I have this code:
  typedef struct { gint dummy; } S;
  S *sp = g_new0 (S, 1);
  S *new_val = g_new0 (S, 1);
  g_atomic_pointer_compare_and_exchange ((gpointer *)&sp, NULL, new_val);

gcc gives this warning:
  warning: dereferencing type-punned pointer will break strict-aliasing rules

The solutions in this case could be:
- Use a dummy union
- Disable strict aliasing
- Use __attribute__((__may_alias__))
- Ignore the warning

I think that the only acceptable solution is to use the attribute, so I propose to add a G_GNUC_MAY_ALIAS macro for GCC.

This was already discussed in bug #335341 and in the mailing list: http://mail.gnome.org/archives/gtk-devel-list/2005-February/msg00001.html

If you think that G_GNUC_MAY_ALIAS is ok I can write the patch for gmacros.h and for the documentation.