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

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

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.