Revised LLVM Fibonacci example

fibonacci.cpp

//===--- examples/Fibonacci/fibonacci.cpp - An example use of the JIT -----===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This small program provides an example of how to build quickly a small module
// with function Fibonacci and execute it with the JIT.
//
// The goal of this snippet is to create in the memory the LLVM module
// consisting of one function as follow:
//
//   int fib(int x) {
//     if(x<=2) return 1;
//     return fib(x-1)+fib(x-2);
//   }
//
// Once we have this, we compile the module via JIT, then execute the `fib'
// function and return result to a driver, i.e. to a "host program".
//
//===----------------------------------------------------------------------===//

#include "llvm/LLVMContext.h"
#include "llvm/Module.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/Verifier.h"
#include "llvm/ExecutionEngine/JIT.h"
#include "llvm/ExecutionEngine/Interpreter.h"
#include "llvm/ExecutionEngine/GenericValue.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Support/TargetSelect.h"
using namespace llvm;

#define CONST_INT32( n, c ) ( ConstantInt::get( Type::getInt32Ty( c ), n ) )

static BasicBlock *CreateBaseCase( Function *FibFunction, LLVMContext &Context ) {
  BasicBlock *Scope = BasicBlock::Create(Context, "base_case", FibFunction);
  ReturnInst::Create(Context, CONST_INT32( 1, Context ), Scope);

  return Scope;
}

static BasicBlock *CreateRecursiveCase( Function *FibFunction, Argument *ArgX, LLVMContext &Context ) {
  BasicBlock* Scope = BasicBlock::Create(Context, "recursive_case", FibFunction);

  // create fib(x-1)
  Value *Sub = BinaryOperator::CreateSub(ArgX, CONST_INT32( 1, Context ), "arg", Scope);
  CallInst *CallFibX1 = CallInst::Create(FibFunction, Sub, "fibx1", Scope);
  CallFibX1->setTailCall();

  // create fib(x-2)
  Sub = BinaryOperator::CreateSub(ArgX, CONST_INT32( 2, Context ), "arg", Scope);
  CallInst *CallFibX2 = CallInst::Create(FibFunction, Sub, "fibx2", Scope);
  CallFibX2->setTailCall();

  // fib(x-1)+fib(x-2)
  Value *Sum = BinaryOperator::CreateAdd(CallFibX1, CallFibX2, "addresult", Scope);

  ReturnInst::Create(Context, Sum, Scope);

  return Scope;
}


static Function *CreateFibFunction(Module *M, LLVMContext &Context) {
  // Create the fib function and insert it into module M.  This function is said
  // to return an int and take an int parameter.
  Function *FibFunction = cast<Function>(M->getOrInsertFunction("fib", Type::getInt32Ty(Context), Type::getInt32Ty(Context), (Type *)0));

  BasicBlock *FunctionScope = BasicBlock::Create(Context, "EntryBlock", FibFunction);

  // Get pointer to the integer argument (and name it 'x') of the fib function...
  Argument *ArgX = FibFunction->arg_begin();
  ArgX->setName("x");

  // If x <= 2 we have the base case, otherwise the recursive case
  Value *CondInst = new ICmpInst(*FunctionScope, ICmpInst::ICMP_SLE, ArgX, CONST_INT32( 2, Context ), "lt.2");
  BranchInst::Create( CreateBaseCase( FibFunction, Context ), CreateRecursiveCase( FibFunction, ArgX, Context ), CondInst, FunctionScope);

  return FibFunction;
}


int main(int argc, char **argv) {
  int n = argc > 1 ? atol(argv[1]) : 24;

  InitializeNativeTarget();
  LLVMContext Context;
  
  // Create some module to put our function into it.
  OwningPtr<Module> M(new Module("test", Context));

  // We are about to create the "fib" function:
  Function *FibFunction = CreateFibFunction(M.get(), Context);

  // Now we going to create JIT
  std::string errStr;
  ExecutionEngine *EE =
    EngineBuilder(M.get())
    .setErrorStr(&errStr)
    .setEngineKind(EngineKind::JIT)
    .create();

  if (!EE) {
    errs() << argv[0] << ": Failed to construct ExecutionEngine: " << errStr
           << "\n";
    return 1;
  }

  errs() << "verifying... ";
  if (verifyModule(*M)) {
    errs() << argv[0] << ": Error constructing function!\n";
    return 1;
  }

  errs() << "OK\n";
  errs() << "We just constructed this LLVM module:\n\n---------\n" << *M;
  errs() << "---------\nstarting fibonacci(" << n << ") with JIT...\n";

  // Call the Fibonacci function with argument n:
  std::vector<GenericValue> Args(1);
  Args[0].IntVal = APInt(32, n);
  GenericValue GV = EE->runFunction(FibFunction, Args);

  // import result of execution
  outs() << "Result: " << GV.IntVal << "\n";
  
  return 0;
}