On Thu, 17 May 2018 12:04:13 +0200 (CEST)
Christophe Leroy <christophe.le...@c-s.fr> wrote:
> commit 87a156fb18fe1 ("Align hot loops of some string functions")
> degraded the performance of string functions by adding useless
> nops
>
> A simple benchmark on an 8xx calling 100000x a memchr() that
> matches the first byte runs in 41668 TB ticks before this patch
> and in 35986 TB ticks after this patch. So this gives an
> improvement of approx 10%
>
> Another benchmark doing the same with a memchr() matching the 128th
> byte runs in 1011365 TB ticks before this patch and 1005682 TB ticks
> after this patch, so regardless on the number of loops, removing
> those useless nops improves the test by 5683 TB ticks.
>
> Fixes: 87a156fb18fe1 ("Align hot loops of some string functions")
> Signed-off-by: Christophe Leroy <christophe.le...@c-s.fr>
> ---
> Was sent already as part of a serie optimising string functions.
> Resending on itself as it is independent of the other changes in the
> serie
>
> arch/powerpc/lib/string.S | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/arch/powerpc/lib/string.S b/arch/powerpc/lib/string.S
> index a787776822d8..a026d8fa8a99 100644
> --- a/arch/powerpc/lib/string.S
> +++ b/arch/powerpc/lib/string.S
> @@ -23,7 +23,9 @@ _GLOBAL(strncpy)
> mtctr r5
> addi r6,r3,-1
> addi r4,r4,-1
> +#ifdef CONFIG_PPC64
> .balign 16
> +#endif
> 1: lbzu r0,1(r4)
> cmpwi 0,r0,0
> stbu r0,1(r6)
The ifdefs are a bit ugly, but you can't argue with the numbers. These
alignments should be IFETCH_ALIGN_BYTES, which is intended to optimise
the ifetch performance when you have such a loop (although there is
always a tradeoff for a single iteration).
Would it make sense to define that for 32-bit as well, and you could use
it here instead of the ifdefs? Small CPUs could just use 0.
Thanks,
Nick
