The Bit attribute is a GNAT-specific attribute which yields the bit offset
within the byte that contains the first bit of storage allocated for the
object to which it is applied.  However, it was returning 0 for bit-packed
array references.

The fix is to extend the special handling of bit-packed array references
already implemented for the Address attribute.  That's quite natural since
this pair of attributes can be seen as a (/,mod) pair for addresses.

The following program must run quietly:

with System; use System;

procedure Bit_Attribute is

  type Bits is array (1..8) of Boolean;
  pragma Pack (Bits);

  My_Bits : Bits := (Others => False);
  pragma Volatile (My_Bits);

  type Rec is record
    A : Boolean;
    B : Bits;
  end record;
  pragma Pack (Rec);

  My_Rec : Rec := (A => False, B => (Others => False));
  pragma Volatile (My_Rec);

  A : Address;
  N : Natural;

begin

  A := My_Bits(3)'Address;
  if A /= My_Bits'Address then
    raise Program_Error;
  end if;

  N := My_Bits(3)'Bit;
  if N /= 2 then
    raise Program_Error;
  end if;

  A := My_Rec.B(3)'Address;
  if A /= My_Rec'Address then
    raise Program_Error;
  end if;

  N := My_Rec.B(3)'Bit;
  if N /= 3 then
    raise Program_Error;
  end if;

end;

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

2010-06-22  Eric Botcazou  <ebotcazou@adacore.com>

	* exp_attr.adb (Expand_N_Attribute_Reference) <Attribute_Bit>: Deal
	with packed array references specially.
	* exp_ch4.adb (Expand_N_Indexed_Component): Do not convert a reference
	to a component of a bit packed array if it is the prefix of 'Bit.
	* exp_pakd.ads (Expand_Packed_Bit_Reference): Declare.
	* exp_pakd.adb (Expand_Packed_Bit_Reference): New procedure.  Expand a
	'Bit reference, where the prefix involves a packed array reference.
	(Get_Base_And_Bit_Offset): New helper, extracted from...
	(Expand_Packed_Address_Reference): ...here.  Call above procedure to
	get the outer object and offset expression.