[JonChesterfield]

It should be no-builtins for disabling the pattern match in clang and things like no-builtin-memset to disable them individually. However that'll be implemented with branches in the backend codegen and it's totally plausible that'll be buggy.

That at least somewhat works because otherwise the memset implementation in libc is prone to being optimised to a call to memset, and thus to a self call which doesn't terminate, and thus to undef.

I've been bitten by this nonsense on two back ends now, really should move it up the work queue. I think there's an active discussion on discourse about making magic libc functions less hazardous.

Can't comment on gcc. Bare metal broadly works on GPUs on clang. X86 is moderately likely to emit calls to memcpy despite flags, but that would be a bug.

The Linux kernel is tested rather more aggressively than my code but my fear of compiler bugs remains firmly established. Occupational hazard of working on compilers - most bugs I see are in the toolchain, as that's where I'm looking.