Re: [PATCH] x86/crc32: optimize tail handling for crc32c short inputs

[Uros Bizjak]

On Wed, Mar 5, 2025 at 10:26 AM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> For handling the 0 <= len < sizeof(unsigned long) bytes left at the end,
> do a 4-2-1 step-down instead of a byte-at-a-time loop.  This allows
> taking advantage of wider CRC instructions.  Note that crc32c-3way.S
> already uses this same optimization too.
>
> crc_kunit shows an improvement of about 25% for len=127.
>
> Suggested-by: H. Peter Anvin <hpa@xxxxxxxxx>
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>

LGTM.

Acked-by: Uros Bizjak <ubizjak@xxxxxxxxx>

Thanks,
Uros.

> ---
>
> This applies to
> https://web.git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=crc-next
>
>  arch/x86/lib/crc32-glue.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/lib/crc32-glue.c b/arch/x86/lib/crc32-glue.c
> index 4b4721176799a..e3f93b17ac3f1 100644
> --- a/arch/x86/lib/crc32-glue.c
> +++ b/arch/x86/lib/crc32-glue.c
> @@ -55,11 +55,19 @@ u32 crc32c_arch(u32 crc, const u8 *p, size_t len)
>
>         for (num_longs = len / sizeof(unsigned long);
>              num_longs != 0; num_longs--, p += sizeof(unsigned long))
>                 asm(CRC32_INST : "+r" (crc) : ASM_INPUT_RM (*(unsigned long *)p));
>
> -       for (len %= sizeof(unsigned long); len; len--, p++)
> +       if (sizeof(unsigned long) > 4 && (len & 4)) {
> +               asm("crc32l %1, %0" : "+r" (crc) : ASM_INPUT_RM (*(u32 *)p));
> +               p += 4;
> +       }
> +       if (len & 2) {
> +               asm("crc32w %1, %0" : "+r" (crc) : ASM_INPUT_RM (*(u16 *)p));
> +               p += 2;
> +       }
> +       if (len & 1)
>                 asm("crc32b %1, %0" : "+r" (crc) : ASM_INPUT_RM (*p));
>
>         return crc;
>  }
>  EXPORT_SYMBOL(crc32c_arch);
>
> base-commit: 13f3d13d88b5dcba104a204fcbee61c75f8407d0
> --
> 2.48.1
>