Re: Type-punning

[Herman Geza]

> I'll try to make a simple example on which GCC produces bad code.
You can find an example below.  GCC-4.1 generates bad code (GCC-4.2 is 
fine).  Neither version give a type-punning warning, though.  I'm sorry 
that I didn't check gcc-4.2 before I started this thread.

Geza

Here's the code, I couldn't simplify it further to still have bad code 
with gcc-4.1.

gcc version: 4.1.3 20070601 (prerelease) (Debian 4.1.2-12)

#include <time.h>

class Tuple {
        public:
                union {
                        int c[2];
                        struct {
                                int x, y;
                        };
                };
        public:
                Tuple() { }
                Tuple(int x, int y) : x(x), y(y) { }
};

class Vector;

class Point: public Tuple {
        public:
                Point() { }
                Point(int x, int y) : Tuple(x, y) { }

                inline Vector operator-(const Point &other) const;
};

class Vector: public Tuple {
        public:
                Vector() { }
                Vector(int x, int y) : Tuple(x, y) { }

                Point &asPoint() {
                        return reinterpret_cast<Point&>(*this);
                }

                Point toPoint() const {
                        return Point(x, y);
                }
};

Vector Point::operator-(const Point &other) const {
        return Vector(x-other.x, y-other.y);
}

inline bool foo(int x, Point a) {
        if (a.x<=x) { return false; }

        return true;
}

int main() {
        Point a;

        a.x = 1;
        a.y = 2;

        time(0);

        int n = 0;

        for (int y=0; y<=44; y++) {
                if (foo(3, (a-a).asPoint())) {
                        n++;
                }
        }
        return n;
}