[Bounded] Vectors, reference types, and the secondary stack

[Bounded] Vectors, reference types, and the secondary stack

[Simon Wright]

It turns out that (GCC 4.9.1) that if you have

   package Interval_Containers
     is new Ada.Containers.Bounded_Vectors
       (Index_Type   => Natural,
        Element_Type => Ada.Real_Time.Time_Span,
        "="          => Ada.Real_Time."=");

   Intervals : Interval_Containers.Vector (5);

and then

   Intervals.Insert_Space (0, 5);
   Intervals (0) := Ada.Real_Time.Milliseconds (50);

then

   function Reference
     (Container : aliased in out Vector;
      Index     : Index_Type) return Reference_Type;

returns its result on the secondary stack!

Why would it need to do that? given the (private) definition

   type Reference_Type
      (Element : not null access Element_Type) is null record;


You ask why I would care. Well, in my STM32F4 RTS the environment task,
in which elaboration happens, isn't actually a task, and doesn't (yet)
have a secondary stack.

The reason it's not a task is that the way to kick off the FreeRTOS
sceduler is to call FreeRTOS.Tasks.Start_Scheduler (aka
vTaskStartScheduler()), which doesn't return unless the scheduler can't
be started; and I haven't found a way to get this behaviour into the
startup code generated by gnatbind, so the poor user has to call it at
the end of their main program.