Why does n++ execute faster than n=n+1?

Question

In C language, Why does n++ execute faster than n=n+1?

(int n=...;  n++;)
(int n=...;  n=n+1;)

Our instructor asked

Answer 1

Actually, the reason is that the operator is defined differently for post-fix than it is for pre-fix. ++n will increment "n" and return a reference to "n" while n++ will increment "n" will returning a const copy of "n". Hence, the phrase n = n + 1 will be more efficient. But I have to agree with the above posters. Good compilers should optimize away an unused return object.