linux-kernel - Re: [PATCH v3 2/2] powerpc: add support for csum

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Date:	Tue, 26 May 2015 14:42:35 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	'Christophe Leroy' <christophe.leroy@....fr>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Michael Ellerman" <mpe@...erman.id.au>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] powerpc: add support for csum_add()

On Tue, 2015-05-26 at 13:57 +0000, David Laight wrote:
> From: Scott Wood ...
> > > I'd also have thought that the 64bit C version above would be 
> > > generally 'good'.
> > 
> > It doesn't generate the addc/addze sequence.  At least with GCC 
> > 4.8.2,
> > it does something like:
> > 
> >     mr      tmp0, csum
> >     li      tmp1, 0
> >     li      tmp2, 0
> >     addc    tmp3, addend, tmp0
> >     adde    csum, tmp2, tmp1
> >     add     csum, csum, tmp3
> 
> I was thinking of all 64bit targets, not 32bit ones.

Oh, you mean move it out of arch/powerpc?  Sounds reasonable, but 
someone should probably check what the resulting code looks like on 
other common arches.  OTOH, if we're going to modify non-arch code, 
that might be a good opportunity to implement Segher's suggestion and 
move to a 64-bit accumulator.

-Scott

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