[Ada] Spurious warnings about unreferenced entities of a packed type

[Arnaud Charlet <charlet at adacore dot com>]

A reference to an element of a packed array is rewritten through conversions
and masks and shifts. If the prefix of the reference is a source entity we
must generate a reference to it for cross-reference purposes, and to prevent
spurious warnings about unused entities.

Compiling 

   gcc -c -gnatwa case_2.adb

must yield:

   case_2.adb:10:06: warning: unit "System" is not referenced
   case_2.adb:37:13: warning: 1 bits of "Storage_Record_Type" unused

--
-- Test case for anomalous GNAT compiler warning.
-- Compile as follows:
-- gnatmake -f -gnatwf case_2
--
-- This will create a warning that the formal parameter "Storage" of the
-- function Test_Function is not referenced, even though it is.
--

with System;
with Text_IO;

procedure Case_2 is

  --
  -- 32-bit signed integer type
  --
  type My_Long is range -(2 ** 31) .. +(2 ** 31) - 1;
  for My_Long'size use 32;

  --
  -- Changing this to 64 will prevent the warning from being reported.
  --
  Record_Size : constant := 65;

  --
  -- Note: this type is defined as having an extra bit unused for ease of
  -- switching the size to 64 to see the warning go away.
  -- Even adding an extra field to fill in the unused bit, GNAT still
  -- reports the warning.
  --
  type Storage_Record_Type is
    record
      E : My_Long;
      F : My_Long;
    end record with
    Size => Record_Size;

  type My_Storage_Array_Type is
    array (My_Long range <>) of Storage_Record_Type with
    Pack => True;

  --
  -- GNAT gives a warning that "Storage" is not used.
  --
  function Test_Function
   (Storage : in My_Storage_Array_Type;
    Left    : in My_Long;
    Right   : in My_Long) return Boolean
  is
  begin

    return Storage (Left).E < Storage (Right).F;

  end Test_Function;

  --
  -- Dummy variables to support the below code.
  --
  Test : My_Storage_Array_Type (1 .. 1);
  Result : Boolean;

begin

  --
  -- This small bit of code is only here to ensure that none of the above
  -- parts get optimized out during compilation.
  --
  Test (1) := (E => 0, F => 0);
  Result := Test_Function (Test, 1, 1);
  Text_IO.Put_Line (Boolean'image (Result));

end Case_2;

Tested on x86_64-pc-linux-gnu, committed on trunk

2014-10-23  Ed Schonberg  <schonberg@adacore.com>

	* exp_pakd.adb (Expand_Packed_Element_Reference): If the
	prefix is a source entity, generate a reference to it before
	transformation, because rewritten node might not generate a
	proper reference, leading to spurious warnings.

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