Re: Why Git is so fast

From: Dmitry Potapov <dpotapov@gmail.com>
Date: May 1, 2009, 05:24 UTC
Message-ID: <20090501052434.GA4750@dpotapov.dyndns.org>
In-Reply-To: <86iqkllw0c.fsf@broadpark.no>

On Thu, Apr 30, 2009 at 10:36:03PM +0200, Kjetil Barvik wrote:

Show 16 quoted lines

>      4) The "static inline void hashcpy(....)" in cache.h could then
>         maybe be written like this:
> 
>   static inline void hashcpy(unsigned long sha_dst[5], const unsigned long sha_src[5])
>   {
>        sha_dst[0] = sha_src[0];
>        sha_dst[1] = sha_src[1];
>        sha_dst[2] = sha_src[2];
>        sha_dst[3] = sha_src[3];
>        sha_dst[4] = sha_src[4];
>   }
> 
>         And hopefully will be compiled to just 5 store/more
>         instructions, or at least hopefully be faster than the currently
>         memcpy() call. But mabye we get more compiled instructions compared
>         to a single call to memcpy()?

Good compilers can inline memcpy and should produce more efficient code for the target architecture, which can be faster than manually written. On x86_64, memcpy() requires only 3 load/store operations to copy SHA-1 while the above code requires 5 operations.

Dmitry