[Ada] Add support for scalar types with small alignment

[Eric Botcazou <ebotcazou at adacore dot com>]

It is piggybacked on the padding machinery.  This makes it possible to write

  type My_Integer is new Integer;
  for My_Integer'Alignment use 1;

Tested on i586-suse-linux, applied on the mainline.


2008-03-08  Eric Botcazou  <ebotcazou@adacore.com>

	* decl.c (gnat_to_gnu_entity) <E_Signed_Integer_Subtype>: Add suppor
	for scalar types with small alignment.


2008-03-08  Eric Botcazou  <ebotcazou@adacore.com>

        * gnat.dg/small_alignment.adb: New test.


-- 
Eric Botcazou

Index: decl.c
===================================================================
--- decl.c	(revision 133014)
+++ decl.c	(working copy)
@@ -1520,6 +1520,45 @@ gnat_to_gnu_entity (Entity_Id gnat_entit
 	  copy_alias_set (gnu_type, gnu_field_type);
 	}
 
+      /* If the type we are dealing with has got a smaller alignment than the
+	 natural one, we need to wrap it up in a record type and under-align
+	 the latter.  We reuse the padding machinery for this purpose.  */
+      else if (Known_Alignment (gnat_entity)
+	       && UI_Is_In_Int_Range (Alignment (gnat_entity))
+	       && (align = UI_To_Int (Alignment (gnat_entity)) * BITS_PER_UNIT)
+	       && align < TYPE_ALIGN (gnu_type))
+	{
+	  tree gnu_field_type = gnu_type;
+	  tree gnu_field;
+
+	  gnu_type = make_node (RECORD_TYPE);
+	  TYPE_NAME (gnu_type) = create_concat_name (gnat_entity, "PAD");
+
+	  TYPE_ALIGN (gnu_type) = align;
+	  TYPE_PACKED (gnu_type) = 1;
+
+	  /* Create a stripped-down declaration of the original type, mainly
+	     for debugging.  */
+	  create_type_decl (get_entity_name (gnat_entity), gnu_field_type,
+			    NULL, true, debug_info_p, gnat_entity);
+
+	  /* Don't notify the field as "addressable", since we won't be taking
+	     it's address and it would prevent create_field_decl from making a
+	     bitfield.  */
+	  gnu_field = create_field_decl (get_identifier ("OBJECT"),
+					 gnu_field_type, gnu_type, 1, 0, 0, 0);
+
+	  finish_record_type (gnu_type, gnu_field, 0, false);
+	  TYPE_IS_PADDING_P (gnu_type) = 1;
+	  SET_TYPE_ADA_SIZE (gnu_type, bitsize_int (esize));
+
+	  copy_alias_set (gnu_type, gnu_field_type);
+	}
+
+      /* Otherwise reset the alignment lest we computed it above.  */
+      else
+	align = 0;
+
       break;
 
     case E_Floating_Point_Type:

-- { dg-do run }
-- { dg-options "-gnatws" }

procedure Small_Alignment is

  type My_Integer is new Integer;
  for My_Integer'Alignment use 1;

  function Set_A return My_Integer is
  begin
    return 12;
  end;

  function Set_B return My_Integer is
  begin
    return 6;
  end;

  C : Character;
  A : My_Integer := Set_A;
  B : My_Integer := Set_B;

begin
  A := A * B / 2;
  if A /= 36 then
    raise Program_Error;
  end if;
end;