[Bug middle-end/81088] UBSAN: false positive as a result of reassosiation

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81088

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
And the issue is that

associate:
...
          /* Only do something if we found more than two objects.  Otherwise,
             nothing has changed and we risk infinite recursion.  */
          if (ok
              && (2 < ((var0 != 0) + (var1 != 0)
                       + (con0 != 0) + (con1 != 0)
                       + (lit0 != 0) + (lit1 != 0)
                       + (minus_lit0 != 0) + (minus_lit1 != 0))))
            {
              bool any_overflows = false;
              if (lit0) any_overflows |= TREE_OVERFLOW (lit0);
              if (lit1) any_overflows |= TREE_OVERFLOW (lit1);
...
              /* Don't introduce overflows through reassociation.  */
              if (!any_overflows
                  && ((lit0 && TREE_OVERFLOW_P (lit0))
                      || (minus_lit0 && TREE_OVERFLOW_P (minus_lit0))))
                return NULL_TREE;

fails to see the overflow when associating

  (1(OVF) - (int) s) + 2147483647

the 1(OVF) is from (int)0xffffffffU which is implementation defined but
IIRC the FEs require the TREE_OVERFLOW flag to be set.

IMHO this any_overflows code is simply bogus.  To avoid dropping useful
association split_tree should drop pre-existing TREE_OVERFLOW.