linux-kernel - Re: [PATCH] powerpc/lib: Remove .balign inside string functions for PPC32

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Date:   Thu, 17 May 2018 23:46:52 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/lib: Remove .balign inside string functions for PPC32

Nicholas Piggin <npiggin@...il.com> writes:

> On Thu, 17 May 2018 12:04:13 +0200 (CEST)
> Christophe Leroy <christophe.leroy@....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.leroy@....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.

Can we do it with a macro in the header, eg. like:

#ifdef CONFIG_PPC64
#define IFETCH_BALIGN	.balign IFETCH_ALIGN_BYTES
#endif

...

  	addi	r4,r4,-1
  	IFETCH_BALIGN
  1:	lbzu	r0,1(r4)


cheers