Why not always use compiler optimization?

Question

One of the questions that I asked some time ago had undefined behavior, so compiler optimization was actually causing the program to break.

But if there is no undefi

Answer 1

One example is short-circuit boolean evaluation. Something like:

if (someFunc() && otherFunc()) {
  ...
}

A 'smart' compiler might realize that someFunc will always return false for some reason, making the entire statement evaluate to false, and decide to not call otherFunc to save CPU time. But if otherFunc contains some code that directly affects program execution (maybe it resets a global flag or something), it now won't perform that step and you program enters an unknown state.