This is an excellent example of a potentially risky and usually unnecessary optimization.
Any compiler worthy of the name can replace a constant, power-of-two multiplication
with a left shift, or a similar division of an unsigned quantity with a right shift.
Furthermore, a compiler will make these optimizations only when they're correct;
many programmers overlook the fact that shifting a negative value to the right is
not equivalent to division. (Therefore, when you need to make sure that these optimizations
are performed, you may have to declare relevant variables as unsigned.)