BITCOUNT performance improved.
At Redis's default optimization level the command is now much faster, always using a constant-time bit manipualtion technique to count bits instead of GCC builtin popcount, and unrolling the loop. The current implementation performance is 1.5GB/s in a MBA 11" (1.8 Ghz i7) compiled with both GCC and clang. The algorithm used is described here: http://graphics.stanford.edu/~seander/bithacks.html
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