Fibonacci计算是一个非常经典的案例，下面用Fibonacci的两种写法 对比普通C函数和LLVM IR的编写区别。
普通C函数写法
代码语言：javascript复制#include <algorithm>
#include <cstdlib>
#include <memory>
#include <string>
#include <vector>
#include <iostream>
using namespace std;
int fib(int x)
{
if (x <= 2)
return 1;
return fib(x - 1) fib(x - 2);
}
int main(int argc, char **argv)
{
int n = argc > 1 ? atol(argv[1]) : 24;
cout << "Result: " << fib(n) << "n";
}LLVM IR写法
代码语言：javascript复制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.
FunctionType *FibFTy = FunctionType::get(Type::getInt32Ty(Context),
{Type::getInt32Ty(Context)}, false);
Function *FibF =
Function::Create(FibFTy, Function::ExternalLinkage, "fib", M);
// Add a basic block to the function.
BasicBlock *BB = BasicBlock::Create(Context, "EntryBlock", FibF);
// Get pointers to the constants.
Value *One = ConstantInt::get(Type::getInt32Ty(Context), 1);
Value *Two = ConstantInt::get(Type::getInt32Ty(Context), 2);
// Get pointer to the integer argument of the add1 function...
Argument *ArgX = &*FibF->arg_begin(); // Get the arg.
ArgX->setName("AnArg"); // Give it a nice symbolic name for fun.
// Create the true_block.
BasicBlock *RetBB = BasicBlock::Create(Context, "return", FibF);
// Create an exit block.
BasicBlock *RecurseBB = BasicBlock::Create(Context, "recurse", FibF);
// Create the "if (arg <= 2) goto exitbb"
Value *CondInst = new ICmpInst(*BB, ICmpInst::ICMP_SLE, ArgX, Two, "cond");
BranchInst::Create(RetBB, RecurseBB, CondInst, BB);
// Create: ret int 1
ReturnInst::Create(Context, One, RetBB);
// create fib(x-1)
Value *Sub = BinaryOperator::CreateSub(ArgX, One, "arg", RecurseBB);
CallInst *CallFibX1 = CallInst::Create(FibF, Sub, "fibx1", RecurseBB);
CallFibX1->setTailCall();
// create fib(x-2)
Sub = BinaryOperator::CreateSub(ArgX, Two, "arg", RecurseBB);
CallInst *CallFibX2 = CallInst::Create(FibF, Sub, "fibx2", RecurseBB);
CallFibX2->setTailCall();
// fib(x-1) fib(x-2)
Value *Sum = BinaryOperator::CreateAdd(CallFibX1, CallFibX2,
"addresult", RecurseBB);
// Create the return instruction and add it to the basic block
ReturnInst::Create(Context, Sum, RecurseBB);
return FibF;
}对比
完整用例
t1.cpp
代码语言：javascript复制/*
* clang t1.cpp `llvm-config --cxxflags --ldflags --system-libs --libs all` -o t1
*
*
*
*/
//===--- examples/Fibonacci/fibonacci.cpp - An example use of the JIT -----===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// 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/ADT/APInt.h"
#include "llvm/IR/Verifier.h"
#include "llvm/ExecutionEngine/ExecutionEngine.h"
#include "llvm/ExecutionEngine/GenericValue.h"
#include "llvm/ExecutionEngine/MCJIT.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/TargetSelect.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cstdlib>
#include <memory>
#include <string>
#include <vector>
using namespace llvm;
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.
FunctionType *FibFTy = FunctionType::get(Type::getInt32Ty(Context),
{Type::getInt32Ty(Context)}, false);
Function *FibF =
Function::Create(FibFTy, Function::ExternalLinkage, "fib", M);
// Add a basic block to the function.
BasicBlock *BB = BasicBlock::Create(Context, "EntryBlock", FibF);
// Get pointers to the constants.
Value *One = ConstantInt::get(Type::getInt32Ty(Context), 1);
Value *Two = ConstantInt::get(Type::getInt32Ty(Context), 2);
// Get pointer to the integer argument of the add1 function...
Argument *ArgX = &*FibF->arg_begin(); // Get the arg.
ArgX->setName("AnArg"); // Give it a nice symbolic name for fun.
// Create the true_block.
BasicBlock *RetBB = BasicBlock::Create(Context, "return", FibF);
// Create an exit block.
BasicBlock *RecurseBB = BasicBlock::Create(Context, "recurse", FibF);
// Create the "if (arg <= 2) goto exitbb"
Value *CondInst = new ICmpInst(*BB, ICmpInst::ICMP_SLE, ArgX, Two, "cond");
BranchInst::Create(RetBB, RecurseBB, CondInst, BB);
// Create: ret int 1
ReturnInst::Create(Context, One, RetBB);
// create fib(x-1)
Value *Sub = BinaryOperator::CreateSub(ArgX, One, "arg", RecurseBB);
CallInst *CallFibX1 = CallInst::Create(FibF, Sub, "fibx1", RecurseBB);
CallFibX1->setTailCall();
// create fib(x-2)
Sub = BinaryOperator::CreateSub(ArgX, Two, "arg", RecurseBB);
CallInst *CallFibX2 = CallInst::Create(FibF, Sub, "fibx2", RecurseBB);
CallFibX2->setTailCall();
// fib(x-1) fib(x-2)
Value *Sum = BinaryOperator::CreateAdd(CallFibX1, CallFibX2,
"addresult", RecurseBB);
// Create the return instruction and add it to the basic block
ReturnInst::Create(Context, Sum, RecurseBB);
return FibF;
}
int main(int argc, char **argv)
{
int n = argc > 1 ? atol(argv[1]) : 24;
InitializeNativeTarget();
InitializeNativeTargetAsmPrinter();
LLVMContext Context;
// Create some module to put our function into it.
std::unique_ptr<Module> Owner(new Module("test", Context));
Module *M = Owner.get();
// We are about to create the "fib" function:
Function *FibF = CreateFibFunction(M, Context);
// Now we going to create JIT
std::string errStr;
ExecutionEngine *EE =
EngineBuilder(std::move(Owner))
.setErrorStr(&errStr)
.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() << "OKn";
errs() << "We just constructed this LLVM module:nn---------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(FibF, Args);
// import result of execution
outs() << "Result: " << GV.IntVal << "n";
return 0;
}t2.cpp
代码语言：javascript复制#include <algorithm>
#include <cstdlib>
#include <memory>
#include <string>
#include <vector>
#include <iostream>
using namespace std;
int fib(int x)
{
if (x <= 2)
return 1;
return fib(x - 1) fib(x - 2);
}
int main(int argc, char **argv)
{
int n = argc > 1 ? atol(argv[1]) : 24;
cout << "Result: " << fib(n) << "n";
}Makefile
代码语言：javascript复制CC1 = clang -stdlib=libstdc
CC2 = g
# CFLAGS = -g -O0
CFLAGS = -O3
LLVM_C_FLAGS = `llvm-config --cxxflags`
LLVM_L_FLAGS = `llvm-config --ldflags --system-libs --libs all`
.PHONY: t1
t1: t1.o
${CC1} $< ${CFLAGS} ${LLVM_L_FLAGS} -o $@
t2: t2.o
${CC2} $< ${CFLAGS} -o $@
clean:
rm -r t1.o t1 t2.o t2
t1.o: t1.cpp
${CC1} ${CFLAGS} ${LLVM_C_FLAGS} -c $< -o $@
t2.o: t2.cpp
${CC2} -c $< ${CFLAGS} -o $@
# wrong: clang t1.o `llvm-config --cxxflags --ldflags --system-libs --libs all` -o t1
# right: clang `llvm-config --cxxflags --ldflags --system-libs --libs all` t1.o -o t1
