The stupid thing here is, I still don't remember which one is the
generic thing, rounddown() or round_down(). I have to look it up every
single time to be sure. I refuse to believe I'd be the only one.
It's okay to accidentally use the generic version, no harm done. It's
definitely not okay to accidentally use the special pow-2 version, so it
should have a special name. I think _pow2() or _pow_2() is a fine
suffix.