[PATCH] x86: Disable XCHG to MOV optimization

[Jan Beulich]

On 13.07.2026 22:55, H.J. Lu wrote:
> 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.

So you really think committing patches under dispute is acceptable? I, for
one, consider this a process violation. Not that this would have been the
first time, though. Nick, may I ask for your take here?

Jan