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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 06, 2025 at 09:27:07AM +0200, Uros Bizjak wrote:

> Now, to use function call instead of "rep movsb", you can
> use -mstringop-strategy=libcall instead of some objtool substitution
> (-O2 -mno-sse -mstringop-strategy=libcall):
> 
> foo:
>        subq    $8, %rsp
>        movl    $160, %edx
>        leaq    16(%rsp), %rsi
>        call    memcpy
>        addq    $8, %rsp
>        ret
> 
> Which IMO is what you are looking for.

Well, no, because I can't replace the 'call memcpy' with "cs;cs;cs;rep
movsb" on good machines.

We want the "rep movsb" / "rep stosb" interface so we can actually use
those instructions when they're good, but then fall back to an
out-of-line function (with the same calling convention as the
instruction) when they're not so very good.