netdev - RE: [PATCH v3 net-next] net: Implement fast csum_partial for x86

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Date:	Tue, 9 Feb 2016 10:48:14 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'George Spelvin' <linux@...izon.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"tom@...bertland.com" <tom@...bertland.com>
CC:	"mingo@...nel.org" <mingo@...nel.org>
Subject: RE: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64

From: George Spelvin [mailto:linux@...izon.com]
> Sent: 08 February 2016 20:13
> David Laight wrote:
> > I'd need convincing that unrolling the loop like that gives any significant gain.
> > You have a dependency chain on the carry flag so have delays between the 'adcq'
> > instructions (these may be more significant than the memory reads from l1 cache).
> 
> If the carry chain is a bottleneck, on Broadwell+ (feature flag
> X86_FEATURE_ADX), there are the ADCX and ADOX instructions, which use
> separate flag bits for their carry chains and so can be interleaved.
> 
> I don't have such a machine to test on, but if someone who does
> would like to do a little benchmarking, that would be an interesting
> data point.
> 
> Unfortunately, that means yet another version of the main loop,
> but if there's a significant benefit...

Well, the only part actually worth writing in assembler is the 'adc' loop.
So run-time substitution of separate versions (as is done for memcpy())
wouldn't be hard.

Since adcx and adox must execute in parallel I clearly need to re-remember
how dependencies against the flags register work. I'm sure I remember
issues with 'false dependencies' against the flags.

However you still need a loop construct that doesn't modify 'o' or 'c'.
Using leal, jcxz, jmp might work.
(Unless broadwell actually has a fast 'loop' instruction.)

(I've not got a suitable test cpu.)

	David