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.
Let the compiler, and library writers take care of most of the work of translating your intention into good runtime performance and only intervene when they don't get the job done.