[Bug middle-end/30267] folding (~ -x) >= (-2147483647-1) to x != -2147483648

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

------- Comment #1 from rguenth at gcc dot gnu dot org  2006-12-27 16:48 -------
This is folded to

;; Function notneg (notneg)
;; enabled by -tree-original


{
  return x != -2147483648;
}



;; Function negnot (negnot)
;; enabled by -tree-original

{
  return x != 2147483647;
}

via

      /* Convert - (~A) to A + 1.  */
      if (INTEGRAL_TYPE_P (type) && TREE_CODE (arg0) == BIT_NOT_EXPR)
        return fold_build2 (PLUS_EXPR, type, TREE_OPERAND (arg0, 0),
                            build_int_cst (type, 1));

and

      /* Convert ~ (-A) to A - 1.  */
      else if (INTEGRAL_TYPE_P (type) && TREE_CODE (arg0) == NEGATE_EXPR)
        return fold_build2 (MINUS_EXPR, type, TREE_OPERAND (arg0, 0),
                            build_int_cst (type, 1));

and

      /* Transform comparisons of the form X +- C1 CMP C2 to X CMP C2 +- C1. 
*/      if ((TREE_CODE (arg0) == PLUS_EXPR || TREE_CODE (arg0) == MINUS_EXPR)
          && (TREE_CODE (TREE_OPERAND (arg0, 1)) == INTEGER_CST
              && !TREE_OVERFLOW (TREE_OPERAND (arg0, 1))
              && !TYPE_UNSIGNED (TREE_TYPE (arg1))
              && !(flag_wrapv || flag_trapv))
          && (TREE_CODE (arg1) == INTEGER_CST
              && !TREE_OVERFLOW (arg1)))
        {
          tree const1 = TREE_OPERAND (arg0, 1);
          tree const2 = arg1;
          tree variable = TREE_OPERAND (arg0, 0);
          tree lhs;
          int lhs_add;
          lhs_add = TREE_CODE (arg0) != PLUS_EXPR;

          lhs = fold_build2 (lhs_add ? PLUS_EXPR : MINUS_EXPR,
                             TREE_TYPE (arg1), const2, const1);
          if (TREE_CODE (lhs) == TREE_CODE (arg1)
              && (TREE_CODE (lhs) != INTEGER_CST
                  || !TREE_OVERFLOW (lhs)))
            return fold_build2 (code, type, variable, lhs);
        }

(citing from the 4.1 branch)

Note this is actually wrong-code.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |wrong-code
      Known to fail|                            |4.1.2 4.3.0
      Known to work|                            |4.0.3
   Last reconfirmed|0000-00-00 00:00:00         |2006-12-27 16:48:37
               date|                            |
            Summary|missed optimization due to  |folding (~ -x) >= (-
                   |bad range propagation       |2147483647-1) to x != -
                   |without -fwrapv             |2147483648


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30267