We'll keep re-evaluating this over time, but I'm currently not sure it's that serious a problem. For many numerically intensive computations, you'll want to move over to a pure array of 64-bit floating point values before the long-running computation begins, at which point you can adopt NaN == NA semantics if appropriate. I believe the solution I outlined in my updated comment above may already describe the buddy bitmap solution you're referring to. It certainly describes a solution that I would refer to as a struct-of-arrays.