(j3.2006) a question on cobounds

[Jim Xia]

j3-bounces at j3-fortran.org wrote on 07/28/2009 08:34:50 AM:
> > 
> > Same shock today here finding this out.  Although it's not much of an 
> > implementation issue, the simple fact that a piece of static memory 
can 
> > be declared differently from one run to another, from one image to 
> > another, is quite appalling. 
> 
> Do you also find it appalling that a user can specify bounds for a 
> noncoarray explicit-shape dummy argument that are different from the 
> bounds of the corresponding actual argument? 


No, because a dummy arguments meant to be flexible.  I'll find it 
appalling if a dummy is allowed to have a SAVE attribute.



                                                 This does not seem like a 

> new concept in Fortran.



An adjustable variable with SAVE attribute is new in Fortran.



> >  This blows away the symmetrical attribute (or simplicity) I've always 

> > associated with coarrays. 
> 
> But the memory image IS symmetric.  This is the key.


The coshape part is NOT symmetric, that is also a key.


> All in a sudden UPC seems not a bad language
> > at all since the asymmetrical nature of a shared array can be better 
> > regulated than this.  If cobounds provide nothing more than a shrewd 
way 
> > to compute image index, then what is the point to have cobounds and 
> > corank?  It may make things much easier if corank has to be 1 and 
> > colbound must be 1.  Maybe we should stay with MPI after all.
> 
> The basic concept of coarrays is the extension of Fortran's array 
> concepts to the image space. Restricting the corank to 1 is incompatible 

> with this foundational characteristic.



There are almost no longer any analogies can be drawn between arrays and 
coarrays, not much beyond syntax.  Can you allow an adjustable array to 
have SAVE attribute like the following?
subroutine foo (n)
     INTEGER, SAVE :: X(N)



> I'm guessing you have not written code that uses coarrays to represent a 

> distributed array across a 2- or 3-dimensional grid. Coarrays with 
> corank > 1 are enormously useful for program clarity and maintainability 

>   in cases like this, and this is a pretty common case. This is a big 
> advantage we have over UPC and MPI.  There are others as well, of 
course.


No, I haven't written any real coarray programs -- where do I obtain a 
compiler for it?  I bought the original argument for supporting higher 
corank just for these reasons.  Now the fact that you can have adjustable 
cobounds starts to reverse my view on this support.  I cannot 
enthusiastically tell anyone that coarrays are a good thing to use, at 
least not this part of the feature.


Cheers,


Jim Xia

XL Fortran Compiler Test
IBM Toronto Lab at 8200 Warden Ave, Markham, On, L6G 1C7
Phone (905) 413-3444  Tie-line 313-3444
email: jimxia at ca.ibm.com
D2/YF7/8200 /MKM

http://www.ibm.com/software/awdtools/fortran/xlfortran
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://j3-fortran.org/pipermail/j3/attachments/20090728/d3e287b1/attachment-0001.html