Compiling the following code with -O2 or higher in g++ 4.1.2 produces incorrect results.  The problem doesn't occur in version 4.2.1.

I compile like this:
  g++ -W -Wall -Werror        -O2 -fPIC -o gccbug gccbug.cpp

The last line of output should show the number 27, but it shows 0 when optimizing  with -O2.

A tar archive containing the source file and a Makefile to build it is here:
http://dooglus.rincevent.net/random/gccbug.1190912601.tgz

Defining any of FIX0, FIX1 or FIX2 fixes the problem.

Here's the source file:

-----
#include <iostream>
#include <string>
#include <vector>

using namespace std;

struct Vector {
	float _x, _y;
	Vector(): _x(0), _y(0) {};
	float &m(const int &i) { return i?_y:_x; }
};

struct Node {
	vector<Node*> nl;
	Node() {nl.push_back(this);};
	string get_name() const {return "x";};
	vector<Node*> get_children() { return nl;};
};

int six() { return 6; }

Vector parse_vector(Node *element)
{
	Vector vect;
	Node *node = element->get_children()[0];
	if(node->get_name()=="x") { // true
#ifdef FIX0
		cout << "1. vect.m(0) <= 27" << endl;
#else
		printf("1. vect.m(0) <= 27\n");
#endif
		vect.m(0)=27;
#ifdef FIX1			 // printing the value stops the over-optimization
		cout << "2. vect.m(0) == " << vect.m(0) << endl;
#endif
	} else if(six() == 6 && node->get_children().empty()) { // false
		exit(0);
		return Vector();
	}
#ifdef FIX2		// checking the value also stops the over optimization
	if (vect.m(0) + vect.m(1) == 24234.657) cout << "har" << endl;
#endif
	return vect;
}

int main() {
	cout << "3. vect.m(0) == " << parse_vector(new Node).m(0) << " <-- should be 27" << endl;
}
-----