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

[Mark Wielaard]

Hi H.J.

On Tue, Jul 14, 2026 at 07:10:19PM +0800, H.J. Lu wrote:
> On Tue, Jul 14, 2026 at 6:33 PM Mark Wielaard <mark@klomp.org> wrote:
> > On Mon, Jul 13, 2026 at 06:05:56PM +0800, H.J. Lu wrote:
> > > -On/-Os I added to assembler which should be 100% safe.  If you can't
> > > or won't make it 100% safe, you can add another option to perform unsafe
> > > optimization.
> >
> > It looks like you need a clear definition of "safe" and "unsafe". And
> > there are probably multiple different ones depending on context.
> > e.g. you could provide an option that makes it "safe" for the user to
> > rely on the instruction length. Or a guarantee that the code will
> > always use the shortest encoding. Another for not trying to optimize
> > away "redundent" encoding prefixes. Or removing "nop" sequences. Or
> > one that explicitly changes any nop sequences to the most efficient
> > one (for a particular length), etc.
> >
> > Just make sure you agree on what "safe" and "unsafe" mean for
> > particular optimization levels/switches. And document them so the user
> > knows what they can expect when enabling the "optimization".
> >
> By "safe", I mean an optimization won't break applications.   The
> most x86 assembler optimizations are for shorter encodings.   They
> shouldn't make an application stop working as expected.

That is not a very good definition because it doesn't specify what
kind of transformations are done. Every transformation might "break" a
specific application or stop it they don't expect a particular
"optimization". You have to be explicit about the kind of
transformation that is done by a particular optimization so the user
can expect them.

What you could do for example is limit optimizations to specific
instruction sets. If I understand your original motivation it was to
use more efficient/shorter sse encodings. Why not put those under a
specific -Osse option for example? Then it is immediately clear why to
use that particular optimization without mixing it with any others.

Cheers,

Mark