">
+<pre>
+# Compile
+ocamlbuild toy.byte
+# Run
+./toy.byte
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<dl>
+<dt>_tags:</dt>
+<dd class="doc_code">
+<pre>
+&lt;{lexer,parser}.ml&gt;: use_camlp4, pp(camlp4of)
+&lt;*.{byte,native}&gt;: g++, use_llvm, use_llvm_analysis
+&lt;*.{byte,native}&gt;: use_llvm_executionengine, use_llvm_target
+&lt;*.{byte,native}&gt;: use_llvm_scalar_opts, use_bindings
+</pre>
+</dd>
+
+<dt>myocamlbuild.ml:</dt>
+<dd class="doc_code">
+<pre>
+open Ocamlbuild_plugin;;
+
+ocaml_lib ~extern:true "llvm";;
+ocaml_lib ~extern:true "llvm_analysis";;
+ocaml_lib ~extern:true "llvm_executionengine";;
+ocaml_lib ~extern:true "llvm_target";;
+ocaml_lib ~extern:true "llvm_scalar_opts";;
+
+flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"; A"-cclib"; A"-rdynamic"]);;
+dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
+</pre>
+</dd>
+
+<dt>token.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer Tokens
+ *===----------------------------------------------------------------------===*)
+
+(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
+ * these others for known things. *)
+type token =
+  (* commands *)
+  | Def | Extern
+
+  (* primary *)
+  | Ident of string | Number of float
+
+  (* unknown *)
+  | Kwd of char
+
+  (* control *)
+  | If | Then | Else
+  | For | In
+
+  (* operators *)
+  | Binary | Unary
+</pre>
+</dd>
+
+<dt>lexer.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer
+ *===----------------------------------------------------------------------===*)
+
+let rec lex = parser
+  (* Skip any whitespace. *)
+  | [&lt; ' (' ' | '\n' | '\r' | '\t'); stream &gt;] -&gt; lex stream
+
+  (* identifier: [a-zA-Z][a-zA-Z0-9] *)
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+
+  (* number: [0-9.]+ *)
+  | [&lt; ' ('0' .. '9' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+
+  (* Comment until end of line. *)
+  | [&lt; ' ('#'); stream &gt;] -&gt;
+      lex_comment stream
+
+  (* Otherwise, just return the character as its ascii value. *)
+  | [&lt; 'c; stream &gt;] -&gt;
+      [&lt; 'Token.Kwd c; lex stream &gt;]
+
+  (* end of stream. *)
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+
+and lex_number buffer = parser
+  | [&lt; ' ('0' .. '9' | '.' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      [&lt; 'Token.Number (float_of_string (Buffer.contents buffer)); stream &gt;]
+
+and lex_ident buffer = parser
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | "if" -&gt; [&lt; 'Token.If; stream &gt;]
+      | "then" -&gt; [&lt; 'Token.Then; stream &gt;]
+      | "else" -&gt; [&lt; 'Token.Else; stream &gt;]
+      | "for" -&gt; [&lt; 'Token.For; stream &gt;]
+      | "in" -&gt; [&lt; 'Token.In; stream &gt;]
+      | "binary" -&gt; [&lt; 'Token.Binary; stream &gt;]
+      | "unary" -&gt; [&lt; 'Token.Unary; stream &gt;]
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+
+and lex_comment = parser
+  | [&lt; ' ('\n'); stream=lex &gt;] -&gt; stream
+  | [&lt; 'c; e=lex_comment &gt;] -&gt; e
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+</pre>
+</dd>
+
+<dt>ast.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Abstract Syntax Tree (aka Parse Tree)
+ *===----------------------------------------------------------------------===*)
+
+(* expr - Base type for all expression nodes. *)
+type expr =
+  (* variant for numeric literals like "1.0". *)
+  | Number of float
+
+  (* variant for referencing a variable, like "a". *)
+  | Variable of string
+
+  (* variant for a unary operator. *)
+  | Unary of char * expr
+
+  (* variant for a binary operator. *)
+  | Binary of char * expr * expr
+
+  (* variant for function calls. *)
+  | Call of string * expr array
+
+  (* variant for if/then/else. *)
+  | If of expr * expr * expr
+
+  (* variant for for/in. *)
+  | For of string * expr * expr * expr option * expr
+
+(* proto - This type represents the "prototype" for a function, which captures
+ * its name, and its argument names (thus implicitly the number of arguments the
+ * function takes). *)
+type proto =
+  | Prototype of string * string array
+  | BinOpPrototype of string * string array * int
+
+(* func - This type represents a function definition itself. *)
+type func = Function of proto * expr
+</pre>
+</dd>
+
+<dt>parser.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===---------------------------------------------------------------------===
+ * Parser
+ *===---------------------------------------------------------------------===*)
+
+(* binop_precedence - This holds the precedence for each binary operator that is
+ * defined *)
+let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
+
+(* precedence - Get the precedence of the pending binary operator token. *)
+let precedence c = try Hashtbl.find binop_precedence c with Not_found -&gt; -1
+
+(* primary
+ *   ::= identifier
+ *   ::= numberexpr
+ *   ::= parenexpr
+ *   ::= ifexpr
+ *   ::= forexpr *)
+let rec parse_primary = parser
+  (* numberexpr ::= number *)
+  | [&lt; 'Token.Number n &gt;] -&gt; Ast.Number n
+
+  (* parenexpr ::= '(' expression ')' *)
+  | [&lt; 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" &gt;] -&gt; e
+
+  (* identifierexpr
+   *   ::= identifier
+   *   ::= identifier '(' argumentexpr ')' *)
+  | [&lt; 'Token.Ident id; stream &gt;] -&gt;
+      let rec parse_args accumulator = parser
+        | [&lt; e=parse_expr; stream &gt;] -&gt;
+            begin parser
+              | [&lt; 'Token.Kwd ','; e=parse_args (e :: accumulator) &gt;] -&gt; e
+              | [&lt; &gt;] -&gt; e :: accumulator
+            end stream
+        | [&lt; &gt;] -&gt; accumulator
+      in
+      let rec parse_ident id = parser
+        (* Call. *)
+        | [&lt; 'Token.Kwd '(';
+             args=parse_args [];
+             'Token.Kwd ')' ?? "expected ')'"&gt;] -&gt;
+            Ast.Call (id, Array.of_list (List.rev args))
+
+        (* Simple variable ref. *)
+        | [&lt; &gt;] -&gt; Ast.Variable id
+      in
+      parse_ident id stream
+
+  (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
+  | [&lt; 'Token.If; c=parse_expr;
+       'Token.Then ?? "expected 'then'"; t=parse_expr;
+       'Token.Else ?? "expected 'else'"; e=parse_expr &gt;] -&gt;
+      Ast.If (c, t, e)
+
+  (* forexpr
+        ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
+  | [&lt; 'Token.For;
+       'Token.Ident id ?? "expected identifier after for";
+       'Token.Kwd '=' ?? "expected '=' after for";
+       stream &gt;] -&gt;
+      begin parser
+        | [&lt;
+             start=parse_expr;
+             'Token.Kwd ',' ?? "expected ',' after for";
+             end_=parse_expr;
+             stream &gt;] -&gt;
+            let step =
+              begin parser
+              | [&lt; 'Token.Kwd ','; step=parse_expr &gt;] -&gt; Some step
+              | [&lt; &gt;] -&gt; None
+              end stream
+            in
+            begin parser
+            | [&lt; 'Token.In; body=parse_expr &gt;] -&gt;
+                Ast.For (id, start, end_, step, body)
+            | [&lt; &gt;] -&gt;
+                raise (Stream.Error "expected 'in' after for")
+            end stream
+        | [&lt; &gt;] -&gt;
+            raise (Stream.Error "expected '=' after for")
+      end stream
+
+  | [&lt; &gt;] -&gt; raise (Stream.Error "unknown token when expecting an expression.")
+
+(* unary
+ *   ::= primary
+ *   ::= '!' unary *)
+and parse_unary = parser
+  (* If this is a unary operator, read it. *)
+  | [&lt; 'Token.Kwd op when op != '(' &amp;&amp; op != ')'; operand=parse_expr &gt;] -&gt;
+      Ast.Unary (op, operand)
+
+  (* If the current token is not an operator, it must be a primary expr. *)
+  | [&lt; stream &gt;] -&gt; parse_primary stream
+
+(* binoprhs
+ *   ::= ('+' primary)* *)
+and parse_bin_rhs expr_prec lhs stream =
+  match Stream.peek stream with
+  (* If this is a binop, find its precedence. *)
+  | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -&gt;
+      let token_prec = precedence c in
+
+      (* If this is a binop that binds at least as tightly as the current binop,
+       * consume it, otherwise we are done. *)
+      if token_prec &lt; expr_prec then lhs else begin
+        (* Eat the binop. *)
+        Stream.junk stream;
+
+        (* Parse the unary expression after the binary operator. *)
+        let rhs = parse_unary stream in
+
+        (* Okay, we know this is a binop. *)
+        let rhs =
+          match Stream.peek stream with
+          | Some (Token.Kwd c2) -&gt;
+              (* If BinOp binds less tightly with rhs than the operator after
+               * rhs, let the pending operator take rhs as its lhs. *)
+              let next_prec = precedence c2 in
+              if token_prec &lt; next_prec
+              then parse_bin_rhs (token_prec + 1) rhs stream
+              else rhs
+          | _ -&gt; rhs
+        in
+
+        (* Merge lhs/rhs. *)
+        let lhs = Ast.Binary (c, lhs, rhs) in
+        parse_bin_rhs expr_prec lhs stream
+      end
+  | _ -&gt; lhs
+
+(* expression
+ *   ::= primary binoprhs *)
+and parse_expr = parser
+  | [&lt; lhs=parse_unary; stream &gt;] -&gt; parse_bin_rhs 0 lhs stream
+
+(* prototype
+ *   ::= id '(' id* ')'
+ *   ::= binary LETTER number? (id, id)
+ *   ::= unary LETTER number? (id) *)
+let parse_prototype =
+  let rec parse_args accumulator = parser
+    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
+    | [&lt; &gt;] -&gt; accumulator
+  in
+  let parse_operator = parser
+    | [&lt; 'Token.Unary &gt;] -&gt; "unary", 1
+    | [&lt; 'Token.Binary &gt;] -&gt; "binary", 2
+  in
+  let parse_binary_precedence = parser
+    | [&lt; 'Token.Number n &gt;] -&gt; int_of_float n
+    | [&lt; &gt;] -&gt; 30
+  in
+  parser
+  | [&lt; 'Token.Ident id;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+       args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      (* success. *)
+      Ast.Prototype (id, Array.of_list (List.rev args))
+  | [&lt; (prefix, kind)=parse_operator;
+       'Token.Kwd op ?? "expected an operator";
+       (* Read the precedence if present. *)
+       binary_precedence=parse_binary_precedence;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+        args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      let name = prefix ^ (String.make 1 op) in
+      let args = Array.of_list (List.rev args) in
+
+      (* Verify right number of arguments for operator. *)
+      if Array.length args != kind
+      then raise (Stream.Error "invalid number of operands for operator")
+      else
+        if kind == 1 then
+          Ast.Prototype (name, args)
+        else
+          Ast.BinOpPrototype (name, args, binary_precedence)
+  | [&lt; &gt;] -&gt;
+      raise (Stream.Error "expected function name in prototype")
+
+(* definition ::= 'def' prototype expression *)
+let parse_definition = parser
+  | [&lt; 'Token.Def; p=parse_prototype; e=parse_expr &gt;] -&gt;
+      Ast.Function (p, e)
+
+(* toplevelexpr ::= expression *)
+let parse_toplevel = parser
+  | [&lt; e=parse_expr &gt;] -&gt;
+      (* Make an anonymous proto. *)
+      Ast.Function (Ast.Prototype ("", [||]), e)
+
+(*  external ::= 'extern' prototype *)
+let parse_extern = parser
+  | [&lt; 'Token.Extern; e=parse_prototype &gt;] -&gt; e
+</pre>
+</dd>
+
+<dt>codegen.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Code Generation
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+
+exception Error of string
+
+let context = global_context ()
+let the_module = create_module context "my cool jit"
+let builder = builder context
+let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
+let double_type = double_type context
+
+let rec codegen_expr = function
+  | Ast.Number n -&gt; const_float double_type n
+  | Ast.Variable name -&gt;
+      (try Hashtbl.find named_values name with
+        | Not_found -&gt; raise (Error "unknown variable name"))
+  | Ast.Unary (op, operand) -&gt;
+      let operand = codegen_expr operand in
+      let callee = "unary" ^ (String.make 1 op) in
+      let callee =
+        match lookup_function callee the_module with
+        | Some callee -&gt; callee
+        | None -&gt; raise (Error "unknown unary operator")
+      in
+      build_call callee [|operand|] "unop" builder
+  | Ast.Binary (op, lhs, rhs) -&gt;
+      let lhs_val = codegen_expr lhs in
+      let rhs_val = codegen_expr rhs in
+      begin
+        match op with
+        | '+' -&gt; build_add lhs_val rhs_val "addtmp" builder
+        | '-' -&gt; build_sub lhs_val rhs_val "subtmp" builder
+        | '*' -&gt; build_mul lhs_val rhs_val "multmp" builder
+        | '&lt;' -&gt;
+            (* Convert bool 0/1 to double 0.0 or 1.0 *)
+            let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
+            build_uitofp i double_type "booltmp" builder
+        | _ -&gt;
+            (* If it wasn't a builtin binary operator, it must be a user defined
+             * one. Emit a call to it. *)
+            let callee = "binary" ^ (String.make 1 op) in
+            let callee =
+              match lookup_function callee the_module with
+              | Some callee -&gt; callee
+              | None -&gt; raise (Error "binary operator not found!")
+            in
+            build_call callee [|lhs_val; rhs_val|] "binop" builder
+      end
+  | Ast.Call (callee, args) -&gt;
+      (* Look up the name in the module table. *)
+      let callee =
+        match lookup_function callee the_module with
+        | Some callee -&gt; callee
+        | None -&gt; raise (Error "unknown function referenced")
+      in
+      let params = params callee in
+
+      (* If argument mismatch error. *)
+      if Array.length params == Array.length args then () else
+        raise (Error "incorrect # arguments passed");
+      let args = Array.map codegen_expr args in
+      build_call callee args "calltmp" builder
+  | Ast.If (cond, then_, else_) -&gt;
+      let cond = codegen_expr cond in
+
+      (* Convert condition to a bool by comparing equal to 0.0 *)
+      let zero = const_float double_type 0.0 in
+      let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
+
+      (* Grab the first block so that we might later add the conditional branch
+       * to it at the end of the function. *)
+      let start_bb = insertion_block builder in
+      let the_function = block_parent start_bb in
+
+      let then_bb = append_block context "then" the_function in
+
+      (* Emit 'then' value. *)
+      position_at_end then_bb builder;
+      let then_val = codegen_expr then_ in
+
+      (* Codegen of 'then' can change the current block, update then_bb for the
+       * phi. We create a new name because one is used for the phi node, and the
+       * other is used for the conditional branch. *)
+      let new_then_bb = insertion_block builder in
+
+      (* Emit 'else' value. *)
+      let else_bb = append_block context "else" the_function in
+      position_at_end else_bb builder;
+      let else_val = codegen_expr else_ in
+
+      (* Codegen of 'else' can change the current block, update else_bb for the
+       * phi. *)
+      let new_else_bb = insertion_block builder in
+
+      (* Emit merge block. *)
+      let merge_bb = append_block context "ifcont" the_function in
+      position_at_end merge_bb builder;
+      let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
+      let phi = build_phi incoming "iftmp" builder in
+
+      (* Return to the start block to add the conditional branch. *)
+      position_at_end start_bb builder;
+      ignore (build_cond_br cond_val then_bb else_bb builder);
+
+      (* Set a unconditional branch at the end of the 'then' block and the
+       * 'else' block to the 'merge' block. *)
+      position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
+      position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
+
+      (* Finally, set the builder to the end of the merge block. *)
+      position_at_end merge_bb builder;
+
+      phi
+  | Ast.For (var_name, start, end_, step, body) -&gt;
+      (* Emit the start code first, without 'variable' in scope. *)
+      let start_val = codegen_expr start in
+
+      (* Make the new basic block for the loop header, inserting after current
+       * block. *)
+      let preheader_bb = insertion_block builder in
+      let the_function = block_parent preheader_bb in
+      let loop_bb = append_block context "loop" the_function in
+
+      (* Insert an explicit fall through from the current block to the
+       * loop_bb. *)
+      ignore (build_br loop_bb builder);
+
+      (* Start insertion in loop_bb. *)
+      position_at_end loop_bb builder;
+
+      (* Start the PHI node with an entry for start. *)
+      let variable = build_phi [(start_val, preheader_bb)] var_name builder in
+
+      (* Within the loop, the variable is defined equal to the PHI node. If it
+       * shadows an existing variable, we have to restore it, so save it
+       * now. *)
+      let old_val =
+        try Some (Hashtbl.find named_values var_name) with Not_found -&gt; None
+      in
+      Hashtbl.add named_values var_name variable;
+
+      (* Emit the body of the loop.  This, like any other expr, can change the
+       * current BB.  Note that we ignore the value computed by the body, but
+       * don't allow an error *)
+      ignore (codegen_expr body);
+
+      (* Emit the step value. *)
+      let step_val =
+        match step with
+        | Some step -&gt; codegen_expr step
+        (* If not specified, use 1.0. *)
+        | None -&gt; const_float double_type 1.0
+      in
+
+      let next_var = build_add variable step_val "nextvar" builder in
+
+      (* Compute the end condition. *)
+      let end_cond = codegen_expr end_ in
+
+      (* Convert condition to a bool by comparing equal to 0.0. *)
+      let zero = const_float double_type 0.0 in
+      let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
+
+      (* Create the "after loop" block and insert it. *)
+      let loop_end_bb = insertion_block builder in
+      let after_bb = append_block context "afterloop" the_function in
+
+      (* Insert the conditional branch into the end of loop_end_bb. *)
+      ignore (build_cond_br end_cond loop_bb after_bb builder);
+
+      (* Any new code will be inserted in after_bb. *)
+      position_at_end after_bb builder;
+
+      (* Add a new entry to the PHI node for the backedge. *)
+      add_incoming (next_var, loop_end_bb) variable;
+
+      (* Restore the unshadowed variable. *)
+      begin match old_val with
+      | Some old_val -&gt; Hashtbl.add named_values var_name old_val
+      | None -&gt; ()
+      end;
+
+      (* for expr always returns 0.0. *)
+      const_null double_type
+
+let codegen_proto = function
+  | Ast.Prototype (name, args) | Ast.BinOpPrototype (name, args, _) -&gt;
+      (* Make the function type: double(double,double) etc. *)
+      let doubles = Array.make (Array.length args) double_type in
+      let ft = function_type double_type doubles in
+      let f =
+        match lookup_function name the_module with
+        | None -&gt; declare_function name ft the_module
+
+        (* If 'f' conflicted, there was already something named 'name'. If it
+         * has a body, don't allow redefinition or reextern. *)
+        | Some f -&gt;
+            (* If 'f' already has a body, reject this. *)
+            if block_begin f &lt;&gt; At_end f then
+              raise (Error "redefinition of function");
+
+            (* If 'f' took a different number of arguments, reject. *)
+            if element_type (type_of f) &lt;&gt; ft then
+              raise (Error "redefinition of function with different # args");
+            f
+      in
+
+      (* Set names for all arguments. *)
+      Array.iteri (fun i a -&gt;
+        let n = args.(i) in
+        set_value_name n a;
+        Hashtbl.add named_values n a;
+      ) (params f);
+      f
+
+let codegen_func the_fpm = function
+  | Ast.Function (proto, body) -&gt;
+      Hashtbl.clear named_values;
+      let the_function = codegen_proto proto in
+
+      (* If this is an operator, install it. *)
+      begin match proto with
+      | Ast.BinOpPrototype (name, args, prec) -&gt;
+          let op = name.[String.length name - 1] in
+          Hashtbl.add Parser.binop_precedence op prec;
+      | _ -&gt; ()
+      end;
+
+      (* Create a new basic block to start insertion into. *)
+      let bb = append_block context "entry" the_function in
+      position_at_end bb builder;
+
+      try
+        let ret_val = codegen_expr body in
+
+        (* Finish off the function. *)
+        let _ = build_ret ret_val builder in
+
+        (* Validate the generated code, checking for consistency. *)
+        Llvm_analysis.assert_valid_function the_function;
+
+        (* Optimize the function. *)
+        let _ = PassManager.run_function the_function the_fpm in
+
+        the_function
+      with e -&gt;
+        delete_function the_function;
+        raise e
+</pre>
+</dd>
+
+<dt>toplevel.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Top-Level parsing and JIT Driver
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+open Llvm_executionengine
+
+(* top ::= definition | external | expression | ';' *)
+let rec main_loop the_fpm the_execution_engine stream =
+  match Stream.peek stream with
+  | None -&gt; ()
+
+  (* ignore top-level semicolons. *)
+  | Some (Token.Kwd ';') -&gt;
+      Stream.junk stream;
+      main_loop the_fpm the_execution_engine stream
+
+  | Some token -&gt;
+      begin
+        try match token with
+        | Token.Def -&gt;
+            let e = Parser.parse_definition stream in
+            print_endline "parsed a function definition.";
+            dump_value (Codegen.codegen_func the_fpm e);
+        | Token.Extern -&gt;
+            let e = Parser.parse_extern stream in
+            print_endline "parsed an extern.";
+            dump_value (Codegen.codegen_proto e);
+        | _ -&gt;
+            (* Evaluate a top-level expression into an anonymous function. *)
+            let e = Parser.parse_toplevel stream in
+            print_endline "parsed a top-level expr";
+            let the_function = Codegen.codegen_func the_fpm e in
+            dump_value the_function;
+
+            (* JIT the function, returning a function pointer. *)
+            let result = ExecutionEngine.run_function the_function [||]
+              the_execution_engine in
+
+            print_string "Evaluated to ";
+            print_float (GenericValue.as_float Codegen.double_type result);
+            print_newline ();
+        with Stream.Error s | Codegen.Error s -&gt;
+          (* Skip token for error recovery. *)
+          Stream.junk stream;
+          print_endline s;
+      end;
+      print_string "ready&gt; "; flush stdout;
+      main_loop the_fpm the_execution_engine stream
+</pre>
+</dd>
+
+<dt>toy.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Main driver code.
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+open Llvm_executionengine
+open Llvm_target
+open Llvm_scalar_opts
+
+let main () =
+  ignore (initialize_native_target ());
+
+  (* Install standard binary operators.
+   * 1 is the lowest precedence. *)
+  Hashtbl.add Parser.binop_precedence '&lt;' 10;
+  Hashtbl.add Parser.binop_precedence '+' 20;
+  Hashtbl.add Parser.binop_precedence '-' 20;
+  Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
+
+  (* Prime the first token. *)
+  print_string "ready&gt; "; flush stdout;
+  let stream = Lexer.lex (Stream.of_channel stdin) in
+
+  (* Create the JIT. *)
+  let the_execution_engine = ExecutionEngine.create Codegen.the_module in
+  let the_fpm = PassManager.create_function Codegen.the_module in
+
+  (* Set up the optimizer pipeline.  Start with registering info about how the
+   * target lays out data structures. *)
+  TargetData.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
+
+  (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
+  add_instruction_combination the_fpm;
+
+  (* reassociate expressions. *)
+  add_reassociation the_fpm;
+
+  (* Eliminate Common SubExpressions. *)
+  add_gvn the_fpm;
+
+  (* Simplify the control flow graph (deleting unreachable blocks, etc). *)
+  add_cfg_simplification the_fpm;
+
+  ignore (PassManager.initialize the_fpm);
+
+  (* Run the main "interpreter loop" now. *)
+  Toplevel.main_loop the_fpm the_execution_engine stream;
+
+  (* Print out all the generated code. *)
+  dump_module Codegen.the_module
+;;
+
+main ()
+</pre>
+</dd>
+
+<dt>bindings.c</dt>
+<dd class="doc_code">
+<pre>
+#include &lt;stdio.h&gt;
+
+/* putchard - putchar that takes a double and returns 0. */
+extern double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+
+/* printd - printf that takes a double prints it as "%f\n", returning 0. */
+extern double printd(double X) {
+  printf("%f\n", X);
+  return 0;
+}
+</pre>
+</dd>
+</dl>
+
+<a href="OCamlLangImpl7.html">Next: Extending the language: mutable variables /
+SSA construction</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/OCamlLangImpl7.html b/docs/tutorial/OCamlLangImpl7.html
new file mode 100644
index 0000000000..c140888626
--- /dev/null
+++ b/docs/tutorial/OCamlLangImpl7.html
@@ -0,0 +1,1907 @@
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Extending the Language: Mutable Variables / SSA
+         construction</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <meta name="author" content="Erick Tryzelaar">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Extending the Language: Mutable Variables</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 7
+  <ol>
+    <li><a href="#intro">Chapter 7 Introduction</a></li>
+    <li><a href="#why">Why is this a hard problem?</a></li>
+    <li><a href="#memory">Memory in LLVM</a></li>
+    <li><a href="#kalvars">Mutable Variables in Kaleidoscope</a></li>
+    <li><a href="#adjustments">Adjusting Existing Variables for
+     Mutation</a></li>
+    <li><a href="#assignment">New Assignment Operator</a></li>
+    <li><a href="#localvars">User-defined Local Variables</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="LangImpl8.html">Chapter 8</a>: Conclusion and other useful LLVM
+ tidbits</li>
+</ul>
+
+<div class="doc_author">
+	<p>
+		Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
+		and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
+	</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 7 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 7 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  In chapters 1 through 6, we've built a very
+respectable, albeit simple, <a
+href="http://en.wikipedia.org/wiki/Functional_programming">functional
+programming language</a>.  In our journey, we learned some parsing techniques,
+how to build and represent an AST, how to build LLVM IR, and how to optimize
+the resultant code as well as JIT compile it.</p>
+
+<p>While Kaleidoscope is interesting as a functional language, the fact that it
+is functional makes it "too easy" to generate LLVM IR for it.  In particular, a
+functional language makes it very easy to build LLVM IR directly in <a
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">SSA form</a>.
+Since LLVM requires that the input code be in SSA form, this is a very nice
+property and it is often unclear to newcomers how to generate code for an
+imperative language with mutable variables.</p>
+
+<p>The short (and happy) summary of this chapter is that there is no need for
+your front-end to build SSA form: LLVM provides highly tuned and well tested
+support for this, though the way it works is a bit unexpected for some.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="why">Why is this a hard problem?</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+To understand why mutable variables cause complexities in SSA construction,
+consider this extremely simple C example:
+</p>
+
+<div class="doc_code">
+<pre>
+int G, H;
+int test(_Bool Condition) {
+  int X;
+  if (Condition)
+    X = G;
+  else
+    X = H;
+  return X;
+}
+</pre>
+</div>
+
+<p>In this case, we have the variable "X", whose value depends on the path
+executed in the program.  Because there are two different possible values for X
+before the return instruction, a PHI node is inserted to merge the two values.
+The LLVM IR that we want for this example looks like this:</p>
+
+<div class="doc_code">
+<pre>
+@G = weak global i32 0   ; type of @G is i32*
+@H = weak global i32 0   ; type of @H is i32*
+
+define i32 @test(i1 %Condition) {
+entry:
+  br i1 %Condition, label %cond_true, label %cond_false
+
+cond_true:
+  %X.0 = load i32* @G
+  br label %cond_next
+
+cond_false:
+  %X.1 = load i32* @H
+  br label %cond_next
+
+cond_next:
+  %X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
+  ret i32 %X.2
+}
+</pre>
+</div>
+
+<p>In this example, the loads from the G and H global variables are explicit in
+the LLVM IR, and they live in the then/else branches of the if statement
+(cond_true/cond_false).  In order to merge the incoming values, the X.2 phi node
+in the cond_next block selects the right value to use based on where control
+flow is coming from: if control flow comes from the cond_false block, X.2 gets
+the value of X.1.  Alternatively, if control flow comes from cond_true, it gets
+the value of X.0.  The intent of this chapter is not to explain the details of
+SSA form.  For more information, see one of the many <a
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">online
+references</a>.</p>
+
+<p>The question for this article is "who places the phi nodes when lowering
+assignments to mutable variables?".  The issue here is that LLVM
+<em>requires</em> that its IR be in SSA form: there is no "non-ssa" mode for it.
+However, SSA construction requires non-trivial algorithms and data structures,
+so it is inconvenient and wasteful for every front-end to have to reproduce this
+logic.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="memory">Memory in LLVM</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>The 'trick' here is that while LLVM does require all register values to be
+in SSA form, it does not require (or permit) memory objects to be in SSA form.
+In the example above, note that the loads from G and H are direct accesses to
+G and H: they are not renamed or versioned.  This differs from some other
+compiler systems, which do try to version memory objects.  In LLVM, instead of
+encoding dataflow analysis of memory into the LLVM IR, it is handled with <a
+href="../WritingAnLLVMPass.html">Analysis Passes</a> which are computed on
+demand.</p>
+
+<p>
+With this in mind, the high-level idea is that we want to make a stack variable
+(which lives in memory, because it is on the stack) for each mutable object in
+a function.  To take advantage of this trick, we need to talk about how LLVM
+represents stack variables.
+</p>
+
+<p>In LLVM, all memory accesses are explicit with load/store instructions, and
+it is carefully designed not to have (or need) an "address-of" operator.  Notice
+how the type of the @G/@H global variables is actually "i32*" even though the
+variable is defined as "i32".  What this means is that @G defines <em>space</em>
+for an i32 in the global data area, but its <em>name</em> actually refers to the
+address for that space.  Stack variables work the same way, except that instead of
+being declared with global variable definitions, they are declared with the
+<a href="../LangRef.html#i_alloca">LLVM alloca instruction</a>:</p>
+
+<div class="doc_code">
+<pre>
+define i32 @example() {
+entry:
+  %X = alloca i32           ; type of %X is i32*.
+  ...
+  %tmp = load i32* %X       ; load the stack value %X from the stack.
+  %tmp2 = add i32 %tmp, 1   ; increment it
+  store i32 %tmp2, i32* %X  ; store it back
+  ...
+</pre>
+</div>
+
+<p>This code shows an example of how you can declare and manipulate a stack
+variable in the LLVM IR.  Stack memory allocated with the alloca instruction is
+fully general: you can pass the address of the stack slot to functions, you can
+store it in other variables, etc.  In our example above, we could rewrite the
+example to use the alloca technique to avoid using a PHI node:</p>
+
+<div class="doc_code">
+<pre>
+@G = weak global i32 0   ; type of @G is i32*
+@H = weak global i32 0   ; type of @H is i32*
+
+define i32 @test(i1 %Condition) {
+entry:
+  %X = alloca i32           ; type of %X is i32*.
+  br i1 %Condition, label %cond_true, label %cond_false
+
+cond_true:
+  %X.0 = load i32* @G
+        store i32 %X.0, i32* %X   ; Update X
+  br label %cond_next
+
+cond_false:
+  %X.1 = load i32* @H
+        store i32 %X.1, i32* %X   ; Update X
+  br label %cond_next
+
+cond_next:
+  %X.2 = load i32* %X       ; Read X
+  ret i32 %X.2
+}
+</pre>
+</div>
+
+<p>With this, we have discovered a way to handle arbitrary mutable variables
+without the need to create Phi nodes at all:</p>
+
+<ol>
+<li>Each mutable variable becomes a stack allocation.</li>
+<li>Each read of the variable becomes a load from the stack.</li>
+<li>Each update of the variable becomes a store to the stack.</li>
+<li>Taking the address of a variable just uses the stack address directly.</li>
+</ol>
+
+<p>While this solution has solved our immediate problem, it introduced another
+one: we have now apparently introduced a lot of stack traffic for very simple
+and common operations, a major performance problem.  Fortunately for us, the
+LLVM optimizer has a highly-tuned optimization pass named "mem2reg" that handles
+this case, promoting allocas like this into SSA registers, inserting Phi nodes
+as appropriate.  If you run this example through the pass, for example, you'll
+get:</p>
+
+<div class="doc_code">
+<pre>
+$ <b>llvm-as &lt; example.ll | opt -mem2reg | llvm-dis</b>
+@G = weak global i32 0
+@H = weak global i32 0
+
+define i32 @test(i1 %Condition) {
+entry:
+  br i1 %Condition, label %cond_true, label %cond_false
+
+cond_true:
+  %X.0 = load i32* @G
+  br label %cond_next
+
+cond_false:
+  %X.1 = load i32* @H
+  br label %cond_next
+
+cond_next:
+  %X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
+  ret i32 %X.01
+}
+</pre>
+</div>
+
+<p>The mem2reg pass implements the standard "iterated dominance frontier"
+algorithm for constructing SSA form and has a number of optimizations that speed
+up (very common) degenerate cases. The mem2reg optimization pass is the answer
+to dealing with mutable variables, and we highly recommend that you depend on
+it.  Note that mem2reg only works on variables in certain circumstances:</p>
+
+<ol>
+<li>mem2reg is alloca-driven: it looks for allocas and if it can handle them, it
+promotes them.  It does not apply to global variables or heap allocations.</li>
+
+<li>mem2reg only looks for alloca instructions in the entry block of the
+function.  Being in the entry block guarantees that the alloca is only executed
+once, which makes analysis simpler.</li>
+
+<li>mem2reg only promotes allocas whose uses are direct loads and stores.  If
+the address of the stack object is passed to a function, or if any funny pointer
+arithmetic is involved, the alloca will not be promoted.</li>
+
+<li>mem2reg only works on allocas of <a
+href="../LangRef.html#t_classifications">first class</a>
+values (such as pointers, scalars and vectors), and only if the array size
+of the allocation is 1 (or missing in the .ll file).  mem2reg is not capable of
+promoting structs or arrays to registers.  Note that the "scalarrepl" pass is
+more powerful and can promote structs, "unions", and arrays in many cases.</li>
+
+</ol>
+
+<p>
+All of these properties are easy to satisfy for most imperative languages, and
+we'll illustrate it below with Kaleidoscope.  The final question you may be
+asking is: should I bother with this nonsense for my front-end?  Wouldn't it be
+better if I just did SSA construction directly, avoiding use of the mem2reg
+optimization pass?  In short, we strongly recommend that you use this technique
+for building SSA form, unless there is an extremely good reason not to.  Using
+this technique is:</p>
+
+<ul>
+<li>Proven and well tested: llvm-gcc and clang both use this technique for local
+mutable variables.  As such, the most common clients of LLVM are using this to
+handle a bulk of their variables.  You can be sure that bugs are found fast and
+fixed early.</li>
+
+<li>Extremely Fast: mem2reg has a number of special cases that make it fast in
+common cases as well as fully general.  For example, it has fast-paths for
+variables that are only used in a single block, variables that only have one
+assignment point, good heuristics to avoid insertion of unneeded phi nodes, etc.
+</li>
+
+<li>Needed for debug info generation: <a href="../SourceLevelDebugging.html">
+Debug information in LLVM</a> relies on having the address of the variable
+exposed so that debug info can be attached to it.  This technique dovetails
+very naturally with this style of debug info.</li>
+</ul>
+
+<p>If nothing else, this makes it much easier to get your front-end up and
+running, and is very simple to implement.  Lets extend Kaleidoscope with mutable
+variables now!
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="kalvars">Mutable Variables in
+Kaleidoscope</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Now that we know the sort of problem we want to tackle, lets see what this
+looks like in the context of our little Kaleidoscope language.  We're going to
+add two features:</p>
+
+<ol>
+<li>The ability to mutate variables with the '=' operator.</li>
+<li>The ability to define new variables.</li>
+</ol>
+
+<p>While the first item is really what this is about, we only have variables
+for incoming arguments as well as for induction variables, and redefining those only
+goes so far :).  Also, the ability to define new variables is a
+useful thing regardless of whether you will be mutating them.  Here's a
+motivating example that shows how we could use these:</p>
+
+<div class="doc_code">
+<pre>
+# Define ':' for sequencing: as a low-precedence operator that ignores operands
+# and just returns the RHS.
+def binary : 1 (x y) y;
+
+# Recursive fib, we could do this before.
+def fib(x)
+  if (x &lt; 3) then
+    1
+  else
+    fib(x-1)+fib(x-2);
+
+# Iterative fib.
+def fibi(x)
+  <b>var a = 1, b = 1, c in</b>
+  (for i = 3, i &lt; x in
+     <b>c = a + b</b> :
+     <b>a = b</b> :
+     <b>b = c</b>) :
+  b;
+
+# Call it.
+fibi(10);
+</pre>
+</div>
+
+<p>
+In order to mutate variables, we have to change our existing variables to use
+the "alloca trick".  Once we have that, we'll add our new operator, then extend
+Kaleidoscope to support new variable definitions.
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="adjustments">Adjusting Existing Variables for
+Mutation</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+The symbol table in Kaleidoscope is managed at code generation time by the
+'<tt>named_values</tt>' map.  This map currently keeps track of the LLVM
+"Value*" that holds the double value for the named variable.  In order to
+support mutation, we need to change this slightly, so that it
+<tt>named_values</tt> holds the <em>memory location</em> of the variable in
+question.  Note that this change is a refactoring: it changes the structure of
+the code, but does not (by itself) change the behavior of the compiler.  All of
+these changes are isolated in the Kaleidoscope code generator.</p>
+
+<p>
+At this point in Kaleidoscope's development, it only supports variables for two
+things: incoming arguments to functions and the induction variable of 'for'
+loops.  For consistency, we'll allow mutation of these variables in addition to
+other user-defined variables.  This means that these will both need memory
+locations.
+</p>
+
+<p>To start our transformation of Kaleidoscope, we'll change the
+<tt>named_values</tt> map so that it maps to AllocaInst* instead of Value*.
+Once we do this, the C++ compiler will tell us what parts of the code we need to
+update:</p>
+
+<p><b>Note:</b> the ocaml bindings currently model both <tt>Value*</tt>s and
+<tt>AllocInst*</tt>s as <tt>Llvm.llvalue</tt>s, but this may change in the
+future to be more type safe.</p>
+
+<div class="doc_code">
+<pre>
+let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
+</pre>
+</div>
+
+<p>Also, since we will need to create these alloca's, we'll use a helper
+function that ensures that the allocas are created in the entry block of the
+function:</p>
+
+<div class="doc_code">
+<pre>
+(* Create an alloca instruction in the entry block of the function. This
+ * is used for mutable variables etc. *)
+let create_entry_block_alloca the_function var_name =
+  let builder = builder_at (instr_begin (entry_block the_function)) in
+  build_alloca double_type var_name builder
+</pre>
+</div>
+
+<p>This funny looking code creates an <tt>Llvm.llbuilder</tt> object that is
+pointing at the first instruction of the entry block.  It then creates an alloca
+with the expected name and returns it.  Because all values in Kaleidoscope are
+doubles, there is no need to pass in a type to use.</p>
+
+<p>With this in place, the first functionality change we want to make is to
+variable references.  In our new scheme, variables live on the stack, so code
+generating a reference to them actually needs to produce a load from the stack
+slot:</p>
+
+<div class="doc_code">
+<pre>
+let rec codegen_expr = function
+  ...
+  | Ast.Variable name -&gt;
+      let v = try Hashtbl.find named_values name with
+        | Not_found -&gt; raise (Error "unknown variable name")
+      in
+      <b>(* Load the value. *)
+      build_load v name builder</b>
+</pre>
+</div>
+
+<p>As you can see, this is pretty straightforward.  Now we need to update the
+things that define the variables to set up the alloca.  We'll start with
+<tt>codegen_expr Ast.For ...</tt> (see the <a href="#code">full code listing</a>
+for the unabridged code):</p>
+
+<div class="doc_code">
+<pre>
+  | Ast.For (var_name, start, end_, step, body) -&gt;
+      let the_function = block_parent (insertion_block builder) in
+
+      (* Create an alloca for the variable in the entry block. *)
+      <b>let alloca = create_entry_block_alloca the_function var_name in</b>
+
+      (* Emit the start code first, without 'variable' in scope. *)
+      let start_val = codegen_expr start in
+
+      <b>(* Store the value into the alloca. *)
+      ignore(build_store start_val alloca builder);</b>
+
+      ...
+
+      (* Within the loop, the variable is defined equal to the PHI node. If it
+       * shadows an existing variable, we have to restore it, so save it
+       * now. *)
+      let old_val =
+        try Some (Hashtbl.find named_values var_name) with Not_found -&gt; None
+      in
+      <b>Hashtbl.add named_values var_name alloca;</b>
+
+      ...
+
+      (* Compute the end condition. *)
+      let end_cond = codegen_expr end_ in
+
+      <b>(* Reload, increment, and restore the alloca. This handles the case where
+       * the body of the loop mutates the variable. *)
+      let cur_var = build_load alloca var_name builder in
+      let next_var = build_add cur_var step_val "nextvar" builder in
+      ignore(build_store next_var alloca builder);</b>
+      ...
+</pre>
+</div>
+
+<p>This code is virtually identical to the code <a
+href="OCamlLangImpl5.html#forcodegen">before we allowed mutable variables</a>.
+The big difference is that we no longer have to construct a PHI node, and we use
+load/store to access the variable as needed.</p>
+
+<p>To support mutable argument variables, we need to also make allocas for them.
+The code for this is also pretty simple:</p>
+
+<div class="doc_code">
+<pre>
+(* Create an alloca for each argument and register the argument in the symbol
+ * table so that references to it will succeed. *)
+let create_argument_allocas the_function proto =
+  let args = match proto with
+    | Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -&gt; args
+  in
+  Array.iteri (fun i ai -&gt;
+    let var_name = args.(i) in
+    (* Create an alloca for this variable. *)
+    let alloca = create_entry_block_alloca the_function var_name in
+
+    (* Store the initial value into the alloca. *)
+    ignore(build_store ai alloca builder);
+
+    (* Add arguments to variable symbol table. *)
+    Hashtbl.add named_values var_name alloca;
+  ) (params the_function)
+</pre>
+</div>
+
+<p>For each argument, we make an alloca, store the input value to the function
+into the alloca, and register the alloca as the memory location for the
+argument.  This method gets invoked by <tt>Codegen.codegen_func</tt> right after
+it sets up the entry block for the function.</p>
+
+<p>The final missing piece is adding the mem2reg pass, which allows us to get
+good codegen once again:</p>
+
+<div class="doc_code">
+<pre>
+let main () =
+  ...
+  let the_fpm = PassManager.create_function Codegen.the_module in
+
+  (* Set up the optimizer pipeline.  Start with registering info about how the
+   * target lays out data structures. *)
+  TargetData.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
+
+  <b>(* Promote allocas to registers. *)
+  add_memory_to_register_promotion the_fpm;</b>
+
+  (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
+  add_instruction_combining the_fpm;
+
+  (* reassociate expressions. *)
+  add_reassociation the_fpm;
+</pre>
+</div>
+
+<p>It is interesting to see what the code looks like before and after the
+mem2reg optimization runs.  For example, this is the before/after code for our
+recursive fib function.  Before the optimization:</p>
+
+<div class="doc_code">
+<pre>
+define double @fib(double %x) {
+entry:
+  <b>%x1 = alloca double
+  store double %x, double* %x1
+  %x2 = load double* %x1</b>
+  %cmptmp = fcmp ult double %x2, 3.000000e+00
+  %booltmp = uitofp i1 %cmptmp to double
+  %ifcond = fcmp one double %booltmp, 0.000000e+00
+  br i1 %ifcond, label %then, label %else
+
+then:    ; preds = %entry
+  br label %ifcont
+
+else:    ; preds = %entry
+  <b>%x3 = load double* %x1</b>
+  %subtmp = fsub double %x3, 1.000000e+00
+  %calltmp = call double @fib( double %subtmp )
+  <b>%x4 = load double* %x1</b>
+  %subtmp5 = fsub double %x4, 2.000000e+00
+  %calltmp6 = call double @fib( double %subtmp5 )
+  %addtmp = fadd double %calltmp, %calltmp6
+  br label %ifcont
+
+ifcont:    ; preds = %else, %then
+  %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
+  ret double %iftmp
+}
+</pre>
+</div>
+
+<p>Here there is only one variable (x, the input argument) but you can still
+see the extremely simple-minded code generation strategy we are using.  In the
+entry block, an alloca is created, and the initial input value is stored into
+it.  Each reference to the variable does a reload from the stack.  Also, note
+that we didn't modify the if/then/else expression, so it still inserts a PHI
+node.  While we could make an alloca for it, it is actually easier to create a
+PHI node for it, so we still just make the PHI.</p>
+
+<p>Here is the code after the mem2reg pass runs:</p>
+
+<div class="doc_code">
+<pre>
+define double @fib(double %x) {
+entry:
+  %cmptmp = fcmp ult double <b>%x</b>, 3.000000e+00
+  %booltmp = uitofp i1 %cmptmp to double
+  %ifcond = fcmp one double %booltmp, 0.000000e+00
+  br i1 %ifcond, label %then, label %else
+
+then:
+  br label %ifcont
+
+else:
+  %subtmp = fsub double <b>%x</b>, 1.000000e+00
+  %calltmp = call double @fib( double %subtmp )
+  %subtmp5 = fsub double <b>%x</b>, 2.000000e+00
+  %calltmp6 = call double @fib( double %subtmp5 )
+  %addtmp = fadd double %calltmp, %calltmp6
+  br label %ifcont
+
+ifcont:    ; preds = %else, %then
+  %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
+  ret double %iftmp
+}
+</pre>
+</div>
+
+<p>This is a trivial case for mem2reg, since there are no redefinitions of the
+variable.  The point of showing this is to calm your tension about inserting
+such blatent inefficiencies :).</p>
+
+<p>After the rest of the optimizers run, we get:</p>
+
+<div class="doc_code">
+<pre>
+define double @fib(double %x) {
+entry:
+  %cmptmp = fcmp ult double %x, 3.000000e+00
+  %booltmp = uitofp i1 %cmptmp to double
+  %ifcond = fcmp ueq double %booltmp, 0.000000e+00
+  br i1 %ifcond, label %else, label %ifcont
+
+else:
+  %subtmp = fsub double %x, 1.000000e+00
+  %calltmp = call double @fib( double %subtmp )
+  %subtmp5 = fsub double %x, 2.000000e+00
+  %calltmp6 = call double @fib( double %subtmp5 )
+  %addtmp = fadd double %calltmp, %calltmp6
+  ret double %addtmp
+
+ifcont:
+  ret double 1.000000e+00
+}
+</pre>
+</div>
+
+<p>Here we see that the simplifycfg pass decided to clone the return instruction
+into the end of the 'else' block.  This allowed it to eliminate some branches
+and the PHI node.</p>
+
+<p>Now that all symbol table references are updated to use stack variables,
+we'll add the assignment operator.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="assignment">New Assignment Operator</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>With our current framework, adding a new assignment operator is really
+simple.  We will parse it just like any other binary operator, but handle it
+internally (instead of allowing the user to define it).  The first step is to
+set a precedence:</p>
+
+<div class="doc_code">
+<pre>
+let main () =
+  (* Install standard binary operators.
+   * 1 is the lowest precedence. *)
+  <b>Hashtbl.add Parser.binop_precedence '=' 2;</b>
+  Hashtbl.add Parser.binop_precedence '&lt;' 10;
+  Hashtbl.add Parser.binop_precedence '+' 20;
+  Hashtbl.add Parser.binop_precedence '-' 20;
+  ...
+</pre>
+</div>
+
+<p>Now that the parser knows the precedence of the binary operator, it takes
+care of all the parsing and AST generation.  We just need to implement codegen
+for the assignment operator.  This looks like:</p>
+
+<div class="doc_code">
+<pre>
+let rec codegen_expr = function
+      begin match op with
+      | '=' -&gt;
+          (* Special case '=' because we don't want to emit the LHS as an
+           * expression. *)
+          let name =
+            match lhs with
+            | Ast.Variable name -&gt; name
+            | _ -&gt; raise (Error "destination of '=' must be a variable")
+          in
+</pre>
+</div>
+
+<p>Unlike the rest of the binary operators, our assignment operator doesn't
+follow the "emit LHS, emit RHS, do computation" model.  As such, it is handled
+as a special case before the other binary operators are handled.  The other
+strange thing is that it requires the LHS to be a variable.  It is invalid to
+have "(x+1) = expr" - only things like "x = expr" are allowed.
+</p>
+
+
+<div class="doc_code">
+<pre>
+          (* Codegen the rhs. *)
+          let val_ = codegen_expr rhs in
+
+          (* Lookup the name. *)
+          let variable = try Hashtbl.find named_values name with
+          | Not_found -&gt; raise (Error "unknown variable name")
+          in
+          ignore(build_store val_ variable builder);
+          val_
+      | _ -&gt;
+			...
+</pre>
+</div>
+
+<p>Once we have the variable, codegen'ing the assignment is straightforward:
+we emit the RHS of the assignment, create a store, and return the computed
+value.  Returning a value allows for chained assignments like "X = (Y = Z)".</p>
+
+<p>Now that we have an assignment operator, we can mutate loop variables and
+arguments.  For example, we can now run code like this:</p>
+
+<div class="doc_code">
+<pre>
+# Function to print a double.
+extern printd(x);
+
+# Define ':' for sequencing: as a low-precedence operator that ignores operands
+# and just returns the RHS.
+def binary : 1 (x y) y;
+
+def test(x)
+  printd(x) :
+  x = 4 :
+  printd(x);
+
+test(123);
+</pre>
+</div>
+
+<p>When run, this example prints "123" and then "4", showing that we did
+actually mutate the value!  Okay, we have now officially implemented our goal:
+getting this to work requires SSA construction in the general case.  However,
+to be really useful, we want the ability to define our own local variables, lets
+add this next!
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="localvars">User-defined Local
+Variables</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Adding var/in is just like any other other extensions we made to
+Kaleidoscope: we extend the lexer, the parser, the AST and the code generator.
+The first step for adding our new 'var/in' construct is to extend the lexer.
+As before, this is pretty trivial, the code looks like this:</p>
+
+<div class="doc_code">
+<pre>
+type token =
+  ...
+  <b>(* var definition *)
+  | Var</b>
+
+...
+
+and lex_ident buffer = parser
+      ...
+      | "in" -&gt; [&lt; 'Token.In; stream &gt;]
+      | "binary" -&gt; [&lt; 'Token.Binary; stream &gt;]
+      | "unary" -&gt; [&lt; 'Token.Unary; stream &gt;]
+      <b>| "var" -&gt; [&lt; 'Token.Var; stream &gt;]</b>
+      ...
+</pre>
+</div>
+
+<p>The next step is to define the AST node that we will construct.  For var/in,
+it looks like this:</p>
+
+<div class="doc_code">
+<pre>
+type expr =
+  ...
+  (* variant for var/in. *)
+  | Var of (string * expr option) array * expr
+  ...
+</pre>
+</div>
+
+<p>var/in allows a list of names to be defined all at once, and each name can
+optionally have an initializer value.  As such, we capture this information in
+the VarNames vector.  Also, var/in has a body, this body is allowed to access
+the variables defined by the var/in.</p>
+
+<p>With this in place, we can define the parser pieces.  The first thing we do
+is add it as a primary expression:</p>
+
+<div class="doc_code">
+<pre>
+(* primary
+ *   ::= identifier
+ *   ::= numberexpr
+ *   ::= parenexpr
+ *   ::= ifexpr
+ *   ::= forexpr
+ <b>*   ::= varexpr</b> *)
+let rec parse_primary = parser
+  ...
+  <b>(* varexpr
+   *   ::= 'var' identifier ('=' expression?
+   *             (',' identifier ('=' expression)?)* 'in' expression *)
+  | [&lt; 'Token.Var;
+       (* At least one variable name is required. *)
+       'Token.Ident id ?? "expected identifier after var";
+       init=parse_var_init;
+       var_names=parse_var_names [(id, init)];
+       (* At this point, we have to have 'in'. *)
+       'Token.In ?? "expected 'in' keyword after 'var'";
+       body=parse_expr &gt;] -&gt;
+      Ast.Var (Array.of_list (List.rev var_names), body)</b>
+
+...
+
+and parse_var_init = parser
+  (* read in the optional initializer. *)
+  | [&lt; 'Token.Kwd '='; e=parse_expr &gt;] -&gt; Some e
+  | [&lt; &gt;] -&gt; None
+
+and parse_var_names accumulator = parser
+  | [&lt; 'Token.Kwd ',';
+       'Token.Ident id ?? "expected identifier list after var";
+       init=parse_var_init;
+       e=parse_var_names ((id, init) :: accumulator) &gt;] -&gt; e
+  | [&lt; &gt;] -&gt; accumulator
+</pre>
+</div>
+
+<p>Now that we can parse and represent the code, we need to support emission of
+LLVM IR for it.  This code starts out with:</p>
+
+<div class="doc_code">
+<pre>
+let rec codegen_expr = function
+  ...
+  | Ast.Var (var_names, body)
+      let old_bindings = ref [] in
+
+      let the_function = block_parent (insertion_block builder) in
+
+      (* Register all variables and emit their initializer. *)
+      Array.iter (fun (var_name, init) -&gt;
+</pre>
+</div>
+
+<p>Basically it loops over all the variables, installing them one at a time.
+For each variable we put into the symbol table, we remember the previous value
+that we replace in OldBindings.</p>
+
+<div class="doc_code">
+<pre>
+        (* Emit the initializer before adding the variable to scope, this
+         * prevents the initializer from referencing the variable itself, and
+         * permits stuff like this:
+         *   var a = 1 in
+         *     var a = a in ...   # refers to outer 'a'. *)
+        let init_val =
+          match init with
+          | Some init -&gt; codegen_expr init
+          (* If not specified, use 0.0. *)
+          | None -&gt; const_float double_type 0.0
+        in
+
+        let alloca = create_entry_block_alloca the_function var_name in
+        ignore(build_store init_val alloca builder);
+
+        (* Remember the old variable binding so that we can restore the binding
+         * when we unrecurse. *)
+
+        begin
+          try
+            let old_value = Hashtbl.find named_values var_name in
+            old_bindings := (var_name, old_value) :: !old_bindings;
+          with Not_found &gt; ()
+        end;
+
+        (* Remember this binding. *)
+        Hashtbl.add named_values var_name alloca;
+      ) var_names;
+</pre>
+</div>
+
+<p>There are more comments here than code.  The basic idea is that we emit the
+initializer, create the alloca, then update the symbol table to point to it.
+Once all the variables are installed in the symbol table, we evaluate the body
+of the var/in expression:</p>
+
+<div class="doc_code">
+<pre>
+      (* Codegen the body, now that all vars are in scope. *)
+      let body_val = codegen_expr body in
+</pre>
+</div>
+
+<p>Finally, before returning, we restore the previous variable bindings:</p>
+
+<div class="doc_code">
+<pre>
+      (* Pop all our variables from scope. *)
+      List.iter (fun (var_name, old_value) -&gt;
+        Hashtbl.add named_values var_name old_value
+      ) !old_bindings;
+
+      (* Return the body computation. *)
+      body_val
+</pre>
+</div>
+
+<p>The end result of all of this is that we get properly scoped variable
+definitions, and we even (trivially) allow mutation of them :).</p>
+
+<p>With this, we completed what we set out to do.  Our nice iterative fib
+example from the intro compiles and runs just fine.  The mem2reg pass optimizes
+all of our stack variables into SSA registers, inserting PHI nodes where needed,
+and our front-end remains simple: no "iterated dominance frontier" computation
+anywhere in sight.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with mutable
+variables and var/in support.  To build this example, use:
+</p>
+
+<div class="doc_code">
+<pre>
+# Compile
+ocamlbuild toy.byte
+# Run
+./toy.byte
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<dl>
+<dt>_tags:</dt>
+<dd class="doc_code">
+<pre>
+&lt;{lexer,parser}.ml&gt;: use_camlp4, pp(camlp4of)
+&lt;*.{byte,native}&gt;: g++, use_llvm, use_llvm_analysis
+&lt;*.{byte,native}&gt;: use_llvm_executionengine, use_llvm_target
+&lt;*.{byte,native}&gt;: use_llvm_scalar_opts, use_bindings
+</pre>
+</dd>
+
+<dt>myocamlbuild.ml:</dt>
+<dd class="doc_code">
+<pre>
+open Ocamlbuild_plugin;;
+
+ocaml_lib ~extern:true "llvm";;
+ocaml_lib ~extern:true "llvm_analysis";;
+ocaml_lib ~extern:true "llvm_executionengine";;
+ocaml_lib ~extern:true "llvm_target";;
+ocaml_lib ~extern:true "llvm_scalar_opts";;
+
+flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"; A"-cclib"; A"-rdynamic"]);;
+dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
+</pre>
+</dd>
+
+<dt>token.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer Tokens
+ *===----------------------------------------------------------------------===*)
+
+(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
+ * these others for known things. *)
+type token =
+  (* commands *)
+  | Def | Extern
+
+  (* primary *)
+  | Ident of string | Number of float
+
+  (* unknown *)
+  | Kwd of char
+
+  (* control *)
+  | If | Then | Else
+  | For | In
+
+  (* operators *)
+  | Binary | Unary
+
+  (* var definition *)
+  | Var
+</pre>
+</dd>
+
+<dt>lexer.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer
+ *===----------------------------------------------------------------------===*)
+
+let rec lex = parser
+  (* Skip any whitespace. *)
+  | [&lt; ' (' ' | '\n' | '\r' | '\t'); stream &gt;] -&gt; lex stream
+
+  (* identifier: [a-zA-Z][a-zA-Z0-9] *)
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+
+  (* number: [0-9.]+ *)
+  | [&lt; ' ('0' .. '9' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+
+  (* Comment until end of line. *)
+  | [&lt; ' ('#'); stream &gt;] -&gt;
+      lex_comment stream
+
+  (* Otherwise, just return the character as its ascii value. *)
+  | [&lt; 'c; stream &gt;] -&gt;
+      [&lt; 'Token.Kwd c; lex stream &gt;]
+
+  (* end of stream. *)
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+
+and lex_number buffer = parser
+  | [&lt; ' ('0' .. '9' | '.' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      [&lt; 'Token.Number (float_of_string (Buffer.contents buffer)); stream &gt;]
+
+and lex_ident buffer = parser
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | "if" -&gt; [&lt; 'Token.If; stream &gt;]
+      | "then" -&gt; [&lt; 'Token.Then; stream &gt;]
+      | "else" -&gt; [&lt; 'Token.Else; stream &gt;]
+      | "for" -&gt; [&lt; 'Token.For; stream &gt;]
+      | "in" -&gt; [&lt; 'Token.In; stream &gt;]
+      | "binary" -&gt; [&lt; 'Token.Binary; stream &gt;]
+      | "unary" -&gt; [&lt; 'Token.Unary; stream &gt;]
+      | "var" -&gt; [&lt; 'Token.Var; stream &gt;]
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+
+and lex_comment = parser
+  | [&lt; ' ('\n'); stream=lex &gt;] -&gt; stream
+  | [&lt; 'c; e=lex_comment &gt;] -&gt; e
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+</pre>
+</dd>
+
+<dt>ast.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Abstract Syntax Tree (aka Parse Tree)
+ *===----------------------------------------------------------------------===*)
+
+(* expr - Base type for all expression nodes. *)
+type expr =
+  (* variant for numeric literals like "1.0". *)
+  | Number of float
+
+  (* variant for referencing a variable, like "a". *)
+  | Variable of string
+
+  (* variant for a unary operator. *)
+  | Unary of char * expr
+
+  (* variant for a binary operator. *)
+  | Binary of char * expr * expr
+
+  (* variant for function calls. *)
+  | Call of string * expr array
+
+  (* variant for if/then/else. *)
+  | If of expr * expr * expr
+
+  (* variant for for/in. *)
+  | For of string * expr * expr * expr option * expr
+
+  (* variant for var/in. *)
+  | Var of (string * expr option) array * expr
+
+(* proto - This type represents the "prototype" for a function, which captures
+ * its name, and its argument names (thus implicitly the number of arguments the
+ * function takes). *)
+type proto =
+  | Prototype of string * string array
+  | BinOpPrototype of string * string array * int
+
+(* func - This type represents a function definition itself. *)
+type func = Function of proto * expr
+</pre>
+</dd>
+
+<dt>parser.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===---------------------------------------------------------------------===
+ * Parser
+ *===---------------------------------------------------------------------===*)
+
+(* binop_precedence - This holds the precedence for each binary operator that is
+ * defined *)
+let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
+
+(* precedence - Get the precedence of the pending binary operator token. *)
+let precedence c = try Hashtbl.find binop_precedence c with Not_found -&gt; -1
+
+(* primary
+ *   ::= identifier
+ *   ::= numberexpr
+ *   ::= parenexpr
+ *   ::= ifexpr
+ *   ::= forexpr
+ *   ::= varexpr *)
+let rec parse_primary = parser
+  (* numberexpr ::= number *)
+  | [&lt; 'Token.Number n &gt;] -&gt; Ast.Number n
+
+  (* parenexpr ::= '(' expression ')' *)
+  | [&lt; 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" &gt;] -&gt; e
+
+  (* identifierexpr
+   *   ::= identifier
+   *   ::= identifier '(' argumentexpr ')' *)
+  | [&lt; 'Token.Ident id; stream &gt;] -&gt;
+      let rec parse_args accumulator = parser
+        | [&lt; e=parse_expr; stream &gt;] -&gt;
+            begin parser
+              | [&lt; 'Token.Kwd ','; e=parse_args (e :: accumulator) &gt;] -&gt; e
+              | [&lt; &gt;] -&gt; e :: accumulator
+            end stream
+        | [&lt; &gt;] -&gt; accumulator
+      in
+      let rec parse_ident id = parser
+        (* Call. *)
+        | [&lt; 'Token.Kwd '(';
+             args=parse_args [];
+             'Token.Kwd ')' ?? "expected ')'"&gt;] -&gt;
+            Ast.Call (id, Array.of_list (List.rev args))
+
+        (* Simple variable ref. *)
+        | [&lt; &gt;] -&gt; Ast.Variable id
+      in
+      parse_ident id stream
+
+  (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
+  | [&lt; 'Token.If; c=parse_expr;
+       'Token.Then ?? "expected 'then'"; t=parse_expr;
+       'Token.Else ?? "expected 'else'"; e=parse_expr &gt;] -&gt;
+      Ast.If (c, t, e)
+
+  (* forexpr
+        ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
+  | [&lt; 'Token.For;
+       'Token.Ident id ?? "expected identifier after for";
+       'Token.Kwd '=' ?? "expected '=' after for";
+       stream &gt;] -&gt;
+      begin parser
+        | [&lt;
+             start=parse_expr;
+             'Token.Kwd ',' ?? "expected ',' after for";
+             end_=parse_expr;
+             stream &gt;] -&gt;
+            let step =
+              begin parser
+              | [&lt; 'Token.Kwd ','; step=parse_expr &gt;] -&gt; Some step
+              | [&lt; &gt;] -&gt; None
+              end stream
+            in
+            begin parser
+            | [&lt; 'Token.In; body=parse_expr &gt;] -&gt;
+                Ast.For (id, start, end_, step, body)
+            | [&lt; &gt;] -&gt;
+                raise (Stream.Error "expected 'in' after for")
+            end stream
+        | [&lt; &gt;] -&gt;
+            raise (Stream.Error "expected '=' after for")
+      end stream
+
+  (* varexpr
+   *   ::= 'var' identifier ('=' expression?
+   *             (',' identifier ('=' expression)?)* 'in' expression *)
+  | [&lt; 'Token.Var;
+       (* At least one variable name is required. *)
+       'Token.Ident id ?? "expected identifier after var";
+       init=parse_var_init;
+       var_names=parse_var_names [(id, init)];
+       (* At this point, we have to have 'in'. *)
+       'Token.In ?? "expected 'in' keyword after 'var'";
+       body=parse_expr &gt;] -&gt;
+      Ast.Var (Array.of_list (List.rev var_names), body)
+
+  | [&lt; &gt;] -&gt; raise (Stream.Error "unknown token when expecting an expression.")
+
+(* unary
+ *   ::= primary
+ *   ::= '!' unary *)
+and parse_unary = parser
+  (* If this is a unary operator, read it. *)
+  | [&lt; 'Token.Kwd op when op != '(' &amp;&amp; op != ')'; operand=parse_expr &gt;] -&gt;
+      Ast.Unary (op, operand)
+
+  (* If the current token is not an operator, it must be a primary expr. *)
+  | [&lt; stream &gt;] -&gt; parse_primary stream
+
+(* binoprhs
+ *   ::= ('+' primary)* *)
+and parse_bin_rhs expr_prec lhs stream =
+  match Stream.peek stream with
+  (* If this is a binop, find its precedence. *)
+  | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -&gt;
+      let token_prec = precedence c in
+
+      (* If this is a binop that binds at least as tightly as the current binop,
+       * consume it, otherwise we are done. *)
+      if token_prec &lt; expr_prec then lhs else begin
+        (* Eat the binop. *)
+        Stream.junk stream;
+
+        (* Parse the primary expression after the binary operator. *)
+        let rhs = parse_unary stream in
+
+        (* Okay, we know this is a binop. *)
+        let rhs =
+          match Stream.peek stream with
+          | Some (Token.Kwd c2) -&gt;
+              (* If BinOp binds less tightly with rhs than the operator after
+               * rhs, let the pending operator take rhs as its lhs. *)
+              let next_prec = precedence c2 in
+              if token_prec &lt; next_prec
+              then parse_bin_rhs (token_prec + 1) rhs stream
+              else rhs
+          | _ -&gt; rhs
+        in
+
+        (* Merge lhs/rhs. *)
+        let lhs = Ast.Binary (c, lhs, rhs) in
+        parse_bin_rhs expr_prec lhs stream
+      end
+  | _ -&gt; lhs
+
+and parse_var_init = parser
+  (* read in the optional initializer. *)
+  | [&lt; 'Token.Kwd '='; e=parse_expr &gt;] -&gt; Some e
+  | [&lt; &gt;] -&gt; None
+
+and parse_var_names accumulator = parser
+  | [&lt; 'Token.Kwd ',';
+       'Token.Ident id ?? "expected identifier list after var";
+       init=parse_var_init;
+       e=parse_var_names ((id, init) :: accumulator) &gt;] -&gt; e
+  | [&lt; &gt;] -&gt; accumulator
+
+(* expression
+ *   ::= primary binoprhs *)
+and parse_expr = parser
+  | [&lt; lhs=parse_unary; stream &gt;] -&gt; parse_bin_rhs 0 lhs stream
+
+(* prototype
+ *   ::= id '(' id* ')'
+ *   ::= binary LETTER number? (id, id)
+ *   ::= unary LETTER number? (id) *)
+let parse_prototype =
+  let rec parse_args accumulator = parser
+    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
+    | [&lt; &gt;] -&gt; accumulator
+  in
+  let parse_operator = parser
+    | [&lt; 'Token.Unary &gt;] -&gt; "unary", 1
+    | [&lt; 'Token.Binary &gt;] -&gt; "binary", 2
+  in
+  let parse_binary_precedence = parser
+    | [&lt; 'Token.Number n &gt;] -&gt; int_of_float n
+    | [&lt; &gt;] -&gt; 30
+  in
+  parser
+  | [&lt; 'Token.Ident id;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+       args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      (* success. *)
+      Ast.Prototype (id, Array.of_list (List.rev args))
+  | [&lt; (prefix, kind)=parse_operator;
+       'Token.Kwd op ?? "expected an operator";
+       (* Read the precedence if present. *)
+       binary_precedence=parse_binary_precedence;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+        args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      let name = prefix ^ (String.make 1 op) in
+      let args = Array.of_list (List.rev args) in
+
+      (* Verify right number of arguments for operator. *)
+      if Array.length args != kind
+      then raise (Stream.Error "invalid number of operands for operator")
+      else
+        if kind == 1 then
+          Ast.Prototype (name, args)
+        else
+          Ast.BinOpPrototype (name, args, binary_precedence)
+  | [&lt; &gt;] -&gt;
+      raise (Stream.Error "expected function name in prototype")
+
+(* definition ::= 'def' prototype expression *)
+let parse_definition = parser
+  | [&lt; 'Token.Def; p=parse_prototype; e=parse_expr &gt;] -&gt;
+      Ast.Function (p, e)
+
+(* toplevelexpr ::= expression *)
+let parse_toplevel = parser
+  | [&lt; e=parse_expr &gt;] -&gt;
+      (* Make an anonymous proto. *)
+      Ast.Function (Ast.Prototype ("", [||]), e)
+
+(*  external ::= 'extern' prototype *)
+let parse_extern = parser
+  | [&lt; 'Token.Extern; e=parse_prototype &gt;] -&gt; e
+</pre>
+</dd>
+
+<dt>codegen.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Code Generation
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+
+exception Error of string
+
+let context = global_context ()
+let the_module = create_module context "my cool jit"
+let builder = builder context
+let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
+let double_type = double_type context
+
+(* Create an alloca instruction in the entry block of the function. This
+ * is used for mutable variables etc. *)
+let create_entry_block_alloca the_function var_name =
+  let builder = builder_at context (instr_begin (entry_block the_function)) in
+  build_alloca double_type var_name builder
+
+let rec codegen_expr = function
+  | Ast.Number n -&gt; const_float double_type n
+  | Ast.Variable name -&gt;
+      let v = try Hashtbl.find named_values name with
+        | Not_found -&gt; raise (Error "unknown variable name")
+      in
+      (* Load the value. *)
+      build_load v name builder
+  | Ast.Unary (op, operand) -&gt;
+      let operand = codegen_expr operand in
+      let callee = "unary" ^ (String.make 1 op) in
+      let callee =
+        match lookup_function callee the_module with
+        | Some callee -&gt; callee
+        | None -&gt; raise (Error "unknown unary operator")
+      in
+      build_call callee [|operand|] "unop" builder
+  | Ast.Binary (op, lhs, rhs) -&gt;
+      begin match op with
+      | '=' -&gt;
+          (* Special case '=' because we don't want to emit the LHS as an
+           * expression. *)
+          let name =
+            match lhs with
+            | Ast.Variable name -&gt; name
+            | _ -&gt; raise (Error "destination of '=' must be a variable")
+          in
+
+          (* Codegen the rhs. *)
+          let val_ = codegen_expr rhs in
+
+          (* Lookup the name. *)
+          let variable = try Hashtbl.find named_values name with
+          | Not_found -&gt; raise (Error "unknown variable name")
+          in
+          ignore(build_store val_ variable builder);
+          val_
+      | _ -&gt;
+          let lhs_val = codegen_expr lhs in
+          let rhs_val = codegen_expr rhs in
+          begin
+            match op with
+            | '+' -&gt; build_add lhs_val rhs_val "addtmp" builder
+            | '-' -&gt; build_sub lhs_val rhs_val "subtmp" builder
+            | '*' -&gt; build_mul lhs_val rhs_val "multmp" builder
+            | '&lt;' -&gt;
+                (* Convert bool 0/1 to double 0.0 or 1.0 *)
+                let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
+                build_uitofp i double_type "booltmp" builder
+            | _ -&gt;
+                (* If it wasn't a builtin binary operator, it must be a user defined
+                 * one. Emit a call to it. *)
+                let callee = "binary" ^ (String.make 1 op) in
+                let callee =
+                  match lookup_function callee the_module with
+                  | Some callee -&gt; callee
+                  | None -&gt; raise (Error "binary operator not found!")
+                in
+                build_call callee [|lhs_val; rhs_val|] "binop" builder
+          end
+      end
+  | Ast.Call (callee, args) -&gt;
+      (* Look up the name in the module table. *)
+      let callee =
+        match lookup_function callee the_module with
+        | Some callee -&gt; callee
+        | None -&gt; raise (Error "unknown function referenced")
+      in
+      let params = params callee in
+
+      (* If argument mismatch error. *)
+      if Array.length params == Array.length args then () else
+        raise (Error "incorrect # arguments passed");
+      let args = Array.map codegen_expr args in
+      build_call callee args "calltmp" builder
+  | Ast.If (cond, then_, else_) -&gt;
+      let cond = codegen_expr cond in
+
+      (* Convert condition to a bool by comparing equal to 0.0 *)
+      let zero = const_float double_type 0.0 in
+      let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
+
+      (* Grab the first block so that we might later add the conditional branch
+       * to it at the end of the function. *)
+      let start_bb = insertion_block builder in
+      let the_function = block_parent start_bb in
+
+      let then_bb = append_block context "then" the_function in
+
+      (* Emit 'then' value. *)
+      position_at_end then_bb builder;
+      let then_val = codegen_expr then_ in
+
+      (* Codegen of 'then' can change the current block, update then_bb for the
+       * phi. We create a new name because one is used for the phi node, and the
+       * other is used for the conditional branch. *)
+      let new_then_bb = insertion_block builder in
+
+      (* Emit 'else' value. *)
+      let else_bb = append_block context "else" the_function in
+      position_at_end else_bb builder;
+      let else_val = codegen_expr else_ in
+
+      (* Codegen of 'else' can change the current block, update else_bb for the
+       * phi. *)
+      let new_else_bb = insertion_block builder in
+
+      (* Emit merge block. *)
+      let merge_bb = append_block context "ifcont" the_function in
+      position_at_end merge_bb builder;
+      let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
+      let phi = build_phi incoming "iftmp" builder in
+
+      (* Return to the start block to add the conditional branch. *)
+      position_at_end start_bb builder;
+      ignore (build_cond_br cond_val then_bb else_bb builder);
+
+      (* Set a unconditional branch at the end of the 'then' block and the
+       * 'else' block to the 'merge' block. *)
+      position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
+      position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
+
+      (* Finally, set the builder to the end of the merge block. *)
+      position_at_end merge_bb builder;
+
+      phi
+  | Ast.For (var_name, start, end_, step, body) -&gt;
+      (* Output this as:
+       *   var = alloca double
+       *   ...
+       *   start = startexpr
+       *   store start -&gt; var
+       *   goto loop
+       * loop:
+       *   ...
+       *   bodyexpr
+       *   ...
+       * loopend:
+       *   step = stepexpr
+       *   endcond = endexpr
+       *
+       *   curvar = load var
+       *   nextvar = curvar + step
+       *   store nextvar -&gt; var
+       *   br endcond, loop, endloop
+       * outloop: *)
+
+      let the_function = block_parent (insertion_block builder) in
+
+      (* Create an alloca for the variable in the entry block. *)
+      let alloca = create_entry_block_alloca the_function var_name in
+
+      (* Emit the start code first, without 'variable' in scope. *)
+      let start_val = codegen_expr start in
+
+      (* Store the value into the alloca. *)
+      ignore(build_store start_val alloca builder);
+
+      (* Make the new basic block for the loop header, inserting after current
+       * block. *)
+      let loop_bb = append_block context "loop" the_function in
+
+      (* Insert an explicit fall through from the current block to the
+       * loop_bb. *)
+      ignore (build_br loop_bb builder);
+
+      (* Start insertion in loop_bb. *)
+      position_at_end loop_bb builder;
+
+      (* Within the loop, the variable is defined equal to the PHI node. If it
+       * shadows an existing variable, we have to restore it, so save it
+       * now. *)
+      let old_val =
+        try Some (Hashtbl.find named_values var_name) with Not_found -&gt; None
+      in
+      Hashtbl.add named_values var_name alloca;
+
+      (* Emit the body of the loop.  This, like any other expr, can change the
+       * current BB.  Note that we ignore the value computed by the body, but
+       * don't allow an error *)
+      ignore (codegen_expr body);
+
+      (* Emit the step value. *)
+      let step_val =
+        match step with
+        | Some step -&gt; codegen_expr step
+        (* If not specified, use 1.0. *)
+        | None -&gt; const_float double_type 1.0
+      in
+
+      (* Compute the end condition. *)
+      let end_cond = codegen_expr end_ in
+
+      (* Reload, increment, and restore the alloca. This handles the case where
+       * the body of the loop mutates the variable. *)
+      let cur_var = build_load alloca var_name builder in
+      let next_var = build_add cur_var step_val "nextvar" builder in
+      ignore(build_store next_var alloca builder);
+
+      (* Convert condition to a bool by comparing equal to 0.0. *)
+      let zero = const_float double_type 0.0 in
+      let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
+
+      (* Create the "after loop" block and insert it. *)
+      let after_bb = append_block context "afterloop" the_function in
+
+      (* Insert the conditional branch into the end of loop_end_bb. *)
+      ignore (build_cond_br end_cond loop_bb after_bb builder);
+
+      (* Any new code will be inserted in after_bb. *)
+      position_at_end after_bb builder;
+
+      (* Restore the unshadowed variable. *)
+      begin match old_val with
+      | Some old_val -&gt; Hashtbl.add named_values var_name old_val
+      | None -&gt; ()
+      end;
+
+      (* for expr always returns 0.0. *)
+      const_null double_type
+  | Ast.Var (var_names, body) -&gt;
+      let old_bindings = ref [] in
+
+      let the_function = block_parent (insertion_block builder) in
+
+      (* Register all variables and emit their initializer. *)
+      Array.iter (fun (var_name, init) -&gt;
+        (* Emit the initializer before adding the variable to scope, this
+         * prevents the initializer from referencing the variable itself, and
+         * permits stuff like this:
+         *   var a = 1 in
+         *     var a = a in ...   # refers to outer 'a'. *)
+        let init_val =
+          match init with
+          | Some init -&gt; codegen_expr init
+          (* If not specified, use 0.0. *)
+          | None -&gt; const_float double_type 0.0
+        in
+
+        let alloca = create_entry_block_alloca the_function var_name in
+        ignore(build_store init_val alloca builder);
+
+        (* Remember the old variable binding so that we can restore the binding
+         * when we unrecurse. *)
+        begin
+          try
+            let old_value = Hashtbl.find named_values var_name in
+            old_bindings := (var_name, old_value) :: !old_bindings;
+          with Not_found -&gt; ()
+        end;
+
+        (* Remember this binding. *)
+        Hashtbl.add named_values var_name alloca;
+      ) var_names;
+
+      (* Codegen the body, now that all vars are in scope. *)
+      let body_val = codegen_expr body in
+
+      (* Pop all our variables from scope. *)
+      List.iter (fun (var_name, old_value) -&gt;
+        Hashtbl.add named_values var_name old_value
+      ) !old_bindings;
+
+      (* Return the body computation. *)
+      body_val
+
+let codegen_proto = function
+  | Ast.Prototype (name, args) | Ast.BinOpPrototype (name, args, _) -&gt;
+      (* Make the function type: double(double,double) etc. *)
+      let doubles = Array.make (Array.length args) double_type in
+      let ft = function_type double_type doubles in
+      let f =
+        match lookup_function name the_module with
+        | None -&gt; declare_function name ft the_module
+
+        (* If 'f' conflicted, there was already something named 'name'. If it
+         * has a body, don't allow redefinition or reextern. *)
+        | Some f -&gt;
+            (* If 'f' already has a body, reject this. *)
+            if block_begin f &lt;&gt; At_end f then
+              raise (Error "redefinition of function");
+
+            (* If 'f' took a different number of arguments, reject. *)
+            if element_type (type_of f) &lt;&gt; ft then
+              raise (Error "redefinition of function with different # args");
+            f
+      in
+
+      (* Set names for all arguments. *)
+      Array.iteri (fun i a -&gt;
+        let n = args.(i) in
+        set_value_name n a;
+        Hashtbl.add named_values n a;
+      ) (params f);
+      f
+
+(* Create an alloca for each argument and register the argument in the symbol
+ * table so that references to it will succeed. *)
+let create_argument_allocas the_function proto =
+  let args = match proto with
+    | Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -&gt; args
+  in
+  Array.iteri (fun i ai -&gt;
+    let var_name = args.(i) in
+    (* Create an alloca for this variable. *)
+    let alloca = create_entry_block_alloca the_function var_name in
+
+    (* Store the initial value into the alloca. *)
+    ignore(build_store ai alloca builder);
+
+    (* Add arguments to variable symbol table. *)
+    Hashtbl.add named_values var_name alloca;
+  ) (params the_function)
+
+let codegen_func the_fpm = function
+  | Ast.Function (proto, body) -&gt;
+      Hashtbl.clear named_values;
+      let the_function = codegen_proto proto in
+
+      (* If this is an operator, install it. *)
+      begin match proto with
+      | Ast.BinOpPrototype (name, args, prec) -&gt;
+          let op = name.[String.length name - 1] in
+          Hashtbl.add Parser.binop_precedence op prec;
+      | _ -&gt; ()
+      end;
+
+      (* Create a new basic block to start insertion into. *)
+      let bb = append_block context "entry" the_function in
+      position_at_end bb builder;
+
+      try
+        (* Add all arguments to the symbol table and create their allocas. *)
+        create_argument_allocas the_function proto;
+
+        let ret_val = codegen_expr body in
+
+        (* Finish off the function. *)
+        let _ = build_ret ret_val builder in
+
+        (* Validate the generated code, checking for consistency. *)
+        Llvm_analysis.assert_valid_function the_function;
+
+        (* Optimize the function. *)
+        let _ = PassManager.run_function the_function the_fpm in
+
+        the_function
+      with e -&gt;
+        delete_function the_function;
+        raise e
+</pre>
+</dd>
+
+<dt>toplevel.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Top-Level parsing and JIT Driver
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+open Llvm_executionengine
+
+(* top ::= definition | external | expression | ';' *)
+let rec main_loop the_fpm the_execution_engine stream =
+  match Stream.peek stream with
+  | None -&gt; ()
+
+  (* ignore top-level semicolons. *)
+  | Some (Token.Kwd ';') -&gt;
+      Stream.junk stream;
+      main_loop the_fpm the_execution_engine stream
+
+  | Some token -&gt;
+      begin
+        try match token with
+        | Token.Def -&gt;
+            let e = Parser.parse_definition stream in
+            print_endline "parsed a function definition.";
+            dump_value (Codegen.codegen_func the_fpm e);
+        | Token.Extern -&gt;
+            let e = Parser.parse_extern stream in
+            print_endline "parsed an extern.";
+            dump_value (Codegen.codegen_proto e);
+        | _ -&gt;
+            (* Evaluate a top-level expression into an anonymous function. *)
+            let e = Parser.parse_toplevel stream in
+            print_endline "parsed a top-level expr";
+            let the_function = Codegen.codegen_func the_fpm e in
+            dump_value the_function;
+
+            (* JIT the function, returning a function pointer. *)
+            let result = ExecutionEngine.run_function the_function [||]
+              the_execution_engine in
+
+            print_string "Evaluated to ";
+            print_float (GenericValue.as_float Codegen.double_type result);
+            print_newline ();
+        with Stream.Error s | Codegen.Error s -&gt;
+          (* Skip token for error recovery. *)
+          Stream.junk stream;
+          print_endline s;
+      end;
+      print_string "ready&gt; "; flush stdout;
+      main_loop the_fpm the_execution_engine stream
+</pre>
+</dd>
+
+<dt>toy.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Main driver code.
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+open Llvm_executionengine
+open Llvm_target
+open Llvm_scalar_opts
+
+let main () =
+  ignore (initialize_native_target ());
+
+  (* Install standard binary operators.
+   * 1 is the lowest precedence. *)
+  Hashtbl.add Parser.binop_precedence '=' 2;
+  Hashtbl.add Parser.binop_precedence '&lt;' 10;
+  Hashtbl.add Parser.binop_precedence '+' 20;
+  Hashtbl.add Parser.binop_precedence '-' 20;
+  Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
+
+  (* Prime the first token. *)
+  print_string "ready&gt; "; flush stdout;
+  let stream = Lexer.lex (Stream.of_channel stdin) in
+
+  (* Create the JIT. *)
+  let the_execution_engine = ExecutionEngine.create Codegen.the_module in
+  let the_fpm = PassManager.create_function Codegen.the_module in
+
+  (* Set up the optimizer pipeline.  Start with registering info about how the
+   * target lays out data structures. *)
+  TargetData.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
+
+  (* Promote allocas to registers. *)
+  add_memory_to_register_promotion the_fpm;
+
+  (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
+  add_instruction_combination the_fpm;
+
+  (* reassociate expressions. *)
+  add_reassociation the_fpm;
+
+  (* Eliminate Common SubExpressions. *)
+  add_gvn the_fpm;
+
+  (* Simplify the control flow graph (deleting unreachable blocks, etc). *)
+  add_cfg_simplification the_fpm;
+
+  ignore (PassManager.initialize the_fpm);
+
+  (* Run the main "interpreter loop" now. *)
+  Toplevel.main_loop the_fpm the_execution_engine stream;
+
+  (* Print out all the generated code. *)
+  dump_module Codegen.the_module
+;;
+
+main ()
+</pre>
+</dd>
+
+<dt>bindings.c</dt>
+<dd class="doc_code">
+<pre>
+#include &lt;stdio.h&gt;
+
+/* putchard - putchar that takes a double and returns 0. */
+extern double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+
+/* printd - printf that takes a double prints it as "%f\n", returning 0. */
+extern double printd(double X) {
+  printf("%f\n", X);
+  return 0;
+}
+</pre>
+</dd>
+</dl>
+
+<a href="LangImpl8.html">Next: Conclusion and other useful LLVM tidbits</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
+  Last modified: $Date$
+</address>
+</body>
+</html>
diff --git a/docs/tutorial/index.html b/docs/tutorial/index.html
new file mode 100644
index 0000000000..250b533f3f
--- /dev/null
+++ b/docs/tutorial/index.html
@@ -0,0 +1,48 @@
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+<html>
+<head>
+  <title>LLVM Tutorial: Table of Contents</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Owen Anderson">
+  <meta name="description" 
+  content="LLVM Tutorial: Table of Contents.">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title"> LLVM Tutorial: Table of Contents </div>
+
+<ol>
+  <li>Kaleidoscope: Implementing a Language with LLVM
+  <ol>
+    <li><a href="LangImpl1.html">Tutorial Introduction and the Lexer</a></li>
+    <li><a href="LangImpl2.html">Implementing a Parser and AST</a></li>
+    <li><a href="LangImpl3.html">Implementing Code Generation to LLVM IR</a></li>
+    <li><a href="LangImpl4.html">Adding JIT and Optimizer Support</a></li>
+    <li><a href="LangImpl5.html">Extending the language: control flow</a></li>
+    <li><a href="LangImpl6.html">Extending the language: user-defined operators</a></li>
+    <li><a href="LangImpl7.html">Extending the language: mutable variables / SSA construction</a></li>
+    <li><a href="LangImpl8.html">Conclusion and other useful LLVM tidbits</a></li>
+  </ol></li>
+  <li>Kaleidoscope: Implementing a Language with LLVM in Objective Caml
+  <ol>
+    <li><a href="OCamlLangImpl1.html">Tutorial Introduction and the Lexer</a></li>
+    <li><a href="OCamlLangImpl2.html">Implementing a Parser and AST</a></li>
+    <li><a href="OCamlLangImpl3.html">Implementing Code Generation to LLVM IR</a></li>
+    <li><a href="OCamlLangImpl4.html">Adding JIT and Optimizer Support</a></li>
+    <li><a href="OCamlLangImpl5.html">Extending the language: control flow</a></li>
+    <li><a href="OCamlLangImpl6.html">Extending the language: user-defined operators</a></li>
+    <li><a href="OCamlLangImpl7.html">Extending the language: mutable variables / SSA construction</a></li>
+    <li><a href="LangImpl8.html">Conclusion and other useful LLVM tidbits</a></li>
+  </ol></li>
+  <li>Advanced Topics
+  <ol>
+    <li><a href="http://llvm.org/pubs/2004-09-22-LCPCLLVMTutorial.html">Writing
+        an Optimization for LLVM</a></li>
+  </ol></li>
+</ol>
+
+</body>
+</html>
author	mike-m <mikem.llvm@gmail.com>	2010-05-07 00:28:04 +0000
committer	mike-m <mikem.llvm@gmail.com>	2010-05-07 00:28:04 +0000
commit	e2c3a49c8029ebd9ef530101cc24c66562e3dff5 (patch)
tree	91bf9600cc8df90cf99751a8f8bafc317cffc91e /docs/tutorial
parent	c10b5afbe8138b0fdf3af4ed3e1ddf96cf3cb4cb (diff)