netdev - Re: [PATCH v5 net-next] net: Implement fast csum_partial for x86

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Date:	Thu, 3 Mar 2016 10:43:37 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Laight <David.Laight@...lab.com>
Cc:	Tom Herbert <tom@...bertland.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
	"kernel-team@...com" <kernel-team@...com>
Subject: Re: [PATCH v5 net-next] net: Implement fast csum_partial for x86_64

On Thu, Mar 3, 2016 at 8:12 AM, David Laight <David.Laight@...lab.com> wrote:
>
> Did you try the asm loop that used 'leax %rcx..., jcxz... jmps..'
> without any unrolling?

Is that actually supposed to work ok these days? jcxz used to be quite
slow, and is historically *never* used.

Now, in theory, loop constructs can actually do better on branch
prediction etc, but Intel seems to have never really tried to push
them, and has instead pretty much discouraged them in favor of making
the normal jumps go faster (including all the instruction fusion etc)

>From what I have seen, the whole "don't use LOOP or JRCXZ" has not changed.

           Linus