From: Jonathan Nieder <jrnieder@gmail.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: 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: Wed, 27 Apr 2011 18:18:12 -0500 [thread overview]
Message-ID: <20110427231748.GA26632@elie> (raw)
In-Reply-To: <20110427225114.GA16765@elte.hu>
Ingo Molnar wrote:
> Most overhead is in hashcmp(), which uses memcmp(), which falls back to
> assembly string operations.
>
> But we know that hashcmp() compares hashes, which if they do not match, the first byte
> will differ in 99% of the cases.
>
> So i tried the patch below: instead of relying on GCC putting in the string
> ops, i used an open-coded loop for this relatively short comparison, which does
> not go beyond the first byte in 99% of the cases.
[...]
> --- a/cache.h
> +++ b/cache.h
> @@ -675,14 +675,33 @@ extern char *sha1_pack_name(const unsigned char *sha1);
[...]
> +static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
> {
> - return memcmp(sha1, sha2, 20);
> + int i;
> +
> + for (i = 0; i < 20; i++, sha1++, sha2++) {
Hm. This would be very sensitive to the compiler, since a too-smart
optimizer could take this loop and rewrite it back to memcmp! So I
wonder if it's possible to convey this to the compiler more precisely:
return memcmp_probably_differs_early(sha1, sha2, 20);
E.g., how would something like
const unsigned int *start1 = (const unsigned int *) sha1;
const unsigned int *start2 = (const unsigned int *) sha2;
if (likely(*start1 != *start2)) {
if (*start1 < *start2)
return -1;
return +1;
}
return memcmp(sha1 + 4, sha2 + 4, 16);
perform?
I suspect we don't have to worry about endianness as long as hashcmp
yields a consistent ordering, but I haven't checked.
Thanks, that was interesting.
Regards,
Jonathan
next prev parent reply other threads:[~2011-04-27 23:18 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 [this message]
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
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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