utils.c (unchecked_convert): When the result type is a non-biased integral type with size 0...

	* 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.

From-SVN: r251701

gcc/ada/ChangeLog

 2017-09-05  Eric Botcazou  <ebotcazou@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  <ebotcazou@adacore.com>
+
 	* gcc-interface/gigi.h (renaming_from_generic_instantiation_p): Turn to
 	(renaming_from_instantiation_p): ...this.
 	* gcc-interface/decl.c (gnat_to_gnu_entity): Use inline predicate

gcc/ada/gcc-interface/utils.c

@@ -5257,20 +5257,26 @@ unchecked_convert (tree type, tree expr, bool notrunc_p)
 					? 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

gcc/testsuite/ChangeLog

 2017-09-05  Eric Botcazou  <ebotcazou@adacore.com>
 
+	* gnat.dg/specs/uc2.ads: New test.
+
+2017-09-05  Eric Botcazou  <ebotcazou@adacore.com>
+
 	* gnat.dg/taft_type4.adb: New test.
 	* gnat.dg/taft_type4_pkg.ad[sb]: New helper.
 

gcc/testsuite/gnat.dg/specs/uc2.ads

+-- { 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;