mpi: in-place alltoall

src/impl/KokkosComm_alltoall.hpp

@@ -88,4 +88,38 @@ void alltoall(const ExecSpace &space, const SendView &sv,
 
   Kokkos::Tools::popRegion();
 }
-}  // namespace KokkosComm::Impl
+
+// in-place alltoall
+template <KokkosExecutionSpace ExecSpace, KokkosView RecvView>
+void alltoall(const ExecSpace &space, const RecvView &rv,
+              const size_t recvCount, MPI_Comm comm) {
+  Kokkos::Tools::pushRegion("KokkosComm::Impl::alltoall");
+
+  using RT         = KokkosComm::Traits<RecvView>;
+  using RecvScalar = typename RecvView::value_type;
+
+  static_assert(RT::rank() <= 1,
+                "alltoall for RecvView::rank > 1 not supported");
+
+  if (KokkosComm::PackTraits<RecvView>::needs_pack(rv)) {
+    throw std::runtime_error(
+        "alltoall for non-contiguous views not implemented");
+  } else {
+    int size;
+    MPI_Comm_size(comm, &size);
+
+    if (recvCount * size > RT::extent(rv, 0)) {
+      std::stringstream ss;
+      ss << "alltoall recvCount * communicator size (" << recvCount << " * "
+         << size << ") is greater than recv view size";
+      throw std::runtime_error(ss.str());
+    }
+
+    MPI_Alltoall(MPI_IN_PLACE, 0 /*ignored*/, MPI_BYTE /*ignored*/,
+                 RT::data_handle(rv), recvCount, mpi_type_v<RecvScalar>, comm);
+  }
+
+  Kokkos::Tools::popRegion();
+}
+
+}  // namespace KokkosComm::Impl

unit_tests/test_alltoall.cpp

@@ -63,4 +63,35 @@ TYPED_TEST(Alltoall, 1D_contig) {
   EXPECT_EQ(errs, 0);
 }
 
+TYPED_TEST(Alltoall, 1D_inplace_contig) {
+  using TestScalar = typename TestFixture::Scalar;
+
+  int rank, size;
+  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+  MPI_Comm_size(MPI_COMM_WORLD, &size);
+
+  const int nContrib = 10;
+
+  Kokkos::View<TestScalar *> rv("rv", size * nContrib);
+
+  // fill send buffer
+  Kokkos::parallel_for(
+      rv.extent(0), KOKKOS_LAMBDA(const int i) { rv(i) = rank + i; });
+
+  KokkosComm::Impl::alltoall(Kokkos::DefaultExecutionSpace(), rv, nContrib,
+                             MPI_COMM_WORLD);
+
+  int errs;
+  Kokkos::parallel_reduce(
+      rv.extent(0),
+      KOKKOS_LAMBDA(const int &i, int &lsum) {
+        const int src = i / nContrib;  // who sent this data
+        const int j =
+            rank * nContrib + (i % nContrib);  // what index i was at the source
+        lsum += rv(i) != src + j;
+      },
+      errs);
+  EXPECT_EQ(errs, 0);
+}
+
 }  // namespace