Seeing the project name EAStdC I thought for a moment this would be the C counterpart to EASTL (e.g. a modern replacement for the C standard library), but alas, it's all written in C++ and intended to be used from C++ (e.g. no C interfaces).

For anyone interested in this sort of stuff and isn't yet aware of the 'bit twiddling hacks' collection:

https://graphics.stanford.edu/~seander/bithacks.html

Also regarding the bit counting function:

    inline int CountBits(uint32_t x) // Branchless version
    { 
        #if defined(EASTDC_SSE_POPCNT)
            return _mm_popcnt_u32(x);
        #else
            x = x - ((x >> 1) & 0x55555555);
            x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
            x = (x + (x >> 4)) & 0x0F0F0F0F;
            return (int)((x * 0x01010101) >> 24);
        #endif
    }

Some compilers go as far as detecting the 'bit twiddling' version and replace that with a popcnt instruction:

https://www.godbolt.org/z/bExMvfzEx