How to find the position of the only-set-bit in a 64-bit value using bit manipulation efficiently?

Question

Just say I have a value of type uint64_t seen as sequence of octets (1 octet = 8-bit). The uint64_t value is known containing only one set bit<

Answer 1

Modern hardware has specialized instructions for that (LZCNT, TZCNT on Intel processors).

Most compilers have intrinsics to easily generate them. See the following wikipedia page.