linux-kernel - Re: [PATCH v5 3/3] powerpc/lib: optimise PPC32 memcmp

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Message-ID: <CA+7wUszdfG+u4YSnxjFaujy3s_Vu92POPamK+=zjYe6Pbu+kxw@mail.gmail.com>
Date:   Tue, 29 May 2018 22:03:01 +0200
From:   Mathieu Malaterre <malat@...ian.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3/3] powerpc/lib: optimise PPC32 memcmp

On Mon, May 28, 2018 at 12:49 PM, Christophe Leroy
<christophe.leroy@....fr> wrote:
> At the time being, memcmp() compares two chunks of memory
> byte per byte.
>
> This patch optimises the comparison by comparing word by word.
>
> A small benchmark performed on an 8xx comparing two chuncks
> of 512 bytes performed 100000 times gives:
>
> Before : 5852274 TB ticks
> After:   1488638 TB ticks
>
> This is almost 4 times faster
>
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
>  arch/powerpc/lib/string_32.S | 37 +++++++++++++++++++++++++++----------
>  1 file changed, 27 insertions(+), 10 deletions(-)

Would it possible for you to move the actual code instead to:

./arch/powerpc/lib/memcmp_32.S

This will seat right next to memcmp_64.S implementation.

> diff --git a/arch/powerpc/lib/string_32.S b/arch/powerpc/lib/string_32.S
> index 40a576d56ac7..4fbaa046aa84 100644
> --- a/arch/powerpc/lib/string_32.S
> +++ b/arch/powerpc/lib/string_32.S
> @@ -16,17 +16,34 @@
>         .text
>
>  _GLOBAL(memcmp)
> -       cmpwi   cr0, r5, 0
> -       beq-    2f
> -       mtctr   r5
> -       addi    r6,r3,-1
> -       addi    r4,r4,-1
> -1:     lbzu    r3,1(r6)
> -       lbzu    r0,1(r4)
> -       subf.   r3,r0,r3
> -       bdnzt   2,1b
> +       srawi.  r7, r5, 2               /* Divide len by 4 */
> +       mr      r6, r3
> +       beq-    3f
> +       mtctr   r7
> +       li      r7, 0
> +1:     lwzx    r3, r6, r7
> +       lwzx    r0, r4, r7
> +       addi    r7, r7, 4
> +       cmplw   cr0, r3, r0
> +       bdnzt   eq, 1b
> +       bne     5f
> +3:     andi.   r3, r5, 3
> +       beqlr
> +       cmplwi  cr1, r3, 2
> +       blt-    cr1, 4f
> +       lhzx    r3, r6, r7
> +       lhzx    r0, r4, r7
> +       addi    r7, r7, 2
> +       subf.   r3, r0, r3
> +       beqlr   cr1
> +       bnelr
> +4:     lbzx    r3, r6, r7
> +       lbzx    r0, r4, r7
> +       subf.   r3, r0, r3
>         blr
> -2:     li      r3,0
> +5:     li      r3, 1
> +       bgtlr
> +       li      r3, -1
>         blr
>  EXPORT_SYMBOL(memcmp)
>
> --
> 2.13.3
>