[PATCH 01/10] Support Intel AVX-IFMA

[Jan Beulich]

On 18.10.2022 23:28, H.J. Lu wrote:
> On Mon, Oct 17, 2022 at 10:33 PM Jan Beulich <jbeulich@suse.com> wrote:
>>
>> On 18.10.2022 00:23, H.J. Lu wrote:
>>> On Sat, Oct 15, 2022 at 11:39 PM Jan Beulich <jbeulich@suse.com> wrote:
>>>>
>>>> On 14.10.2022 20:10, H.J. Lu wrote:
>>>>> On Fri, Oct 14, 2022 at 2:52 AM Jan Beulich <jbeulich@suse.com> wrote:
>>>>>>
>>>>>> On 14.10.2022 11:12, Haochen Jiang wrote:
>>>>>>> From: wwwhhhyyy <hongyu.wang@intel.com>
>>>>>>>
>>>>>>> x86: Support Intel AVX-IFMA
>>>>>>>
>>>>>>> Intel AVX IFMA instructions are marked with CpuVEX_PREFIX, which is
>>>>>>> cleared by default.  Without {vex} pseudo prefix, Intel IFMA instructions
>>>>>>> are encoded with EVEX prefix.  {vex} pseudo prefix will turn on VEX
>>>>>>> encoding for Intel IFMA instructions.
>>>>>>
>>>>>> I firmly object to the proliferation of this mis-feature. As expressed
>>>>>> before for AVX-VNNI, as long as the user has disabled AVX512 (or
>>>>>> respective sub-features thereof), there should be no need to use {vex} in
>>>>>> the source code. There's also no reason at all to make the disassembler
>>>>>> print {vex} prefixes - we don't do so for any other insns (apart from
>>>>>> AVX-VNNI) where an ambiguity exists between their VEX and EVEX encodings
>>>>>> (when none of the EVEX-specific features is used).
>>>>>
>>>>> The {vex} prefix is used with AVX-IFMA instructions so that IFMA instructions
>>>>> without a prefix, which are generated by compilers or handwritten, will be
>>>>> always encoded with EVEX.
>>>>
>>>> So again: Why is this necessary when a programmer disabled AVX512? I fully
>>>> agree we need to pick the EVEX encoding by default if available, but I see
>>>> no reason whatsoever to insist on a {vex} prefix when the EVEX variant is
>>>> unavailable anyway. As you said back at the time for AVX-VNNI - this was a
>>>> design decision taken at Intel. Which is fine for a draft implementation.
>>>> But decisions for an open source project should be taken in the open, and
>>>> opinions of others should not simply be put off.
>>>>
>>>
>>> We can discuss how to initialize i.vec_encoding.  But it is orthogonal to
>>> this patch.
>>
>> One can view it as orthogonal, yes, but if we change the model then doing
>> so before more code and testcases need changing is imo preferable.
>>
> 
> We can skip the pseudo VEX prefix check when AVX512F is disabled.

Let me see if I can pull ahead the patch I have (right now it's at the end
of the 3rd series I have pending, when the 1st one continues to be debated),
so the new cases in this series could then come on top.

> There should be no testcase changes.

Well - existing tests ought it continue to work, yes, but the prefix-less
forms then also will want testing.

Jan