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

Jan Beulich <jbeulich-IBi9RG/[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.

>
> 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

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