c – 如何获取LLVM全局变量常量值？

我试图从全局变量中获取浮点值并将其设置为指令的操作数.

这是我想要做的：

@a = private constant float 0x3FB99999A0000000

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

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

以下是我到目前为止所尝试的内容：

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,当我尝试运行代码时,它会导致分段错误.

我确信我已经访问了我想要的全局变量和指令
通过打印的价值

gv_fpval是0.1,因为0x3FB99999A0000000等于0.10000000149011612双

精确.似乎程序在setOperand()崩溃了.

解决方法

请考虑以下示例

HELLO.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;
}

模块传递将循环浮点全局变量,程序中的任何使用都将被常量FP值替换. LLVM传递如下：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);

关键点：-

>在对指令进行任何修改之前.首先收集工作清单.
>在工作清单上执行修改.
>使用迭代器时无法进行修改.

我成功测试了上面的源代码hello.cpp后传递的相应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,... !dbg !18
store float 0x401147AE20000000,float* %c,!dbg !18
call void @llvm.dbg.declare(metadata float* %d,metadata !19,... !dbg !20
%0 = load float,!dbg !21
%mul = fmul float 0x3FF6666660000000,%0,!dbg !22
store float %mul,float* %d,!dbg !20
%1 = load float,!dbg !23
ret float %1,!dbg !24

也许使用-O3优化标志会消灭一切……

希望这可以帮助..