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

[Jan Beulich]

On 13.07.2026 11:28, Mark Wielaard wrote:
> Hi Jan,
> 
> On Mon, Jul 13, 2026 at 08:09:23AM +0200, Jan Beulich wrote:
>> On 12.07.2026 17:37, Mark Wielaard wrote:
>>> 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."
>>
>> Well. You've seen H.J.'s reply, haven't you? His take is that -O very much
>> is for compiler generated code. As just indicated on another thread, what
>> is needed here first is an agreed upon understanding what we want to
>> optimize (for). Intentions originally weren't made sufficiently clear, and
>> hence views looks to have heavily diverged.
> 
> OK, but lets document that then so people don't accidentially use -O
> without knowing that the option doesn't have clear semantics. How
> about simply:
> 
> "Assembler optimization is currently an experimental feature that
> might result in changing instruction encoding, replacement of
> instructions by equivalent ones or removal of instructions that could
> be seen as nops. Don't use it when you have code that depends on the
> exact assembler instruction encodings."

I could live with this (yet I'd still like to understand what the criteria
would be to drop this experimental status). H.J. - I fear you may not like
it in this shape?

Jan