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: Fri, 29 Apr 2011 09:24:08 -0700
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References: <20110427225114.GA16765@elte.hu> <7voc3r5kzn.fsf@alter.siamese.dyndns.org> <20110428062717.GA952@elte.hu> <BANLkTik_2sHZ0OTgQeHpRnpmNsAmT=sAcA@mail.gmail.com> <20110428093703.GB15349@elte.hu> <BANLkTim+Kk_ah_4+pQKCi8bXtA8thRVRjQ@mail.gmail.com> <4DB93D16.4000603@cs.helsinki.fi> <BANLkTimD7KZz4fS0QynPui7-JQS10AkLtg@mail.gmail.com> <4DB941CD.2050403@cs.helsinki.fi> <BANLkTik-uk-mpdHZxcz8Nem=nEzED_tuJg@mail.gmail.com> <BANLkTikt0CU87maPs65WGi0oopD+g0uVDA@mail.gmail.com>
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Cc: kusmabite@gmail.com, Pekka Enberg <penberg@cs.helsinki.fi>,
	Ingo Molnar <mingo@elte.hu>,
	Junio C Hamano <gitster@pobox.com>, git@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	=?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>
To: Alex Riesen <raa.lkml@gmail.com>
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On 04/29/2011 12:05 AM, Alex Riesen wrote:
> 
> Can one take advantage of common expression optimization here?
> Like this:
> 
> +       if (*sha1 - *sha2)
> +               return *sha1 - *sha2;
> 

Or even clue the compiler in explicitly:

	delta = *sha1 - *sha2;
	if (delta)
		return delta;

For newer x86-specific optimization there are a bunch of newer SSE
instructions which can be used to do bulk compares which may be faster
if optimized for the fixed length compare; of course it requires a new
enough processor.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.