From: Segher Boessenkool <segher@kernel•crashing.org>
To: Scott Wood <scottwood@freescale•com>
Cc: Christophe Leroy <christophe.leroy@c-s•fr>,
Benjamin Herrenschmidt <benh@kernel•crashing.org>,
Paul Mackerras <paulus@samba•org>,
Michael Ellerman <mpe@ellerman•id.au>,
linuxppc-dev@lists•ozlabs.org, linux-kernel@vger•kernel.org
Subject: Re: [PATCH v2 2/2] powerpc32: optimise csum_partial() loop
Date: Wed, 5 Aug 2015 23:39:38 -0500 [thread overview]
Message-ID: <20150806043938.GE18479@gate.crashing.org> (raw)
In-Reply-To: <1438828301.2097.126.camel@freescale.com>
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
next prev parent reply other threads:[~2015-08-06 4:40 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-05 13:29 [PATCH v2 0/2] powerpc32: Optimise csum_partial() Christophe Leroy
2015-08-05 13:29 ` [PATCH v2 1/2] powerpc32: optimise a few instructions in csum_partial() Christophe Leroy
2015-08-05 13:29 ` [PATCH v2 2/2] powerpc32: optimise csum_partial() loop Christophe Leroy
2015-08-06 0:30 ` Segher Boessenkool
2015-08-06 2:31 ` Scott Wood
2015-08-06 4:39 ` Segher Boessenkool [this message]
2015-08-06 22:45 ` Scott Wood
2015-08-06 23:25 ` Segher Boessenkool
2015-08-17 10:56 ` leroy christophe
2015-08-17 11:00 ` leroy christophe
2015-08-17 13:05 ` leroy christophe
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