c – 如何获取LLVM全局变量常量值？

概述我试图从全局变量中获取浮点值并将其设置为指令的 *** 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, % 我试图从全局变量中获取浮点值并将其设置为指令的 *** 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 valuefloat 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 4call voID @llvm.dbg.declare(Metadata float* %b,Metadata !14,Metadata !15),... !dbg !16store float 0x40019999A0000000,float* %b,align 4,!dbg !16call voID @llvm.dbg.declare(Metadata float* %c,Metadata !17,... !dbg !18store float 0x401147AE20000000,float* %c,!dbg !18call voID @llvm.dbg.declare(Metadata float* %d,Metadata !19,... !dbg !20%0 = load float,!dbg !21%mul = fmul float 0x3FF6666660000000,%0,!dbg !22store float %mul,float* %d,!dbg !20%1 = load float,!dbg !23ret 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..

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