Essentially, C++ does not consider looping forever to be a side effect. You have to do something like writing to a volatile variable to ensure that the compiler does what you want.

"The implementation may assume that any thread will eventually do one of the following: — terminate, — make a call to a library I/O function, — access or modify a volatile object, or — perform a synchronization operation or an atomic operation. [ Note: This is intended to allow compiler transformations such as removal of empty loops, even when termination cannot be proven. — end note ]"