Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons

From: Junio C Hamano <gitster@pobox.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 16:32:12 -0700	[thread overview]
Message-ID: <7voc3r5kzn.fsf@alter.siamese.dyndns.org> (raw)
In-Reply-To: <20110427225114.GA16765@elte.hu> (Ingo Molnar's message of "Thu, 28 Apr 2011 00:51:14 +0200")

Ingo Molnar <mingo@elte.hu> writes:

> +static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
>  {
> -	return !memcmp(sha1, null_sha1, 20);
> +	int i;
> +
> +	for (i = 0; i < 20; i++, sha1++, sha2++) {
> +		if (*sha1 != *sha2) {
> +			if (*sha1 < *sha2)
> +				return -1;
> +			return +1;
> +		}
> +	}
> +
> +	return 0;

This is very unfortunate, as it is so trivially correct and we shouldn't
have to do it.  If the compiler does not use a good inlined memcmp(), this
patch may fly, but I fear it may hurt other compilers, no?

> +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?