Re: Why Git is so fast

[Mike Hommey <mh@glandium.org>]

On Fri, May 01, 2009 at 09:24:34AM +0400, Dmitry Potapov wrote:
> 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.

I guess, though, that some enforced alignment could help produce
slightly more efficient code on some architectures (most notably sparc,
which really doesn't like to deal with unaligned words).

Mike