[PATCH] Fix avx2 strncmp offset compare condition check [BZ #25933]

[H.J. Lu]

On Sat, Jun 13, 2020 at 4:23 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> From: Sunil K Pandey <skpgkp1@gmail.com>
>
> strcmp-avx2.S: In avx2 strncmp function, strings are compared in
> chunks of 4 vector size(i.e. 32x4=128 byte for avx2). After first 4
> vector size comparison, code must check whether it already passed
> the given offset. This patch implement avx2 offset check condition
> for strncmp function, if both string compare same for first 4 vector
> size.
> ---
>  sysdeps/x86_64/multiarch/strcmp-avx2.S | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/sysdeps/x86_64/multiarch/strcmp-avx2.S b/sysdeps/x86_64/multiarch/strcmp-avx2.S
> index 48d03a9f46..ee82fa3e19 100644
> --- a/sysdeps/x86_64/multiarch/strcmp-avx2.S
> +++ b/sysdeps/x86_64/multiarch/strcmp-avx2.S
> @@ -591,7 +591,14 @@ L(loop_cross_page_2_vec):
>         movl    $(PAGE_SIZE / (VEC_SIZE * 4) - 1), %esi
>
>         testq   %rdi, %rdi
> +# ifdef USE_AS_STRNCMP
> +       /* At this point, if %rdi value is 0, it already tested
> +          VEC_SIZE*4+%r10 byte starting from %rax. This label
> +          checks whether strncmp maximum offset reached or not.  */
> +       je      L(string_nbyte_offset_check)
> +# else
>         je      L(back_to_loop)
> +# endif
>         tzcntq  %rdi, %rcx
>         addq    %r10, %rcx
>         /* Adjust for number of bytes skipped.  */
> @@ -627,6 +634,14 @@ L(loop_cross_page_2_vec):
>         VZEROUPPER
>         ret
>
> +# ifdef USE_AS_STRNCMP
> +L(string_nbyte_offset_check):
> +       leaq    (VEC_SIZE * 4)(%r10), %r10
> +       cmpq    %r10, %r11
> +       jbe     L(zero)
> +       jmp     L(back_to_loop)
> +# endif
> +
>         .p2align 4
>  L(cross_page_loop):
>         /* Check one byte/dword at a time.  */
> --
> 2.26.2
>

I am checking it in and backport it to the active branches.

-- 
H.J.