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

On Mon, Jul 6, 2026 at 2:16 PM Jan Beulich <[email protected]> wrote:
>
> On 03.07.2026 15:32, H.J. Lu wrote:
> > On Fri, Jul 3, 2026 at 8:28 PM Jan Beulich <[email protected]> wrote:
> >>
> >> On 03.07.2026 13:39, H.J. Lu wrote:
> >>> On Fri, Jul 3, 2026 at 6:04 PM Sam James <[email protected]> wrote:
> >>>>
> >>>> Jan Beulich <[email protected]> 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.
> >>
> >> The question isn't about "being safe". -O should be safe on whatever input.
> >> If it's not, it's a bug.
> >>
> >> The question is whether it is plausible to use -O... at all for compiler
> >> generated code. I causes extra overhead in the assembler, after all. If the
> >> compiler did a decent job, all of that extra overhead is going to be in
> >> vein. (As said elsewhere, the situation is different for code coming from
> >> asm() - that's not really compiler generated code.)
> >>
> >
> > From what we have learned so far, "XCHG REG,REG" has been done
> > on purpose and compilers never generate them automatically.   Assembler
> > should leave them alone even with encoding optimization.
>
> No, why? Optimization is specifically for hand-coded assembly, so what a
> compiler emits doesn't matter here. Following this argumentation of yours,
> we should remove all optimization again from gas. People can use any
> particular encoding "on purpose", after all. As said elsewhere, if you're
> after particular encodings, don't engage optimization in the first place.
>

When we are informed that an assembler optimization breaks the
application, we should consider disabling such optimization.


-- 
H.J.


_______________________________________________
Valgrind-developers mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/valgrind-developers