examples/Kaleidoscope/Chapter4/toy.cpp

#include "llvm/ADT/STLExtras.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/Passes.h"
#include "llvm/ExecutionEngine/ExecutionEngine.h"
#include "llvm/ExecutionEngine/MCJIT.h"
#include "llvm/ExecutionEngine/SectionMemoryManager.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/LegacyPassManager.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Verifier.h"
#include "llvm/Support/TargetSelect.h"
#include "llvm/Transforms/Scalar.h"
#include <cctype>
#include <cstdio>
#include <map>
#include <string>
#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
};

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 && LastChar != '\n' && 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)
//===----------------------------------------------------------------------===//
namespace {
/// 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) {}
  Value *Codegen() override;
};

/// VariableExprAST - Expression class for referencing a variable, like "a".
class VariableExprAST : public ExprAST {
  std::string Name;

public:
  VariableExprAST(const std::string &Name) : Name(Name) {}
  Value *Codegen() override;
};

/// BinaryExprAST - Expression class for a binary operator.
class BinaryExprAST : public ExprAST {
  char Op;
  std::unique_ptr<ExprAST> LHS, RHS;

public:
  BinaryExprAST(char Op, std::unique_ptr<ExprAST> LHS,
                std::unique_ptr<ExprAST> RHS)
      : Op(Op), LHS(std::move(LHS)), RHS(std::move(RHS)) {}
  Value *Codegen() override;
};

/// CallExprAST - Expression class for function calls.
class CallExprAST : public ExprAST {
  std::string Callee;
  std::vector<std::unique_ptr<ExprAST>> Args;

public:
  CallExprAST(const std::string &Callee,
              std::vector<std::unique_ptr<ExprAST>> Args)
      : Callee(Callee), Args(std::move(Args)) {}
  Value *Codegen() override;
};

/// 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<std::string> Args;

public:
  PrototypeAST(const std::string &Name, std::vector<std::string> Args)
      : Name(Name), Args(std::move(Args)) {}
  Function *Codegen();
};

/// FunctionAST - This class represents a function definition itself.
class FunctionAST {
  std::unique_ptr<PrototypeAST> Proto;
  std::unique_ptr<ExprAST> Body;

public:
  FunctionAST(std::unique_ptr<PrototypeAST> Proto,
              std::unique_ptr<ExprAST> Body)
      : Proto(std::move(Proto)), Body(std::move(Body)) {}
  Function *Codegen();
};
} // end anonymous namespace

//===----------------------------------------------------------------------===//
// 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<char, int> 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 <= 0)
    return -1;
  return TokPrec;
}

/// Error* - These are little helper functions for error handling.
std::unique_ptr<ExprAST> Error(const char *Str) {
  fprintf(stderr, "Error: %s\n", Str);
  return nullptr;
}
std::unique_ptr<PrototypeAST> ErrorP(const char *Str) {
  Error(Str);
  return nullptr;
}
std::unique_ptr<FunctionAST> ErrorF(const char *Str) {
  Error(Str);
  return nullptr;
}

static std::unique_ptr<ExprAST> ParseExpression();

/// numberexpr ::= number
static std::unique_ptr<ExprAST> ParseNumberExpr() {
  auto Result = llvm::make_unique<NumberExprAST>(NumVal);
  getNextToken(); // consume the number
  return std::move(Result);
}

/// parenexpr ::= '(' expression ')'
static std::unique_ptr<ExprAST> ParseParenExpr() {
  getNextToken(); // eat (.
  auto V = ParseExpression();
  if (!V)
    return nullptr;

  if (CurTok != ')')
    return Error("expected ')'");
  getNextToken(); // eat ).
  return V;
}

/// identifierexpr
///   ::= identifier
///   ::= identifier '(' expression* ')'
static std::unique_ptr<ExprAST> ParseIdentifierExpr() {
  std::string IdName = IdentifierStr;

  getNextToken(); // eat identifier.

  if (CurTok != '(') // Simple variable ref.
    return llvm::make_unique<VariableExprAST>(IdName);

  // Call.
  getNextToken(); // eat (
  std::vector<std::unique_ptr<ExprAST>> Args;
  if (CurTok != ')') {
    while (1) {
      if (auto Arg = ParseExpression())
        Args.push_back(std::move(Arg));
      else
        return nullptr;

      if (CurTok == ')')
        break;

      if (CurTok != ',')
        return Error("Expected ')' or ',' in argument list");
      getNextToken();
    }
  }

  // Eat the ')'.
  getNextToken();

  return llvm::make_unique<CallExprAST>(IdName, std::move(Args));
}

/// primary
///   ::= identifierexpr
///   ::= numberexpr
///   ::= parenexpr
static std::unique_ptr<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 std::unique_ptr<ExprAST> ParseBinOpRHS(int ExprPrec,
                                              std::unique_ptr<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 < ExprPrec)
      return LHS;

    // Okay, we know this is a binop.
    int BinOp = CurTok;
    getNextToken(); // eat binop

    // Parse the primary expression after the binary operator.
    auto RHS = ParsePrimary();
    if (!RHS)
      return nullptr;

    // 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 < NextPrec) {
      RHS = ParseBinOpRHS(TokPrec + 1, std::move(RHS));
      if (!RHS)
        return nullptr;
    }

    // Merge LHS/RHS.
    LHS =
        llvm::make_unique<BinaryExprAST>(BinOp, std::move(LHS), std::move(RHS));
  }
}

/// expression
///   ::= primary binoprhs
///
static std::unique_ptr<ExprAST> ParseExpression() {
  auto LHS = ParsePrimary();
  if (!LHS)
    return nullptr;

  return ParseBinOpRHS(0, std::move(LHS));
}

/// prototype
///   ::= id '(' id* ')'
static std::unique_ptr<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<std::string> ArgNames;
  while (getNextToken() == tok_identifier)
    ArgNames.push_back(IdentifierStr);
  if (CurTok != ')')
    return ErrorP("Expected ')' in prototype");

  // success.
  getNextToken(); // eat ')'.

  return llvm::make_unique<PrototypeAST>(FnName, std::move(ArgNames));
}

/// definition ::= 'def' prototype expression
static std::unique_ptr<FunctionAST> ParseDefinition() {
  getNextToken(); // eat def.
  auto Proto = ParsePrototype();
  if (!Proto)
    return nullptr;

  if (auto E = ParseExpression())
    return llvm::make_unique<FunctionAST>(std::move(Proto), std::move(E));
  return nullptr;
}

/// toplevelexpr ::= expression
static std::unique_ptr<FunctionAST> ParseTopLevelExpr() {
  if (auto E = ParseExpression()) {
    // Make an anonymous proto.
    auto Proto =
        llvm::make_unique<PrototypeAST>("", std::vector<std::string>());
    return llvm::make_unique<FunctionAST>(std::move(Proto), std::move(E));
  }
  return nullptr;
}

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

//===----------------------------------------------------------------------===//
// Quick and dirty hack
//===----------------------------------------------------------------------===//

// FIXME: Obviously we can do better than this
std::string GenerateUniqueName(const char *root) {
  static int i = 0;
  char s[16];
  sprintf(s, "%s%d", root, i++);
  std::string S = s;
  return S;
}

std::string MakeLegalFunctionName(std::string Name) {
  std::string NewName;
  if (!Name.length())
    return GenerateUniqueName("anon_func_");

  // Start with what we have
  NewName = Name;

  // Look for a numberic first character
  if (NewName.find_first_of("0123456789") == 0) {
    NewName.insert(0, 1, 'n');
  }

  // Replace illegal characters with their ASCII equivalent
  std::string legal_elements =
      "_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
  size_t pos;
  while ((pos = NewName.find_first_not_of(legal_elements)) !=
         std::string::npos) {
    char old_c = NewName.at(pos);
    char new_str[16];
    sprintf(new_str, "%d", (int)old_c);
    NewName = NewName.replace(pos, 1, new_str);
  }

  return NewName;
}

//===----------------------------------------------------------------------===//
// MCJIT helper class
//===----------------------------------------------------------------------===//

class MCJITHelper {
public:
  MCJITHelper(LLVMContext &C) : Context(C), OpenModule(NULL) {}
  ~MCJITHelper();

  Function *getFunction(const std::string FnName);
  Module *getModuleForNewFunction();
  void *getPointerToFunction(Function *F);
  void *getSymbolAddress(const std::string &Name);
  void dump();

private:
  typedef std::vector<Module *> ModuleVector;
  typedef std::vector<ExecutionEngine *> EngineVector;

  LLVMContext &Context;
  Module *OpenModule;
  ModuleVector Modules;
  EngineVector Engines;
};

class HelpingMemoryManager : public SectionMemoryManager {
  HelpingMemoryManager(const HelpingMemoryManager &) = delete;
  void operator=(const HelpingMemoryManager &) = delete;

public:
  HelpingMemoryManager(MCJITHelper *Helper) : MasterHelper(Helper) {}
  ~HelpingMemoryManager() override {}

  /// This method returns the address of the specified symbol.
  /// Our implementation will attempt to find symbols in other
  /// modules associated with the MCJITHelper to cross link symbols
  /// from one generated module to another.
  uint64_t getSymbolAddress(const std::string &Name) override;

private:
  MCJITHelper *MasterHelper;
};

uint64_t HelpingMemoryManager::getSymbolAddress(const std::string &Name) {
  uint64_t FnAddr = SectionMemoryManager::getSymbolAddress(Name);
  if (FnAddr)
    return FnAddr;

  uint64_t HelperFun = (uint64_t)MasterHelper->getSymbolAddress(Name);
  if (!HelperFun)
    report_fatal_error("Program used extern function '" + Name +
                       "' which could not be resolved!");

  return HelperFun;
}

MCJITHelper::~MCJITHelper() {
  if (OpenModule)
    delete OpenModule;
  EngineVector::iterator begin = Engines.begin();
  EngineVector::iterator end = Engines.end();
  EngineVector::iterator it;
  for (it = begin; it != end; ++it)
    delete *it;
}

Function *MCJITHelper::getFunction(const std::string FnName) {
  ModuleVector::iterator begin = Modules.begin();
  ModuleVector::iterator end = Modules.end();
  ModuleVector::iterator it;
  for (it = begin; it != end; ++it) {
    Function *F = (*it)->getFunction(FnName);
    if (F) {
      if (*it == OpenModule)
        return F;

      assert(OpenModule != NULL);

      // This function is in a module that has already been JITed.
      // We need to generate a new prototype for external linkage.
      Function *PF = OpenModule->getFunction(FnName);
      if (PF && !PF->empty()) {
        ErrorF("redefinition of function across modules");
        return nullptr;
      }

      // If we don't have a prototype yet, create one.
      if (!PF)
        PF = Function::Create(F->getFunctionType(), Function::ExternalLinkage,
                              FnName, OpenModule);
      return PF;
    }
  }
  return NULL;
}

Module *MCJITHelper::getModuleForNewFunction() {
  // If we have a Module that hasn't been JITed, use that.
  if (OpenModule)
    return OpenModule;

  // Otherwise create a new Module.
  std::string ModName = GenerateUniqueName("mcjit_module_");
  Module *M = new Module(ModName, Context);
  Modules.push_back(M);
  OpenModule = M;
  return M;
}

void *MCJITHelper::getPointerToFunction(Function *F) {
  // See if an existing instance of MCJIT has this function.
  EngineVector::iterator begin = Engines.begin();
  EngineVector::iterator end = Engines.end();
  EngineVector::iterator it;
  for (it = begin; it != end; ++it) {
    void *P = (*it)->getPointerToFunction(F);
    if (P)
      return P;
  }

  // If we didn't find the function, see if we can generate it.
  if (OpenModule) {
    std::string ErrStr;
    ExecutionEngine *NewEngine =
        EngineBuilder(std::unique_ptr<Module>(OpenModule))
            .setErrorStr(&ErrStr)
            .setMCJITMemoryManager(std::unique_ptr<HelpingMemoryManager>(
                new HelpingMemoryManager(this)))
            .create();
    if (!NewEngine) {
      fprintf(stderr, "Could not create ExecutionEngine: %s\n", ErrStr.c_str());
      exit(1);
    }

    // Create a function pass manager for this engine
    auto *FPM = new legacy::FunctionPassManager(OpenModule);

    // Set up the optimizer pipeline.  Start with registering info about how the
    // target lays out data structures.
    OpenModule->setDataLayout(NewEngine->getDataLayout());
    // Provide basic AliasAnalysis support for GVN.
    FPM->add(createBasicAliasAnalysisPass());
    // Promote allocas to registers.
    FPM->add(createPromoteMemoryToRegisterPass());
    // Do simple "peephole" optimizations and bit-twiddling optzns.
    FPM->add(createInstructionCombiningPass());
    // Reassociate expressions.
    FPM->add(createReassociatePass());
    // Eliminate Common SubExpressions.
    FPM->add(createGVNPass());
    // Simplify the control flow graph (deleting unreachable blocks, etc).
    FPM->add(createCFGSimplificationPass());
    FPM->doInitialization();

    // For each function in the module
    Module::iterator it;
    Module::iterator end = OpenModule->end();
    for (it = OpenModule->begin(); it != end; ++it) {
      // Run the FPM on this function
      FPM->run(*it);
    }

    // We don't need this anymore
    delete FPM;

    OpenModule = NULL;
    Engines.push_back(NewEngine);
    NewEngine->finalizeObject();
    return NewEngine->getPointerToFunction(F);
  }
  return NULL;
}

void *MCJITHelper::getSymbolAddress(const std::string &Name) {
  // Look for the symbol in each of our execution engines.
  EngineVector::iterator begin = Engines.begin();
  EngineVector::iterator end = Engines.end();
  EngineVector::iterator it;
  for (it = begin; it != end; ++it) {
    uint64_t FAddr = (*it)->getFunctionAddress(Name);
    if (FAddr) {
      return (void *)FAddr;
    }
  }
  return NULL;
}

void MCJITHelper::dump() {
  ModuleVector::iterator begin = Modules.begin();
  ModuleVector::iterator end = Modules.end();
  ModuleVector::iterator it;
  for (it = begin; it != end; ++it)
    (*it)->dump();
}
//===----------------------------------------------------------------------===//
// Code Generation
//===----------------------------------------------------------------------===//

static MCJITHelper *JITHelper;
static IRBuilder<> Builder(getGlobalContext());
static std::map<std::string, Value *> NamedValues;

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

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->Codegen();
  Value *R = RHS->Codegen();
  if (!L || !R)
    return nullptr;

  switch (Op) {
  case '+':
    return Builder.CreateFAdd(L, R, "addtmp");
  case '-':
    return Builder.CreateFSub(L, R, "subtmp");
  case '*':
    return Builder.CreateFMul(L, R, "multmp");
  case '<':
    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 = JITHelper->getFunction(Callee);
  if (!CalleeF)
    return ErrorV("Unknown function referenced");

  // If argument mismatch error.
  if (CalleeF->arg_size() != Args.size())
    return ErrorV("Incorrect # arguments passed");

  std::vector<Value *> ArgsV;
  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
    ArgsV.push_back(Args[i]->Codegen());
    if (!ArgsV.back())
      return nullptr;
  }

  return Builder.CreateCall(CalleeF, ArgsV, "calltmp");
}

Function *PrototypeAST::Codegen() {
  // Make the function type:  double(double,double) etc.
  std::vector<Type *> Doubles(Args.size(),
                              Type::getDoubleTy(getGlobalContext()));
  FunctionType *FT =
      FunctionType::get(Type::getDoubleTy(getGlobalContext()), Doubles, false);

  std::string FnName = MakeLegalFunctionName(Name);

  Module *M = JITHelper->getModuleForNewFunction();

  Function *F = Function::Create(FT, Function::ExternalLinkage, FnName, M);

  // If F conflicted, there was already something named 'Name'.  If it has a
  // body, don't allow redefinition or reextern.
  if (F->getName() != FnName) {
    // Delete the one we just made and get the existing one.
    F->eraseFromParent();
    F = JITHelper->getFunction(Name);
    // If F already has a body, reject this.
    if (!F->empty()) {
      ErrorF("redefinition of function");
      return nullptr;
    }

    // If F took a different number of args, reject.
    if (F->arg_size() != Args.size()) {
      ErrorF("redefinition of function with different # args");
      return nullptr;
    }
  }

  // Set names for all arguments.
  unsigned Idx = 0;
  for (Function::arg_iterator AI = F->arg_begin(); Idx != Args.size();
       ++AI, ++Idx) {
    AI->setName(Args[Idx]);

    // Add arguments to variable symbol table.
    NamedValues[Args[Idx]] = AI;
  }

  return F;
}

Function *FunctionAST::Codegen() {
  NamedValues.clear();

  Function *TheFunction = Proto->Codegen();
  if (!TheFunction)
    return nullptr;

  // Create a new basic block to start insertion into.
  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
  Builder.SetInsertPoint(BB);

  if (Value *RetVal = Body->Codegen()) {
    // Finish off the function.
    Builder.CreateRet(RetVal);

    // Validate the generated code, checking for consistency.
    verifyFunction(*TheFunction);

    return TheFunction;
  }

  // Error reading body, remove function.
  TheFunction->eraseFromParent();
  return nullptr;
}

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

static void HandleDefinition() {
  if (auto FnAST = ParseDefinition()) {
    if (auto *FnIR = FnAST->Codegen()) {
      fprintf(stderr, "Read function definition:");
      FnIR->dump();
    }
  } else {
    // Skip token for error recovery.
    getNextToken();
  }
}

static void HandleExtern() {
  if (auto ProtoAST = ParseExtern()) {
    if (auto *FnIR = ProtoAST->Codegen()) {
      fprintf(stderr, "Read extern: ");
      FnIR->dump();
    }
  } else {
    // Skip token for error recovery.
    getNextToken();
  }
}

static void HandleTopLevelExpression() {
  // Evaluate a top-level expression into an anonymous function.
  if (auto FnAST = ParseTopLevelExpr()) {
    if (auto *FnIR = FnAST->Codegen()) {
      // JIT the function, returning a function pointer.
      void *FPtr = JITHelper->getPointerToFunction(FnIR);

      // 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> ");
    switch (CurTok) {
    case tok_eof:
      return;
    case ';': // ignore top-level semicolons.
      getNextToken();
      break;
    case tok_def:
      HandleDefinition();
      break;
    case tok_extern:
      HandleExtern();
      break;
    default:
      HandleTopLevelExpression();
      break;
    }
  }
}

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

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

/// printd - printf that takes a double prints it as "%f\n", returning 0.
extern "C" double printd(double X) {
  printf("%f\n", X);
  return 0;
}

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

int main() {
  InitializeNativeTarget();
  InitializeNativeTargetAsmPrinter();
  InitializeNativeTargetAsmParser();
  LLVMContext &Context = getGlobalContext();
  JITHelper = new MCJITHelper(Context);

  // Install standard binary operators.
  // 1 is lowest precedence.
  BinopPrecedence['<'] = 10;
  BinopPrecedence['+'] = 20;
  BinopPrecedence['-'] = 20;
  BinopPrecedence['*'] = 40; // highest.

  // Prime the first token.
  fprintf(stderr, "ready> ");
  getNextToken();

  // Run the main "interpreter loop" now.
  MainLoop();

  // Print out all of the generated code.
  JITHelper->dump();

  return 0;
}