The builtin POPCNT that came with Intel's SSE4 (SSE4a for AMD) is much faster. However, at a certain point, using AVX2 (and AVX-512 if present) is actually fast

The builtin POPCNT that came with Intel's SSE4 (SSE4a for AMD) is much faster. However, at a certain point, using AVX2 (and AVX-512 if present) is actually faster yet [1] - at least for 512 byte inputs or larger. [1]: https://github.com/WojciechMula/sse-popcount https://github.com/WojciechMula/sse-popcount