CODEWITHSUNDEEP
llvmllvm-irllvm-c++-api
Jay
Jay

Reputation: 394

How to obtain a pointer(?) to a BitCast instruction?

let's assume I have a simple IR that looks like this:

%1 = alloca i8*, align 8
%2 = bitcast i8* %1 to i8*
%3 = load i8*, i8** %1, align 8

So I am trying to replace the use of %1 in %3 = load i8*, %1, align 8 with the instruction %2 = bitcast i8* %1 to i8*. I understand how to replace using the replaceUsesWithIf API, but the problem is that after using that API, I end up with the following

%3 = load i8*, i8* %2, align 8

This is incorrect because the type that I am loading from should be i8** rather than i8*.

Therefore, I think I need to obtain a pointer to a BitCast instruction. I believe one way could be to create a new BitCast instruction of %2 = bitcast i8* %1 to i8* instruction with i8** type?

Here is my attempt at doing so:

for (auto &Inst : BB) {
        if (auto bitcast_I = dyn_cast<BitCastInst>(&Inst)) {
           /* %2 = bitcast i8* %1 to i8* will be matched */
            
           Value *bitcast_v = bitcastI->getOperand(0);
           BitCastInst *newBitcast = new BitCastInst(bitcast_v, 
                       PointerType::get(IntegerType::get(context, 8), 0), "newBit", bitcast_I);
        }

Unfortunately, what I attempted above doesn't yield the correct code, but at the same time, I'm not even too sure whether this is right, so I wanted to ask whether this approach makes sense.

Thank you in advance,

Upvotes: 0

Views: 486

Answers (1)

Nick Lewycky
Nick Lewycky

Reputation: 1342

Your values have different types, %1 being i8** and %2 being i8* so you cannot replace one with the other. If you want to make your load use the bitcast in %2, then you will need to delete the load and build a new LoadInst.

Upvotes: 1

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