> In CPython itself: See count_set_bits in Modules/mathmodule.c

Python/hamt.c contains an optimized function:

static inline uint32_t
hamt_bitcount(uint32_t i)
{
    /* We could use native popcount instruction but that would
       require to either add configure flags to enable SSE4.2
       support or to detect it dynamically.  Otherwise, we have
       a risk of CPython not working properly on older hardware.

       In practice, there's no observable difference in
       performance between using a popcount instruction or the
       following fallback code.

       The algorithm is copied from:
       https://graphics.stanford.edu/~seander/bithacks.html
    */
    i = i - ((i >> 1) & 0x55555555);
    i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
    return (((i + (i >> 4)) & 0xF0F0F0F) * 0x1010101) >> 24;
}


Python/pymath.c provides a "unsigned int _Py_bit_length(unsigned long d)" function used by math.factorial, _PyLong_NumBits(), int.__format__(), long / long, _PyLong_Frexp() and PyLong_AsDouble(), etc.

Maybe we could add a _Py_bit_count().

See also bpo-29782: "Use __builtin_clzl for bits_in_digit if available" which proposes to micro-optimize _Py_bit_length().

--

In the meanwhile, I also added pycore_byteswap.h *internal* header which provides static inline function which *do* use builtin functions like __builtin_bswap32().