[Bug sanitizer/81088] UBSAN: false positive as a result of reassosiation

[rguenth at gcc dot gnu.org]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-06-14
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We associate to

(1 - (int) s) + 2147483647

without -fsanitize=undefined which is fine.  But with -fsanitize=undefined
we end up with:

;; Function main (null)
;; enabled by -tree-original


{
  i = if (SAVE_EXPR <(int) y>;, SAVE_EXPR <(int) y> == 0 || (SAVE_EXPR <(int)
y>;, 0);)
    {
      __builtin___ubsan_handle_divrem_overflow (&*.Lubsan_data0, 0, (unsigned
long) SAVE_EXPR <(int) y>);
    }
  else
    {
      <<< Unknown tree: void_cst >>>
    }, SAVE_EXPR <(int) y>;, -(int) s + -2147483648(OVF);;
  return 0;
}


this goes wrong somewhere when fully folding

-((int) s + (int) ~(if (SAVE_EXPR <(int) y>;, SAVE_EXPR <(int) y> == 0 ||
(SAVE_EXPR <(int) y>;, 0);)
  {
    __builtin___ubsan_handle_divrem_overflow (&*.Lubsan_data0, 0, (unsigned
long) SAVE_EXPR <(int) y>);
  }
else
  {
    <<< Unknown tree: void_cst >>>
  }, (unsigned int) (0 / SAVE_EXPR <(int) y>);)) + 2147483647