[llvm-dev] Finding nearest store to a variable

[Jajoo, Malhar via llvm-dev]

Hey guys ,


I have question : How to find nearest store to a alloca variable in LLVM ?


I tried to write a LLVM pass for the following C++ program -


#include <stdio.h>
int main()
{
int a ;
int datasize;

datasize = 5;
datasize=10; // most recent definition for the (useless)  "datasize=datasize" below.

datasize = datasize;

// Just for verification of result. should be 10.
printf("value=%d",a+datasize);
return a+datasize;

}


//==================


I wrote the following LLVM Function Pass for it ( Have not included unecessary details )




for(Function::iterator BB = F.begin() ; BB != F.end() ; ++BB)
{
for(BasicBlock::iterator I = BB->begin(); I != BB->end() ; ++I)
{
if(StoreInst* st = dyn_cast<StoreInst>(I))
{
User* user1 = st;
std::string str = user1 ->getOperand(1)->getName();

// it is a store to datasize
if(str=="datasize)
{
Use& use1 = user1->getOperandUse(0) ;
// trying to find nearest assignment/store for this.
Value* v = use1 .get();
errs() << *v << "\n" ;
// this means that it was datasize = datasize
                                      // since it is translated to the following in IR -
                                     // %0 = load i32, i32* %datasize, align 4
                                    // store i32 %0, i32* %datasize, align 4
if(LoadInst* loadInst = dyn_cast<LoadInst>(v))
{
User* user2= loadInst ;
Use& use2= user21->getOperandUse(0);

// find nearest defiinion/store
Value* v2 = use2.get();
errs() << "found nearest definiton" << *v2 << "\n" ;
}
}
}
}

}


//===========================================

The result I get -


i32 5 ----------------> ignore,not useful
i32 10 ------------------> ignore,not useful
  %0 = load i32, i32* %datasize, align 4
found nearest  %datasize = alloca i32, align 4

My Issue :  The nearest "def" found for the usage of datasize=datasize is an
alloca instruction while I wanted to find the IR -
store i32 10, i32* %datasize, align 4
(In C++ - "datasize=10" instruction. as seen below).


// ... c++ code

datasize = 5;
datasize=10; // most recent definition for the (useless)  "datasize=datasize" below.

datasize = datasize;

// ... C++ code


Thank you for keeping patience and reading uptil here.
My Question : Can anyone explain how I can find the nearest store to a variable  in LLVM IR ?




Thanks,

Malhar
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