Re: Strange warning message

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 17-12-02 10:47 , Simon Wright wrote:
> Robert Eachus <rieachus@comcast.net> writes:
>
>> For tasks, we have this additional constraint.  You can't leave the
>> scope in which a task is declared because the task can still access
>> those objects.  So functions, procedures, tasks and even packages can
>> need to wait for all the tasks declared locally to be completed (or
>> waiting at a terminate alternative).  Again declaring library level
>> tasks, if you don't need this complex behavior, eliminates run-time
>> code to make it work.
>
> In my experience, when GNAT (with the default static elaboration) has
> been unable to find a valid elaboration order, it's almost always been
> because of library-level tasks; the binder can't tell whether the task
> (which starts executing on completion of its package's elaboration)
> actually makes any calls to with'd packages at that point, so assumes
> the worst.

That is also my experience, and I therefore isolate library-level tasks 
in their own packages which do _not_ provide any public operations. For 
example, if a subsystem is implemented by the data and operations in 
package Foo, but also needs a library-level task, I put the task in a 
child package, Foo.Tasks, which provides nothing else (so a pragma 
Elaborate_Body is usually needed in the declaration of Foo.Tasks).

Only the main subprogram has context clauses "with Xxx.Tasks". No other 
package refers to the Xxx.Tasks packages.

GNAT can then put all the Xxx.Tasks packages at the end of the 
elaboration order (before the main subprogram) and they cause no 
elaboration circularities.

Another benefit of this separation is that it becomes easy to select the 
(sub)set of tasks that are included in the program. For example, unit 
testing of package Foo can be performed in a sequential way by not 
including Foo.Tasks in the unit-testing program.

Another approach to the elaboration problem is to use pragma 
Partition_Elaboration_Policy (Sequential) to delay the activation of 
library-level tasks until the end of the library elaboration. I haven't 
tried that yet.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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