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

[Erik <sigra at home dot se>]

Peter Doerfler wrote:

> 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?

This is exactly what I was looking for. It seems to work great. Many 
thanks! Why did I not use it? I was just not smart enough.  I tried to 
use a union like

union T {
  Coords c;
  Uint32 i;
};

in the ordering functor, but I had to ditch that idea because it was not 
allowed to have a type with constructor in a union. (And Coords has a 
constructor, although it was omitted in the sample code here.)

> 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;
> }