Modifying ConstantExpr

IR & Optimizations
1

Hi All,
I have the below statement stored as const_expr of type ConstantExpr.
Statement : i8 getelementptr inbounds ([3 x i8], [3 x i8]* @.str, i64 0, i64 0)

I am looking to change [3 x i8] to say [5 x i8].

If I do the below, I am able to get [3 x i8], but I am not able to modify. Should I try differently? Any inputs please?
GEPOperator *gep = cast(const_expr)
*(gep->getSourceElementType()) //This prints it properly

Thanks!

2

Hi!

First of all, please be aware that LLVM is now transitioning to opaque pointer types for real. Very soon, you won’t have [3 x i8]* types anymore, it’ll all just be ptr types.

That said: the common approach to doing this sort of thing is to build new instructions (or constantexprs – but more likely instructions that are automatically constant-folded, depending on your context). That is, you’d first create a bitcast of the GEP operand @str to the desired type and then create a new GEP. Then you’d typically do replaceAllUsesWith (RAUW).

In your case, the GEP itself is a constant, so the details of the RAUW may differ. It depends on the larger context of what you’re trying to do.

Thanks,
Nicolai

3

Thank you. Your suggestion seems to work. Just to share, the below is what I did.

Constant *GEPIndices[2] = {
ConstantInt::get(Type::getInt64Ty(F.getContext()), 0),
ConstantInt::get(Type::getInt64Ty(F.getContext()), 0)};
Constant *GEP = ConstantExpr::getGetElementPtr(s1Ty, g1, GEPIndices);
i->setOperand(0,GEP);