(j3.2006) Integration of co-arrays with the intrinsic shift functions
Thu Jul 19 13:08:02 EDT 2007
On Wednesday 18 July 2007 19:42, Malcolm Cohen wrote:
> Right, and this means that the argument about whether the CO_whatever is a
> function or a subroutine could be somewhat moot.
Just because we allow programmers to write programs whose behavior depends on
the phase of the moon does not imply we should give up on keeping some basic
standards in designing new language features (the whole business about
functions with side-effects being a very old one).
Another important point here for Craig is that we do not have functions
C525 A co-array shall be a component or a variable that is not a function
Lifting this restriction requires some work. In the data-parallel world it may
be obvious what
means and there is no problems. Also in F90. But if you think about it you'll
see that co_shift cannot return an array (his paper says "Result
Characteristics. The result is an array of the type and type parameters of
CO_ARRAY, and has the shape of CO_ARRAY"). You don't want it to, since that
completely destroys the collective nature of the call.
CAF is not a data-parallel model. Most importantly, it has independent and
multiple execution sequences. Interpreting what co_shift(x,1) really means is
difficult in such a context. Bill is absolutely right that you can write your
co_shift(x,1) in Fortran 2008. Yes, the compiler won't optimize it. But the
compiler won't optimize
either unless (in this order):
1) it is guaranteed that all images execute this statement
2) x is a co-array
3) co_shift returns a co-array, that is, returns an array of the same shape on
We don't have mechanisms in CAF for specifying 1 or 3! Yes, they can be added.
With lots of work and significant complication. I personally do not feel this
is the right time or place to make such extensions.
This is not to say I think data parallel is a "bad direction" in general. I
happen to like ZPL and Chapel. It's just not the place of Fortran, IMO.
Aleksandar Donev, Ph.D.
Lawrence Postdoctoral Fellow @ LLNL
High Performance Computational Materials Science and Chemistry
E-mail: donev1 at llnl.gov
Phone: (925) 424-6816 Fax: (925) 423-0785
Address: P.O.Box 808, L-367, Livermore, CA 94551-9900
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