permute optimization

[John W. Eaton]

On 16-Dec-2008, Jaroslav Hajek wrote:

| using a similar technique as for dense indexing, I've rewritten
| permute, using a recursive, possibly parallelizable algorithm.
| As usual, a simplistic benchmark follows (set n to suitable value):
| n = 80;
| a = ones (n, 2*n, 2*n, 2, 5);
| tic; b = permute (a, [1, 2, 3, 5, 4]); toc
| clear b
| tic; b = permute (a, [3, 4, 1, 2, 5]); toc
| clear b
| tic; b = permute (a, [4, 3, 1, 2, 5]); toc
| 
| 
| before patch:
| 
| Elapsed time is 0.202561 seconds.
| Elapsed time is 0.290111 seconds.
| Elapsed time is 1.05752 seconds.
| 
| after:
| 
| Elapsed time is 0.10301 seconds.
| Elapsed time is 0.212579 seconds.
| Elapsed time is 0.299735 seconds.
| 
| I.e. speed-up by 96.6%, 36.5%, 252.8% (measured as old time / new time - 1).

Thanks for working on this.

| there is a further optimization possible - a blocked generalized
| transpose analogous to David's transpose optimization. Given the
| relative importance of transpose vs. permute, I don't think it's
| necessary, but certainly possible.

I don't see it as a high priority.  Of course, it is up to you whether
you would like to work on it.

Thanks,

jwe