[Arm] Crashed when I compile this code

[Zhenhang1213]

Crashed when I compile this code， I find ARMTargetLowering::getRegisterByName doesn't support Specified Register, should we fix this bug?

llvm-project/llvm/lib/Target/ARM/ARMISelLowering.cpp

Lines 6176 to 6185 in b90ea5c

	Register ARMTargetLowering::getRegisterByName(const char* RegName, LLT VT,
	const MachineFunction &MF) const {
	Register Reg = StringSwitch<unsigned>(RegName)
	.Case("sp", ARM::SP)
	.Default(0);
	if (Reg)
	return Reg;
	report_fatal_error(Twine("Invalid register name \""
	+ StringRef(RegName) + "\"."));
	}

code:https://godbolt.org/z/bbh95vsEW

[Zhenhang1213]

I find a patch may be related to this problem.
link: https://reviews.llvm.org/D68862, and it was reverted

[Zhenhang1213]

@davemgreen do you have some ideas?

[llvmbot]

@llvm/issue-subscribers-backend-arm

Author: Austin (Zhenhang1213)

Crashed when I compile this code， I find ARMTargetLowering::getRegisterByName doesn't support Specified Register, should we fix this bug? https://github.com/llvm/llvm-project/blob/b90ea5caade7b92796276937467a0dabc355a62e/llvm/lib/Target/ARM/ARMISelLowering.cpp#L6176-L6185

code:https://godbolt.org/z/bbh95vsEW

[davemgreen]

I believe it is not supported, but I don't remember the details of why. Maybe @efriedma-quic can remember more info? I think some of the registers (for example frame pointers) are difficult to use correctly and can change on different targets.

[efriedma-quic]

We don't support global register variables for registers which are allocatable (i.e. regular registers that are caller-save or callee-save). It's a hard feature to implement due to the weird ABI implications. Specifically, gcc reserves the register for some uses, but not all uses, so there are inconsistencies, and it's not clear which inconsistencies are intentional.

Some LLVM targets support "-ffixed-*" to turn allocatable registers into unallocatable registers, which allows them to be used in a global register variable. I don't think we currently have any flags like that for 32-bit Arm.

Currently, we print fatal error: error in backend: Invalid register name "r8". We can probably improve that to something more useful.