linux-kernel - Re: [PATCH v4 3/3] powerpc/lib: optimise PPC32 memcmp

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Date:   Thu, 24 May 2018 12:24:58 -0500
From:   Segher Boessenkool <segher@...nel.crashing.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>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] powerpc/lib: optimise PPC32 memcmp

On Wed, May 23, 2018 at 09:47:32AM +0200, Christophe Leroy 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

> diff --git a/arch/powerpc/lib/string_32.S b/arch/powerpc/lib/string_32.S
> index 40a576d56ac7..542e6cecbcaf 100644
> --- a/arch/powerpc/lib/string_32.S
> +++ b/arch/powerpc/lib/string_32.S
> @@ -16,17 +16,45 @@
>  	.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:
> +#ifdef __LITTLE_ENDIAN__
> +	lwbrx	r3, r6, r7
> +	lwbrx	r0, r4, r7
> +#else
> +	lwzx	r3, r6, r7
> +	lwzx	r0, r4, r7
> +#endif

You don't test whether the pointers are word-aligned.  Does that work?
Say, when a load is crossing a page boundary, or segment boundary.


Segher