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

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Date:	Fri, 22 May 2015 16:39:56 -0500
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Scott Wood <scottwood@...escale.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v3 2/2] powerpc: add support for csum_add()

On Fri, May 22, 2015 at 02:32:42PM -0500, Scott Wood wrote:
> > 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

Right.  Don't expect older compilers to do sane things here.

All this begs a question...  If it is worth spending so much time
micro-optimising this, why not pick the low-hanging fruit first?
Having a 32-bit accumulator for ones' complement sums, on a 64-bit
system, is not such a great idea.


Segher
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