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

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Date:	8 Feb 2016 15:12:34 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	David.Laight@...LAB.COM, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, tom@...bertland.com
Cc:	mingo@...nel.org
Subject: Re: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64

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...