From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons
Date: Thu, 28 Apr 2011 02:35:41 +0200
Message-ID: <20110428003541.GA18382@linux-mips.org>
References: <20110427225114.GA16765@elte.hu>
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Cc: Ingo Molnar <mingo@elte.hu>, git@vger.kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	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?= Weisbecker <fweisbec@gmail.com>,
	Pekka Enberg <penberg@cs.helsinki.fi>
To: Junio C Hamano <gitster@pobox.com>
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On Wed, Apr 27, 2011 at 04:32:12PM -0700, Junio C Hamano wrote:

> > +static inline int is_null_sha1(const unsigned char *sha1)
> >  {
> > -	return memcmp(sha1, sha2, 20);
> > +	const unsigned long long *sha1_64 = (void *)sha1;
> > +	const unsigned int *sha1_32 = (void *)sha1;
> 
> Can everybody do unaligned accesses just fine?

Misaligned accesses cause exceptions on some architectures which then
are fixed up in software making these accesses _very_ slow.  You can
use __attribute__((packed)) to work around that but that will on the
affected architectures make gcc generate code pessimistically that is
slower than not using __attribute__((packed)) in case of proper
alignment.  And __attribute__((packed)) only works with GCC.

  Ralf