Re: [PATCH 6/9] powerpc32: optimise a few instructions in csum_partial()

[Christophe Leroy]

Le 23/10/2015 05:30, Scott Wood a Ãcrit :
> On Tue, 2015-09-22 at 16:34 +0200, Christophe Leroy wrote:
> > r5 does contain the value to be updated, so lets use r5 all way long
> > for that. It makes the code more readable.
> >
> > To avoid confusion, it is better to use adde instead of addc
> >
> > The first addition is useless. Its only purpose is to clear carry.
> > As r4 is a signed int that is always positive, this can be done by
> > using srawi instead of srwi
> >
> > Let's also remove the comment about bdnz having no overhead as it
> > is not correct on all powerpc, at least on MPC8xx
> >
> > In the last part, in our situation, the remaining quantity of bytes
> > to be proceeded is between 0 and 3. Therefore, we can base that part
> > on the value of bit 31 and bit 30 of r4 instead of anding r4 with 3
> > then proceding on comparisons and substractions.
> >
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> > ---
> >   arch/powerpc/lib/checksum_32.S | 37 +++++++++++++++++--------------------
> >   1 file changed, 17 insertions(+), 20 deletions(-)
> Do you have benchmarks for these optimizations?
>
> -Scott
Using mftbl() to get timebase just before and after call to 
csum_partial(), I get the following on an MPC885:
* 78 bytes packets: 9% faster (11,5 to 10,4 tb ticks)
* 328 bytes packets: 3% faster (47,9 to 46,5 tb ticks)

Christophe
> > diff --git a/arch/powerpc/lib/checksum_32.S b/arch/powerpc/lib/checksum_32.S
> > index 3472372..9c12602 100644
> > --- a/arch/powerpc/lib/checksum_32.S
> > +++ b/arch/powerpc/lib/checksum_32.S
> > @@ -27,35 +27,32 @@
> >    * csum_partial(buff, len, sum)
> >    */
> >   _GLOBAL(csum_partial)
> > -     addic   r0,r5,0
> >        subi    r3,r3,4
> > -     srwi.   r6,r4,2
> > +     srawi.  r6,r4,2         /* Divide len by 4 and also clear carry */
> >        beq     3f              /* if we're doing < 4 bytes */
> > -     andi.   r5,r3,2         /* Align buffer to longword boundary */
> > +     andi.   r0,r3,2         /* Align buffer to longword boundary */
> >        beq+    1f
> > -     lhz     r5,4(r3)        /* do 2 bytes to get aligned */
> > -     addi    r3,r3,2
> > +     lhz     r0,4(r3)        /* do 2 bytes to get aligned */
> >        subi    r4,r4,2
> > -     addc    r0,r0,r5
> > +     addi    r3,r3,2
> >        srwi.   r6,r4,2         /* # words to do */
> > +     adde    r5,r5,r0
> >        beq     3f
> >   1:   mtctr   r6
> > -2:   lwzu    r5,4(r3)        /* the bdnz has zero overhead, so it should */
> > -     adde    r0,r0,r5        /* be unnecessary to unroll this loop */
> > +2:   lwzu    r0,4(r3)
> > +     adde    r5,r5,r0
> >        bdnz    2b
> > -     andi.   r4,r4,3
> > -3:   cmpwi   0,r4,2
> > -     blt+    4f
> > -     lhz     r5,4(r3)
> > +3:   andi.   r0,r4,2
> > +     beq+    4f
> > +     lhz     r0,4(r3)
> >        addi    r3,r3,2
> > -     subi    r4,r4,2
> > -     adde    r0,r0,r5
> > -4:   cmpwi   0,r4,1
> > -     bne+    5f
> > -     lbz     r5,4(r3)
> > -     slwi    r5,r5,8         /* Upper byte of word */
> > -     adde    r0,r0,r5
> > -5:   addze   r3,r0           /* add in final carry */
> > +     adde    r5,r5,r0
> > +4:   andi.   r0,r4,1
> > +     beq+    5f
> > +     lbz     r0,4(r3)
> > +     slwi    r0,r0,8         /* Upper byte of word */
> > +     adde    r5,r5,r0
> > +5:   addze   r3,r5           /* add in final carry */
> >        blr
> >   
> >   /*