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

From: Jonathan Nieder <[email-masked]>
Date: Thu, 28 Apr 2011 07:36:17 -0500

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).

Anyway, assuming it does not kill performance for some reason, the
above sounds good to me.  Thanks for spelling it out.

Jonathan