方法之一如下

#include <stdint.h>
#include <stdio.h>

size_t bit_count( uint64_t n )
{
    size_t count = 0;

    for (; n != 0; n &= n - 1)
    {
        ++count;
    }

    return count;
}

int main( void )
{
    printf( "%zu\n", bit_count( 0x123456789abcdef0 ) );
}

程序输出为

32