[Ada] Fix ICE on type witn zero precision

[Eric Botcazou]

This is a regression present on the mainline, 7 and 6 branches, in the form of 
an ICE during tree-ccp, which is confused by a type witn zero precision.

Tested on x86_64-suse-linux, applied on mainline, 7 and 6 branches.


2017-09-05  Eric Botcazou  <ebotca...@adacore.com>

        * gcc-interface/utils.c (unchecked_convert): When the result type is a
        non-biased integral type with size 0, set the result to 0 directly.


2017-09-05  Eric Botcazou  <ebotca...@adacore.com>

        * gnat.dg/specs/uc2.ads: New test.

-- 
Eric Botcazou
Index: gcc-interface/utils.c
===================================================================
--- gcc-interface/utils.c	(revision 251700)
+++ gcc-interface/utils.c	(working copy)
@@ -5257,20 +5257,26 @@ unchecked_convert (tree type, tree expr,
 					? TYPE_RM_SIZE (etype)
 					: TYPE_SIZE (etype)) == 0)))
     {
-      tree base_type
-	= gnat_type_for_size (TREE_INT_CST_LOW (TYPE_SIZE (type)),
-			      type_unsigned_for_rm (type));
-      tree shift_expr
-	= convert (base_type,
-		   size_binop (MINUS_EXPR,
-			       TYPE_SIZE (type), TYPE_RM_SIZE (type)));
-      expr
-	= convert (type,
-		   build_binary_op (RSHIFT_EXPR, base_type,
-				    build_binary_op (LSHIFT_EXPR, base_type,
-						     convert (base_type, expr),
-						     shift_expr),
-				    shift_expr));
+      if (integer_zerop (TYPE_RM_SIZE (type)))
+	expr = build_int_cst (type, 0);
+      else
+	{
+	  tree base_type
+	    = gnat_type_for_size (TREE_INT_CST_LOW (TYPE_SIZE (type)),
+				  type_unsigned_for_rm (type));
+	  tree shift_expr
+	    = convert (base_type,
+		       size_binop (MINUS_EXPR,
+				   TYPE_SIZE (type), TYPE_RM_SIZE (type)));
+	  expr
+	    = convert (type,
+		       build_binary_op (RSHIFT_EXPR, base_type,
+				        build_binary_op (LSHIFT_EXPR, base_type,
+							 convert (base_type,
+								  expr),
+							 shift_expr),
+				        shift_expr));
+	}
     }
 
   /* An unchecked conversion should never raise Constraint_Error.  The code

-- { dg-do compile }
-- { dg-options "-O" }

with Ada.Unchecked_Conversion;

package UC2 is

  subtype Word_Type is Integer range 0 .. 0;
  type Arr is array (1 .. Word_Type'Size) of Boolean;
  pragma Pack(Arr);

  function Conv is
     new Ada.Unchecked_Conversion (Source => Arr, Target => Word_Type);

  A : Arr;
  W : Word_Type := Conv(A);

end UC2;