[PATCH] x86: Disable XCHG to MOV optimization

[H.J. Lu]

On Mon, Jul 13, 2026 at 11:42 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 13.07.2026 17:31, H.J. Lu wrote:
> > On Mon, Jul 13, 2026 at 11:20 PM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 13.07.2026 17:10, H.J. Lu wrote:
> >>> On Mon, Jul 13, 2026 at 11:03 PM Jan Beulich <jbeulich@suse.com> 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.
>
> Again - no. It simply can't. (Ftaod when I say -O in this discussion, I
> generally mean all its forms.)
>

My original design goal of -O is 100% safe for any applications.  It
is too bad we don't agree on it.  Here is the final patch I am checking
in.

-- 
H.J.
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