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!standard D.7 (19.1/2)          10-10-31 AI05-0189-1/04
!standard H.4(23.3/2)
!standard C.7.1(3/1)
!standard C.7.1(4)
!standard C.7.1(8)
!standard C.7.1(10)
!class amendment 09-11-03
!status work item 09-11-03
!status received 09-11-03
!priority Low
!difficulty Easy
!subject Restriction No_Standard_Storage_Pools_After_Elaboration
!summary
Provide a restriction that disallows use of allocators from standard storage pools after library unit elaboration is complete. Also provide additional operations in Task_Identification to permit identifying the environment task, and whether a task has completed its activation, to enable user-defined storage managers to enforce a corresponding restriction.
!problem
It is quite common that a resource-constrained long-lived application wants to avoid any heap allocation after library unit elaboration is complete, to ensure that there is no heap growth during the ongoing execution of the program.
!proposal
(see summary)
!wording
Modify C.7.1(3/1):
function Current_Task return Task_Id;
{ function Environment_Task return Task_Id;}
procedure Abort_Task(T : in Task_Id);
Add after C.7.1(4):
function Activation_Is_Complete(T : Task_Id) return Boolean;
Add after C.7.1(8):
The function Environment_Task returns a value that identifies the environment task.
Add after C.7.1(10):
The function Activation_Is_Complete returns True if the task identified by T has completed its activation (whether successfully or not). It returns False otherwise. If T identifies the environment task, Activation_Is_Complete returns True after the elaboration of the library_items of the partition has completed.
Add after D.7(19.1/2):
No_Standard_Storage_Pools_After_Elaboration
Specifies that an allocator using a standard storage pool (see 13.11) shall not occur within a parameterless library subprogram, nor within the handled_sequence_of_statements of a task body. For the purposes of this rule, an allocator of a type derived from a formal access type does not use a standard storage pool.
At run-time, Storage_Error is raised if an allocator using a standard storage pool is evaluated after the elaboration of the library_items of the partition has completed.
Revise H.4(23.3/2) as follows:
* the following restrictions defined in D.7: No_Task_Hierarchy, No_Abort_Statement, No_Implicit_Heap_Allocation {, No_Standard_Storage_Pools_After_Elaboration}; and
!discussion
We considered restricting allocators to the environment task, but felt it was important to allow non-environment tasks to perform allocators during their activation. Note that allowing non-environment tasks to perform allocators means that synchronization is still required during heap allocation.
We do not require this restriction to be entirely enforced prior to execution. We perhaps could do so, but to do it completely could require significant static analysis.
The original proposal disallowed all allocators after elaboration, but it seemed preferable to allow user-defined storage pool implementations to enforce the restriction themselves by providing enough information via additional Task_Identification queries.
!examples
** TBD **
!ACATS test
ACATS B and C tests are needed.
!appendix

From: Tucker Taft
Sent: Friday, June 12, 2009  6:23 AM

This is an official request from Ada Switzerland for a restriction that
disallows allocation after library-unit elaboration is complete.

[Editor's note: Not sure when Tucker becaome Swiss! I presume he meant to
say that he is relaying an official request from Ada Switzerland, or some
such thing.]

****************************************************************

From: Jean-Pierre Rosen
Sent: Friday, June 12, 2009  12:05 PM

No_Local_Allocator seems to come close...

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From: Tucker Taft
Sent: Friday, June 12, 2009  3:44 PM

True, but it is too limiting.  It means that you can't use allocators in
subprograms, even if they are called only during library-unit elaboration.
That pretty much defeats any kind of abstraction.

Many embedded systems have the model of allowing dynamic allocation during
start up, but not thereafter. Ada really ought to support that. What I would
imagine is a restriction that disallows use of a default storage pool after
elaboration is complete, while allowing user-defined storage pools to check
in the Allocate procedure against some global flag that indicates whether
library-level elaboration is complete.

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From: Tullio Vardanega
Sent: Saturday, June 13, 2009  3:05 AM

Something along the line of Tucker's outline would be very useful indeed.
At present we often find ourselves in the need of cheating ourselves and
the compiler by removing pragma Restrictions to permit the elaboration code
to use dynamic memory allocation, but in that manner we lose the ability to
enforce the restriction for the code executed after elaboration.

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From: Tucker Taft
Sent: Monday, June 7, 2010  10:53 AM

Here is a minor update to the AI on the
restriction No_Allocators_After_Elaboration. [This is version /02 of the AI - Editor.]
Note that the minutes implied that library-level tasks weren't certain to be
activated until the "begin" of the main subprogram.  Actually, they must be
activated before the main subprogram is invoked at all. (See 10.2(20-21) which
indicates that the call on the main subprogram follows the "begin" of the
conceptual environment task body.)

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From: Steve Baird
Sent: Monday, June 7, 2010  1:32 PM

> An allocator shall not occur after the begin of a task body nor in the main subprogram.

This is fundamentally a rule about a runtime check and the (not particularly
common) statically detectable cases listed above don't seem to me to be worth
special treatment.

> If an implementation chooses to detect a violation of this restriction
> at run-time, Storage_Error should be raised;

Why is this optional? Do you think that the performance advantage of this
approach outweighs the portability costs?

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From: Steve Baird
Sent: Monday, June 7, 2010  1:47 PM

> Why is this optional? Do you think that the performance advantage of
> this approach outweighs the portability costs?

It's a significant overhead to require the testing of a global variable on every
single allocation, so it seems reasonable not to require it. I suppose we could
associate a check name with it, so that the check could be specifically
suppressed?

****************************************************************

From: Tucker Taft
Sent: Monday, June 7, 2010  1:52 PM

>> An allocator shall not occur after the begin of a task body nor in
>> the main subprogram.
>
> This is fundamentally a rule about a runtime check and the (not
> particularly common) statically detectable cases listed above don't
> seem to me to be worth special treatment.

The straw vote was 5-3-3 to keep a static check on task bodies.  So that's why
it is there.

>> If an implementation chooses to detect a violation of this
>> restriction at run-time, Storage_Error should be raised;
>
> Why is this optional? Do you think that the performance advantage of
> this approach outweighs the portability costs?

This is the standard way that restrictions are worded.
See, e.g., D.7(20) on Max_Storage_At_Blocking.

****************************************************************

From: Steve Baird
Sent: Monday, June 7, 2010  2:05 PM

> The straw vote was 5-3-3 to keep a static check on task bodies.  So
> that's why it is there.
>

Fine.

> This is the standard way that restrictions are worded.
> See, e.g., D.7(20) on Max_Storage_At_Blocking.

That's a bad example because the meaning of what is being checked there is very
implementation dependent.

Since Max_Asynchronous_Select_Nesting has the same sort of wording, however, I
do see your point.

****************************************************************

From: Jean-Pierre Rosen
Sent: Monday, June 7, 2010  2:42 PM

>> Why is this optional? Do you think that the performance advantage of
>> this approach outweighs the portability costs?
>
> It's a significant overhead to require the testing of a global
> variable on every single allocation, so it seems reasonable not to
> require it. I suppose we could associate a check name with it, so that
> the check could be specifically suppressed?

What good is there in putting a restriction if it not checked neither at
compile-time nor at run-time?

I'd rather require a run-time check, and allow the compiler to reject the
restriction if it is not supported.

****************************************************************

From: Tucker Taft
Sent: Monday, June 7, 2010  2:54 PM

As I told Steve, this is merely a question of consistency with other
restrictions. Exactly how restrictions are enforced has pretty much always been
to some degree implementation defined.

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From: Tucker Taft
Sent: Saturday, October 30, 2010  9:43 PM

Here is an update to the No_Allocators_After_Elaboration restriction AI.
[This is version /03 of the AI - Editor.] At the last ARG meeting, we had
decided to limit this restriction to allocators using a default storage pool,
and add some queries in Task_Identification to enable user-defined storage pool
implementations to enforce the restriction themselves. To correspond to the
change in semantics of the restriction, we have changed the name of the
restriction to No_Default_Storage_Pools_After_Elaboration.

****************************************************************

From: Randy Brukardt
Sent: Saturday, October 30, 2010 11:15 PM

Silly bug: the function is defined as Activation_Is_Completed, but the text
identifies Activation_Is_Complete.

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