Will my compiler ignore useless code?

Question

I\'ve been through a few questions over the network about this subject but I didn\'t find any answer for my question, or it\'s for another language or it doesn\'t answer totally

Answer 1

Obviously your compiler will not ignore useless code, but analyse it carefully and then try to remove it, if it performs optimisations.

In your case, the first interesting thing is whether the variable j is used after the loop or not. The other interesting thing is TestRunTime.Length. The compiler will look at it and check whether it always returns the same result, and if yes whether it has any side effects, and if yes whether calling it once has the same side effect in total as calling it repeatedly.

If TestRunTime.Length has no side effect and j is not used then the loop is removed.

Otherwise, if calling TestRunTime.Length repeatedly has more side effects than calling it once, or if repeated calls return different values, then the loop must be executed.

Otherwise, j = max (0, TestRunTime.Length).

Next, the compiler can determine whether the assignment TestRunTime.Length is needed. It may be replaced with code that just determines what TestRunTime.Length would be.

Then of course your compiler might not try any fancy optimisations, or the language rules might be so that it cannot determine these things, and you are stuck.