[PATCH] AArch64: Optimize memrchr

[Szabolcs Nagy]

The 01/12/2023 15:57, Wilco Dijkstra wrote:
> Optimize the main loop - large strings are 43% faster on modern CPUs.
> Passes regress.

please commit it, thanks.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>


> 
> ---
> 
> diff --git a/sysdeps/aarch64/memrchr.S b/sysdeps/aarch64/memrchr.S
> index 9d2d29a396d46d6c2e74e3ca637091e2f3d68d5e..621fc65109736646b74900db8d15c6f8a7c68895 100644
> --- a/sysdeps/aarch64/memrchr.S
> +++ b/sysdeps/aarch64/memrchr.S
> @@ -26,7 +26,6 @@
>   * MTE compatible.
>   */
> 
> -/* Arguments and results.  */
>  #define srcin          x0
>  #define chrin          w1
>  #define cntin          x2
> @@ -77,31 +76,34 @@ ENTRY (__memrchr)
>         csel    result, result, xzr, hi
>         ret
> 
> +       nop
>  L(start_loop):
> -       sub     tmp, end, src
> -       subs    cntrem, cntin, tmp
> +       subs    cntrem, src, srcin
>         b.ls    L(nomatch)
> 
>         /* Make sure that it won't overread by a 16-byte chunk */
> -       add     tmp, cntrem, 15
> -       tbnz    tmp, 4, L(loop32_2)
> +       sub     cntrem, cntrem, 1
> +       tbz     cntrem, 4, L(loop32_2)
> +       add     src, src, 16
> 
> -       .p2align 4
> +       .p2align 5
>  L(loop32):
> -       ldr     qdata, [src, -16]!
> +       ldr     qdata, [src, -32]!
>         cmeq    vhas_chr.16b, vdata.16b, vrepchr.16b
>         umaxp   vend.16b, vhas_chr.16b, vhas_chr.16b            /* 128->64 */
>         fmov    synd, dend
>         cbnz    synd, L(end)
> 
>  L(loop32_2):
> -       ldr     qdata, [src, -16]!
> +       ldr     qdata, [src, -16]
>         subs    cntrem, cntrem, 32
>         cmeq    vhas_chr.16b, vdata.16b, vrepchr.16b
> -       b.ls    L(end)
> +       b.lo    L(end_2)
>         umaxp   vend.16b, vhas_chr.16b, vhas_chr.16b            /* 128->64 */
>         fmov    synd, dend
>         cbz     synd, L(loop32)
> +L(end_2):
> +       sub     src, src, 16
>  L(end):
>         shrn    vend.8b, vhas_chr.8h, 4         /* 128->64 */
>         fmov    synd, dend