Question on getObjectSize function for pointer Types

Beginners
1

I want to find the size in bytes of a memory location pointed to by a pointer.
I have found a function that does it from MemoryBuiltins.h

bool getObjectSize(const Value *Ptr, uint64_t &Size, const DataLayout &DL,
                   const TargetLibraryInfo *TLI, ObjectSizeOpts Opts = {});

and I used the DeadStoreElimination(DSE) transformation pass (in function getPointerSize) as an example of how to use it. However, when I pass the same IR in my pass, it always returns zero, while DSE returns the actual size. Am I doing something wrong?

How I create my TLI obj inside the function pass

  const TargetLibraryInfo &TLI = AM.getResult<TargetLibraryAnalysis>(F);

The parameters of my transformation pass

static bool MyTransformationPass( Function &F, const TargetLibraryInfo &TLI)

Calling the getPointerSize function

            if (Value *V = dyn_cast<Value>(inst)){
                const DataLayout &DL = inst->getModule()->getDataLayout();
                uint64_t ptrSize = getPointerSize(V, DL, TLI, &F);
                errs() << "SIZE:     " << ptrSize << "\n";
            }

The getPointerSize that I copied from the DeadStoreElimination code

uint64_t getPointerSize(const Value *V, const DataLayout &DL,
                               const TargetLibraryInfo &TLI,
                               const Function *F) {
  uint64_t Size;
  ObjectSizeOpts Opts;
  Opts.NullIsUnknownSize = NullPointerIsDefined(F);
  if (getObjectSize(V, Size, DL, &TLI, Opts))
    return Size;
  return 0;
}

Lastly, is there an easier way of doing this, or its this the best way?

2

Hello. The DataLayout knows the pointer size, I think in this case it can be simplified as DL.getPointerSize() or DL.GetPointerSizeInBits()/8.

3

I changed it to this; however, it’s not the same when I compared the size output from my pass and DeadStoreElimination(DSE). Mine shows 8Bytes while DSE shows 4Bytes even tho they measure the same IR.
const DataLayout &DL = inst->getModule()->getDataLayout();

size = DL.getPointerSize();