From mboxrd@z Thu Jan  1 00:00:00 1970
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons
Date: Thu, 28 Apr 2011 13:19:04 -0700
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References: <20110427225114.GA16765@elte.hu> <7voc3r5kzn.fsf@alter.siamese.dyndns.org> <20110428003541.GA18382@linux-mips.org> <20110428081817.GA29344@pcpool00.mathematik.uni-freiburg.de> <4DB9367B.2050607@op5.se> <BANLkTikU3evfo86WmQeVS_Z41s3xSK1DJw@mail.gmail.com>
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Cc: Andreas Ericsson <ae@op5.se>,
	"Bernhard R. Link" 
	<brl+ccmadness@pcpool00.mathematik.uni-freiburg.de>,
	Ralf Baechle <ralf@linux-mips.org>,
	Junio C Hamano <gitster@pobox.com>,
	Ingo Molnar <mingo@elte.hu>, git@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	=?ISO-8859-1?Q?Fr=E9d=E9ric_?= =?ISO-8859-1?Q?Weisbecker?= 
	<fweisbec@gmail.com>, Pekka Enberg <penberg@cs.helsinki.fi>
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On 04/28/2011 02:55 AM, Erik Faye-Lund wrote:
> 
> I disagree. We have no guarantee that the SHA-1s are aligned on x86
> either, and unaligned accesses are slow on x86.
> 

Not particularly, especially not statistically.  Furthermore, for a
sizable chunk like a SHA-1, not all accesses will have the cross-grain
penalities that you sometimes can have.

> I think it's much much cleaner to add an early-out on the first byte,
> and hope that memcmp is optimized properly. If it's not, those
> platforms can add an override to memcmp in git-compat-util and/or
> compat/*.

Overall, doing an architecture optimization library especially for
widely used architectures like x86 is not a bad idea.

	-hpa