machine learning for loop unrolling

[Ken Raeburn]

>   - compile with the loop unrolled 1x, 2x, 4x, 8x, 16x, 32x and
> measure the time the benchmark takes

The optimal unrolling factor may not be a power of two, depending on  
icache size (11 times the loop body size?), iteration count (13*n for  
some unknown n?), and whether there are actions performed inside the  
loop once or twice every N passes (for N not a power of two).

The powers of two would probably hit a lot of the common cases, but  
you might want to throw in some intermediate values too, if it's too  
costly to check all practical values.

Ken