Update comments.

From-SVN: r134073

gcc/ada/itypes.ads

@@ -6,7 +6,7 @@
 --                                                                          --
 --                                 S p e c                                  --
 --                                                                          --
---          Copyright (C) 1992-2007, Free Software Foundation, Inc.         --
+--          Copyright (C) 1992-2008, Free Software Foundation, Inc.         --
 --                                                                          --
 -- GNAT is free software;  you can  redistribute it  and/or modify it under --
 -- terms of the  GNU General Public License as published  by the Free Soft- --
@@ -38,7 +38,8 @@ package Itypes is
    --  Implicit types (Itypes) are types and subtypes created by the semantic
    --  phase or the expander to reflect the underlying semantics. These could
    --  be generated by building trees for corresponding declarations and then
-   --  analyzing these trees, but there are three reasons for not doing this:
+   --  analyzing these trees, but there are three reasons for not doing this
+   --  in some cases:
 
    --    1. The declarations would require more tree nodes
 
@@ -81,6 +82,25 @@ package Itypes is
    --  and for the array subtype. The associated node of each index subtype
    --  is the corresponding range expression.
 
+   --  Notes on the use of the Parent field of an Itype
+
+   --    In some cases, we do create a declaration node for an itype, and in
+   --    such cases, the Parent field of the Itype points to this declaration
+   --    in the normal manner. This case can be detected by checking for a
+   --    non-empty Parent field referencing a declaration whose Defining_Entity
+   --    is the Itype in question.
+
+   --    In some other cases, where we don't generate such a declaration, as
+   --    described above, the Itype is attached to the tree implicitly by being
+   --    referenced elsewhere, e.g. as the Etype of some object. In this case
+   --    the Parent field may be Empty.
+
+   --    In other cases where we don't generate a declaration for the Itype,
+   --    the Itype may be attached to an arbitrary node in the tree, using
+   --    the Parent field. This Parent field may even reference a declaration
+   --    for a related different entity (hence the description of the tests
+   --    needed for the case where a declaration for the Itype is created).
+
    ------------------
    -- Create_Itype --
    ------------------
@@ -115,8 +135,10 @@ package Itypes is
    --  The Scope_Id parameter specifies the scope of the created type, and
    --  is normally the Current_Scope as shown, but can be set otherwise.
    --
-   --  If Ekind is in Access_Subprogram_Type_Kind, Can_Use_Internal_Rep is set
-   --  True, unless Always_Compatible_Rep_On_Target is True.
+   --  The size/align fields are initialized to unknown (Uint_0).
+   --
+   --  If Ekind is in Access_Subprogram_Kind, Can_Use_Internal_Rep is set True,
+   --  unless Always_Compatible_Rep_On_Target is True.
 
    ---------------------------------
    -- Create_Null_Excluding_Itype --