mpi: contiguous MPI_Allgather

docs/api/core.rst

@@ -97,6 +97,22 @@ Collective
     :tparam RecvView: A Kokkos::View to recv
     :tparam ExecSpace: A Kokkos execution space to operate in
 
+
+.. cpp:function:: template <KokkosExecutionSpace ExecSpace, KokkosView SendView, KokkosView RecvView> \
+                  void KokkosComm::allgather(const ExecSpace &space, const SendView &sv, const RecvView &rv, MPI_Comm comm)
+
+    MPI_Allgather wrapper
+
+    :param space: The execution space to operate in
+    :param sv: The data to send
+    :param rv: The view to receive into
+    :param comm: the MPI communicator
+    :tparam SendView: A Kokkos::View to send. Contiguous and rank less than 2.
+    :tparam RecvView: A Kokkos::View to recv. Contiguous and rank 1.
+    :tparam ExecSpace: A Kokkos execution space to operate in
+
+    If ``sv`` is a rank-0 view, the value from the jth rank will be placed in index j of ``rv``.
+
 Related Types
 -------------
 

src/KokkosComm_collective.hpp

@@ -21,6 +21,7 @@
 #include "KokkosComm_concepts.hpp"
 #include "KokkosComm_alltoall.hpp"
 #include "KokkosComm_reduce.hpp"
+#include "KokkosComm_allgather.hpp"
 
 namespace KokkosComm {
 
@@ -29,4 +30,9 @@ void reduce(const ExecSpace &space, const SendView &sv, const RecvView &rv, MPI_
   return Impl::reduce(space, sv, rv, op, root, comm);
 }
 
+template <KokkosView SendView, KokkosView RecvView, KokkosExecutionSpace ExecSpace>
+void allgather(const ExecSpace &space, const SendView &sv, const RecvView &rv, MPI_Comm comm) {
+  return Impl::allgather(space, sv, rv, comm);
+}
+
 }  // namespace KokkosComm

src/impl/KokkosComm_allgather.hpp

@@ -0,0 +1,70 @@
+//@HEADER
+// ************************************************************************
+//
+//                        Kokkos v. 4.0
+//       Copyright (2022) National Technology & Engineering
+//               Solutions of Sandia, LLC (NTESS).
+//
+// Under the terms of Contract DE-NA0003525 with NTESS,
+// the U.S. Government retains certain rights in this software.
+//
+// Part of Kokkos, under the Apache License v2.0 with LLVM Exceptions.
+// See https://kokkos.org/LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//@HEADER
+
+#pragma once
+
+#include <Kokkos_Core.hpp>
+
+#include "KokkosComm_pack_traits.hpp"
+#include "KokkosComm_traits.hpp"
+
+// impl
+#include "KokkosComm_include_mpi.hpp"
+#include "KokkosComm_types.hpp"
+
+namespace KokkosComm::Impl {
+
+template <KokkosView SendView, KokkosView RecvView>
+void allgather(const SendView &sv, const RecvView &rv, MPI_Comm comm) {
+  Kokkos::Tools::pushRegion("KokkosComm::Impl::allgather");
+
+  using ST         = KokkosComm::Traits<SendView>;
+  using RT         = KokkosComm::Traits<RecvView>;
+  using SendScalar = typename SendView::value_type;
+  using RecvScalar = typename RecvView::value_type;
+
+  static_assert(ST::rank() <= 1, "allgather for SendView::rank > 1 not supported");
+  static_assert(RT::rank() <= 1, "allgather for RecvView::rank > 1 not supported");
+
+  if (!ST::is_contiguous(sv)) {
+    throw std::runtime_error("low-level allgather requires contiguous send view");
+  }
+  if (!RT::is_contiguous(rv)) {
+    throw std::runtime_error("low-level allgather requires contiguous recv view");
+  }
+  const int count = ST::span(sv);  // all ranks send/recv same count
+  MPI_Allgather(ST::data_handle(sv), count, mpi_type_v<SendScalar>, RT::data_handle(rv), count, mpi_type_v<RecvScalar>,
+                comm);
+
+  Kokkos::Tools::popRegion();
+}
+
+template <KokkosExecutionSpace ExecSpace, KokkosView SendView, KokkosView RecvView>
+void allgather(const ExecSpace &space, const SendView &sv, const RecvView &rv, MPI_Comm comm) {
+  Kokkos::Tools::pushRegion("KokkosComm::Impl::allgather");
+  using SPT = KokkosComm::PackTraits<SendView>;
+  using RPT = KokkosComm::PackTraits<RecvView>;
+
+  if (SPT::needs_pack(sv) || RPT::needs_pack(rv)) {
+    throw std::runtime_error("allgather for non-contiguous views not implemented");
+  } else {
+    space.fence();  // work in space may have been used to produce send view data
+    allgather(sv, rv, comm);
+  }
+
+  Kokkos::Tools::popRegion();
+}
+}  // namespace KokkosComm::Impl

unit_tests/CMakeLists.txt

@@ -50,6 +50,7 @@ add_executable(test-main test_main.cpp
   test_barrier.cpp
   test_alltoall.cpp
   test_reduce.cpp
+  test_allgather.cpp
 )
 target_link_libraries(test-main KokkosComm::KokkosComm gtest)
 if(KOKKOSCOMM_ENABLE_TESTS)

unit_tests/test_allgather.cpp

@@ -0,0 +1,82 @@
+//@HEADER
+// ************************************************************************
+//
+//                        Kokkos v. 4.0
+//       Copyright (2022) National Technology & Engineering
+//               Solutions of Sandia, LLC (NTESS).
+//
+// Under the terms of Contract DE-NA0003525 with NTESS,
+// the U.S. Government retains certain rights in this software.
+//
+// Part of Kokkos, under the Apache License v2.0 with LLVM Exceptions.
+// See https://kokkos.org/LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//@HEADER
+
+#include <gtest/gtest.h>
+
+#include "KokkosComm.hpp"
+
+template <typename T>
+class Allgather : public testing::Test {
+ public:
+  using Scalar = T;
+};
+
+using ScalarTypes = ::testing::Types<int, int64_t, float, double, Kokkos::complex<float>, Kokkos::complex<double>>;
+TYPED_TEST_SUITE(Allgather, ScalarTypes);
+
+TYPED_TEST(Allgather, 0D) {
+  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> sv("sv");
+  Kokkos::View<TestScalar *> rv("rv", size);
+
+  // fill send buffer
+  Kokkos::parallel_for(
+      sv.extent(0), KOKKOS_LAMBDA(const int i) { sv() = rank; });
+
+  KokkosComm::allgather(Kokkos::DefaultExecutionSpace(), sv, rv, MPI_COMM_WORLD);
+
+  int errs;
+  Kokkos::parallel_reduce(
+      rv.extent(0), KOKKOS_LAMBDA(const int &src, int &lsum) { lsum += rv(src) != src; }, errs);
+  EXPECT_EQ(errs, 0);
+}
+
+TYPED_TEST(Allgather, 1D_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 *> sv("sv", nContrib);
+  Kokkos::View<TestScalar *> rv("rv", size * nContrib);
+
+  // fill send buffer
+  Kokkos::parallel_for(
+      sv.extent(0), KOKKOS_LAMBDA(const int i) { sv(i) = rank + i; });
+
+  KokkosComm::allgather(Kokkos::DefaultExecutionSpace(), sv, rv, MPI_COMM_WORLD);
+
+  int errs;
+  Kokkos::parallel_reduce(
+      rv.extent(0),
+      KOKKOS_LAMBDA(const int &i, int &lsum) {
+        const int src = i / nContrib;
+        const int j   = i % nContrib;
+        lsum += rv(i) != src + j;
+      },
+      errs);
+  EXPECT_EQ(errs, 0);
+}