Well, actually, in this case, no, POPCNT is unnecessary here, since you don't actually have to count anything. you only need to know if the bitmask changed when you added a new character ;)

But otherwise, yes.

I spent quite a while optimizing GCC's bitmap operations years ago, including implementing some new sparse bitmap types, and the sparse bitmap types i implemented ended up dependent on the speed of popcnt and friends, so yes.