LLVM Backend - calling MachineBasicBlock::getSymbol in postIselHook

Beginners
1

Hey.

I created a post-isel-hook, that create a new MachineBasicBlock and push it after an instruction.

Here is a snippet from the function (only relevant code):

void MyTargetLowering::AdjustInstrPostInstrSelection(...) {
    // Create a new label
    MachineBasicBlock *NewLabel = MF.CreateMachineBasicBlock();

    // Add it to the machine function - after the call instruction
    MF.insert(std::next(MBBI), NewLabel);

    // Build the new instruction
    MachineInstrBuilder newInst = BuildMI(...);

    // Set the label as the operand for the instruction
    newInst.addMBB(NewLabel);

    newInst->setPostInstrSymbol(MF, NewLabel->getSymbol());
}

Now, I encountered a problem when I try to compile the following code:

extern char getc();

int main() {
	char c;

	while (c = getc()) {
		switch (c) {
		case 'a':
			break;
		case 'b':
			break;
		}
	}

	return 0;
}

When trying to run llc on the generated LLVM code, I got the error:

<unknown>:0: error: symbol 'LBB0_7' is already defined

When I tried to debug the code, I saw that there is a function MachineFunction::RenumberBlocks that handles the MBB numbers.

I also noticed that when I call NewLabel->getSymbol it uses its current Number attribute (which gets override by the renumber function), and so the post-instr-symbol seems to be incorrect.

Can someone please explain to me what am I doing wrong?

[Notice: I'm using LLVM v15]

If it's a known bug I would love to know about it.