3 ms·
The question was "how do you do bit-counting of a bunch of numbers". The two canonical answers are "lookup table" and "using bit shifts and multiplying by magi
by powera 10y ago
The question was "how do you do bit-counting of a bunch of numbers". The two canonical answers are "lookup table" and "using bit shifts and multiplying by magic numbers".
The fact that there's a machine instruction for it does makes it a bad question.
- mitchty 10y agoHonestly 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.
- gcp 10y agoPopcnt isn't particularly well optimized in most micro-architectural implementations.
- mitchty 10y agoLooks that way with a quick test. But it looks like there may be a better way with SSE3 PSHUFB: http://wm.ite.pl/articles/sse-popcount.html http://wm.ite.pl/articles/sse-popcount.html
- rayiner 10y agoIs 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).
- gcp 10y agoHmm, 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/ http://danluu.com/assembly-intrinsics/