Hi all, when I was playing around the Boost.DynamicBitset library, I noticed the last lines of the count() function (which returns the number of bits in the bitset that are set): return do_count(m_bits.begin(), num_blocks(), Block(0), static_cast<value_to_type<(bool)mode> *>(0)); Well, this line was modified 10 years ago last time. Let's go deeper. GNU C provides several language features not found in ISO standard C. These include a bunch of bit operations - you can look at them here: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html. For example, *__builtin_popcountll *counts the number of bits in an 'unsigned long long', so we can use this kind of code to calculate bits (don't judge, I wrote this fast and didn't think of coding standards): size_type res = 0; for (size_type i = 0; i < m_bits.size(); i++) { res += __builtin_popcountll(m_bits[i]); } I checked that m_bits[] and popcount() both work with 64-bit values on my machine. It halved the running time of a program that has a bitset and calls count() over and over again - here's the code <https://gist.github.com/Izaron/5005fed4184ee6483699191dc5ef3677> I used. These __builtin_XXX functions can be used everywhere where it's possible, with preprocessor directives. Or is it actually used and longer time means something else? I used the latest version of Boost.