Comment by nayuki

> if you do fflush(stdout); return INT_MAX + 1; the compiler cannot omit the fflush() call merely because the subsequent statement had undefined behavior

False! The expression (INT_MAX + 1) has no side effect (assuming no UB), so according to the rules of the C abstract machine, the compiler is allowed to hoist this calculation above the fflush(). If you run this on a machine that traps on integer overflow (which is allowed behavior), the process could crash before the fflush() is executed. Remember, everyone: With UB, anything can happen.