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diff --git a/docs/tutorial/OCamlLangImpl5.html b/docs/tutorial/OCamlLangImpl5.html deleted file mode 100644 index d25f1dc9bb..0000000000 --- a/docs/tutorial/OCamlLangImpl5.html +++ /dev/null @@ -1,1560 +0,0 @@ -<!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="../_static/llvm.css" type="text/css"> -</head> - -<body> - -<h1>Kaleidoscope: Extending the Language: Control Flow</h1> - -<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> - -<!-- *********************************************************************** --> -<h2><a name="intro">Chapter 5 Introduction</a></h2> -<!-- *********************************************************************** --> - -<div> - -<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> - -<!-- *********************************************************************** --> -<h2><a name="ifthen">If/Then/Else</a></h2> -<!-- *********************************************************************** --> - -<div> - -<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 < 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> - -<!-- ======================================================================= --> -<h4><a name="iflexer">Lexer Extensions for If/Then/Else</a></h4> -<!-- ======================================================================= --> - - -<div> - -<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" -> [< 'Token.Def; stream >] - | "extern" -> [< 'Token.Extern; stream >] - | "if" -> [< 'Token.If; stream >] - | "then" -> [< 'Token.Then; stream >] - | "else" -> [< 'Token.Else; stream >] - | "for" -> [< 'Token.For; stream >] - | "in" -> [< 'Token.In; stream >] - | id -> [< 'Token.Ident id; stream >] -</pre> -</div> - -</div> - -<!-- ======================================================================= --> -<h4><a name="ifast">AST Extensions for If/Then/Else</a></h4> -<!-- ======================================================================= --> - -<div> - -<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> - -<!-- ======================================================================= --> -<h4><a name="ifparser">Parser Extensions for If/Then/Else</a></h4> -<!-- ======================================================================= --> - -<div> - -<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 *) - | [< 'Token.If; c=parse_expr; - 'Token.Then ?? "expected 'then'"; t=parse_expr; - 'Token.Else ?? "expected 'else'"; e=parse_expr >] -> - 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 *) - | [< 'Token.If; c=parse_expr; - 'Token.Then ?? "expected 'then'"; t=parse_expr; - 'Token.Else ?? "expected 'else'"; e=parse_expr >] -> - Ast.If (c, t, e) -</pre> -</div> - -</div> - -<!-- ======================================================================= --> -<h4><a name="ifir">LLVM IR for If/Then/Else</a></h4> -<!-- ======================================================================= --> - -<div> - -<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 < 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> - -<!-- ======================================================================= --> -<h4><a name="ifcodegen">Code Generation for If/Then/Else</a></h4> -<!-- ======================================================================= --> - -<div> - -<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_) -> - 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> - -<!-- *********************************************************************** --> -<h2><a name="for">'for' Loop Expression</a></h2> -<!-- *********************************************************************** --> - -<div> - -<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 < 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 < 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> - -<!-- ======================================================================= --> -<h4><a name="forlexer">Lexer Extensions for the 'for' Loop</a></h4> -<!-- ======================================================================= --> - -<div> - -<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" -> [< 'Token.Def; stream >] - | "extern" -> [< 'Token.Extern; stream >] - | "if" -> [< 'Token.If; stream >] - | "then" -> [< 'Token.Then; stream >] - | "else" -> [< 'Token.Else; stream >] - <b>| "for" -> [< 'Token.For; stream >] - | "in" -> [< 'Token.In; stream >]</b> - | id -> [< 'Token.Ident id; stream >] -</pre> -</div> - -</div> - -<!-- ======================================================================= --> -<h4><a name="forast">AST Extensions for the 'for' Loop</a></h4> -<!-- ======================================================================= --> - -<div> - -<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> - -<!-- ======================================================================= --> -<h4><a name="forparser">Parser Extensions for the 'for' Loop</a></h4> -<!-- ======================================================================= --> - -<div> - -<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 *) - | [< 'Token.For; - 'Token.Ident id ?? "expected identifier after for"; - 'Token.Kwd '=' ?? "expected '=' after for"; - stream >] -> - begin parser - | [< - start=parse_expr; - 'Token.Kwd ',' ?? "expected ',' after for"; - end_=parse_expr; - stream >] -> - let step = - begin parser - | [< 'Token.Kwd ','; step=parse_expr >] -> Some step - | [< >] -> None - end stream - in - begin parser - | [< 'Token.In; body=parse_expr >] -> - Ast.For (id, start, end_, step, body) - | [< >] -> - raise (Stream.Error "expected 'in' after for") - end stream - | [< >] -> - raise (Stream.Error "expected '=' after for") - end stream -</pre> -</div> - -</div> - -<!-- ======================================================================= --> -<h4><a name="forir">LLVM IR for the 'for' Loop</a></h4> -<!-- ======================================================================= --> - -<div> - -<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> - -<!-- ======================================================================= --> -<h4><a name="forcodegen">Code Generation for the 'for' Loop</a></h4> -<!-- ======================================================================= --> - -<div> - -<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) -> - (* 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 -> 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 -> codegen_expr step - (* If not specified, use 1.0. *) - | None -> 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 -> Hashtbl.add named_values var_name old_val - | None -> () - 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> - -<!-- *********************************************************************** --> -<h2><a name="code">Full Code Listing</a></h2> -<!-- *********************************************************************** --> - -<div> - -<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> -<{lexer,parser}.ml>: use_camlp4, pp(camlp4of) -<*.{byte,native}>: g++, use_llvm, use_llvm_analysis -<*.{byte,native}>: use_llvm_executionengine, use_llvm_target -<*.{byte,native}>: 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. *) - | [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream - - (* identifier: [a-zA-Z][a-zA-Z0-9] *) - | [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] -> - let buffer = Buffer.create 1 in - Buffer.add_char buffer c; - lex_ident buffer stream - - (* number: [0-9.]+ *) - | [< ' ('0' .. '9' as c); stream >] -> - let buffer = Buffer.create 1 in - Buffer.add_char buffer c; - lex_number buffer stream - - (* Comment until end of line. *) - | [< ' ('#'); stream >] -> - lex_comment stream - - (* Otherwise, just return the character as its ascii value. *) - | [< 'c; stream >] -> - [< 'Token.Kwd c; lex stream >] - - (* end of stream. *) - | [< >] -> [< >] - -and lex_number buffer = parser - | [< ' ('0' .. '9' | '.' as c); stream >] -> - Buffer.add_char buffer c; - lex_number buffer stream - | [< st |