[Bug string/28354] For x86_64 string/memory functions use of EVEX registers sets HI16_ZMM_state adding context switch overhead

["goldstein.w.n at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28354

Noah Goldstein <goldstein.w.n at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |goldstein.w.n at gmail dot com
             Target|                            |x86_64-linux

--- Comment #1 from Noah Goldstein <goldstein.w.n at gmail dot com> ---
My general opinion is that we should move the current evex function to evex-rtm
and add a new class of evex function which may use avx512 functions but stay in
the ymm0-ymm15 register range.

Benefits:

1) evex instructions cost more code size (+2 bytes at least)
2) Its impossible to encode certain useful instructions with the evex prefix
(i.e `vpcmpeq`)
3) We may be adding 1024 bytes to uses context switches.


Costs:

1) vzeroupper is not free (in terms of code size or execution).
2) more total code size consumer by the library (this is limited by the fact
that they will be in their own section and users will generally only stay in
one section for all string/memory functions)

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