Re: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64

From	"George Spelvin" <linux@horizon.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64
Date	2016-02-08 21:20 +0100
Message-ID	<r00rT-1Mq-5@gated-at.bofh.it> (permalink)
Organization	linux.* mail to news gateway

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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...

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Re: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64 "George Spelvin" <linux@horizon.com> - 2016-02-08 21:20 +0100
  RE: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64 David Laight <David.Laight@ACULAB.COM> - 2016-02-09 12:00 +0100
    RE: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64 "George Spelvin" <linux@horizon.com> - 2016-02-10 02:00 +0100
      RE: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64 David Laight <David.Laight@ACULAB.COM> - 2016-02-10 12:50 +0100
        RE: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64 "George Spelvin" <linux@horizon.com> - 2016-02-10 15:50 +0100
          RE: [PATCH v3 net-next] net: Implement fast csum_partial for x86_64 David Laight <David.Laight@ACULAB.COM> - 2016-02-10 16:30 +0100

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