Why does this divide-by-zero error only occur in optimized code?

Question

I just found a bug that, strangely, occurred only when optimization was turned on (g++ -O2). It was an Arithmetic exception in the following code, when interval was set to zero (from a command line argument):

for(int i = 0; i < n; ++i) {
  if((i + 1) % interval == 0) { // exception here
    DoSomething();
  }
}

It's obvious that modulo zero operation threw a division-by-zero exception, but why did this only occur when the code was compiled with optimization turned on?

Answer 1

Divide by zero is always undefined behavior. The fact that you get different results with different optimization settings still fits within the definition of undefined behavior.

Answer 2

You don't show us where 'interval' gets set. The optimizer may be doing something that sets 'interval' to 0. Change your code to

for(int i = 0; i < n; ++i) {
  if (0==interval) { break; }
  if((i + 1) % interval == 0) { // exception here
    DoSomething();
  }
}

And see if you still get the error. Or better yet, show us where 'interval' is getting its value.

Answer 3

Constant folding.

You declared interval as a global const int and compiler took you at your word.