Index: sem_ch13.adb
===================================================================
--- sem_ch13.adb	(revision 230229)
+++ sem_ch13.adb	(working copy)
@@ -4724,10 +4724,24 @@
 
                   Find_Overlaid_Entity (N, O_Ent, Off);
 
-                  --  If the object overlays a constant view, mark it so
+                  if Present (O_Ent) then
+                     --  If the object overlays a constant object, mark it so
 
-                  if Present (O_Ent) and then Is_Constant_Object (O_Ent) then
-                     Set_Overlays_Constant (U_Ent);
+                     if Is_Constant_Object (O_Ent) then
+                        Set_Overlays_Constant (U_Ent);
+                     end if;
+                  else
+                     --  If this is not an overlay, mark a variable as being
+                     --  volatile to prevent unwanted optimizations. It's a
+                     --  conservative interpretation of RM 13.3(19) for the
+                     --  cases where the compiler cannot detect potential
+                     --  aliasing issues easily and it also covers the case
+                     --  of an absolute address where the volatile aspect is
+                     --  kind of implicit.
+
+                     if Ekind (U_Ent) = E_Variable then
+                        Set_Treat_As_Volatile (U_Ent);
+                     end if;
                   end if;
 
                   --  Overlaying controlled objects is erroneous.
Index: gcc-interface/decl.c
===================================================================
--- gcc-interface/decl.c	(revision 230229)
+++ gcc-interface/decl.c	(working copy)
@@ -1068,14 +1068,12 @@
 	  }
 
 	/* Make a volatile version of this object's type if we are to make
-	   the object volatile.  We also interpret 13.3(19) conservatively
-	   and disallow any optimizations for such a non-constant object.  */
+	   the object volatile.  We also implement RM 13.3(19) for exported
+	   and imported (non-constant) objects by making them volatile.  */
 	if ((Treat_As_Volatile (gnat_entity)
 	     || (!const_flag
 		 && gnu_type != except_type_node
-		 && (Is_Exported (gnat_entity)
-		     || imported_p
-		     || Present (Address_Clause (gnat_entity)))))
+		 && (Is_Exported (gnat_entity) || imported_p)))
 	    && !TYPE_VOLATILE (gnu_type))
 	  {
 	    const int quals
@@ -1118,7 +1116,8 @@
 	  gnu_expr = convert (gnu_type, gnu_expr);
 
 	/* If this is a pointer that doesn't have an initializing expression,
-	   initialize it to NULL, unless the object is imported.  */
+	   initialize it to NULL, unless the object is declared imported as
+	   per RM B.1(24).  */
 	if (definition
 	    && (POINTER_TYPE_P (gnu_type) || TYPE_IS_FAT_POINTER_P (gnu_type))
 	    && !gnu_expr
@@ -1141,7 +1140,7 @@
 	    save_gnu_tree (gnat_entity, NULL_TREE, false);
 
 	    /* Convert the type of the object to a reference type that can
-	       alias everything as per 13.3(19).  */
+	       alias everything as per RM 13.3(19).  */
 	    gnu_type
 	      = build_reference_type_for_mode (gnu_type, ptr_mode, true);
 	    gnu_address = convert (gnu_type, gnu_address);
@@ -1206,11 +1205,10 @@
 	   as an indirect object.  Likewise for Stdcall objects that are
 	   imported.  */
 	if ((!definition && Present (Address_Clause (gnat_entity)))
-	    || (Is_Imported (gnat_entity)
-		&& Has_Stdcall_Convention (gnat_entity)))
+	    || (imported_p && Has_Stdcall_Convention (gnat_entity)))
 	  {
 	    /* Convert the type of the object to a reference type that can
-	       alias everything as per 13.3(19).  */
+	       alias everything as per RM 13.3(19).  */
 	    gnu_type
 	      = build_reference_type_for_mode (gnu_type, ptr_mode, true);
 	    used_by_ref = true;
@@ -1402,10 +1400,9 @@
 
 	/* If this name is external or a name was specified, use it, but don't
 	   use the Interface_Name with an address clause (see cd30005).  */
-	if ((Present (Interface_Name (gnat_entity))
-	     && No (Address_Clause (gnat_entity)))
-	    || (Is_Public (gnat_entity)
-		&& (!Is_Imported (gnat_entity) || Is_Exported (gnat_entity))))
+	if ((Is_Public (gnat_entity) && !Is_Imported (gnat_entity))
+	    || (Present (Interface_Name (gnat_entity))
+		&& No (Address_Clause (gnat_entity))))
 	  gnu_ext_name = create_concat_name (gnat_entity, NULL);
 
 	/* If this is an aggregate constant initialized to a constant, force it
@@ -4618,7 +4615,7 @@
 	    save_gnu_tree (gnat_entity, NULL_TREE, false);
 
 	    /* Convert the type of the object to a reference type that can
-	       alias everything as per 13.3(19).  */
+	       alias everything as per RM 13.3(19).  */
 	    gnu_type
 	      = build_reference_type_for_mode (gnu_type, ptr_mode, true);
 	    if (gnu_address)