Re: Why Git is so fast

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:

quoted

     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