[PATCH] Fix PR51750

[Richard Guenther]

Another manifestation about the issue we have wrt sizetypes
implicitly sign-extending but types-compatible types not.

Bootstrapped and tested on x86_64-unknown-linux-gnu, applied to trunk.

Richard.

2012-01-04  Richard Guenther  <rguent...@suse.de>

        PR middle-end/51750
        * tree.c (size_low_cst): New function.
        * tree.h (size_low_cst): Declare.
        * fold-const.c (fold_comparison): Use it to extract the low
        part of the POINTER_PLUS_EXPR offset.

Index: gcc/tree.c
===================================================================
--- gcc/tree.c  (revision 182867)
+++ gcc/tree.c  (working copy)
@@ -6509,6 +6509,17 @@ tree_low_cst (const_tree t, int pos)
   return TREE_INT_CST_LOW (t);
 }
 
+/* Return the HOST_WIDE_INT least significant bits of T, a sizetype
+   kind INTEGER_CST.  This makes sure to properly sign-extend the
+   constant.  */
+
+HOST_WIDE_INT
+size_low_cst (const_tree t)
+{
+  double_int d = tree_to_double_int (t);
+  return double_int_sext (d, TYPE_PRECISION (TREE_TYPE (t))).low;
+}
+
 /* Return the most significant (sign) bit of T.  */
 
 int
Index: gcc/tree.h
===================================================================
--- gcc/tree.h  (revision 182867)
+++ gcc/tree.h  (working copy)
@@ -4428,6 +4428,7 @@ tree_low_cst (const_tree t, int pos)
   return TREE_INT_CST_LOW (t);
 }
 #endif
+extern HOST_WIDE_INT size_low_cst (const_tree);
 extern int tree_int_cst_sgn (const_tree);
 extern int tree_int_cst_sign_bit (const_tree);
 extern unsigned int tree_int_cst_min_precision (tree, bool);
Index: gcc/fold-const.c
===================================================================
--- gcc/fold-const.c    (revision 182867)
+++ gcc/fold-const.c    (working copy)
@@ -8886,13 +8886,16 @@ fold_comparison (location_t loc, enum tr
              indirect_base0 = true;
            }
          offset0 = TREE_OPERAND (arg0, 1);
-         if (host_integerp (offset0, 0)
-             && ((HOST_WIDE_INT) (TREE_INT_CST_LOW (offset0) * BITS_PER_UNIT)
-                 / BITS_PER_UNIT
-                 == (HOST_WIDE_INT) TREE_INT_CST_LOW (offset0)))
+         if (host_integerp (offset0, 0))
            {
-             bitpos0 = TREE_INT_CST_LOW (offset0) * BITS_PER_UNIT;
-             offset0 = NULL_TREE;
+             HOST_WIDE_INT off = size_low_cst (offset0);
+             if ((HOST_WIDE_INT) (((unsigned HOST_WIDE_INT) off)
+                                  * BITS_PER_UNIT)
+                 / BITS_PER_UNIT == (HOST_WIDE_INT) off)
+               {
+                 bitpos0 = off * BITS_PER_UNIT;
+                 offset0 = NULL_TREE;
+               }
            }
        }
 
@@ -8917,13 +8920,16 @@ fold_comparison (location_t loc, enum tr
              indirect_base1 = true;
            }
          offset1 = TREE_OPERAND (arg1, 1);
-         if (host_integerp (offset1, 0)
-             && ((HOST_WIDE_INT) (TREE_INT_CST_LOW (offset1) * BITS_PER_UNIT)
-                 / BITS_PER_UNIT
-                 == (HOST_WIDE_INT) TREE_INT_CST_LOW (offset1)))
+         if (host_integerp (offset1, 0))
            {
-             bitpos1 = TREE_INT_CST_LOW (offset1) * BITS_PER_UNIT;
-             offset1 = NULL_TREE;
+             HOST_WIDE_INT off = size_low_cst (offset1);
+             if ((HOST_WIDE_INT) (((unsigned HOST_WIDE_INT) off)
+                                  * BITS_PER_UNIT)
+                 / BITS_PER_UNIT == (HOST_WIDE_INT) off)
+               {
+                 bitpos1 = off * BITS_PER_UNIT;
+                 offset1 = NULL_TREE;
+               }
            }
        }