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 :/).
