netdev - Re: [net-next PATCH] csum: Update csum_block_add to use rotate instead of byteswap

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Date:	Tue, 08 Mar 2016 21:50:47 -0800
From:	Joe Perches <joe@...ches.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Alexander Duyck <aduyck@...antis.com>,
	Netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [net-next PATCH] csum: Update csum_block_add to use rotate
 instead of byteswap

On Tue, 2016-03-08 at 21:23 -0800, Alexander Duyck wrote:
> On Tue, Mar 8, 2016 at 3:25 PM, Joe Perches <joe@...ches.com> wrote:
> > On Tue, 2016-03-08 at 14:42 -0800, Alexander Duyck wrote:
> > > The code for csum_block_add was doing a funky byteswap to swap the even and
> > > odd bytes of the checksum if the offset was odd.  Instead of doing this we
> > > can save ourselves some trouble and just shift by 8 as this should have the
> > > same effect in terms of the final checksum value and only requires one
> > > instruction.
> > 3 instructions?
> I was talking about just the one ror vs mov, shl, shr, and ,and, add.
> 
> I assume when you say 3 you are including the test and either some
> form of conditional move or jump?

Yeah, instruction count also depends on architecture (arm/x86/ppc...)

> > > diff --git a/include/net/checksum.h b/include/net/checksum.h
[]
> > > @@ -88,8 +88,10 @@ static inline __wsum
> > >  csum_block_add(__wsum csum, __wsum csum2, int offset)
> > >  {
> > >       u32 sum = (__force u32)csum2;
> > > -     if (offset&1)
> > > -             sum = ((sum&0xFF00FF)<<8)+((sum>>8)&0xFF00FF);
> > > +
> > > +     if (offset & 1)
> > > +             sum = (sum << 24) + (sum >> 8);
> > Maybe use ror32(sum, 8);
> I was actually thinking I could use something like this.  I didn't
> realize it was even available.

Now you know: bitops.h

> > or maybe something like:
> > 
> > {
> >         u32 sum;
> > 
> >         /* rotated csum2 of odd offset will be the right checksum */
> >         if (offset & 1)
> >                 sum = ror32((__force u32)csum2, 8);
> >         else
> >                 sum = (__force u32)csum2;
> > 
> Any specific reason for breaking it up like this?  It seems like it
> was easier to just have sum be assigned first and then rotating it if
> needed.  What is gained by splitting the assignment up over two
> different calls?

It's only for reader clarity where a comment could be useful.
The compiler output shouldn't change.