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

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Date:	Wed, 5 Aug 2015 23:39:38 -0500
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Scott Wood <scottwood@...escale.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, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop

On Wed, Aug 05, 2015 at 09:31:41PM -0500, Scott Wood wrote:
> 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?

The original loop was already optimal, as the comment said.  The new
code adds extra instructions and a mispredicted branch.  You also
might get less overlap between the loads and adde (I didn't check
if there is any originally): those instructions are no longer
interleaved.

I think it is a stupid idea to optimise code for all 32-bit PowerPC
CPUs based on solely what is best for a particularly simple, slow
implementation; and that is what this patch is doing.


Segher
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