Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons
- From
Jonathan Nieder <jrnieder@gmail.com>
- Date
- Apr 28, 2011, 12:36 UTC
- Message-ID
- <20110428123617.GA2062@elie>
- In-Reply-To
- <BANLkTik-uk-mpdHZxcz8Nem=nEzED_tuJg@mail.gmail.com>
Hi,
A side note for amusement.
Erik Faye-Lund wrote:
Show 16 quoted lines
> --- 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