[Bug tree-optimization/77291] False positive for -Warray-bounds

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77291

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic, wrong-code
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2016-08-19
          Component|c++                         |tree-optimization
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  VRP does

static void
check_array_ref (location_t location, tree ref, bool ignore_off_by_one)
{
...
  up_bound = array_ref_up_bound (ref);
...
  /* Accesses to trailing arrays via pointers may access storage
     beyond the types array bounds.  */
  if (warn_array_bounds < 2
      && array_at_struct_end_p (ref))
    return;

but here array_at_struct_end_p returns false because the size is still
constrained
by the DECL size.  _But_ VRP fails to adjust up_bound accordingly, that is,
it is relying on array_at_struct_end_p to return true if the array bounds do
not
cover all possible elements towards its extent.

It looks like idx_infer_loop_bounds may have the same issue (wrong-code).
bounds_are_valid in graphite is even worse.

I'll think about a good solution.