Why does this function count the number of set bits in an integer

I was asked the following question in an interview:

int foofoo(unsigned int u) {
    unsigned int foo = u;
    do{
        u = u/2;
        foo -= u;
    }while(u > 0);
    return foo;
}

I was asked to tell what does this function do and I was able to find that it counts the number of set bits in an unsigned int value, but I was not able to prove that, maybe someone can?