Re: [PATCH 01/10] Support Intel AVX-IFMA

["H.J. Lu" <hjl.tools@gmail.com>]

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.
There should be no testcase changes.


-- 
H.J.