Re: [PATCH v1 2/3] x86: Move and slightly improve memset_erms

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

On Tue, Jun 28, 2022 at 8:27 AM 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 L(memset_{chk}_erms) to its own file.  Leaving it in between the

 It is ENTRY, not L.   Did you mean to move them to the end of file?

> memset_{impl}_unaligned both adds unnecessary complexity to the
> file and wastes space in a relatively hot cache section.
> ---
>  .../multiarch/memset-vec-unaligned-erms.S     | 54 ++++++++-----------
>  1 file changed, 23 insertions(+), 31 deletions(-)
>
> diff --git a/sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S b/sysdeps/x86_64/multiarch/memset-vec-unaligned-erms.S
> index abc12d9cda..d98c613651 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
> @@ -461,3 +430,26 @@ L(between_2_3):
>  #endif
>         ret
>  END (MEMSET_SYMBOL (__memset, unaligned_erms))
> +
> +#if defined USE_MULTIARCH && IS_IN (libc) && 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
> +       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
> --
> 2.34.1
>


-- 
H.J.