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

[H.J. Lu]

On Tue, Jul 14, 2026 at 6:33 PM Mark Wielaard <mark@klomp.org> wrote:
>
> Hi,
>
> 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.

-- 
H.J.