[Openmp-commits] [PATCH] D97085: [OpenMP] libomp: implement OpenMP 5.1 inoutset task dependence type

[Joachim Protze via Phabricator via Openmp-commits]

protze.joachim added inline comments.


================
Comment at: openmp/runtime/src/kmp_taskdeps.cpp:344-346
+        // link node as successor of all nodes in the prev_set if any
+        npredecessors +=
+            __kmp_depnode_link_successor(gtid, thread, task, node, prev_set);
----------------
If I understand this code right, this has O(n^2) complexity for two sets of size n?

Consider:
```
for (int i=0; i<n; i++) {
#pragma omp task depend(in:A)
work(A,i);
}
for (int i=0; i<n; i++) {
#pragma omp task depend(inoutset:A)
work(A,i);
}
```
All n tasks in the first loop would be predecessor for each of the tasks in the second loop. This will also result in n^2 releases/notifications.


I'd suggest to model the dependencies like:
```
for (int i=0; i<n; i++) {
#pragma omp task depend(in:A)
work(A,i);
}
#pragma omp taskwait depend(inout:A) nowait
for (int i=0; i<n; i++) {
#pragma omp task depend(inoutset:A)
work(A,i);
}
```
I.e., add a dummy dependency node between the two sets, which has n predecessors and n successors. You probably understand the depnode code better, than I do, but I think you would only need to add some code in line 357, where `last_set` is moved to `prev_set`.
This dummy node would reduce linking and releasing complexity to O(n).


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D97085/new/

https://reviews.llvm.org/D97085