Re: [PATCH] x86: Disable XCHG to MOV optimization

On Mon, Jul 13, 2026 at 11:20 PM Jan Beulich <[email protected]> wrote:
>
> On 13.07.2026 17:10, H.J. Lu wrote:
> > On Mon, Jul 13, 2026 at 11:03 PM Jan Beulich <[email protected]> wrote:
> >>
> >> On 13.07.2026 14:08, H.J. Lu wrote:
> >>> I am going to check this patch into master as well as 2.47 branch.
> >>> I added optimize_for_unsafe, which is 0, and moved XCHG to MOV
> >>> optimization under it.  We can add something like -Ounsafe later.
> >>
> >> But this is wrong, the optimization itself isn't unsafe. Please can we
> >
> > You can change it to a different name.  But -O on master must work with
> > today's valgrind.
>
> That's your position. I continue to fail to see why -O needs to work on
> anything (valgrind or not) that depends on getting to see specific
> encodings for certain insns. Such uses of -O are simply wrong. Undoing

You can add a different option.  -O should work for all applications today.

> the change on the branch is, as previously indicated, merely to give them
> some time to adjust their machinery.
>
> Jan



-- 
H.J.