[Ada] Couple of adjustments to special return mechanism

[Eric Botcazou <ebotcazou@adacore.com>]

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Functions that return a discriminated record types with default discriminant 
are handled specially, in the sense that the caller and the callee don't see 
the same size for the return value (the former seeing the larger one).  The 
attached patch makes a couple of adjustments to this support.

Tested on x86_64-suse-linux, applied on the mainline.


2015-06-19  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Function>: Make sure
	the size of a padding type built around the return type is positive.
	* gcc-interface/trans.c (gnat_to_gnu) <N_Simple_Return_Statement>:
	Use INIT_EXPR instead of MODIFY_EXPR to assign to the return object.


-- 
Eric Botcazou

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Index: gcc-interface/decl.c
===================================================================
--- gcc-interface/decl.c	(revision 224602)
+++ gcc-interface/decl.c	(working copy)
@@ -4242,12 +4242,23 @@ gnat_to_gnu_entity (Entity_Id gnat_entit
 	    if (CONTAINS_PLACEHOLDER_P (TYPE_SIZE (gnu_return_type)))
 	      {
 		tree orig_type = gnu_return_type;
+		tree max_return_size
+		  = max_size (TYPE_SIZE (gnu_return_type), true);
+
+		/* If the size overflows to 0, set it to an arbitrary positive
+		   value so that assignments in the type are preserved.  Their
+		   actual size is independent of this positive value.  */
+		if (TREE_CODE (max_return_size) == INTEGER_CST
+		    && TREE_OVERFLOW (max_return_size)
+		    && integer_zerop (max_return_size))
+		  {
+		    max_return_size = copy_node (bitsize_unit_node);
+		    TREE_OVERFLOW (max_return_size) = 1;
+		  }
 
 		gnu_return_type
-		  = maybe_pad_type (gnu_return_type,
-				    max_size (TYPE_SIZE (gnu_return_type),
-					      true),
-				    0, gnat_entity, false, false, definition,
+		  = maybe_pad_type (gnu_return_type, max_return_size, 0,
+				    gnat_entity, false, false, definition,
 				    true);
 
 		/* Declare it now since it will never be declared otherwise.
Index: gcc-interface/trans.c
===================================================================
--- gcc-interface/trans.c	(revision 224602)
+++ gcc-interface/trans.c	(working copy)
@@ -6861,7 +6861,7 @@ gnat_to_gnu (Node_Id gnat_node)
 		tree gnu_ret_deref
 		  = build_unary_op (INDIRECT_REF, TREE_TYPE (gnu_ret_val),
 				    gnu_ret_obj);
-		gnu_result = build2 (MODIFY_EXPR, void_type_node,
+		gnu_result = build2 (INIT_EXPR, void_type_node,
 				     gnu_ret_deref, gnu_ret_val);
 		add_stmt_with_node (gnu_result, gnat_node);
 		gnu_ret_val = NULL_TREE;