GCC 12 miscompilation of volatile asm (was: Re: [PATCH] arm64/io: Remind compiler that there is a memory side effect)

[Andrew Cooper]

On 05/04/2022 14:04, Mark Rutland wrote:
> On Tue, Apr 05, 2022 at 01:51:30PM +0100, Mark Rutland wrote:
> My x86_64 test case is:
>
> Per compiler explorer (https://godbolt.org/z/cveff9hq5) GCC trunk currently
> compiles this as:
>
> | msr_rmw_set_bits:
> |         mov     rcx, rdi
> |         rdmsr
> |         sal     rdx, 32
> |         mov     eax, eax
> |         or      rax, rsi
> |         or      rax, rdx
> |         mov     rdx, rax
> |         shr     rdx, 32
> |         wrmsr
> |         ret
> | func_with_msr_side_effects:
> |         ret
>

Yeah... that code gen is very broken for func_with_msr_side_effects().

~Andrew