Bitmap Count provide a function that calculates the number of 1-bits in a bitmap. Function is optimized for the case where the array size is divisable by size of int, then it uses the compiler builtin function: __builtin_popcount (available in gcc and clang).
If a different compiler need to be used, this project: https://github.com/Noctune/bitcount provide a more general implementation of the popcount function.
Use the folowing command to build the test: g++ -Wall -o test_bitcount test_bitcount.cpp bitcount.cpp