Intel C++ compiler bug? (pointers aliasing)

Question

I am using Intel C++ compiler 12.0, and working on a program similar to the following, which is very simple and straightforward. The while loop should stop in the first run. However, when I build the code using /O2 flag with the Intel compiler, the while loop never stops. If I disable the optimization, or use visual C++, the loop exits normally. If I change pt->flag to p.flag, which I suppose is the same thing, the loop exits normally too. I think it has something to do with Intel's optimization. Is this a bug in the Intel compiler? or I missed something here?

#include <iostream>

using namespace std;

struct para {
    int i;  
    int flag;
};

int main(int argc, char **argv)
{
    para p;
    p.i = 0;
    p.flag = 1;

    para * pt = &p;
    cout << "loop started" << endl;

    int i;
    while (p.flag) {
        if (p.i == 0) {
            for (i=0; i<1; i++) {
                if (p.flag != 1)
                    break;
            }
            if (i==1) {
                pt->flag = 0;
            }
        }
    }
    cout << "loop stopped" << endl;
    return 1;
}

Updates: Thanks for everybody's answer. I am confused by the explanation of "pointer aliasing". If p is in the registers and pt can't access it, why the following code will break the while loop with the intel compiler? Firstly "pt->flag = 0;" should never take effect because i=0. Secondly even if it's in effect, shouldn't "pointer aliasing" prevent p being modified?

BTW: Can anybody let me know how to turn on/off the pointer aliasing in Visual Studio with intel compiler? Thanks!

#include <iostream>
using namespace std;
struct para {
    int i;  
    int flag;
};

int main(int argc, char **argv)
{
    para p;
    p.i = 0;
    p.flag = 1;

    para * pt = &p;
    cout << "loop started" << endl;

    int i=0;
    while (p.flag) {
        if (p.i == 0) {
            //for (i=0; i<1; i++) {
            //  if (p.flag != 1)
            //      break;
            //}
            if (i==1) {
                pt->flag = 0;
            }
        }
    }
    cout << "loop stopped" << endl;
    return 1;
}

Answer 1

This is a situation known as pointer aliasing. You have a variable p and a pointer pt that points to the same thing as p. Many optimisations can be used if the compiler can assume that there is only one "name" for a given variable, and /O2 for your compiler may enable this assumption. For example, the compiler could keep all the member variables for p in registers during the loop, which obviously can't be accessed through a memory pointer.

Check your compiler documentation to find out how to tell it to not assume there are no pointer aliases. This could be a compiler switch or a #pragma.