hweight.c

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#include <linux/bitops.h>

unsigned int hweight32(unsigned int w)
{
    unsigned int res = w - ((w >> 1) & 0x55555555);
    res = (res & 0x33333333) + ((res >> 2) & 0x33333333);
    res = (res + (res >> 4)) & 0x0F0F0F0F;
    res = res + (res >> 8);
    return (res + (res >> 16)) & 0x000000FF;
}

unsigned long hweight64(__u64 w)
{
#if BITS_PER_LONG == 32
    return hweight32((unsigned int)(w >> 32)) + hweight32((unsigned int)w);
#elif BITS_PER_LONG == 64
    __u64 res = w - ((w >> 1) & 0x5555555555555555ul);
    res = (res & 0x3333333333333333ul) + ((res >> 2) & 0x3333333333333333ul);
    res = (res + (res >> 4)) & 0x0F0F0F0F0F0F0F0Ful;
    res = res + (res >> 8);
    res = res + (res >> 16);
    return (res + (res >> 32)) & 0x00000000000000FFul;
#endif
}