Optimizing away a “while(1);” in C++0x

Question

Updated, see below!

I have heard and read that C++0x allows an compiler to print "Hello" for the following snippet

#include <iostream>

int main() {
  while(1) 
    ;
  std::cout << "Hello" << std::endl;
}

It apparently has something to do with threads and optimization capabilities. It looks to me that this can surprise many people though.

Does someone have a good explanation of why this was necessary to allow? For reference, the most recent C++0x draft says at 6.5/5

A loop that, outside of the for-init-statement in the case of a for statement,

• makes no calls to library I/O functions, and
• does not access or modify volatile objects, and
• performs no synchronization operations (1.10) or atomic operations (Clause 29)

may be assumed by the implementation to terminate. [ Note: This is intended to allow compiler transfor- mations, such as removal of empty loops, even when termination cannot be proven. — end note ]

Edit:

This insightful article says about that Standards text

Unfortunately, the words "undefined behavior" are not used. However, anytime the standard says "the compiler may assume P," it is implied that a program which has the property not-P has undefined semantics.

Is that correct, and is the compiler allowed to print "Bye" for the above program?

---

There is an even more insightful thread here, which is about an analogous change to C, started off by the Guy done the above linked article. Among other useful facts, they present a solution that seems to also apply to C++0x (Update: This won't work anymore with n3225 - see below!)

endless:
  goto endless;

A compiler is not allowed to optimize that away, it seems, because it's not a loop, but a jump. Another guy summarizes the proposed change in C++0x and C201X

By writing a loop, the programmer is asserting either that the loop does something with visible behavior (performs I/O, accesses volatile objects, or performs synchronization or atomic operations), or that it eventually terminates. If I violate that assumption by writing an infinite loop with no side effects, I am lying to the compiler, and my program's behavior is undefined. (If I'm lucky, the compiler might warn me about it.) The language doesn't provide (no longer provides?) a way to express an infinite loop without visible behavior.

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Update on 3.1.2011 with n3225: Committee moved the text to 1.10/24 and say

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.

The goto trick will not work anymore!

Answer 1

I think this is along the lines of the this type of question, which references another thread. Optimization can occasionally remove empty loops.

Answer 2

I think it's worth pointing out that loops which would be infinite except for the fact that they interact with other threads via non-volatile, non-synchronised variables can now yield incorrect behaviour with a new compiler.

I other words, make your globals volatile -- as well as arguments passed into such a loop via pointer/reference.