[Leszek]

Agreed, NuN tagging is an awesome use of the NaN space, and it has the same benefits of being a no-op for pointers. I would only consider this for cases where NuN tagging is impossible, e.g. some system that needs more pointer bits than NuN tagging allows.

I'd add that the claim that this could be implemented in V8 doesn't take into account pointer compression, where on-heap V8 tagged pointers are 32-bit, not 64-bit.

[dgreensp]

It’s also worth noting that engines with JIT, like V8, don’t box intermediate floating-point numbers in calculations after optimization has been performed. Arrays of (only) numbers also don’t box the numbers (though all numeric object property values that aren’t small integers are now boxed in V8). This means you can read floats from arrays, do math on them, and write them to arrays, without boxing or unboxing. This wouldn’t necessarily be true in a less sophisticated runtime that lacks an optimizing compiler (of which V8 has two or three IIRC).