http://stackoverflow.com/a/24192681/827263

sample.c

#include <stdio.h>
#include <stdint.h>
int main(void) {

    int64_t u = 0;
    int i;

    for(i=0;i<44;i++)
        u |= (uint64_t)1 << i;

    for(;i<64;i++)
        u |= (uint64_t)0 << i;

    int t = __builtin_popcountl(u);

    printf("u = 0x%llx, t = %d\n", (unsigned long long)u, t);
}

// Output:
// u = 0xfffffffffff, t = 44