g++ bug demo

README.md

This small C++ program demonstrates an apparent optimization bug in g++.

It's based on a question on Stack Overflow: Does C++ allow an optimizing compiler to ignore side effects on the for-condition?. (I'm reasonably sure the answer in this case is no.)

bug.cpp

// http://stackoverflow.com/q/26067473/827263
// Using g++ 4.8.2 or 4.9.1, prints "ok" with "-O0" or "-O1", "BUG!" with "_O2" or "-O3".
// Apparently the problem also occurs with Microsoft Visual C++ 11.00.60610.1.
// clang++ 3.5.1 does not exhibit the problem; it prints "ok" at all optimization levels.

#include <iostream>

static int count = 0;
static bool bug = false;

void bar() {
    extern int upper_bound;
    upper_bound--;
    count ++;
}

void foo()
{
    extern int upper_bound;
    for (int i = 0; i < upper_bound; ) {
        bar();
        count ++;
        if (count > 20) {
            bug = true;
            return;
        }
    }
}

int main() {
    foo();
    if (bug) {
        std::cout << "BUG!\n";
    }
    else {
        std::cout << "ok\n";
    }
}

int upper_bound = 10;