From: Alex Riesen <raa.lkml@gmail.com>
Date: Fri, 29 Apr 2011 07:05:46 GMT
Subject: Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons
Message-ID: <BANLkTikt0CU87maPs65WGi0oopD+g0uVDA@mail.gmail.com>
In-Reply-To: <BANLkTik-uk-mpdHZxcz8Nem=nEzED_tuJg@mail.gmail.com>

On Thu, Apr 28, 2011 at 13:59, Erik Faye-Lund <kusmabite@gmail.com> wrote:
> 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;

Can one take advantage of common expression optimization here?
Like this:

+       if (*sha1 - *sha2)
+               return *sha1 - *sha2;