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

[Jan Beulich]

On 03.07.2026 13:39, H.J. Lu wrote:
> 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.

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.)

Jan