ARM: error in backend: Cannot select: intrinsic %llvm.arm.hint

[AdrianBunk]

Reduced testcase for https://bugs.debian.org/1005871 for the code in https://github.com/google/filament/blob/main/libs/utils/include/utils/compiler.h#L188

$ cat test.c
void a() { __builtin_arm_sev(); }
$ clang -c --target=arm -mcpu=arm946e-s -mfloat-abi=soft test.c
fatal error: error in backend: Cannot select: intrinsic %llvm.arm.hint
...

The bug exists at least with LLVM versions 11-14 from Debian unstable (other versions were not tested).

Note that this is an armv5 CPU where the instruction in question does not exist, using -mcpu=cortex-a9 instead works and emits the instruction.

https://developer.arm.com/documentation/101028/0012/7--Synchronization--barrier--and-hint-intrinsics

The intrinsics in this section are available for all targets. They may be no-ops (i.e. generate no code, but possibly act as a code motion barrier in compilers) on targets where the relevant instructions do not exist.

The frontend is correct in accepting __builtin_arm_sev() despite the instruction not existing, but treating it as a no-op in the backend seems to go wrong or is not implemented.

[llvmbot]

@llvm/issue-subscribers-backend-arm