The point is that writing "__builtin_popcount" makes your intentions clearer than implementing popcount as a loop. The guaranteed performance is downstream of t

The point is that writing "__builtin_popcount" makes your intentions clearer than implementing popcount as a loop. The guaranteed performance is downstream of the clarity: since the code is more explicit, the compiler doesn't have to rely on heuristics to guess that you want popcount, so it will always do the right thing.