Re: optimal encoding of SIMD insns

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

On Mon, Nov 20, 2017 at 4:08 AM, Jan Beulich <JBeulich@suse.com> wrote:
> H.J.,
>
> one more sub-optimal thing I've come across, int the context of my
> analysis of whether Vec_Disp8 really is fully redundant with
> DispMemShift: Certain instructions, for example
>
>         vaddps  xmm0, xmm0, [eax+0x80]
>
> are encodable with both VEX and EVEX. Generally the assembler
> tries to pick the shortest encoding. Obviously the VEX encoding,
> due to requiring the Disp32 ModR/M form, is longer than the
> EVEX one in the specific example above. Clearly such a
> conversion can't be done unilaterally, as that could break code
> assuming to be run on AVX-512-incapable hardware. However,
> does anything speak against doing so after having seen a
> command line option or directive explicitly enabling AVX-512
> insns? Or should this instead be made even more explicit, by
> introducing something paralleling the automatic SSE->AVX
> conversion?
>

Have you looked at

commit 86fa6981e7487e2c2df4337aa75ed2d93c32eaf2
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Thu Mar 9 09:58:46 2017 -0800

    X86: Add pseudo prefixes to control encoding

    Many x86 instructions have more than one encodings.  Assembler picks
    the default one, usually the shortest one.  Although the ".s", ".d8"
    and ".d32" suffixes can be used to swap register operands or specify
    displacement size, they aren't very flexible.  This patch adds pseudo
    prefixes, {xxx}, to control instruction encoding.  The available
    pseudo prefixes are {disp8}, {disp32}, {load}, {store}, {vex2}, {vex3}
    and {evex}.  Pseudo prefixes are preferred over the ".s", ".d8" and
    ".d32" suffixes, which are deprecated.


-- 
H.J.