[keithwhor]

Nice! Would love to see it implemented - though that popcnt instruction isn't accessible via JavaScript as far as I'm aware. ;)

[DannyBee]

Also note - your while loop isn't 0 cycles, in fact, it's probably worse than the bit-twiddling depending on sparseness.

This is because the test and branch always happens, and is essentially guaranteed to be mispredicted a lot.

These branch mispredictions are likely to cost a lot more than the n cycles it theoretically saves.