Re: [PATCH v2] x86/lib: Use REP MOVSB for large memcpy() on ERMS CPUs without FSRM

[H. Peter Anvin]

On November 10, 2026 9:53:07 AM PST, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>On Wed, 4 Nov 2026 at 14:28, Usama Arif <usama.arif@xxxxxxxxx> wrote:
>>
>> +       ALTERNATIVE __stringify(cmpq $0x20, %rcx; jb .Lhandle_tail), \
>> +                   __stringify(rep movsb; RET), X86_FEATURE_FSRM
>
>Let's not do that.
>
>Putting magic things like RET in alternatives is just a bad bad idea.
>
>               Linus

Why is that, *given that it is in assembly*?

For C (or Rust, or C++, ...) it is obviously bad chicken.