When I was coding a solution for 633E, I noticed that using

int log(int x) {
    int ct = 0;
    int k = 1;
    while (k <= x) {
        k *= 2; ct++;
    }
    return ct-1;
}

over floor(log2(x)) allowed my program to run in time. Could someone please explain to me why one is faster than the other? Thanks!

See here for my code and use compare to see that nothing else is different:

log(x): http://codeforces.net/contest/633/submission/18990265

floor(log2(x)): http://codeforces.net/contest/633/submission/18990181