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

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Date:   Fri, 25 May 2018 07:55:51 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
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



Le 24/05/2018 à 19:24, Segher Boessenkool a écrit :
> 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?

copy_tofrom_user() word-aligns the store address and doesn't take care 
of the load address, so I believe it works.

Now, I just read in the MPC885 Ref Manual that unaligned access 
generates alignment exception when the processor is running in LE mode.

Ref. made to the discussion on patch "powerpc/32be: use stmw/lmw for 
registers save/restore in asm" 
(https://patchwork.ozlabs.org/patch/899465/), I will drop the handling 
for LE mode.

Christophe

> Say, when a load is crossing a page boundary, or segment boundary.
> 
> 
> Segher
>