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

[Daniel Carrera <dcarrera at gmail dot com>]

On 07/16/2011 06:43 PM, Mikael Morin wrote:
> > Well, the current implementation supports effectively only a single
> > image - for -fcoarray=single on purpose and for -fcoarray=lib because it
> > has not yet been implemented.
> >
> > Later, one has to add some function call for "scalar[<image_numer>]"
> > while "scalar" itself is the local variable and can be handled as above.
> 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 (possibly out of date) copy of remote images was available locally, like a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?

I am no expert, but I'll try to answer:   Yes.


Yes, network traffic happens whenever there are square brackets and no 
copies are stored locally. However, you have no guarantee of how far 
ahead other images are. For example:

real :: foo[:]

foo = this_image()

if (this_image() == 1) then
    foo = foo + foo[2]
end if
if (this_image() == 2) then
    foo = foo + foo[1]
end if


This program could do all sorts of crazy things. As you said, the SYNC 
is a idle wait, just to make processes wait for each other. The 
following program is predictable:

real :: foo[:]

foo = this_image()

sync all

if (this_image() == 1) then
    foo = foo + foo[2]
end if

sync all

if (this_image() == 2) then
    foo = foo + foo[1]
end if


Cheers,
Daniel.
--
I'm not overweight, I'm undertall.