linux-kernel - Re: [PATCH] x86: Run checksumming in parallel accross multiple alu's

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Message-ID: <1383322612.1737.6.camel@bwh-desktop.uk.level5networks.com>
Date:	Fri, 1 Nov 2013 16:16:52 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Neil Horman <nhorman@...driver.com>
CC:	Doug Ledford <dledford@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	"Eric Dumazet" <eric.dumazet@...il.com>,
	<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
	David Laight <David.Laight@...LAB.COM>
Subject: Re: [PATCH] x86: Run checksumming in parallel accross multiple alu's

On Fri, 2013-11-01 at 12:08 -0400, Neil Horman wrote:
> On Fri, Nov 01, 2013 at 03:42:46PM +0000, Ben Hutchings wrote:
> > On Thu, 2013-10-31 at 14:30 -0400, Neil Horman wrote:
> > [...]
> > > It
> > > functions, but unfortunately the performance lost to the completely broken
> > > branch prediction that this inflicts makes it a non starter:
> > [...]
> > 
> > Conditional branches are no good but conditional moves might be worth a shot.
> > 
> > Ben.
> > 
> How would you suggest replacing the jumps in this case?  I agree it would be
> faster here, but I'm not sure how I would implement an increment using a single
> conditional move.

You can't, but it lets you use additional registers as carry flags.
Whether there are enough registers and enough parallelism to cancel out
the extra additions required, I don't know.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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