Re: [Patch, Fortran] Support allocatable *scalar* coarrays

[Tobias Burnus <burnus@net-b.de>]

Mikael Morin wrote:
> On Saturday 16 July 2011 17:25:36 Tobias Burnus wrote:
>>       integer, save :: A[4:6, 7:*]
>> is a scalar variable on each image, but it has a coarank of 2 with
>> lcobound(A) == [4, 7] and ucobound(A, dim=1) == 7.
> ucobound(A, dim=1) == 6 ? Otherwise I'm even more confused.

Sorry for the typo. It's indeed 6.

> Ah, OK; that's where I was misunderstanding coarrays. I was thinking that a
> (possibly out of date) copy of remote images was available locally, like a
> normal array; and with any network exchanges happening during the SYNC* calls
> only. In fact network traffic happens anywhere there are square brackets, and SYNC*
> are mere iddle waits, right?

In terms of the Fortran standard: Yes. In terms of the implementation: 
It depends. For the front end: It simply requests to receive (or send) 
remote data when it sees a "[...]" - for pushing data to an remote 
image, it might even be asynchrnous.

However, the current plan for libcaf_mpi is that one has two-sided 
communication; the image which wants to have the content of a remote 
image sends a request - and waits for the answer while continuing to 
process incoming requests. Thus, if the image is unlucky, it has to wait 
until the other image hits a SYNC and can then answer requests. If it is 
lucky, the other image also has some remove access and can directly 
process the request.

Via a helper process, the answer could be provided faster - or via 
one-sided communication - or in case of a shared memory implementation.

> While it seems to work well, we would probably have gained some clarity by
> using a separate struct for coarray references.
> For example with the current scheme, array[1,2] has type ARRAY_FULL, but some
> dimen_type are of type DIMEN_ELEMENT. Odd.

Presumably. The problem is that codimensions act on one hand like normal 
dimensions but on the other hand they are different. When declaring 
them, "rank + corank <= 15", adding them as extra dimension is also 
logical etc. On the other hand, when referencing a local coarray, one 
has no brackets and if there is a bracket, one can only give an element 
(single coarray) and not a range or vector.

> Thanks for all the clarifications. Patch is OK (I guess). 

Thanks for the review!

Tobias