linux-kernel - Re: [PATCH v2 2/2] powerpc32: optimise csum

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Date:	Wed, 5 Aug 2015 21:31:41 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
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>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop

On Wed, 2015-08-05 at 19:30 -0500, Segher Boessenkool wrote:
> On Wed, Aug 05, 2015 at 03:29:35PM +0200, Christophe Leroy wrote:
> > On the 8xx, load latency is 2 cycles and taking branches also takes
> > 2 cycles. So let's unroll the loop.
> 
> This is not true for most other 32-bit PowerPC; this patch makes
> performance worse on e.g. 6xx/7xx/7xxx.  Let's not!

Chips with a load latency greater than 2 cycles should also benefit from the 
unrolling.  Have you benchmarked this somewhere and seen it reduce 
performance?  Do you know of any 32-bit PPC chips with a load latency less 
than 2 cycles?

-Scott

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