!standard 4.5.10(3/5)                                  20-08-28  AI12-0355-2/01
!standard 4.5.10(21/5)
!standard 5.5(3/5)
!standard 5.5(9/5)
!standard 5.5.3(25/5)
!standard 13.1.1(37/3)
!priority Medium
!difficulty Medium
!subject Aspect specifications for parallel constructs
!summary

All constructs that include the word "parallel" now permit an
aspect_specification as part of their syntax.

!problem

We need a mechanism to pass information through to a lightweight
parallelism scheduler. Something like an aspect specification might be a
good way to specify such options.

!proposal

We propose to add an aspect_specification to every place the PARALLEL
keyword is envisioned currently

    PARALLEL [(chunk_specification)] [aspect_specification] FOR ...

This allows specifying options to control the scheduling of 
parallel constructs.

!wording

Replace 4.5.10 (3/5) with:

  value_sequence ::=
    '[' [parallel[(chunk_specification)] [aspect_specification]]
        iterated_element_association ']'

Modify 4.5.10 (21/5):

  For the evaluation of a value_sequence, the
  iterated_element_association{, the chunk_specification, and the
  aspect_specification, if any, are elaborated in an arbitrary order}
  [is elaborated], then an iteration is performed, and for each value
  conditionally produced by the iteration (see 5.5 and 5.5.2), the
  associated expression is evaluated with the loop parameter having this
  value, to produce a result that is converted to Value_Type, and used
  to define the next value in the sequence.

Replace 5.5 (3/5) with:

  iteration_scheme ::= while condition
     | for loop_parameter_specification
     | for iterator_specification
     | [parallel [aspect_specification]]
       for procedural_iterator
     | parallel [(chunk_specification)] [aspect_specification]
       for loop_parameter_specification
     | parallel [(chunk_specification)] [aspect_specification]
       for iterator_specification

Modify 5.5 (9/5):

  For the execution of a loop_statement that has an iteration_scheme
  including a loop_parameter_specification, after elaborating the
  chunk_specification {and aspect_specification}, if any, the
  loop_parameter_specification is elaborated. ...

Add to the end of 5.5.3 (25/5):

  ... The aspect_specification, if any, is elaborated prior to
  the invocation of the iterating procedure.

Modify 13.1.1 (37/3):

  At the freezing point of the associated entity, the
  aspect_specification is elaborated. {When appearing in a construct
  other than a declaration, an aspect_specification is elaborated as
  part of the execution of the construct.}  The elaboration of the
  aspect_specification {consists of the elaboration of each
  aspect_definiion in an arbitrary order.  The elaboration of an
  aspect_definition} includes the evaluation of [the]{any} name or
  expression[, if any,]{that is part of the aspect_definition} unless
  the [aspect itself is]{part is itself} an expression. If the
  corresponding aspect{ (or part thereof)} represents an expression (as
  in a precondition), the elaboration {of that part} has no effect; the
  expression is evaluated later at points within the execution as
  specified elsewhere in this International Standard for the particular
  aspect.

!examples

An implementation could support an aspect specification associated with
a parallel loop such as:

   parallel with Collapse => 2, Grainsize => 100_000
   for I in 1 .. 10_000_000 loop
       ...
   end loop;

!discussion

To allow passing tuning parameters through an implementation-defined API
to an underlying lightweight thread scheduler to control lightweight
parallelism, it is useful to permit aspect specifications on all
parallel constructs.

!ASIS

[Not sure. It seems like some new capabilities might be needed,
but I didn't check - Editor.]

!ACATS test

ACATS B- and C-Tests are needed to check that the new capabilities are
supported.


!appendix

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