If he's really going for optimization, he might consider passing in the 'ret' array rather than creating a new one each time. (Or even better, using multiple-values return to pass them stack allocated).

It also might be good to use a macro or compiler macro to unroll the loops, then you don't have to worry about manipulating 2 looping pointers. All you'd end up with is a bunch of constant arefs and addition/multiplication.

He also might want to set debug to 0, although I don't have any way to test that (or any of this) at the moment.

Then you get something that looks like this:

http://paste.lisp.org/display/120946

I haven't tested it (I don't even know if it works!), but the disassembly looks faster...

/rampant speculation.