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/LangImpl5.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>
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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>
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/LangImpl5.html
parent	c10b5afbe8138b0fdf3af4ed3e1ddf96cf3cb4cb (diff)
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