netdev - RE: [PATCH 3.12 065/122] lib/checksum.c: fix carry in csum_tcpudp

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Date:	Wed, 18 Feb 2015 09:40:23 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Karl Beldan' <karl.beldan@...il.com>
CC:	'Jiri Slaby' <jslaby@...e.cz>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Karl Beldan <karl.beldan@...ierawaves.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	Mike Frysinger <vapier@...too.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: RE: [PATCH 3.12 065/122] lib/checksum.c: fix carry in
 csum_tcpudp_nofold

From: Karl Beldan 
> On Tue, Feb 17, 2015 at 12:04:22PM +0000, David Laight wrote:
> > > +static inline u32 from64to32(u64 x)
> > > +{
> > > +	/* add up 32-bit and 32-bit for 32+c bit */
> > > +	x = (x & 0xffffffff) + (x >> 32);
> > > +	/* add up carry.. */
> > > +	x = (x & 0xffffffff) + (x >> 32);
> > > +	return (u32)x;
> > > +}
> >
> > As a matter of interest, does the compiler optimise away the
> > second (x & 0xffffffff) ?
> > The code could just be:
> > 	x = (x & 0xffffffff) + (x >> 32);
> > 	return x + (x >> 32);
> >
>
> On my side, from what I've seen so far, your version results in better
> assembly, esp. with clang, but my first version
> http://article.gmane.org/gmane.linux.kernel/1875407:
>         x += (x << 32) + (x >> 32);
> 	return (__force __wsum)(x >> 32);
> resulted in even better assembly, I just verified with gcc/clang,
> x86_64/ARM and -O1,2,3.

The latter looks to have a shorter dependency chain as well.
Although I'd definitely include a comment saying that it is equivalent
to the two lines in the current patch.

Does either compiler manage to use a rotate for the two shifts?
Using '(x << 32) | (x >> 32)' might convince it to do so.
That would reduce it to three 'real' instructions and a register rename.

	David