Re: [PATCH 3/4] diffcore-pickaxe: further refactor count_match()

René Scharfe [off-list ref] writes:

I get this (Ubuntu 8.10 x64, Fedora 10 x64 using the same Linux repo,
Windows Vista x64 using a different Linux repo with the same HEAD on
NTFS and msysgit, numbers are the elapsed time in seconds, best of five
runs):

                           Ubuntu  Fedora  Windows
   v1.6.2-rc2                8.14    8.16    9.236
   v1.6.2-rc2+[1-4]          2.43    2.45    2.995
   v1.6.2-rc2+[1-4]+memmem   1.31    1.25    2.917
   v1.6.2-rc2+[1-3]+memmem   1.51    1.16    8.455

Ubuntu has glibc 2.8, while Fedora 10 has glibc 2.9, with a new and more
efficient memmem() implementation.  On Windows, we use our own naive
memmem() implementation.

Shoot, I was not being careful enough.

So using memmem() is worthwhile.  And providing a better fall-back
version in compat/ can speed up this particular case to the point where
the fourth patch becomes moot.

Are these numbers telling me that with a good memmem() implementation,
patch 4/4 is not just moot but actively detrimental?

With a long enough needle, it entirely is possible that scanning the whole
image with sublinear string search algorithm would perform much better
than the preprocessing patch 4/4 does which has to scan all the bytes in
the common parts.