From: Ingo Molnar <mingo@elte•hu>
To: Jonathan Nieder <jrnieder@gmail•com>
Cc: "Erik Faye-Lund" <kusmabite@gmail•com>,
"Pekka Enberg" <penberg@cs•helsinki.fi>,
"Junio C Hamano" <gitster@pobox•com>,
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>
Subject: Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons
Date: Thu, 28 Apr 2011 15:37:08 +0200 [thread overview]
Message-ID: <20110428133708.GA31383@elte.hu> (raw)
In-Reply-To: <20110428123617.GA2062@elie>
* Jonathan Nieder <jrnieder@gmail•com> wrote:
> Hi,
>
> A side note for amusement.
>
> Erik Faye-Lund wrote:
>
> > --- a/cache.h
> > +++ b/cache.h
> > @@ -681,13 +681,17 @@ extern char *sha1_pack_name(const unsigned char *sha1);
> > extern char *sha1_pack_index_name(const unsigned char *sha1);
> > extern const char *find_unique_abbrev(const unsigned char *sha1, int);
> > extern const unsigned char null_sha1[20];
> > -static inline int is_null_sha1(const unsigned char *sha1)
> > +static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
> > {
> > - return !memcmp(sha1, null_sha1, 20);
> > + /* early out for fast mis-match */
> > + if (*sha1 != *sha2)
> > + return *sha1 - *sha2;
> > +
> > + return memcmp(sha1 + 1, sha2 + 1, 19);
> > }
>
> On the off-chance that sha1 and sha2 are nicely aligned, a more
> redundant
>
> if (*sha1 != *sha2)
> return *sha1 - *sha2;
>
> return memcmp(sha1, sha2, 20);
>
> would take advantage of that (yes, this is just superstition, but it
> somehow seems comforting anyway).
Your variant also makes the code slightly more compact as the sha1+1 and sha2+1
addresses do not have to be computed. I'll re-test and resend this variant.
Thanks,
Ingo
next prev parent reply other threads:[~2011-04-28 13:38 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
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 [this message]
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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