[Ada] Fix spurious 'noreturn' function does return warning at -O0 (1)

[Eric Botcazou <ebotcazou at adacore dot com>]

This fixes a spurious 'noreturn' function does return warning at -O0 on code 
involving controlled types.

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


2012-07-18  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/trans.c (stmt_group_may_fallthru): New function.
	(gnat_to_gnu) <N_Block_Statement>: Use it to find out whether the
	block needs to be translated.


2012-07-18  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/noreturn4.ad[sb]: New test.
	* gnat.dg/noreturn4_pkg.ads: New helper.


-- 
Eric Botcazou

Index: gcc-interface/trans.c
===================================================================
--- gcc-interface/trans.c	(revision 189607)
+++ gcc-interface/trans.c	(working copy)
@@ -244,6 +244,7 @@ static void add_cleanup (tree, Node_Id);
 static void add_stmt_list (List_Id);
 static void push_exception_label_stack (VEC(tree,gc) **, Entity_Id);
 static tree build_stmt_group (List_Id, bool);
+static inline bool stmt_group_may_fallthru (void);
 static enum gimplify_status gnat_gimplify_stmt (tree *);
 static void elaborate_all_entities (Node_Id);
 static void process_freeze_entity (Node_Id);
@@ -6197,12 +6198,18 @@ gnat_to_gnu (Node_Id gnat_node)
       break;
 
     case N_Block_Statement:
-      start_stmt_group ();
-      gnat_pushlevel ();
-      process_decls (Declarations (gnat_node), Empty, Empty, true, true);
-      add_stmt (gnat_to_gnu (Handled_Statement_Sequence (gnat_node)));
-      gnat_poplevel ();
-      gnu_result = end_stmt_group ();
+      /* The only way to enter the block is to fall through to it.  */
+      if (stmt_group_may_fallthru ())
+	{
+	  start_stmt_group ();
+	  gnat_pushlevel ();
+	  process_decls (Declarations (gnat_node), Empty, Empty, true, true);
+	  add_stmt (gnat_to_gnu (Handled_Statement_Sequence (gnat_node)));
+	  gnat_poplevel ();
+	  gnu_result = end_stmt_group ();
+	}
+      else
+	gnu_result = alloc_stmt_list ();
       break;
 
     case N_Exit_Statement:
@@ -7240,6 +7247,17 @@ end_stmt_group (void)
   return gnu_retval;
 }
 
+/* Return whether the current statement group may fall through.  */
+
+static inline bool
+stmt_group_may_fallthru (void)
+{
+  if (current_stmt_group->stmt_list)
+    return block_may_fallthru (current_stmt_group->stmt_list);
+  else
+    return true;
+}
+
 /* Add a list of statements from GNAT_LIST, a possibly-empty list of
    statements.*/
 

-- { dg-do compile }

with Noreturn4_Pkg; use Noreturn4_Pkg;

package body Noreturn4 is

  procedure P1 (Msg : String) is
  begin
     P1 (Msg, 0);
  end;
  procedure P1 (Msg : String; Val : Integer) is
  begin
     Fatal_Error (Value (It));
  end;

  procedure Fatal_Error (X : Integer) is
  begin
     raise PRogram_Error;
  end;

end Noreturn4;

with Ada.Finalization; use Ada.Finalization;

package Noreturn4_Pkg is

  type Priv is private;
  function It return Priv;
  function Value (Obj : Priv) return Integer;
  function OK (Obj : Priv) return Boolean;

private
  type Priv is new Controlled with record
     Value : Integer := 15;
  end record;

  procedure Adjust   (Obj : in out Priv);
  procedure Finalize (Obj : in out Priv);

end Noreturn4_Pkg;

package Noreturn4 is

  procedure P1 (Msg : String);
  procedure P1 (Msg : String; Val : Integer);
  pragma No_Return (P1);

  procedure Fatal_Error (X : Integer);
  pragma No_Return (Fatal_Error);

end Noreturn4;