[Valgrind-developers] [PATCH 1/2] x86: optimize XCHG to MOV for same-register forms

[H.J. Lu]

On Fri, Jul 3, 2026 at 6:04 PM Sam James <sam@gentoo.org> wrote:
>
> Jan Beulich <jbeulich@suse.com> writes:
>
> > On 03.07.2026 08:09, Paul Floyd wrote:
> >> On 2026-07-03 08:00, Jan Beulich wrote:
> >>> On 03.07.2026 06:55, Paul Floyd wrote:
> >>>> Would it be possible for gas to only do this transformation if the source and destination registers are different?
> >>> When the registers are different, this transformation is invalid to do.
> >>
> >> OK so you are optimising a no-op. Does GCC use it as a no-op?
> >
> > I don't expect so. In fact, my take is that -O... should not be used on
> > compiler generated code. The compiler should do whatever optimizations
> > are possible / sensible, and it should not emit code which can (easily)
> > further be optimized. (Easily because the assembler really only does
> > very simple and pretty obvious transformations.)
>
> Yes, that's reasonable. We should document it though.

-O should be safe for compiler generated codes.

> >
> > I can see that there may be a desire to optimize asm()-generated code.
> > That imo shouldn't be done by passing -Wa,-O... but rather by some
> > other, to be invented machinery. (We could tie it to the #APP / #NO_APP
> > markers that the compiler emits, for example.)
> >
> > Jan



-- 
H.J.