conv2 performance

[John W. Eaton]

I recieved a complaint that Octave's conv2 function is about 5 times
slower than Matlab's.  By making a few simple changes, I was able to
improve the perfromance some, but not by a factor of 5.  My changes
are here:

  http://hg.savannah.gnu.org/hgweb/octave/rev/dc8637fd7a76

On my system, I see the following improvement after these changes:

  > n = 1024; m = 64; x = rand(n,n); k = rand(m,m); tic; y = conv2(x,k); toc

  old: Elapsed time is 26.2576 seconds.
  new: Elapsed time is 13.6104 seconds.

and

  > n = 1024; m = 512; x = rand(n,1); k = rand(m,1); m = rand (m,n);
  > tic; y = conv2(x,k,m); toc

  old: Elapsed time is 8.53273 seconds.
  new: Elapsed time is 3.27103 seconds.

Maybe there is still room for improvement here.  I would happily use a
free software library with a GPL-compatible license to implement this
function, but I don't know whether one is available.

jwe