Re: [PATCH v3 2/3] x86: Move and slightly improve memset_erms

[Sunil Pandey <skpgkp2@gmail.com>]

On Wed, Jun 29, 2022 at 4:30 PM H.J. Lu via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On Wed, Jun 29, 2022 at 4:07 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > Implementation wise:
> >     1. Remove the VZEROUPPER as memset_{impl}_unaligned_erms does not
> >        use the L(stosb) label that was previously defined.
> >
> >     2. Don't give the hotpath (fallthrough) to zero size.
> >
> > Code positioning wise:
> >
> > Move memset_{chk}_erms to its own file.  Leaving it in between the
> > memset_{impl}_unaligned both adds unnecessary complexity to the
> > file and wastes space in a relatively hot cache section.
> > ---
> >  sysdeps/x86_64/multiarch/Makefile             |  1 +
> >  sysdeps/x86_64/multiarch/memset-erms.S        | 44 +++++++++++++++++++
> >  .../multiarch/memset-vec-unaligned-erms.S     | 31 -------------
> >  3 files changed, 45 insertions(+), 31 deletions(-)
> >  create mode 100644 sysdeps/x86_64/multiarch/memset-erms.S
> >
> > diff --git a/sysdeps/x86_64/multiarch/Makefile b/sysdeps/x86_64/multiarch/Makefile
> > index 62a4d96fb8..18cea04423 100644
> > --- a/sysdeps/x86_64/multiarch/Makefile
> > +++ b/sysdeps/x86_64/multiarch/Makefile
> > @@ -30,6 +30,7 @@ sysdep_routines += \
> >    memset-avx2-unaligned-erms-rtm \
> >    memset-avx512-no-vzeroupper \
> >    memset-avx512-unaligned-erms \
> > +  memset-erms \
> >    memset-evex-unaligned-erms \
> >    memset-sse2-unaligned-erms \
> >    rawmemchr-avx2 \
> > diff --git a/sysdeps/x86_64/multiarch/memset-erms.S b/sysdeps/x86_64/multiarch/memset-erms.S
> > new file mode 100644
> > index 0000000000..e83cccc731
> > --- /dev/null
> > +++ b/sysdeps/x86_64/multiarch/memset-erms.S
> > @@ -0,0 +1,44 @@
> > +/* memset implement with rep stosb
> > +   Copyright (C) 2022 Free Software Foundation, Inc.
> > +   This file is part of the GNU C Library.
> > +
> > +   The GNU C Library is free software; you can redistribute it and/or
> > +   modify it under the terms of the GNU Lesser General Public
> > +   License as published by the Free Software Foundation; either
> > +   version 2.1 of the License, or (at your option) any later version.
> > +
> > +   The GNU C Library is distributed in the hope that it will be useful,
> > +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> > +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> > +   Lesser General Public License for more details.
> > +
> > +   You should have received a copy of the GNU Lesser General Public
> > +   License along with the GNU C Library; if not, see
> > +   <https://www.gnu.org/licenses/>.  */
> > +
> > +
> > +#include <sysdep.h>
> > +
> > +#if defined USE_MULTIARCH && IS_IN (libc)
> > +       .text
> > +ENTRY (__memset_chk_erms)
> > +       cmp     %RDX_LP, %RCX_LP
> > +       jb      HIDDEN_JUMPTARGET (__chk_fail)
> > +END (__memset_chk_erms)
> > +
> > +/* Only used to measure performance of REP STOSB.  */
> > +ENTRY (__memset_erms)
> > +       /* Skip zero length.  */
> > +       test    %RDX_LP, %RDX_LP
> > +       jz       L(stosb_return_zero)
> > +       mov     %RDX_LP, %RCX_LP
> > +       movzbl  %sil, %eax
> > +       mov     %RDI_LP, %RDX_LP
> > +       rep stosb
> > +       mov     %RDX_LP, %RAX_LP
> > +       ret
> > +L(stosb_return_zero):
> > +       movq    %rdi, %rax
> > +       ret
> > +END (__memset_erms)
> > +#endif
> > diff --git a/sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S b/sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S
> > index abc12d9cda..905d0fa464 100644
> > --- a/sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S
> > +++ b/sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S
> > @@ -156,37 +156,6 @@ L(entry_from_wmemset):
> >  #if defined USE_MULTIARCH && IS_IN (libc)
> >  END (MEMSET_SYMBOL (__memset, unaligned))
> >
> > -# if VEC_SIZE == 16
> > -ENTRY (__memset_chk_erms)
> > -       cmp     %RDX_LP, %RCX_LP
> > -       jb      HIDDEN_JUMPTARGET (__chk_fail)
> > -END (__memset_chk_erms)
> > -
> > -/* Only used to measure performance of REP STOSB.  */
> > -ENTRY (__memset_erms)
> > -       /* Skip zero length.  */
> > -       test    %RDX_LP, %RDX_LP
> > -       jnz      L(stosb)
> > -       movq    %rdi, %rax
> > -       ret
> > -# else
> > -/* Provide a hidden symbol to debugger.  */
> > -       .hidden MEMSET_SYMBOL (__memset, erms)
> > -ENTRY (MEMSET_SYMBOL (__memset, erms))
> > -# endif
> > -L(stosb):
> > -       mov     %RDX_LP, %RCX_LP
> > -       movzbl  %sil, %eax
> > -       mov     %RDI_LP, %RDX_LP
> > -       rep stosb
> > -       mov     %RDX_LP, %RAX_LP
> > -       VZEROUPPER_RETURN
> > -# if VEC_SIZE == 16
> > -END (__memset_erms)
> > -# else
> > -END (MEMSET_SYMBOL (__memset, erms))
> > -# endif
> > -
> >  # if defined SHARED && IS_IN (libc)
> >  ENTRY_CHK (MEMSET_CHK_SYMBOL (__memset_chk, unaligned_erms))
> >         cmp     %RDX_LP, %RCX_LP
> > --
> > 2.34.1
> >
>
> LGTM.
>
> Thanks.
>
> --
> H.J.

I would like to backport this patch to release branches.
Any comments or objections?

--Sunil