From: Ralf Baechle <ralf@linux-mips.org>
To: Junio C Hamano <gitster@pobox.com>
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>,
"Frédéric Weisbecker" <fweisbec@gmail.com>,
"Pekka Enberg" <penberg@cs.helsinki.fi>
Subject: Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons
Date: Thu, 28 Apr 2011 02:35:41 +0200 [thread overview]
Message-ID: <20110428003541.GA18382@linux-mips.org> (raw)
In-Reply-To: <7voc3r5kzn.fsf@alter.siamese.dyndns.org>
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
next prev parent reply other threads:[~2011-04-28 0:35 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-27 22:51 [PATCH] git gc: Speed it up by 18% via faster hash comparisons Ingo Molnar
2011-04-27 23:10 ` Ingo Molnar
2011-04-27 23:18 ` Jonathan Nieder
2011-04-28 6:36 ` Ingo Molnar
2011-04-28 9:31 ` Jonathan Nieder
2011-04-28 10:36 ` Ingo Molnar
2011-04-28 9:32 ` Dmitry Potapov
2011-04-27 23:32 ` Junio C Hamano
2011-04-28 0:35 ` Ralf Baechle [this message]
2011-04-28 8:18 ` Bernhard R. Link
2011-04-28 9:42 ` Andreas Ericsson
2011-04-28 9:55 ` Erik Faye-Lund
2011-04-28 20:19 ` H. Peter Anvin
2011-04-28 6:27 ` Ingo Molnar
2011-04-28 9:17 ` Erik Faye-Lund
2011-04-28 9:33 ` Ingo Molnar
2011-04-28 9:37 ` Ingo Molnar
2011-04-28 9:50 ` Erik Faye-Lund
2011-04-28 10:10 ` Pekka Enberg
2011-04-28 10:19 ` Erik Faye-Lund
2011-04-28 10:30 ` Pekka Enberg
2011-04-28 11:59 ` Erik Faye-Lund
2011-04-28 12:12 ` Pekka Enberg
2011-04-28 12:36 ` Jonathan Nieder
2011-04-28 12:40 ` Erik Faye-Lund
2011-04-28 13:37 ` Ingo Molnar
2011-04-28 15:14 ` Ingo Molnar
2011-04-28 16:00 ` Erik Faye-Lund
2011-04-28 20:32 ` Ingo Molnar
2011-04-29 7:05 ` Alex Riesen
2011-04-29 16:24 ` H. Peter Anvin
2011-04-28 12:16 ` Tor Arntsen
2011-04-28 20:23 ` H. Peter Anvin
2011-04-28 12:17 ` Andreas Ericsson
2011-04-28 12:28 ` Erik Faye-Lund
2011-04-28 10:19 ` Ingo Molnar
2011-04-28 12:02 ` Nguyen Thai Ngoc Duy
2011-04-28 12:18 ` Erik Faye-Lund
2011-04-28 20:20 ` Junio C Hamano
2011-04-28 16:36 ` Dmitry Potapov
2011-04-28 8:52 ` Dmitry Potapov
2011-04-28 9:11 ` Ingo Molnar
2011-04-28 9:31 ` Dmitry Potapov
2011-04-28 9:44 ` Ingo Molnar
2011-04-28 9:38 ` Ingo Molnar
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