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References: <bug-31130-10053@http.gcc.gnu.org/bugzilla/>
Subject: [Bug tree-optimization/31130] [4.3 Regression] VRP no longer derives range for division after negation
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From: "ian at airs dot com" <gcc-bugzilla@gcc.gnu.org>
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------- Comment #1 from ian at airs dot com  2007-03-11 20:39 -------
I am testing this patch.

Index: gcc/tree-vrp.c
===================================================================
--- gcc/tree-vrp.c      (revision 122820)
+++ gcc/tree-vrp.c      (working copy)
@@ -2142,13 +2142,11 @@ extract_range_from_unary_expr (value_ran
        min = fold_unary_to_constant (code, TREE_TYPE (expr), vr0.max);
       else if (needs_overflow_infinity (TREE_TYPE (expr)))
        {
-         if (supports_overflow_infinity (TREE_TYPE (expr)))
-           min = positive_overflow_infinity (TREE_TYPE (expr));
-         else
-           {
-             set_value_range_to_varying (vr);
-             return;
-           }
+         /* Negating TYPE_MIN_VALUE gives us a minimum value of
+            positive overflow infinity, and there is nothing useful
+            we can do with such a range.  */
+         set_value_range_to_varying (vr);
+         return;
        }
       else
        min = TYPE_MIN_VALUE (TREE_TYPE (expr));
@@ -2161,8 +2159,16 @@ extract_range_from_unary_expr (value_ran
        max = fold_unary_to_constant (code, TREE_TYPE (expr), vr0.min);
       else if (needs_overflow_infinity (TREE_TYPE (expr)))
        {
-         if (supports_overflow_infinity (TREE_TYPE (expr)))
-           max = positive_overflow_infinity (TREE_TYPE (expr));
+         /* We have a non-overflowed TYPE_MIN as the minimum value.
+            If TYPE_MIN is also the maximum value, then negating this
+            gives us a positive overflow, and we just go straight to
+            varying since we will get there anyhow at the bottom of
+            this function.  Otherwise TYPE_MIN is a half-range
+            [TYPE_MIN, X] without overflow, so we flip it to a
+            half-range [-X, TYPE_MAX] without overflow.  */
+         if (vr0.max != TYPE_MIN_VALUE (TREE_TYPE (expr))
+             && !is_negative_overflow_infinity (vr0.max))
+           max = TYPE_MAX_VALUE (TREE_TYPE (expr));
          else
            {
              set_value_range_to_varying (vr);


-- 

ian at airs dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |ian at airs dot com
                   |dot org                     |
             Status|UNCONFIRMED                 |ASSIGNED
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-03-11 20:39:57
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31130