Re: [PATCH v6 7/8] x86: Shrink code size of memchr-avx2.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: 59 bytes
>
> There are no major changes in the benchmarks.
> Geometric Mean of all benchmarks New / Old: 0.967
>
> Full xcheck passes on x86_64.
> ---
>  sysdeps/x86_64/multiarch/memchr-avx2-rtm.S |   1 +
>  sysdeps/x86_64/multiarch/memchr-avx2.S     | 109 +++++++++++----------
>  2 files changed, 60 insertions(+), 50 deletions(-)
>
> diff --git a/sysdeps/x86_64/multiarch/memchr-avx2-rtm.S b/sysdeps/x86_64/multiarch/memchr-avx2-rtm.S
> index 87b076c7c4..c4d71938c5 100644
> --- a/sysdeps/x86_64/multiarch/memchr-avx2-rtm.S
> +++ b/sysdeps/x86_64/multiarch/memchr-avx2-rtm.S
> @@ -2,6 +2,7 @@
>  # define MEMCHR __memchr_avx2_rtm
>  #endif
>
> +#define COND_VZEROUPPER        COND_VZEROUPPER_XTEST
>  #define ZERO_UPPER_VEC_REGISTERS_RETURN \
>    ZERO_UPPER_VEC_REGISTERS_RETURN_XTEST
>
> diff --git a/sysdeps/x86_64/multiarch/memchr-avx2.S b/sysdeps/x86_64/multiarch/memchr-avx2.S
> index 75bd7262e0..28a01280ec 100644
> --- a/sysdeps/x86_64/multiarch/memchr-avx2.S
> +++ b/sysdeps/x86_64/multiarch/memchr-avx2.S
> @@ -57,7 +57,7 @@
>  # define CHAR_PER_VEC  (VEC_SIZE / CHAR_SIZE)
>
>         .section SECTION(.text),"ax",@progbits
> -ENTRY (MEMCHR)
> +ENTRY_P2ALIGN (MEMCHR, 5)
>  # ifndef USE_AS_RAWMEMCHR
>         /* Check for zero length.  */
>  #  ifdef __ILP32__
> @@ -87,12 +87,14 @@ ENTRY (MEMCHR)
>  # endif
>         testl   %eax, %eax
>         jz      L(aligned_more)
> -       tzcntl  %eax, %eax
> +       bsfl    %eax, %eax
>         addq    %rdi, %rax
> -       VZEROUPPER_RETURN
> +L(return_vzeroupper):
> +       ZERO_UPPER_VEC_REGISTERS_RETURN
> +
>
>  # ifndef USE_AS_RAWMEMCHR
> -       .p2align 5
> +       .p2align 4
>  L(first_vec_x0):
>         /* Check if first match was before length.  */
>         tzcntl  %eax, %eax
> @@ -100,58 +102,31 @@ L(first_vec_x0):
>         /* NB: Multiply length by 4 to get byte count.  */
>         sall    $2, %edx
>  #  endif
> -       xorl    %ecx, %ecx
> +    COND_VZEROUPPER
> +       /* Use branch instead of cmovcc so L(first_vec_x0) fits in one fetch
> +          block. branch here as opposed to cmovcc is not that costly. Common
> +          usage of memchr is to check if the return was NULL (if string was
> +          known to contain CHAR user would use rawmemchr). This branch will be
> +          highly correlated with the user branch and can be used by most
> +          modern branch predictors to predict the user branch.  */
>         cmpl    %eax, %edx
> -       leaq    (%rdi, %rax), %rax
> -       cmovle  %rcx, %rax
> -       VZEROUPPER_RETURN
> -
> -L(null):
> -       xorl    %eax, %eax
> -       ret
> -# 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
> -          adjusting length if it is not and this is memchr.  */
> -       movq    %rdi, %rcx
> -       /* Align data to VEC_SIZE - 1. ALGN_PTR_REG is rcx for memchr
> -          and rdi for rawmemchr.  */
> -       orq     $(VEC_SIZE - 1), %ALGN_PTR_REG
> -       VPCMPEQ -(VEC_SIZE - 1)(%ALGN_PTR_REG), %ymm0, %ymm1
> -       vpmovmskb %ymm1, %eax
> -# ifndef USE_AS_RAWMEMCHR
> -       /* Calculate length until end of page (length checked for a
> -          match).  */
> -       leaq    1(%ALGN_PTR_REG), %rsi
> -       subq    %RRAW_PTR_REG, %rsi
> -#  ifdef USE_AS_WMEMCHR
> -       /* NB: Divide bytes by 4 to get wchar_t count.  */
> -       shrl    $2, %esi
> -#  endif
> -# endif
> -       /* Remove the leading bytes.  */
> -       sarxl   %ERAW_PTR_REG, %eax, %eax
> -# ifndef USE_AS_RAWMEMCHR
> -       /* Check the end of data.  */
> -       cmpq    %rsi, %rdx
> -       jbe     L(first_vec_x0)
> +    jle  L(null)
> +       addq    %rdi, %rax
> +    ret
>  # endif
> -       testl   %eax, %eax
> -       jz      L(cross_page_continue)
> -       tzcntl  %eax, %eax
> -       addq    %RRAW_PTR_REG, %rax
> -L(return_vzeroupper):
> -       ZERO_UPPER_VEC_REGISTERS_RETURN
>
> -       .p2align 4
> +       .p2align 4,, 10
>  L(first_vec_x1):
> -       tzcntl  %eax, %eax
> +       bsfl    %eax, %eax
>         incq    %rdi
>         addq    %rdi, %rax
>         VZEROUPPER_RETURN
> -
> +# ifndef USE_AS_RAWMEMCHR
> +       /* First in aligning bytes here.  */
> +L(null):
> +       xorl    %eax, %eax
> +       ret
> +# endif
>         .p2align 4
>  L(first_vec_x2):
>         tzcntl  %eax, %eax
> @@ -340,7 +315,7 @@ L(first_vec_x1_check):
>         incq    %rdi
>         addq    %rdi, %rax
>         VZEROUPPER_RETURN
> -       .p2align 4
> +       .p2align 4,, 6
>  L(set_zero_end):
>         xorl    %eax, %eax
>         VZEROUPPER_RETURN
> @@ -428,5 +403,39 @@ L(last_vec_x3):
>         VZEROUPPER_RETURN
>  # 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 adjusting length if it
> +          is not and this is memchr.  */
> +       movq    %rdi, %rcx
> +       /* Align data to VEC_SIZE. ALGN_PTR_REG is rcx for memchr and rdi for
> +          rawmemchr.  */
> +       andq    $-VEC_SIZE, %ALGN_PTR_REG
> +       VPCMPEQ (%ALGN_PTR_REG), %ymm0, %ymm1
> +       vpmovmskb %ymm1, %eax
> +# ifndef USE_AS_RAWMEMCHR
> +       /* Calculate length until end of page (length checked for a match).  */
> +       leal    VEC_SIZE(%ALGN_PTR_REG), %esi
> +       subl    %ERAW_PTR_REG, %esi
> +# ifdef USE_AS_WMEMCHR
> +       /* NB: Divide bytes by 4 to get wchar_t count.  */
> +       shrl    $2, %esi
> +# endif
> +# endif
> +       /* Remove the leading bytes.  */
> +       sarxl   %ERAW_PTR_REG, %eax, %eax
> +# ifndef USE_AS_RAWMEMCHR
> +       /* Check the end of data.  */
> +       cmpq    %rsi, %rdx
> +       jbe     L(first_vec_x0)
> +# endif
> +       testl   %eax, %eax
> +       jz      L(cross_page_continue)
> +       bsfl    %eax, %eax
> +       addq    %RRAW_PTR_REG, %rax
> +       VZEROUPPER_RETURN
> +
> +
>  END (MEMCHR)
>  #endif
> --
> 2.34.1
>

LGTM.

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

Thanks.

-- 
H.J.