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

[Erik]

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;
> }