c++ - 如何获取LLVM全局变量常量值？

Question

我正在尝试从全局变量中获取浮点值并将其设置为指令的操作数。

这是我想做的:

@a = private constant float 0x3FB99999A0000000

...
%1 = load float, float* @a ---> removed

%3 = fmul fast %1, %2  ---> %3 = fmul fast float 0x3FB99999A0000000, %2

下面是我到目前为止尝试过的:

for (auto gv_iter = llvm_module.global_begin();gv_iter != llvm_module.global_end(); gv_iter++){ 

    llvm::GlobalVariable* gv = &*gv_iter;

    for(auto user_of_gv : gv->users()){

        llvm::Instruction *instr_ld_gv = llvm::dyn_cast<llvm::Instruction>(user_of_gv);
        llvm::Value *val_gv = llvm::cast<llvm::Value>(instr_ld_gv);

        llvm::Constant *const_gv = gv->getInitializer();    
        llvm::ConstantFP *constfp_gv = llvm::dyn_cast<llvm::ConstantFP>(const_gv);
        float gv_fpval = (constfp_gv->getValueAPF()).convertToFloat(); 

        llvm::Constant *const_gv_opd = llvm::ConstantFP::get(llvm::Type::getFloatTy(llvm_context),gv_fpval); 

        for(auto user_of_load : val_gv->users()){ 

            llvm::Instruction *instr_exe_gv = llvm::dyn_cast<llvm::Instruction>(user_of_load);

            //P
            for(int operand_num = 0;operand_num < instr_exe_gv->getNumOperands();operand_num++){ 

                llvm::Value *val_instr_op =  instr_exe_gv->getOperand(operand_num);

                if(val_instr_op == val_gv){

                    instr_exe_gv->setOperand(operand_num,const_gv_opd);
                    instr_ld_gv->removeFromParent();

                }            
            }
        }
    }
}

但是，当我尝试运行我的代码时，它会导致段错误。

我确定我已经访问了我想要的全局变量和指令通过打印

的值

gv_fpval 是 0.1 因为 0x3FB99999A0000000 等于 0.10000000149011612 的双数

精度。程序似乎在 setOperand() 处崩溃。

Answer 1

考虑下面的例子

你好.cpp

 #include <stdio.h>
// Global Constant value
float a=1.4f;

float Multiply(){
    float b=2.2f;
    float c=4.32f;
    float d= a*c;
    return d;
}

int main(int argc, char const *argv[])
{

    printf("%f\n",Multiply());

    return 0;
}

模块 pass 将循环查找浮点全局变量，程序中的任何使用都将替换为常量 FP 值。 LLVM pass 如下所示 ConstantReplacementPass.cpp:-

#include "llvm/Pass.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/IR/LegacyPassManager.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/Transforms/IPO/PassManagerBuilder.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/DebugInfo/DWARF/DWARFDebugLine.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/DebugInfo.h"
using namespace llvm;

/* StackOverflow : https://stackoverflow.com/questions/48212351/how-to-get-llvm-global-variable-constant-value* /
/**Bernard Nongpoh */

namespace {
    class ConstantReplacementPass : public ModulePass {


    public:
        static char ID;


        ConstantReplacementPass() : ModulePass(ID) {
            srand (time(NULL));
        }

        virtual bool runOnModule(Module &M) {

            // list to collect instruction
            /*
             * You cannot change an iterator while iterating over it
            • To remove instructions or modify, first collect the instructions to remove/modify
            •
             *
             * **/
            // This are the list of load to delete
            SmallVector<Instruction*,128> *WorkListLoad=new SmallVector<Instruction*,128>();
            // This is the list of instruction to modify the source operand
            SmallVector<Instruction*,128> *WorkListUserOfLoad=new SmallVector<Instruction*,128>();


            for (auto gv_iter = M.global_begin();gv_iter != M.global_end(); gv_iter++) {
                   /* GLOBAL DATA INFO*/
                    GlobalVariable *gv = &*gv_iter;
                    Constant *const_gv = gv->getInitializer();
                    ConstantFP *Fvalue;
                    if(!const_gv->isNullValue()) {

                        if (ConstantFP *constfp_gv = llvm::dyn_cast<llvm::ConstantFP>(const_gv)) {
                            float gv_fpval = (constfp_gv->getValueAPF()).convertToFloat();
                            Fvalue = constfp_gv;
                            errs() << gv_fpval; // Value retrieved here
                            // Collect Instruction to modify


                        }

                        for (auto user_of_gv: gv->users()) {
                            // Collect in a worklist
                            if (llvm::Instruction *instr_ld_gv = llvm::dyn_cast<Instruction>(user_of_gv)) {

                                if (LoadInst *loadInst = dyn_cast<LoadInst>(instr_ld_gv)) {

                                    WorkListLoad->push_back(loadInst);
                                    for (auto user_of_load:loadInst->users()) {
                                        user_of_load->dump();
                                        Instruction *instruction1 = dyn_cast<Instruction>(user_of_load);
                                        instruction1->dump();
                                        //instruction1->setOperand(0, Fvalue);
                                        //instruction1->dump();
                                        // if(Instruction *instruction1 = dyn_cast<Instruction>(user_of_load))
                                        WorkListUserOfLoad->push_back(instruction1);
                                        //instruction1->setOperand(0, Fvalue);
                                        //instruction1->dump();
                                    }

                                }
                            }
                        }


                    // Modify Here
                        while (!WorkListUserOfLoad->empty()) {
                            Instruction *instruction = WorkListUserOfLoad->pop_back_val();
                            instruction->setOperand(0, Fvalue);
                            instruction->dump();
                        }

                        // Removing all loads that are used by the global variable
                        while (!WorkListLoad->empty()) {
                            Instruction *instruction = WorkListLoad->pop_back_val();
                            instruction->eraseFromParent();
                        }



                    }
                }








             return true;
    }
  };
}

char ConstantReplacementPass::ID = 0;


static RegisterPass<ConstantReplacementPass> F0("constantREP", "Constant Replacement Pass "
                                         , false,true);

要点:-

1. 在对指令进行任何修改之前。在工作列表中首先收集。

2. 对工作列表进行修改。

3. 您不能在使用迭代器时进行修改。

我在上面的源码hello.cpp上测试成功，pass后对应的IR如下:-

entry:
%b = alloca float, align 4
%c = alloca float, align 4
%d = alloca float, align 4
call void @llvm.dbg.declare(metadata float* %b, metadata !14, metadata !15),
... !dbg !16
store float 0x40019999A0000000, float* %b, align 4, !dbg !16
call void @llvm.dbg.declare(metadata float* %c, metadata !17, metadata !15),
... !dbg !18
store float 0x401147AE20000000, float* %c, align 4, !dbg !18
call void @llvm.dbg.declare(metadata float* %d, metadata !19, metadata !15),
... !dbg !20
%0 = load float, float* %c, align 4, !dbg !21
%mul = fmul float 0x3FF6666660000000, %0, !dbg !22
store float %mul, float* %d, align 4, !dbg !20
%1 = load float, float* %d, align 4, !dbg !23
ret float %1, !dbg !24

也许使用 -O3 优化标志会清除一切...

希望这有帮助..