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 08:27:39AM +0200, Uros Bizjak wrote:
> On Fri, Jun 6, 2025 at 8:13 AM Uros Bizjak <ubizjak@gmail.com> wrote:
> >
> > On Thu, Jun 5, 2025 at 9:00 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > 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)
> >
> > This won't fly, because gas complains:
> >
> > z.s: Assembler messages:
> > z.s:1: Error: same type of prefix used twice
> 
> However, it is possible to use " cs ; cs ; cs ; rep movsb". 

Heh, the way I encoded it was:

	.byte 0x2e, 0x2e, 0x2e
	rep movsb %ds:(%rsi), %es:(%rdi)

GCC compiled it, and then objdump grokked it (although it outputs one CS
too few). Your variant is much nicer though.

> We can add a compile flag to the compiler, and it will be able to emit
> the desired sequence.

Thanks; Linus, this would be acceptable?