The term optimization is a misnomer. The problem of changing code in all cases to the best it can be is an undecidable problem; it cannot be solved an algorithm on a computer.
We can, however, improve code in most cases.
There are machine dependent optimizations such as good allocation of registers and machine independent optimizations such as computing constants; constant computation at compile-time rather than generating code to compute them saves execution time.
Optimization can consume a lot of compile-time, so we want the most payoff for the least effort. In particular, the compiler needs to guess where the most frequently executed parts of the program are.
