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

Hi Jan,

On Tue, Jul 07, 2026 at 04:35:27PM +0200, Jan Beulich wrote:
> On 07.07.2026 16:21, Sam James wrote:
> > "Assembler optimization is intended for hand-written code, not to be
> > used on compiler output." which then gives us licence to dispose with
> > bugs reports like this, as opposed to the ambiguity right now.
> 
> I fear I wouldn't be happy with making such a statement in doc. For one,
> "intended" is weak enough that people may still think the options are
> worthwhile to use on compiler output. Plus there's the issue with inline
> assembly, which imo can plausibly be subject to optimization. Yet at
> this time we have no way to have optimization "kick in" only on those
> portions.
> 
> I could perhaps live with a yet weaker version of what you suggest:
> 
> "Assembler optimization is intended primarily for hand-written code.  If
>  you notice any effects on compiler generated code, please consider
>  raising a bug against the compiler.  Note, however, that this does not
>  extend to there possibly being effects on code originating from inline
>  assembly."

I am not sure I understand why you want inline assembly excluded. Yes,
inline assembly is "hand-written", but only because there was no exact
way to make the compiler produce a specific sequence or encoding of
some instructions. How about something like:

"Assembler optimization is intended primarily for hand-written
assembly files, not for use with compiler generated code (or inline
assembly crafted explicitly to produce a specific instruction sequence
or encoding).  If you notice any effects on compiler generated code,
please consider raising a bug against the compiler about a missed
optimization.  Note, however, that this does not extend to there
possibly being effects on code originating from hand-written assembly
code embedded as a whole into a higher-level language compiler."

Cheers,

Mark