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

[Christophe LEROY]

Le 24/05/2018 Ã 19:24, Segher Boessenkool a ÃcritÂ:
> 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