Re: [PATCH] x86: fold AVX512-VNNI disassembler entries with AVX-VNNI ones

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

On Sun, Oct 16, 2022 at 11:39 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 14.10.2022 19:28, H.J. Lu wrote:
> > On Fri, Oct 14, 2022 at 3:22 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> Make %XV also print the separating blank in the VEX case, while making
> >> it do nothing for EVEX-encoded insns. This way the AVX-VNNI entries
> >> can be re-used for AVX512-VNNI, at the same time fixing the lack of
> >> EVEX.W decoding.
> >>
> >> For the AVX-VNNI ones further make sure only VEX.66 forms are actually
> >> decoded.
> >> ---
> >> Irrespective of this change I continue to disagree with the arbitrary
> >> printing of "{vex}" for the AVX-VNNI insns: If that's meant for
> >> disambiguation purposes, then EVEX-encoded insns not using EVEX-specific
> >> functionality by having VEX counterparts (vaddps %xmm0, %xmm0, %xmm0)
> >> should also be prefixed by "{evex}".
> >
> > This is done to match the assembler.  There are 3 kinds of VNNI processors:
> >
> > 1. AVX512-VNNI only.
> > 2. AVX-VNNI only.
> > 3. AVX512-VNNI and AVX-VNNI.
> >
> > Since AVX512-VNNI came out first, all VNNI instructions without a prefix
> > are encoded as AVX512-VNNI.  The existing VNNI instructions without
> > a prefix, generated by compiler or hand written, are encoded with EVEX.
> > If one needs VNNI with VEX encoding, the {vex} prefix should be used.
>
> ... if, as said, AVX512 wasn't turned off altogether.
>
> With your model, just look at how odd code using both AVX-VNNI and
> AVX-VNNI-INT8 then looks:
>
>         vpdpbssd        %ymm0, %ymm5, %ymm6
>         vpdpbsud        %ymm1, %ymm5, %ymm6
>         {vex} vpdpbusd  %ymm2, %ymm5, %ymm6
>         vpdpbuud        %ymm3, %ymm5, %ymm6
>
> Yes, one could further clutter this and add {vex} to every line. But
> why would anyone want to clutter their code?

This ensures that VNNI instructions without a prefix are encoded
with EVEX.

> Plus the same argument then applies to AVX512VL: This came out later than
> AVX, and the assembler (necessarily) requires {evex} to actually encode
> these (when there are AVX equivalents). Hence to match the assembler, the
> disassembler then also ought to emit {evex} for this subset of encodings.

A patch is welcome.

> Jan
>
> > This applies to any AVX extensions which come after EVEX ones, including
> > AVX-IFMA.
>


-- 
H.J.