[PATCH] x86: adjust AVX-VNNI-INT{8,16} prereqs

[Jan Beulich]

On 06.09.2024 04:51, Jiang, Haochen wrote:
>> From: Jan Beulich <jbeulich@suse.com>
>> Sent: Thursday, September 5, 2024 7:32 PM
>>
>> These are more logical to take AVX-VNNI as prereq, retaining AVX2 as an
>> indirect prereq.
> 
> From HW side, that is ok till now since I don't see a machine with
> AVX-VNNI-INT{8,16} but no AVX-VNNI.
> 
> But this will lead to if someone uses .arch .noavx_vnni, then it will turn
> of AVX-VNNI-INT{8,16}. I don't think this is the expected behavior. If we
> care about this .noxxx behavior, we should not do that. This is not quite
> same as the AVX10.2 I will mention next since IMO, these three ISAs
> are quite independent.

But that's actually one of the points of the change: I _want_ .noavx_vnni
to have said effect. It makes little sense to me to keep AVX-VNNI-INT{8,16}
enabled when AVX-VNNI is off.

>> ---
>> In turn the question arises whether AVX10.2 should consider both to be
>> prereqs as well. The situation here isn't the same as it was with AVX512-VNNI
>> vs AVX-VNNI, after all.
> 
> For AVX10.2, maybe I would like to prefer to imply them since it is weird not
> having VEX part when having EVEX part if the EVEX part is introduced after
> VEX and there is no HW only having EVEX part. Although GCC will not do
> the imply since we are pretty conservative on that and will stick to doc,
> in Binutils, for convenience, I suppose it could be done.

And what are the chances of the doc being updated to mention all AVX-VNNI*
as implied by AVX10.2, just like AVX10.1 documents a fair number of AVX512*
as implied?

Similarly for the earlier remark, and more generally: It would help if the
SDM specified dependencies between extensions more clearly / strictly than
it does right now.

Jan