llvm-6502/docs/tutorial/LangImpl3.html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
                      "http://www.w3.org/TR/html4/strict.dtd">

<html>
<head>
  <title>Kaleidoscope: Implementing code generation to LLVM IR</title>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <meta name="author" content="Chris Lattner">
  <link rel="stylesheet" href="../llvm.css" type="text/css">
</head>

<body>

<div class="doc_title">Kaleidoscope: Code generation to LLVM IR</div>

<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">Part 3 Introduction</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>Welcome to part 3 of the "<a href="index.html">Implementing a language with
LLVM</a>" tutorial.  This chapter shows you how to transform the <a 
href="LangImpl2.html">Abstract Syntax Tree built in Chapter 2</a> into LLVM IR.
This will teach you a little bit about how LLVM does things, as well as
demonstrate how easy it is to use.  It's much more work to build a lexer and
parser than it is to generate LLVM IR code.
</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section"><a name="basics">Code Generation setup</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>
In order to generate LLVM IR, we want some simple setup to get started.  First,
we define virtual codegen methods in each AST class:</p>

<div class="doc_code">
<pre>
/// 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:
  explicit NumberExprAST(double val) : Val(val) {}
  virtual Value *Codegen();
};
...
</pre>
</div>

<p>The Codegen() method says to emit IR for that AST node and all things it
depends on, and they all return an LLVM Value object. 
"Value" is the class used to represent a "<a 
href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Static Single
Assignment (SSA)</a> register" or "SSA value" in LLVM.  The most distinct aspect
of SSA values is that their value is computed as the related instruction
executes, and it does not get a new value until (and if) the instruction
re-executes.  In order words, there is no way to "change" an SSA value.  For
more information, please read up on <a 
href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Static Single
Assignment</a> - the concepts are really quite natural once you grok them.</p>

<p>The
second thing we want is an "Error" method like we used for parser, which will
be used to report errors found during code generation (for example, use of an
undeclared parameter):</p>

<div class="doc_code">
<pre>
Value *ErrorV(const char *Str) { Error(Str); return 0; }

static Module *TheModule;
static LLVMBuilder Builder;
static std::map&lt;std::string, Value*&gt; NamedValues;
</pre>
</div>

<p>The static variables will be used during code generation.  <tt>TheModule</tt>
is the LLVM construct that contains all of the functions and global variables in
a chunk of code.  In many ways, it is the top-level structure that the LLVM IR
uses to contain code.</p>

<p>The <tt>Builder</tt> object is a helper object that makes it easy to generate
LLVM instructions.  The <tt>Builder</tt> keeps track of the current place to
insert instructions and has methods to create new instructions.</p>

<p>The <tt>NamedValues</tt> map keeps track of which values are defined in the
current scope and what their LLVM representation is.  In this form of
Kaleidoscope, the only things that can be referenced are function parameters.
As such, function parameters will be in this map when generating code for their
function body.</p>

<p>
With these basics in place, we can start talking about how to generate code for
each expression.  Note that this assumes that the <tt>Builder</tt> has been set
up to generate code <em>into</em> something.  For now, we'll assume that this
has already been done, and we'll just use it to emit code.
</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section"><a name="exprs">Expression Code Generation</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>Generating LLVM code for expression nodes is very straight-forward: less
than 45 lines of commented code for all four of our expression nodes.  First,
we'll do numeric literals:</p>

<div class="doc_code">
<pre>
Value *NumberExprAST::Codegen() {
  return ConstantFP::get(Type::DoubleTy, APFloat(Val));
}
</pre>
</div>

<p>In the LLVM IR, numeric constants are represented with the
<tt>ConstantFP</tt> class, which holds the numeric value in an <tt>APFloat</tt>
internally (<tt>APFloat</tt> has the capability of holding floating point
constants of <em>A</em>rbitrary <em>P</em>recision).  This code basically just
creates and returns a <tt>ConstantFP</tt>.  Note that in the LLVM IR
that constants are all uniqued together and shared.  For this reason, the API
uses "the foo::get(..)" idiom instead of "new foo(..)" or "foo::create(..).</p>

<div class="doc_code">
<pre>
Value *VariableExprAST::Codegen() {
  // Look this variable up in the function.
  Value *V = NamedValues[Name];
  return V ? V : ErrorV("Unknown variable name");
}
</pre>
</div>

<p>References to variables is also quite simple here.  In the simple version
of Kaleidoscope, we assume that the variable has already been emited somewhere
and its value is available.  In practice, the only values that can be in the
<tt>NamedValues</tt> map are function arguments.  This
code simply checks to see that the specified name is in the map (if not, an 
unknown variable is being referenced) and returns the value for it.</p>

<div class="doc_code">
<pre>
Value *BinaryExprAST::Codegen() {
  Value *L = LHS-&gt;Codegen();
  Value *R = RHS-&gt;Codegen();
  if (L == 0 || R == 0) return 0;
  
  switch (Op) {
  case '+': return Builder.CreateAdd(L, R, "addtmp");
  case '-': return Builder.CreateSub(L, R, "subtmp");
  case '*': return Builder.CreateMul(L, R, "multmp");
  case '&lt;':
    L = Builder.CreateFCmpULT(L, R, "multmp");
    // Convert bool 0/1 to double 0.0 or 1.0
    return Builder.CreateUIToFP(L, Type::DoubleTy, "booltmp");
  default: return ErrorV("invalid binary operator");
  }
}
</pre>
</div>

<p>Binary operators start to get more interesting.  The basic idea here is that
we recursively emit code for the left-hand side of the expression, then the 
right-hand side, then we compute the result of the binary expression.  In this
code, we do a simple switch on the opcode to create the right LLVM instruction.
</p>

<p>In this example, the LLVM builder class is starting to show its value.  
Because it knows where to insert the newly created instruction, you just have to
specificy what instruction to create (e.g. with <tt>CreateAdd</tt>), which
operands to use (<tt>L</tt> and <tt>R</tt> here) and optionally provide a name
for the generated instruction.  One nice thing about LLVM is that the name is 
just a hint: if there are multiple additions in a single function, the first
will be named "addtmp" and the second will be "autorenamed" by adding a suffix,
giving it a name like "addtmp42".  Local value names for instructions are purely
optional, but it makes it much easier to read the IR dumps.</p>

<p><a href="../LangRef.html#instref">LLVM instructions</a> are constrained to
have very strict type properties: for example, the Left and Right operators of
an <a href="../LangRef.html#i_add">add instruction</a> have to have the same
type, and that the result of the add matches the operands.  Because all values
in Kaleidoscope are doubles, this makes for very simple code for add, sub and
mul.</p>

<p>On the other hand, LLVM specifies that the <a 
href="../LangRef.html#i_fcmp">fcmp instruction</a> always returns an 'i1' value
(a one bit integer).  However, Kaleidoscope wants the value to be a 0.0 or 1.0
value.  In order to get these semantics, we combine the fcmp instruction with
a <a href="../LangRef.html#i_uitofp">uitofp instruction</a>.  This instruction
converts its input integer into a floating point value by treating the input
as an unsigned value.  In contrast, if we used the <a 
href="../LangRef.html#i_sitofp">sitofp instruction</a>, the Kaleidoscope '<'
operator would return 0.0 and -1.0, depending on the input value.</p>

<div class="doc_code">
<pre>
Value *CallExprAST::Codegen() {
  // Look up the name in the global module table.
  Function *CalleeF = TheModule-&gt;getFunction(Callee);
  if (CalleeF == 0)
    return ErrorV("Unknown function referenced");
  
  // If argument mismatch error.
  if (CalleeF-&gt;arg_size() != Args.size())
    return ErrorV("Incorrect # arguments passed");

  std::vector&lt;Value*&gt; ArgsV;
  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
    ArgsV.push_back(Args[i]-&gt;Codegen());
    if (ArgsV.back() == 0) return 0;
  }
  
  return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp");
}
</pre>
</div>

<p>Code generation for function calls is quite straight-forward with LLVM.  The
code above first looks the name of the function up in the LLVM Module's symbol
table.  Recall that the LLVM Module is the container that holds all of the
functions we are JIT'ing.  By giving each function the same name as what the
user specifies, we can use the LLVM symbol table to resolve function names for
us.</p>

<p>Once we have the function to call, we recursively codegen each argument that
is to be passed in, and create an LLVM <a href="../LangRef.html#i_call">call
instruction</a>.  Note that LLVM uses the native C calling conventions by
default, allowing these calls to call into standard library functions like
"sin" and "cos" with no additional effort.</p>

<p>This wraps up our handling of the four basic expressions that we have so far
in Kaleidoscope.  Feel free to go in and add some more.  For example, by 
browsing the <a href="../LangRef.html">LLVM language reference</a> you'll find
several other interesting instructions that are really easy to plug into our
basic framework.</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section"><a name="code">Conclusions and the Full Code</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<div class="doc_code">
<pre>
// To build this:
//  g++ -g toy.cpp `llvm-config --cppflags` `llvm-config --ldflags` \
//                `llvm-config --libs core` -I ~/llvm/include/
//  ./a.out 
// See example below.

#include "llvm/DerivedTypes.h"
#include "llvm/Module.h"
#include "llvm/Support/LLVMBuilder.h"
#include &lt;cstdio&gt;
#include &lt;string&gt;
#include &lt;map&gt;
#include &lt;vector&gt;
using namespace llvm;

//===----------------------------------------------------------------------===//
// Lexer
//===----------------------------------------------------------------------===//

// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
// of these for known things.
enum Token {
  tok_eof = -1,

  // commands
  tok_def = -2, tok_extern = -3,

  // primary
  tok_identifier = -4, tok_number = -5,
};

static std::string IdentifierStr;  // Filled in if tok_identifier
static double NumVal;              // Filled in if tok_number

/// gettok - Return the next token from standard input.
static int gettok() {
  static int LastChar = ' ';

  // Skip any whitespace.
  while (isspace(LastChar))
    LastChar = getchar();

  if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
    IdentifierStr = LastChar;
    while (isalnum((LastChar = getchar())))
      IdentifierStr += LastChar;

    if (IdentifierStr == "def") return tok_def;
    if (IdentifierStr == "extern") return tok_extern;
    return tok_identifier;
  }

  if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
    std::string NumStr;
    do {
      NumStr += LastChar;
      LastChar = getchar();
    } while (isdigit(LastChar) || LastChar == '.');

    NumVal = strtod(NumStr.c_str(), 0);
    return tok_number;
  }

  if (LastChar == '#') {
    // Comment until end of line.
    do LastChar = getchar();
    while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp; 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:
  explicit 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:
  explicit VariableExprAST(const std::string &amp;name) : Name(name) {}
  virtual Value *Codegen();
};

/// BinaryExprAST - Expression class for a binary operator.
class BinaryExprAST : public ExprAST {
  char Op;
  ExprAST *LHS, *RHS;
public:
  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
    : Op(op), LHS(lhs), RHS(rhs) {}
  virtual Value *Codegen();
};

/// CallExprAST - Expression class for function calls.
class CallExprAST : public ExprAST {
  std::string Callee;
  std::vector&lt;ExprAST*&gt; Args;
public:
  CallExprAST(const std::string &amp;callee, std::vector&lt;ExprAST*&gt; &amp;args)
    : Callee(callee), Args(args) {}
  virtual Value *Codegen();
};

/// PrototypeAST - This class represents the "prototype" for a function,
/// which captures its argument names as well as if it is an operator.
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 it 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
///   ::= identifer
///   ::= identifer '(' expression* ')'
static ExprAST *ParseIdentifierExpr() {
  std::string IdName = IdentifierStr;
  
  getNextToken();  // eat identifer.
  
  if (CurTok != '(') // Simple variable ref.
    return new VariableExprAST(IdName);
  
  // Call.
  getNextToken();  // eat (
  std::vector&lt;ExprAST*&gt; Args;
  while (1) {
    ExprAST *Arg = ParseExpression();
    if (!Arg) return 0;
    Args.push_back(Arg);
    
    if (CurTok == ')') break;
    
    if (CurTok != ',')
      return Error("Expected ')'");
    getNextToken();
  }

  // Eat the ')'.
  getNextToken();
  
  return new CallExprAST(IdName, Args);
}

/// numberexpr ::= number
static ExprAST *ParseNumberExpr() {
  ExprAST *Result = new NumberExprAST(NumVal);
  getNextToken(); // consume the number
  return Result;
}

/// parenexpr ::= '(' expression ')'
static ExprAST *ParseParenExpr() {
  getNextToken();  // eat (.
  ExprAST *V = ParseExpression();
  if (!V) return 0;
  
  if (CurTok != ')')
    return Error("expected ')'");
  getNextToken();  // eat ).
  return V;
}

/// primary
///   ::= identifierexpr
///   ::= numberexpr
///   ::= parenexpr
static ExprAST *ParsePrimary() {
  switch (CurTok) {
  default: return Error("unknown token when expecting an expression");
  case tok_identifier: return ParseIdentifierExpr();
  case tok_number:     return ParseNumberExpr();
  case '(':            return ParseParenExpr();
  }
}

/// binoprhs
///   ::= ('+' primary)*
static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
  // If this is a binop, find its precedence.
  while (1) {
    int TokPrec = GetTokPrecedence();
    
    // If this is a binop that binds at least as tightly as the current binop,
    // consume it, otherwise we are done.
    if (TokPrec &lt; ExprPrec)
      return LHS;
    
    // Okay, we know this is a binop.
    int BinOp = CurTok;
    getNextToken();  // eat binop
    
    // Parse the primary expression after the binary operator.
    ExprAST *RHS = ParsePrimary();
    if (!RHS) return 0;
    
    // If BinOp binds less tightly with RHS than the operator after RHS, let
    // the pending operator take RHS as its LHS.
    int NextPrec = GetTokPrecedence();
    if (TokPrec &lt; NextPrec) {
      RHS = ParseBinOpRHS(TokPrec+1, RHS);
      if (RHS == 0) return 0;
    }
    
    // Merge LHS/RHS.
    LHS = new BinaryExprAST(BinOp, LHS, RHS);
  }
}

/// expression
///   ::= primary binoprhs
///
static ExprAST *ParseExpression() {
  ExprAST *LHS = ParsePrimary();
  if (!LHS) return 0;
  
  return ParseBinOpRHS(0, LHS);
}

/// prototype
///   ::= id '(' id* ')'
static PrototypeAST *ParsePrototype() {
  if (CurTok != tok_identifier)
    return ErrorP("Expected function name in prototype");

  std::string FnName = IdentifierStr;
  getNextToken();
  
  if (CurTok != '(')
    return ErrorP("Expected '(' in prototype");
  
  std::vector&lt;std::string&gt; ArgNames;
  while (getNextToken() == tok_identifier)
    ArgNames.push_back(IdentifierStr);
  if (CurTok != ')')
    return ErrorP("Expected ')' in prototype");
  
  // success.
  getNextToken();  // eat ')'.
  
  return new PrototypeAST(FnName, ArgNames);
}

/// definition ::= 'def' prototype expression
static FunctionAST *ParseDefinition() {
  getNextToken();  // eat def.
  PrototypeAST *Proto = ParsePrototype();
  if (Proto == 0) return 0;

  if (ExprAST *E = ParseExpression())
    return new FunctionAST(Proto, E);
  return 0;
}

/// toplevelexpr ::= expression
static FunctionAST *ParseTopLevelExpr() {
  if (ExprAST *E = ParseExpression()) {
    // Make an anonymous proto.
    PrototypeAST *Proto = new PrototypeAST("", std::vector&lt;std::string&gt;());
    return new FunctionAST(Proto, E);
  }
  return 0;
}

/// external ::= 'extern' prototype
static PrototypeAST *ParseExtern() {
  getNextToken();  // eat extern.
  return ParsePrototype();
}

//===----------------------------------------------------------------------===//
// Code Generation
//===----------------------------------------------------------------------===//

static Module *TheModule;
static LLVMBuilder Builder;
static std::map&lt;std::string, Value*&gt; NamedValues;

Value *ErrorV(const char *Str) { Error(Str); return 0; }

Value *NumberExprAST::Codegen() {
  return ConstantFP::get(Type::DoubleTy, 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, "multmp");
    // Convert bool 0/1 to double 0.0 or 1.0
    return Builder.CreateUIToFP(L, Type::DoubleTy, "booltmp");
  default: return ErrorV("invalid binary operator");
  }
}

Value *CallExprAST::Codegen() {
  // Look up the name in the global module table.
  Function *CalleeF = TheModule-&gt;getFunction(Callee);
  if (CalleeF == 0)
    return ErrorV("Unknown function referenced");
  
  // If argument mismatch error.
  if (CalleeF-&gt;arg_size() != Args.size())
    return ErrorV("Incorrect # arguments passed");

  std::vector&lt;Value*&gt; ArgsV;
  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
    ArgsV.push_back(Args[i]-&gt;Codegen());
    if (ArgsV.back() == 0) return 0;
  }
  
  return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp");
}

Function *PrototypeAST::Codegen() {
  // Make the function type:  double(double,double) etc.
  FunctionType *FT = 
    FunctionType::get(Type::DoubleTy, std::vector&lt;const Type*&gt;(Args.size(),
                                                               Type::DoubleTy),
                      false);
  
  Function *F = new Function(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.
  Builder.SetInsertPoint(new BasicBlock("entry", TheFunction));
  
  if (Value *RetVal = Body-&gt;Codegen()) {
    // Finish off the function.
    Builder.CreateRet(RetVal);
    return TheFunction;
  }
  
  // Error reading body, remove function.
  TheFunction-&gt;eraseFromParent();
  return 0;
}

//===----------------------------------------------------------------------===//
// Top-Level parsing and JIT Driver
//===----------------------------------------------------------------------===//

static void HandleDefinition() {
  if (FunctionAST *F = ParseDefinition()) {
    if (Function *LF = F-&gt;Codegen()) {
      fprintf(stderr, "Read function definition:");
      LF-&gt;dump();
    }
  } else {
    // Skip token for error recovery.
    getNextToken();
  }
}

static void HandleExtern() {
  if (PrototypeAST *P = ParseExtern()) {
    if (Function *F = P-&gt;Codegen()) {
      fprintf(stderr, "Read extern: ");
      F-&gt;dump();
    }
  } else {
    // Skip token for error recovery.
    getNextToken();
  }
}

static void HandleTopLevelExpression() {
  // Evaluate a top level expression into an anonymous function.
  if (FunctionAST *F = ParseTopLevelExpr()) {
    if (Function *LF = F-&gt;Codegen()) {
      fprintf(stderr, "Read top-level expression:");
      LF-&gt;dump();
    }
  } else {
    // Skip token for error recovery.
    getNextToken();
  }
}

/// top ::= definition | external | expression | ';'
static void MainLoop() {
  while (1) {
    fprintf(stderr, "ready&gt; ");
    switch (CurTok) {
    case tok_eof:    return;
    case ';':        getNextToken(); break;  // ignore top level semicolons.
    case tok_def:    HandleDefinition(); break;
    case tok_extern: HandleExtern(); break;
    default:         HandleTopLevelExpression(); break;
    }
  }
}


//===----------------------------------------------------------------------===//
// "Library" functions that can be "extern'd" from user code.
//===----------------------------------------------------------------------===//

/// putchard - putchar that takes a double and returns 0.
extern "C" 
double putchard(double X) {
  putchar((char)X);
  return 0;
}

//===----------------------------------------------------------------------===//
// Main driver code.
//===----------------------------------------------------------------------===//

int main() {
  TheModule = new Module("my cool jit");

  // 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();

  MainLoop();
  TheModule-&gt;dump();
  return 0;
}

/* Examples:

def fib(x)
  if (x &lt; 3) then
    1
  else
    fib(x-1)+fib(x-2);

fib(10);

*/
</pre>
</div>
</div>

<!-- *********************************************************************** -->
<hr>
<address>
  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
  <a href="http://validator.w3.org/check/referer"><img
  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!" /></a>

  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
  Last modified: $Date: 2007-10-17 11:05:13 -0700 (Wed, 17 Oct 2007) $
</address>
</body>
</html>