Re: [PATCH v2] x86: prevent gcc from emitting rep movsq/stosq for inlined ops

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 05, 2025 at 06:47:33PM +0200, Mateusz Guzik wrote:
> gcc is over eager to use rep movsq/stosq (starts above 40 bytes), which
> comes with a significant penalty on CPUs without the respective fast
> short ops bits (FSRM/FSRS).

I don't suppose there's a magic compiler toggle to make it emit prefix
padded 'rep movs'/'rep stos' variants such that they are 5 bytes each,
right?

Something like:

   2e 2e 2e f3 a4          cs cs rep movsb %ds:(%rsi),%es:(%rdi)

because if we can get the compilers to do this; then I can get objtool
to collect all these locations and then we can runtime patch them to be:

   call rep_movs_alternative / rep_stos_alternative

or whatever other crap we want really.