(j3.2006) [ukfortran] (SC22WG5.5025) Canada's national body report to WG 9 (Ada)

[N.M. Maclaren]

On Jul 10 2013, Van Snyder wrote:
>
>> > I believe that it would be straightforward to add a tasking facility 
>> > to Fortran, if restricted to PURE procedures, and feasible if not. 
>> > Defining asynchronous procedures would not be trivial, of course. 
>> > However, I cannot see how to fit it in with coarrays.
>> 
>> We already have one form of tasking - asynchronous I/O.  ...
>
>Ada has had tasking, and not limited to the local SMP threaded form,
>since 1983.  ...
>
>I have proposed at least three asynchronous computation and data
>transfer mechanisms: ...
>
>Bill has suggested that this is syntactic sugar for
>
>  DO CONCURRENT ( I = 1:3 )
>  SELECT CASE ( I )
>  CASE ( 1 )
>    BLOCK
>      [ <specification-part> ]
>      <block>
>    END BLOCK
>  CASE ( 2 )
>    BLOCK
>      [ <specification-part> ]
>      <block>
>    END BLOCK
>  CASE ( 3 )
>    BLOCK
>      [ <specification-part> ]
>      <block>
>    END BLOCK
>  END SELECT
>  END DO

Yes - but really only for PURE procedures.

>perhaps with embedded IF constructs, but there is an important
>difference:  A task/thread/activity is not created if "IF
>( <logical-expr> )" appears and the <logical-expr> is false.
>
>(see 00-317 and 06-187).
>
>In either case, a variable that is not declared within a
><specification-part> within the construct should not be allowed in a
>variable definition context, except within a CRITICAL construct, which
>needs to be extended beyond images to sequences of execution. Similarly,
>an impure procedure can only be invoked from within a CRITICAL
>construct. An alternative is a new PARALLEL attribute for procedures,
>which indicates that any alteration to a nonlocal variable or dummy
>argument occurs within a CRITICAL construct.

Yes.  It is this sort of thing that is the issue.  Fortran already has
a lot of mechanism for asynchronous PURE procedure calls, which would
need only syntactic sugar, and the real problem is impure ones.  I am
sure that it could be done, and cleanly, but is a significant amount
of effort in checking for gotchas and wordsmithing.


Regards,
Nick Maclaren.