linux-kernel - Re: Big git diff speedup by avoiding x86 "fast string" memcmp

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Date:	Thu, 16 Dec 2010 11:53:09 +0200
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	David Miller <davem@...emloft.net>
CC:	npiggin@...il.com, hooanon05@...oo.co.jp, npiggin@...nel.dk,
	torvalds@...ux-foundation.org, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: Big git diff speedup by avoiding x86 "fast string" memcmp

On 12/15/2010 08:00 PM, David Miller wrote:
> From: Boaz Harrosh <bharrosh@...asas.com>
> Date: Wed, 15 Dec 2010 15:15:09 +0200
> 
>> I agree that the byte-compare or long-compare should give you very close
>> results in modern pipeline CPUs. But surly 12 increments-and-test should
>> show up against 3 (or even 2). I would say it must be a better plan.
> 
> For strings of these lengths the setup code necessary to initialize
> the inner loop and the tail code to handle the sub-word ending cases
> eliminate whatever gains there are.
> 

You miss understood me. I'm saying that we know the beggining of the
string is aligned and Nick offered to pad the last long, so surly
a shift by 2 (or 3) + the reduction of the 12 dec-and-test to 3
should give you an optimization?

> I know this as I've been hacking on assembler optimized strcmp() and
> memcmp() in my spare time over the past year or so.

Boaz
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