[PATCH] x86: Disable XCHG to MOV optimization

[Jan Beulich]

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.)

Jan