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

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Message-ID: <4D08BF5D.1060509@panasas.com>
Date:	Wed, 15 Dec 2010 15:15:09 +0200
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Nick Piggin <npiggin@...il.com>
CC:	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	Nick Piggin <npiggin@...nel.dk>,
	Linus Torvalds <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 06:06 AM, Nick Piggin wrote:
> 
> However instead of a normal memcmp, we could actually pad dentry
> names out to sizeof(long) with zeros, and take advantage of that with
> a memcmp that does not have to handle tails -- it would operate
> entirely with longs.
> 
> That would avoid icache and branch regressions, and might speed up
> the operation on some architectures. I just doubted whether it would
> show an improvement to be worth doing at all. If it does, I'm all for it.
> 

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.

BTW the long approach if you assume that the beginning of the string
is long aligned than it is only a matter of comparing the last byte
with a mask, no branches. But I'm not saying, just make sure they are
padded.

Just my $0.017
Thanks
Boaz
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