RFR: 8234331: Add robust and optimized utility for rounding up to next power of two+

[John Rose]

On Nov 27, 2019, at 11:34 PM, Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
> 
> In the end, I wonder whether we should have two kind of APIs, or a
> parameter, distinguishing between "next power of 2" and "next power of 2
> unless input value is already power of 2”.

Naming is important for clarity.  “Round up” means if it’s already “rounded”
(whatever that means) the input is returned unchanged.

The other notion is a true “next up”, because it always increases —
barring overflow.  The possibility of overflow makes the “next up” function
more bug-prone than the “round up” function.

The usual trick for deriving that second function is to add one to the argument
to the first function, ensuring that the result will always increase.

If (for clarity) we implement a “next power of two” function, rather than ask
coders to use the +1 trick, the second function should be implemented in terms of
the first function using the +1 trick, maybe with an assert added against overflow.

My $0.02.

— John