Honestly I would have said popcnt as well. Lookup table or bit shifts when I can have the cpu count the 1's? I guess i'd need to benchmark it to be sure. Either way I can't say its a good question.
Is it? It looks like on most recent Intel CPUs, it's 3 cycle latency, 1 cycle throughput on a 64-bit register. A 8-bit LUT solution is going to less than 16-bits per cycle on any recent Intel/AMD CPU (maximum of two load ports).
Hmm, much better than I remember. I guess this goes a long way to explain why this wasn't always seen in practice: http://danluu.com/assembly-intrinsics/
