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SubjectRe: [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

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