g++-4.0.3: program output is different when compiled with -O3 (did not happen with gcc-3.4.5)

[Peter Doerfler]

Erik wrote:
> I just installed g++-4.0.3 on a Pentium Mobile. I have a program that 
> behaves as expected when compiled with -O0 but gives different output 
> when compiled with -O3. I do not know what to do about it. The program 
> is called prov. Here is the result of compiling and running it:
> $ g++-4.0.3 -O0 -Wall -o prov prov.cc -I/usr/include/SDL && ./prov; echo $?
> 1
> $ g++-4.0.3 -O3 -Wall -o prov prov.cc -I/usr/include/SDL && ./prov; echo $?
> 0
> 
> The program looks like this:
> ______________________________________________________
> #include <SDL_types.h>
> struct Coords {
>    bool operator<(const Coords other) const {
>        return
>            *reinterpret_cast<const Uint32 * const>(this)
>            <
>            *reinterpret_cast<const Uint32 * const>(&other);
>    }
>      int x : 16, y : 16;
> };
> int main() {
>    Coords a = {0, 1}, b = {1, 0};
>    return b < a and a < b;
> }
> ______________________________________________________
> 
> 
> The operator< is for use with standard containers, so it has to work. It 
> should of course never happen that b is less than a and a is less than 
> b, so the program should always return 0. I can see that the assembly 
> output is very different, but I admitt that I do not understand much of 
> it. With g++-3.4.5 the program produces identical output for the 
> different optimization levels.

Why don't you use an anonymous union, like so?

struct Coords {

   bool operator<(const Coords other) const;

   union {
     struct {
       int x : 16, y : 16;
     };

     unsigned int z;
   };
};

bool Coords::operator<(const Coords other) const {
   return z < other.z;
}


HTH, Peter