Re: Why Git is so fast

[Dmitry Potapov <dpotapov@gmail.com>]

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