[PATCH v1 2/2] x86: Small improvements for wcslen

[H.J. Lu]

On Fri, Mar 25, 2022 at 3:13 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> Just a few QOL changes.
>     1. Prefer `add` > `lea` as it has high execution units it can run
>        on.
>     2. Don't break macro-fusion between `test` and `jcc`
>     3. Reduce code size by removing gratuitous padding bytes (-90
>        bytes).
>
> geometric_mean(N=20) of all benchmarks New / Original: 0.959
>
> All string/memory tests pass.
> ---
>  sysdeps/x86_64/wcslen.S | 86 ++++++++++++++++++++---------------------
>  1 file changed, 41 insertions(+), 45 deletions(-)
>
> diff --git a/sysdeps/x86_64/wcslen.S b/sysdeps/x86_64/wcslen.S
> index c9165dbf03..d641141d75 100644
> --- a/sysdeps/x86_64/wcslen.S
> +++ b/sysdeps/x86_64/wcslen.S
> @@ -40,82 +40,82 @@ ENTRY (__wcslen)
>         pxor    %xmm0, %xmm0
>
>         lea     32(%rdi), %rax
> -       lea     16(%rdi), %rcx
> +       addq    $16, %rdi
>         and     $-16, %rax
>
>         pcmpeqd (%rax), %xmm0
>         pmovmskb %xmm0, %edx
>         pxor    %xmm1, %xmm1
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm1
>         pmovmskb %xmm1, %edx
>         pxor    %xmm2, %xmm2
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm2
>         pmovmskb %xmm2, %edx
>         pxor    %xmm3, %xmm3
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm3
>         pmovmskb %xmm3, %edx
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm0
>         pmovmskb %xmm0, %edx
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm1
>         pmovmskb %xmm1, %edx
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm2
>         pmovmskb %xmm2, %edx
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm3
>         pmovmskb %xmm3, %edx
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm0
>         pmovmskb %xmm0, %edx
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm1
>         pmovmskb %xmm1, %edx
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm2
>         pmovmskb %xmm2, %edx
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         pcmpeqd (%rax), %xmm3
>         pmovmskb %xmm3, %edx
> +       addq    $16, %rax
>         test    %edx, %edx
> -       lea     16(%rax), %rax
>         jnz     L(exit)
>
>         and     $-0x40, %rax
> @@ -132,104 +132,100 @@ L(aligned_64_loop):
>         pminub  %xmm0, %xmm2
>         pcmpeqd %xmm3, %xmm2
>         pmovmskb %xmm2, %edx
> +       addq    $64, %rax
>         test    %edx, %edx
> -       lea     64(%rax), %rax
>         jz      L(aligned_64_loop)
>
>         pcmpeqd -64(%rax), %xmm3
>         pmovmskb %xmm3, %edx
> +    addq       $48, %rdi
>         test    %edx, %edx
> -       lea     48(%rcx), %rcx
>         jnz     L(exit)
>
>         pcmpeqd %xmm1, %xmm3
>         pmovmskb %xmm3, %edx
> +    addq       $-16, %rdi
>         test    %edx, %edx
> -       lea     -16(%rcx), %rcx
>         jnz     L(exit)
>
>         pcmpeqd -32(%rax), %xmm3
>         pmovmskb %xmm3, %edx
> +    addq       $-16, %rdi
>         test    %edx, %edx
> -       lea     -16(%rcx), %rcx
>         jnz     L(exit)
>
>         pcmpeqd %xmm6, %xmm3
>         pmovmskb %xmm3, %edx
> +    addq       $-16, %rdi
>         test    %edx, %edx
> -       lea     -16(%rcx), %rcx
> -       jnz     L(exit)
> -
> -       jmp     L(aligned_64_loop)
> +       jz      L(aligned_64_loop)
>
>         .p2align 4
>  L(exit):
> -       sub     %rcx, %rax
> +       sub     %rdi, %rax
>         shr     $2, %rax
>         test    %dl, %dl
>         jz      L(exit_high)
>
> -       mov     %dl, %cl
> -       and     $15, %cl
> +       andl    $15, %edx
>         jz      L(exit_1)
>         ret
>
> -       .p2align 4
> +       /* No align here. Naturally aligned % 16 == 1.  */
>  L(exit_high):
> -       mov     %dh, %ch
> -       and     $15, %ch
> +       andl    $(15 << 8), %edx
>         jz      L(exit_3)
>         add     $2, %rax
>         ret
>
> -       .p2align 4
> +       .p2align 3
>  L(exit_1):
>         add     $1, %rax
>         ret
>
> -       .p2align 4
> +       .p2align 3
>  L(exit_3):
>         add     $3, %rax
>         ret
>
> -       .p2align 4
> +       .p2align 3
>  L(exit_tail0):
> -       xor     %rax, %rax
> +       xorl    %eax, %eax
>         ret
>
> -       .p2align 4
> +       .p2align 3
>  L(exit_tail1):
> -       mov     $1, %rax
> +       movl    $1, %eax
>         ret
>
> -       .p2align 4
> +       .p2align 3
>  L(exit_tail2):
> -       mov     $2, %rax
> +       movl    $2, %eax
>         ret
>
> -       .p2align 4
> +       .p2align 3
>  L(exit_tail3):
> -       mov     $3, %rax
> +       movl    $3, %eax
>         ret
>
> -       .p2align 4
> +       .p2align 3
>  L(exit_tail4):
> -       mov     $4, %rax
> +       movl    $4, %eax
>         ret
>
> -       .p2align 4
> +       .p2align 3
>  L(exit_tail5):
> -       mov     $5, %rax
> +       movl    $5, %eax
>         ret
>
> -       .p2align 4
> +       .p2align 3
>  L(exit_tail6):
> -       mov     $6, %rax
> +       movl    $6, %eax
>         ret
>
> -       .p2align 4
> +       .p2align 3
>  L(exit_tail7):
> -       mov     $7, %rax
> +       movl    $7, %eax
>         ret
>
>  END (__wcslen)
> --
> 2.25.1
>

LGTM.

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

Thanks.

-- 
H.J.