On 25.09.2024 10:52, Hongtao Liu wrote:
> On Wed, Sep 25, 2024 at 3:55 PM Jan Beulich <jbeul...@suse.com> wrote:
>>
>> On 25.09.2024 09:38, Hongtao Liu wrote:
>>> On Wed, Sep 25, 2024 at 2:56 PM Jan Beulich <jbeul...@suse.com> wrote:
>>>>
>>>> Commit a79d13a01f8c ("i386: Fix aes/vaes patterns [PR114576]") correctly
>>>> said "..., but we need to emit {evex} prefix in the assembly if AES ISA
>>>> is not enabled". Yet it did so only for the TARGET_AES insns. Going from
>>>> the alternative chosen in the TARGET_VAES insns is wrong for two
>>>> reasons:
>>>> - if, with AES disabled, the latter alternative was chosen despite no
>>>> "high" XMM register nor any eGPR in use, gas would still pick the AES
>>> w/o EVEX SSE REG or EGPR, the first alternative will always be
>>> matched(alternative 0).
>>> That is how it works(match from left to right).
>>
>> Well, if that's guaranteed to always be the case, then ...
>>
>>>> (VEX) encoding when no {evex} pseudo-prefix is in use (which is
>>>> against - as stated by the description of said commit - AES presently
>>>> not being considered a prereq of VAES in gcc);
>>>
>>>> - if AES is (also) enabled, EVEX encoding would needlessly be forced.
>>> So it's more like an optimization that use VEX encoding when AES is enabled?
>>
>> ... in a way it's an optimization, yes. I can adjust the description
>> accordingly. However, it's not _just_ an optimization, it also is a
>> fix for compilation (really: assembly) failing in ...
>>
>>>> ---
>>>> As an aside - {evex} (and other) pseudo-prefixes would better be avoided
>>>> anyway whenever possible, as those are getting in the way of code
>>>> putting in place macro overrides for certain insns: gas 2.43 rejects
>>>> such bogus placement of pseudo-prefixes.
> So it sounds like a walkaround in GCC to avoid the gas bug?
There's no gas bug here. Gas 2.43 is validly complaining; older gas was
buggy not to complain.
> In general, I'm ok for the patch since we already did that in
> TARGET_AES patterns.
IOW AIUI I'm to re-submit then with an adjusted (shortened) description.
Okay.
Jan
