Hi Andre, > Gesendet: Mittwoch, 23. Juli 2025 um 10:06 > Von: "Andre Vehreschild" <ve...@gmx.de> > An: "Harald Anlauf" <anl...@gmx.de> > CC: fortran@gcc.gnu.org > Betreff: Re: Add: [Bug fortran/121043] [16 Regression] Tests of OpenCoarray > fail to pass, works on 15.1.1 20250712 > > Hi Harald, > > > I spent some time understanding why Intel fails on your example. > > Playing around with your code below, adding print statements, > > rearranging code, etc., it appears that ifx has a problem with: > > I came to that conclusion, too. But we both agree that independent of > rewriting > the change team to an associate the mapping is valid, right? I mean `row` just > is an alias for a slice of the `caf` array. This should be like an array with > a > pointer or allocatable attribute, that must not be reallocated, because it is > sharing memory. > > > > associate(row => caf(:, team_number(row_team))) > > ... > > > col_t: change team(column_team) > > > associate(cell => row(this_image(row_team))) > > > cell = team_number(row_team) > > > if (this_image(row_team) /= 1) > > > row(this_image(row_team))[1, > > > team=row_team] = cell end associate > > > end team col_t > > > > If I understand correctly, there is a very complicated dependency > > of lhs and rhs for the assignment > > > > if (this_image(row_team) /= 1) row(this_image(row_team))[1, team=row_team] > > = cell > > > > due to the associates. Trying to rewrite it gives weird runtime errors. > > Well, parallel program development usually does not make things easier.
But faster programs, if done right! ;-) > > This is beyond my paygrade. > > > > Regarding the previous version: > > > > > Anyhow to answer your initial question: The line: > > > > > > if (this_image() == 1) caf(:, team_number(row_team))[1, team_number = -1] > > > = > > > row > > > > Looking at it with my MPI knowledge, this does feel unusual. What would be > > the communicator here? Is there an equivalent to MPI communicators with > > Coarrays? (and does form_team create that communicator, change_team remap > > coarray descriptors suitably, etc.?) Or am I completely off? > > MPI-Communicator wise: Every form team creates a new MPI-communicator, that > has > only the images/processes participating in it. Every change team changes to > that communicator on the current image, limiting the "world" view when current > team is addressed. But the parent communicators are not unavailable! By using > for example team_number = -1 one can address the initial communicator/team. In > MPI terms MPI_COMM_WORLD. > > A coarray itself does not know in which communicator/team it lives, because > coarrays created in parent teams are available in their children. Both > libraries (OpenCoarrays as well as caf_shmem) just make sure, that coarrays > that where allocated in a child team are unallocated when that team ends. > > I totally understand that it feels unusual. Thinking about a Red-Black-SSOR > over > MPI one would be writing into the "wrong" communicator here. But well, this is > mixing up algorithms with techniques. Because one algo makes this feel wrong, > it must not mean, that another algorithm can not do a reduce over all > processes. > > Did that help? Actually this discussion is quite helpful to me, so I (and maybe others) understand more of the underlying stuff. I now spent some time looking thru portions of the F2023 standard, and I think that it answers many questions in that respect: - 10.2 Assignment, esp. 10.2.1.3 Interpretation of intrinsic assignments - 11.1.3 ASSOCIATE construct - Transformational intrinsics: this_image, team_number, ... It seems to be clear in most cases on which image something is evaluated, and which order. > I mean, we are way off of the original question, which was if it > is ok to always compute a function result on the image initiating a > communication instead of in the caf_accessor. I am still confused what you mean by "initiating a communication". The function you are talking about takes an argument, interpreted in the way defined by the standard, and each image evalutes its portion. In code such as > if (this_image() == 1) caf(:, team_number(row_team))[1, team_number = -1] = > row team_number is a transformational function, which I expect to get evaluated on each image where the condition is fulfilled. I don't see any communication involved. Then there is the assignment, which is difficult. I haven't thought long enough about the consistency between the condition which refers to the current team, and coindex 1 of the initial team. (This is why I asked about communicators and alike, as this assignment might be correct only under very special conditions, or I just don't understand it.) > That patch has still to be merged > or joined to the pr88076 series. So can you clarify that your code evaluates in the standard-defined way? Cheers, Harald > Thanks for coming back to this. > > Regards, > Andre > -- > Andre Vehreschild * Email: vehre ad gmx dot de >
