Problem taking 'Size of large array (gnat)

[Dr Adrian Wrigley <amtw@linuxchip.demon.co.uk>]

I have encountered a problem running code for handling large records and arrays.

Although the ARM says the 'Size attribute is a universal integer, I
cannot use the attribute when the expected value would exceed 31 bits.
This becomes a problem when the size of the object exceeds 256MB.
I was trying to memory map (mmap) a file containing a large record.

I am using gnat 3.12p on x86/Linux

Questions:

1)  Does this problem exist on the other popular Ada implementations on x86?
2)  How can I compute the size in bytes of objects > 256MB?

Example code is below...
--------------
with Text_IO; use Text_IO;

procedure ShowSize is

   type Big is array (0 .. 70_000_000) of Float;
   type Long is range 0 .. 2**63 - 1;
   X : Long;

begin

   X := Big'Size; -- Constraint error here!

   Text_IO.Put_Line ("X got " & Long'Image (X));

end ShowSize;
---------------
--
Dr Adrian Wrigley, (currently in Vancouver, BC)