Compiler doesn't respect 'Size for atomic object

[Simon Wright <simon@pushface.org>]

The code below is an abstract of one for a Cortex-M processor; it
generates an external interrupt, useful for testing.

All released GNAT compilers I have access to and which accept the syntax
(GPL 2015+, FSF GCC 5+) produce the result that the hardware needs and
that I intended, i.e. 16#1#.

GCC 8, which is still under development, produces 16#FFFFFF01#, i.e. has
only written the least significant byte.

I'm pretty sure this is an error, rather than an unhelpful
interpretation: can anyone confirm one way or the other, before I raise
a bug, please?

========================================================================
with Ada.Text_IO;
with Interfaces;

procedure Full_Word is

   Register : Interfaces.Unsigned_32;

   type Interrupt_ID is range 0 .. 44;

   procedure Trigger_Interrupt (IRQ : Interrupt_ID) is
      --  The Software Trigger Interrupt Register.
      NVIC_STIR : Interrupt_ID
      with
        Import,
        Atomic,
        Size    => 32,
        Address => Register'Address;

   begin
      NVIC_STIR := IRQ;                   -- h/w requires 32-bit write
   end Trigger_Interrupt;

   package Unsigned_32_IO
     is new Ada.Text_IO.Modular_IO (Interfaces.Unsigned_32);

begin
   Register := 16#ffff_ffff#;
   Trigger_Interrupt (1);
   Unsigned_32_IO.Put (Register, Base => 16);
   Ada.Text_IO.New_Line;
end Full_Word;