[Bug c/70143] [6 Regression] false strict-aliasing warning

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70143

--- Comment #5 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 9 Mar 2016, jakub at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70143
> 
> --- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> Note even GCC 5.x and earlier warn here with -Wstrict-aliasing=2 or
> -Wstrict-aliasing=1, it is just whether we warn about this with the most common
> one -Wstrict-aliasing=3.

Sure.  I can trivially re-instantiate previous behavior with

Index: gcc/c-family/c-common.c
===================================================================
--- gcc/c-family/c-common.c     (revision 234025)
+++ gcc/c-family/c-common.c     (working copy)
@@ -1568,7 +1568,9 @@ strict_aliasing_warning (tree otype, tre
           alias_set_type set2 = get_alias_set (TREE_TYPE (type));

           if (set1 != set2 && set2 != 0
-             && (set1 == 0 || !alias_set_subset_of (set2, set1)))
+             && (set1 == 0
+                 || (!alias_set_subset_of (set2, set1)
+                     && !alias_set_subset_of (set1, set2))))
            {
              warning (OPT_Wstrict_aliasing, "dereferencing type-punned "
                       "pointer will break strict-aliasing rules");

or with the suggested && !alias_sets_conflict_p (set1, set2).

Though that will FAIL the gcc.dg/Wstrict-aliasing-struct-member.c
I added.

Note that we then also don't warn for

struct a {
  int i;
  int k;
};
struct b {
  struct a a;
  int j;
};
int main(void) {
  static struct b b;
  struct a *ap=(struct a *)&b;
  return ((struct b *)&ap->k)->j;
}

then either.  Doing this kind of warnings strictly based on alias
sets only is going to be "broken" - though I consider our strict-aliasing
warnings broken anyway (and I don't think we can ever implement something
sensible :/).