>> Also, any floating-point operation performed on a NaN will give back the same NaN. > This is not entirely true. As I say two sentences prior: > a couple o

>> Also, any floating-point operation performed on a NaN will give back the same NaN. > This is not entirely true. As I say two sentences prior: > a couple of qualities near-ubiquitous on modern processors and architectures Obviously, this won't work if you don't do nan-propagation. It also won't work if you don't have vector ops at all, or if they're much slower than the scalar ops. I will add: if you are guaranteed a particular nan, you can just use that as your tag. (Although it looks like riscv uses a nan with an all-zero mantissa, which is unfortunate, as it means using it as the low-bits tag slows down integer math.) > you also have to worry about whether or not the compiler will guarantee NaN payload preservation The target audience is authors of compilers for dynamically-typed programming languages.