Change default of non-overlapping address space conversion

The current default of making all undefined coversions being
set to null is not useful.  It has caused all users to lie
and say that spaces are subsets when they are not, just so
that they can override the conversion.

	* expr.c (expand_expr_real_2): Use convert_modes on disjoint
	address spaces.

From-SVN: r229142

gcc/ChangeLog

+2015-10-21  Richard Henderson  <rth@redhat.com>
+
+	* expr.c (expand_expr_real_2): Use convert_modes on disjoint
+	address spaces.
+
 2015-10-21  Richard Sandiford  <richard.sandiford@arm.com>
 
 	* builtins.c (fold_builtin_cabs): Delete.

gcc/expr.c

@@ -8171,34 +8171,40 @@ expand_expr_real_2 (sepops ops, rtx target, machine_mode tmode,
     case ADDR_SPACE_CONVERT_EXPR:
       {
 	tree treeop0_type = TREE_TYPE (treeop0);
-	addr_space_t as_to;
-	addr_space_t as_from;
 
 	gcc_assert (POINTER_TYPE_P (type));
 	gcc_assert (POINTER_TYPE_P (treeop0_type));
 
-	as_to = TYPE_ADDR_SPACE (TREE_TYPE (type));
-	as_from = TYPE_ADDR_SPACE (TREE_TYPE (treeop0_type));
+	addr_space_t as_to = TYPE_ADDR_SPACE (TREE_TYPE (type));
+	addr_space_t as_from = TYPE_ADDR_SPACE (TREE_TYPE (treeop0_type));
 
         /* Conversions between pointers to the same address space should
 	   have been implemented via CONVERT_EXPR / NOP_EXPR.  */
 	gcc_assert (as_to != as_from);
 
+	op0 = expand_expr (treeop0, NULL_RTX, VOIDmode, modifier);
+
         /* Ask target code to handle conversion between pointers
 	   to overlapping address spaces.  */
 	if (targetm.addr_space.subset_p (as_to, as_from)
 	    || targetm.addr_space.subset_p (as_from, as_to))
 	  {
-	    op0 = expand_expr (treeop0, NULL_RTX, VOIDmode, modifier);
 	    op0 = targetm.addr_space.convert (op0, treeop0_type, type);
-	    gcc_assert (op0);
-	    return op0;
 	  }
-
-	/* For disjoint address spaces, converting anything but
-	   a null pointer invokes undefined behaviour.  We simply
-	   always return a null pointer here.  */
-	return CONST0_RTX (mode);
+        else
+          {
+	    /* For disjoint address spaces, converting anything but a null
+	       pointer invokes undefined behaviour.  We truncate or extend the
+	       value as if we'd converted via integers, which handles 0 as
+	       required, and all others as the programmer likely expects.  */
+#ifndef POINTERS_EXTEND_UNSIGNED
+	    const int POINTERS_EXTEND_UNSIGNED = 1;
+#endif
+	    op0 = convert_modes (mode, TYPE_MODE (treeop0_type),
+				 op0, POINTERS_EXTEND_UNSIGNED);
+	  }
+	gcc_assert (op0);
+	return op0;
       }
 
     case POINTER_PLUS_EXPR: