How to get LLVM global variable constant value?

Consider the following example

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;
}

The module pass will loop for Floating point global variable and any use in the program will be replaced by the constant FP value. The LLVM pass are as follow 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);

Key points:-

1. Before doing any modification on the instruction. Collect first in the worklist.

2. perform the modification on the worklist.

3. you cannot do the modification while using the iterator.

I successfully tested on the above source code hello.cpp the corresponding IR after the pass is as follow:-

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

Maybe using -O3 optimization flag will wipe out everything...

Hope this helps..