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

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Message-Id: <20101215.100055.226772943.davem@davemloft.net>
Date:	Wed, 15 Dec 2010 10:00:55 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	bharrosh@...asas.com
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

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.

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