LLVM - 生成局部变量

接上面一篇：https://wonderful.blog.csdn.net/article/details/106902005

上面一篇实现了如下函数，但是 都是对入参a的操作

 double myfor(double a)
 {
     for(i = 0; i < a; i++) {
       a = a + 1
     }
     return a
 }

本例中，我们希望实现是局部变量的加减，然后return，如下

 double myvar(double a)
 {
     double b = 0;
     for(i = 0; i < a; i++) {
       b = a + 1
     }
     return b
 }

运行本例源码后，它生成的llvm ir如下：

define fastcc double @myvar(double %a) {
myentry:
  %cmptmp = fcmp ult double %a, 1.000000e+01
  br i1 %cmptmp, label %then, label %else

then:                                             ; preds = %myentry
  br label %end

else:                                             ; preds = %myentry
  br label %end

end:                                              ; preds = %else, %then
  %la.0 = phi double [ 7.000000e+00, %then ], [ 9.000000e+00, %else ]
  ret double %la.0
}

1、创建函数
2、使用 CreateEntryBlockAlloca(TheFunction, "la");创建名为"la"的变量
3、使用
Value *StartVal = ConstantFP::get(TheContext, APFloat(0.0)); Builder.CreateStore(innerargs[0], Alloca);
初始化局部变量为0

源码：

#include "./llvm-8.0.1.src/examples/Kaleidoscope/include/KaleidoscopeJIT.h"
#include "llvm/ADT/APFloat.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/LegacyPassManager.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Verifier.h"
#include "llvm/Support/TargetSelect.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Transforms/InstCombine/InstCombine.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/GVN.h"
#include "llvm/Transforms/Utils.h"
#include <algorithm>
#include <cassert>
#include <cctype>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <map>
#include <memory>
#include <string>
#include <utility>
#include <vector>
#include <iostream>
using namespace std;
using namespace llvm;
using namespace llvm::orc;

//LLVM items
static LLVMContext TheContext;
static IRBuilder<> Builder(TheContext);
static std::unique_ptr<Module> TheModule;
//JIT
static std::unique_ptr<KaleidoscopeJIT> TheJIT;

static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
                                          const std::string &VarName) {
  IRBuilder<> TmpB(&TheFunction->getEntryBlock(),
                 TheFunction->getEntryBlock().begin());
  return TmpB.CreateAlloca(Type::getDoubleTy(TheContext), 0,
                           VarName.c_str());
}

/*
 *double myvar(double a)
 *{
 *    double b = 0;
 *    for(i = 0; i < a; i++) {
 *      b = a + 1
 *    }
 *    return b
 *}
 *
*/
int main()
{
    InitializeNativeTarget();
    InitializeNativeTargetAsmPrinter();

    //init module
    TheModule = llvm::make_unique<Module>("myjit", TheContext);

    //used to be runned by jit later
    TheJIT = llvm::make_unique<KaleidoscopeJIT>();
    TheModule->setDataLayout(TheJIT->getTargetMachine().createDataLayout());
    static std::unique_ptr<legacy::FunctionPassManager> TheFPM;
    TheFPM = llvm::make_unique<legacy::FunctionPassManager>(TheModule.get());
    TheFPM->add(createPromoteMemoryToRegisterPass());

    //define the args
    vector<std::string> ArgNames;

    //mycfor has 1 args
    ArgNames.push_back(string("a"));

    //make the 1 args attach to LLVM Type::double
    std::vector<Type *> Doubles(ArgNames.size(), Type::getDoubleTy(TheContext));

    //generate llvm function type
    FunctionType *FT = FunctionType::get(Type::getDoubleTy(TheContext), Doubles, false);

    //Create function whose FunctionType is FT
    Function *TheFunction = Function::Create(FT, Function::ExternalLinkage, "myvar", TheModule.get());

    //give the name of Function args and save the args as innerargs
    unsigned Idx = 0;

    std::vector<Value *>innerargs;

    for (auto &Arg : TheFunction->args()) {
        Arg.setName(ArgNames[Idx++]);
        innerargs.push_back(&Arg);
    }

    //this function's basic block
    BasicBlock *BB = BasicBlock::Create(TheContext, "myentry", TheFunction);

    BasicBlock *ThenBB = BasicBlock::Create(TheContext, "then");
    BasicBlock *ElseBB = BasicBlock::Create(TheContext, "else");
    BasicBlock *EndBB  = BasicBlock::Create(TheContext, "end");

    Builder.SetInsertPoint(BB);

    //create local var la and inited to 0
    AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, "la");
    Value *StartVal = ConstantFP::get(TheContext, APFloat(0.0));
    Builder.CreateStore(innerargs[0], Alloca);


    //if a < 0 ThenBB else ElseBB
    Value *CondV = Builder.CreateFCmpULT(innerargs[0], ConstantFP::get(TheContext, APFloat(10.0)), "cmptmp");
    Builder.CreateCondBr(CondV, ThenBB, ElseBB);

    TheFunction->getBasicBlockList().push_back(ThenBB);
    Builder.SetInsertPoint(ThenBB);

    Value *NextVar = ConstantFP::get(TheContext, APFloat(7.0));
    Builder.CreateStore(NextVar, Alloca);
    Builder.CreateBr(EndBB);

    TheFunction->getBasicBlockList().push_back(ElseBB);
    Builder.SetInsertPoint(ElseBB);

    NextVar = ConstantFP::get(TheContext, APFloat(9.0));
    Builder.CreateStore(NextVar, Alloca);
    Builder.CreateBr(EndBB);


    TheFunction->getBasicBlockList().push_back(EndBB);
    Builder.SetInsertPoint(EndBB);
    Builder.CreateRet( Builder.CreateLoad(Alloca, "la") );

    TheFunction->setCallingConv(llvm::CallingConv::Fast);
    verifyFunction(*TheFunction);

    //will opt the code into phi even we haven't use it.
    TheFPM->run(*TheFunction);

    TheFunction->print(errs());

    //using jit to run this code
    auto H = TheJIT->addModule(std::move(TheModule));
    auto ExprSymbol = TheJIT->findSymbol("myvar");
    double (*myvar)(double) = (double (*)(double))(intptr_t)cantFail(ExprSymbol.getAddress());
    cout <<myvar(9)<<endl;
}