Hmm; carry is free, not sure about overflow. But more generally, it's an extra output, for what would otherwise be one-output operations. Put it this way - if

Hmm; carry is free, not sure about overflow. But more generally, it's an extra output, for what would otherwise be one-output operations. Put it this way - if a lot of javascript operations could modify a piece of global state, that would be trivial on a trivial interpreter, but I bet it would add a lot of complexity to your JIT. Same goes for CPUs - no problem for simple implementations, but it would add complexity to an out-of-order or other clever microarchitecture.