Re: [PATCH v1] x86: Add EVEX optimized memchr family not safe for RTM

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

On Wed, May 5, 2021 at 9:25 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Wed, May 5, 2021 at 9:23 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Tue, May 4, 2021 at 4:34 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> > >
> > > No bug.
> > >
> > > This commit adds a new implementation for EVEX memchr that is not safe
> > > for RTM because it uses vzeroupper. The benefit is that by using
> >
> > EVEX memchr won't cause RTM abort if YMM16-YMM31 are used
> > since there is no need to use vzeroupper.  Please remove vzeroupper from
> > EVEX memchr and remove EVEX RTM functions.
>
> That's impossible for this implementation.
>
> The reason ymm0-ymm15 are used is so that we can use vpcmpeq which is
> not encodable with ymm16-ymm31.
>
> This implementation is optimized for CPUs which dont support RTM but
> do support EVEX.
>

Are you seeing something along the line of Prefer_AVX2_STRCMP:

commit 1da50d4bda07f04135dca39f40e79fc9eabed1f8
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Fri Feb 26 05:36:59 2021 -0800

    x86: Set Prefer_No_VZEROUPPER and add Prefer_AVX2_STRCMP

    1. Set Prefer_No_VZEROUPPER if RTM is usable to avoid RTM abort triggered
    by VZEROUPPER inside a transactionally executing RTM region.
    2. Since to compare 2 32-byte strings, 256-bit EVEX strcmp requires 2
    loads, 3 VPCMPs and 2 KORDs while AVX2 strcmp requires 1 load, 2 VPCMPEQs,
    1 VPMINU and 1 VPMOVMSKB, AVX2 strcmp is faster than EVEX strcmp.  Add
    Prefer_AVX2_STRCMP to prefer AVX2 strcmp family functions.


-- 
H.J.