diff options
Diffstat (limited to 'docs/tutorial/LangImpl5.html')
| -rw-r--r-- | docs/tutorial/LangImpl5.html | 1772 |
1 files changed, 0 insertions, 1772 deletions
diff --git a/docs/tutorial/LangImpl5.html b/docs/tutorial/LangImpl5.html deleted file mode 100644 index 768d9a0e11..0000000000 --- a/docs/tutorial/LangImpl5.html +++ /dev/null @@ -1,1772 +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"> - <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="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> - -<!-- *********************************************************************** --> -<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 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 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> - -<!-- ======================================================================= --> -<h4><a name="ifast">AST Extensions for If/Then/Else</a></h4> -<!-- ======================================================================= --> - -<div> - -<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> - -<!-- ======================================================================= --> -<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> -/// 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> - -<!-- ======================================================================= --> -<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>F->viewCFG()</tt>" or -"<tt>F->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> - -<!-- ======================================================================= --> -<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> -Value *IfExprAST::Codegen() { - Value *CondV = Cond->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()->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->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->getBasicBlockList().push_back(ElseBB); - Builder.SetInsertPoint(ElseBB); - - Value *ElseV = Else->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()), 2, - "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> - -<!-- *********************************************************************** --> -<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 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> - -<!-- ======================================================================= --> -<h4><a name="forast">AST Extensions for the 'for' Loop</a></h4> -<!-- ======================================================================= --> - -<div> - -<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 &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> - -<!-- ======================================================================= --> -<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> -/// 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> - -<!-- ======================================================================= --> -<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> -Value *ForExprAST::Codegen() { - // Emit the start code first, without 'variable' in scope. - Value *StartVal = Start->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()->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()), 2, VarName.c_str()); - Variable->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->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->Codegen(); - if (StepVal == 0) return 0; - } else { - // If not specified, use 1.0. - StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0)); - } - - Value *NextVar = Builder.CreateFAdd(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->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->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> - -<!-- *********************************************************************** --> -<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 -clang++ -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/IRBuilder.h" -#include "llvm/LLVMContext.h" -#include "llvm/Module.h" -#include "llvm/PassManager.h" -#include "llvm/Analysis/Verifier.h" -#include "llvm/Analysis/Passes.h" -#include "llvm/DataLayout.h" -#include "llvm/Transforms/Scalar.h" -#include "llvm/Support/TargetSelect.h" -#include <cstdio> -#include <string> -#include <map> -#include <vector> -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 && LastChar != '\n' &am |