How to verify if LLVM "ret" instruction returns void?

Question

I have the following snippet of code:

static LLVMContext TempContext;
Type * RetTy = Type::getVoidTy(TempContext)
for (Instruction *I : ListOfInstructions) {
  if (isa<ReturnInst>(I)) {
    RetTy = I->getOperand(0)->getType();
    break
  }
}

Where I am trying to capture the RetTy of a instruction, void or not, so I can use it on

getOrInsertFunction("TempF", FunctionType::get(RetTy, ArgsTys,false));

This code works as long as the ret instruction is not a ret void.

I tried to add a second if to check the void case, but this does not seem to work, and execution stops on the in the FunctionType::get(...) function, printing a back trace.

for (Instruction *I : ListOfInstructions) {
  if (isa<ReturnInst>(I)) {
    if ( I->getOperand(0)->getType() != Type::getVoidTy(TempContext)) {
      RetTy = I->getOperand(0)->getType();
      break
    }
  }
}

Note that removing the for loop all together works and execution continues, as the function FunctionType::get(...) handles the initialized Type * RetTy = Type::getVoidTy(TempContext) "void" value for RetTy just fine. But then I can't capture when a llvm function returns non-void values.

How do I know when an instruction I is a return instruction and it is returning Void in LLVM IR?

Brian · Accepted Answer · 2017-09-30 00:54:26Z

5

The problem with your current code is that ret void does not have an operand, so calling getOperand(0) accesses invalid data.

Replacing your if with:

if (ReturnInst *ri = dyn_cast<ReturnInst>(I))
{
  if (ri->getNumOperands() == 0)
  {
    errs() << "VOID: " << *ri << "\n";
  }
  else
  {
    errs() << "NON-VOID: " << *ri << "\n";
  }
}

Now, this code will output VOID: ret void, having correctly detected the instruction.

As an alternative, you could retrieve the return type for the function using any instruction by relying on instructions being contained by a function I->getFunction()->getReturnType(); however, this would assume that the function is well-formed and its ReturnInst matches its type and the instruction is part of a function.

answered Sep 30, 2017 at 0:54

Brian

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1 Comment

nico Over a year ago

I did not see this while reading the docs. Now, that you highlighted it, I see that IRBuilder calls ReturnInst::Create method that initializes a TerminatorInst (in InstrTypes.h) with NumOps argument set to 0. Thank you for the answer.

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