Base-2 integer logarithm of 64-bit unsigned integer
C 40 54
Edit Clever recursive trick by @Kyle - that's creative!
int l(uint64_t n){return n?l(n/2)+1:-1;}

(Previous version: That's the bare starting point - creativity level 0)
int l(uint64_t n){int r=-1;for(;n;n&gt;&gt;=1)r++;return r;}

Test: Ideone