dyn_cast<Instruction *> returns NULL where it should return a valid instruction

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1

I have the following prototype for a function:
void bkp_memory(char *, int);

Inside my LLVM IR, I have a callsite looks like the following:

tail call void @bkp_memory(i8* bitcast (i32** @P to i8*), i32 4) nounwind

When I try to obtain its 1st argument and check whether it is a valid instruction, using:
Instruction *Inst = dyn_cast<Instruction *>(I->getOperand(0));

it gives me a NULL Ptr, which is quite a surprise to me.

Am I doing anything wrong here?

Note that in many other cases, the bkp_memory() callsite won’t have the embedded form. They look like:

  store i32* %21, i32** @P, align 4
  %22 = bitcast i32* %21 to i8*
  tail call void @bkp_memory(i8* %22, i32 4) nounwind

And the dyn_cast<> conversion is fine in these cases.

Would that be the problem?

Thank you

Chuck

2

I have the following prototype for a function:
void bkp_memory(char *, int);

Inside my LLVM IR, I have a callsite looks like the following:

tail call void @bkp_memory(i8* bitcast (i32** @P to i8*), i32 4) nounwind

When I try to obtain its 1st argument and check whether it is a valid instruction, using:
Instruction *Inst = dyn_cast<Instruction *>(I->getOperand(0));

it gives me a NULL Ptr, which is quite a surprise to me.

Am I doing anything wrong here?

The bitcast you see above is not an instruction. Rather, it is a constant expression and is represented by a ConstExpr object in the C++ API. That is most likely because @P is a constant (a function pointer, global value, or something similar).

So,

  1. Use the stripPointerCasts() method of llvm::Value to strip away all pointer casts (whether they are cast instructions or ConstExpr casts):

I->getOperand(0)->stripPointerCasts()

  1. Realize that not all operands are instructions. In this case, the operand is probably a global variable. Chances are good that:

dyn_cast(I->getOperand(0)->stripPointerCasts())

… will return NULL because the operand is a constant and not an instruction.

Note that in many other cases, the bkp_memory() callsite won’t have the embedded form. They look like:

  store i32* %21, i32** @P, align 4
  %22 = bitcast i32* %21 to i8*
  tail call void @bkp_memory(i8* %22, i32 4) nounwind

And the dyn_cast<> conversion is fine in these cases.

Would that be the problem?

The “embedded” form is a constant expression. The “unembedded” form is an instruction. LLVM has several constant expressions that are modeled after instructions; the difference is that a constant expression’s operands are all constants, and therefore the constant expression can also be used like a constant.

For more details, please see the LLVM Language Reference manual.

– John T.

3

John,

Thank you for the quick response, that is precisely the problem.
It is a global variable indeed, and the embedded form is not an instruction, but an cast expression :slight_smile:

Based on your suggestion, I worked out a quick fix for the problem.

Thank you very much

Chuck