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

From: Pekka Enberg <[email-masked]>
Date: Thu, 28 Apr 2011 15:12:13 +0300

On 4/28/11 2:59 PM, Erik Faye-Lund wrote:
> So my preference is still something like this. Call me conservative ;)
>
> diff --git a/cache.h b/cache.h
> index c730c58..8bc03c6 100644
> --- 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);
>   }
> -static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
> +static inline int is_null_sha1(const unsigned char *sha1)
>   {
> -	return memcmp(sha1, sha2, 20);
> +	return !hashcmp(sha1, null_sha1);
>   }
>   static inline void hashcpy(unsigned char *sha_dst, const unsigned
> char *sha_src)
>   {

Yup, might be the most reasonable thing to do if it still speeds things up.

			Pekka