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
