Re: [PATCH v6 8/8] x86: Shrink code size of memchr-evex.S

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

On Mon, Jun 6, 2022 at 9:11 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> This is not meant as a performance optimization. The previous code was
> far to liberal in aligning targets and wasted code size unnecissarily.
>
> The total code size saving is: 64 bytes
>
> There are no non-negligible changes in the benchmarks.
> Geometric Mean of all benchmarks New / Old: 1.000
>
> Full xcheck passes on x86_64.
> ---
>  sysdeps/x86_64/multiarch/memchr-evex.S | 46 ++++++++++++++------------
>  1 file changed, 25 insertions(+), 21 deletions(-)
>
> diff --git a/sysdeps/x86_64/multiarch/memchr-evex.S b/sysdeps/x86_64/multiarch/memchr-evex.S
> index cfaf02907d..0fd11b7632 100644
> --- a/sysdeps/x86_64/multiarch/memchr-evex.S
> +++ b/sysdeps/x86_64/multiarch/memchr-evex.S
> @@ -88,7 +88,7 @@
>  # define PAGE_SIZE 4096
>
>         .section SECTION(.text),"ax",@progbits
> -ENTRY (MEMCHR)
> +ENTRY_P2ALIGN (MEMCHR, 6)
>  # ifndef USE_AS_RAWMEMCHR
>         /* Check for zero length.  */
>         test    %RDX_LP, %RDX_LP
> @@ -131,22 +131,24 @@ L(zero):
>         xorl    %eax, %eax
>         ret
>
> -       .p2align 5
> +       .p2align 4
>  L(first_vec_x0):
> -       /* Check if first match was before length.  */
> -       tzcntl  %eax, %eax
> -       xorl    %ecx, %ecx
> -       cmpl    %eax, %edx
> -       leaq    (%rdi, %rax, CHAR_SIZE), %rax
> -       cmovle  %rcx, %rax
> +       /* Check if first match was before length. NB: tzcnt has false data-
> +          dependency on destination. eax already had a data-dependency on esi
> +          so this should have no affect here.  */
> +       tzcntl  %eax, %esi
> +#  ifdef USE_AS_WMEMCHR
> +       leaq    (%rdi, %rsi, CHAR_SIZE), %rdi
> +#  else
> +       addq    %rsi, %rdi
> +#  endif
> +       xorl    %eax, %eax
> +       cmpl    %esi, %edx
> +       cmovg   %rdi, %rax
>         ret
> -# else
> -       /* NB: first_vec_x0 is 17 bytes which will leave
> -          cross_page_boundary (which is relatively cold) close enough
> -          to ideal alignment. So only realign L(cross_page_boundary) if
> -          rawmemchr.  */
> -       .p2align 4
>  # endif
> +
> +       .p2align 4
>  L(cross_page_boundary):
>         /* Save pointer before aligning as its original value is
>            necessary for computer return address if byte is found or
> @@ -400,10 +402,14 @@ L(last_2x_vec):
>  L(zero_end):
>         ret
>
> +L(set_zero_end):
> +       xorl    %eax, %eax
> +       ret
>
>         .p2align 4
>  L(first_vec_x1_check):
> -       tzcntl  %eax, %eax
> +       /* eax must be non-zero. Use bsfl to save code size.  */
> +       bsfl    %eax, %eax
>         /* Adjust length.  */
>         subl    $-(CHAR_PER_VEC * 4), %edx
>         /* Check if match within remaining length.  */
> @@ -412,9 +418,6 @@ L(first_vec_x1_check):
>         /* NB: Multiply bytes by CHAR_SIZE to get the wchar_t count.  */
>         leaq    VEC_SIZE(%rdi, %rax, CHAR_SIZE), %rax
>         ret
> -L(set_zero_end):
> -       xorl    %eax, %eax
> -       ret
>
>         .p2align 4
>  L(loop_4x_vec_end):
> @@ -464,7 +467,7 @@ L(loop_4x_vec_end):
>  # endif
>         ret
>
> -       .p2align 4
> +       .p2align 4,, 10
>  L(last_vec_x1_return):
>         tzcntl  %eax, %eax
>  # if defined USE_AS_WMEMCHR || RET_OFFSET != 0
> @@ -496,6 +499,7 @@ L(last_vec_x3_return):
>  # endif
>
>  # ifndef USE_AS_RAWMEMCHR
> +       .p2align 4,, 5
>  L(last_4x_vec_or_less_cmpeq):
>         VPCMP   $0, (VEC_SIZE * 5)(%rdi), %YMMMATCH, %k0
>         kmovd   %k0, %eax
> @@ -546,7 +550,7 @@ L(last_4x_vec):
>  #  endif
>         andl    %ecx, %eax
>         jz      L(zero_end2)
> -       tzcntl  %eax, %eax
> +       bsfl    %eax, %eax
>         leaq    (VEC_SIZE * 4)(%rdi, %rax, CHAR_SIZE), %rax
>  L(zero_end2):
>         ret
> @@ -562,6 +566,6 @@ L(last_vec_x3):
>         leaq    (VEC_SIZE * 3)(%rdi, %rax, CHAR_SIZE), %rax
>         ret
>  # endif
> -
> +       /* 7 bytes from next cache line.  */
>  END (MEMCHR)
>  #endif
> --
> 2.34.1
>

LGTM.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Thanks.

-- 
H.J.