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
}