Add function to convert mdspan to a const view (#1188)

`make_const_mdspan` is a helper function to convert `mdspan<T>` into `mdspan<const T>`.
I added examples of it's usage
@mhoemmen @Nyrio

Authors:
  - Micka (https://github.com/lowener)
  - Corey J. Nolet (https://github.com/cjnolet)

Approvers:
  - Louis Sugy (https://github.com/Nyrio)
  - Corey J. Nolet (https://github.com/cjnolet)
  - Mark Hoemmen (https://github.com/mhoemmen)

URL: #1188

cpp/bench/matrix/argmin.cu

@@ -46,9 +46,7 @@ struct Argmin : public fixture {
   void run_benchmark(::benchmark::State& state) override
   {
     loop_on_state(state, [this]() {
-      auto matrix_const_view = raft::make_device_matrix_view<const T, IdxT, row_major>(
-        matrix.data_handle(), matrix.extent(0), matrix.extent(1));
-      raft::matrix::argmin(handle, matrix_const_view, indices.view());
+      raft::matrix::argmin(handle, raft::make_const_mdspan(matrix.view()), indices.view());
     });
   }
 

cpp/bench/matrix/gather.cu

@@ -64,14 +64,11 @@ struct Gather : public fixture {
     state.SetLabel(label_stream.str());
 
     loop_on_state(state, [this]() {
-      auto matrix_const_view = raft::make_device_matrix_view<const T, IdxT, row_major>(
-        matrix.data_handle(), matrix.extent(0), matrix.extent(1));
-      auto map_const_view =
-        raft::make_device_vector_view<const MapT, IdxT>(map.data_handle(), map.extent(0));
+      auto matrix_const_view = raft::make_const_mdspan(matrix.view());
+      auto map_const_view    = raft::make_const_mdspan(map.view());
       if constexpr (Conditional) {
-        auto stencil_const_view =
-          raft::make_device_vector_view<const T, IdxT>(stencil.data_handle(), stencil.extent(0));
-        auto pred_op = raft::plug_const_op(T(0.0), raft::greater_op());
+        auto stencil_const_view = raft::make_const_mdspan(stencil.view());
+        auto pred_op            = raft::plug_const_op(T(0.0), raft::greater_op());
         raft::matrix::gather_if(
           handle, matrix_const_view, out.view(), map_const_view, stencil_const_view, pred_op);
       } else {

cpp/include/raft/core/mdspan.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, NVIDIA CORPORATION.
+ * Copyright (c) 2022-2023, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -304,4 +304,52 @@ RAFT_INLINE_FUNCTION auto unravel_index(Idx idx,
   }
 }
 
+/**
+ * @brief Const accessor specialization for default_accessor
+ *
+ * @tparam ElementType
+ * @param a
+ * @return std::experimental::default_accessor<std::add_const_t<ElementType>>
+ */
+template <class ElementType>
+std::experimental::default_accessor<std::add_const_t<ElementType>> accessor_of_const(
+  std::experimental::default_accessor<ElementType> a)
+{
+  return {a};
+}
+
+/**
+ * @brief Const accessor specialization for host_device_accessor
+ *
+ * @tparam ElementType the data type of the mdspan elements
+ * @tparam MemType the type of memory where the elements are stored.
+ * @param a host_device_accessor
+ * @return host_device_accessor<std::experimental::default_accessor<std::add_const_t<ElementType>>,
+ * MemType>
+ */
+template <class ElementType, memory_type MemType>
+host_device_accessor<std::experimental::default_accessor<std::add_const_t<ElementType>>, MemType>
+accessor_of_const(host_device_accessor<std::experimental::default_accessor<ElementType>, MemType> a)
+{
+  return {a};
+}
+
+/**
+ * @brief Create a copy of the given mdspan with const element type
+ *
+ * @tparam ElementType the const-qualified data type of the mdspan elements
+ * @tparam Extents raft::extents for dimensions
+ * @tparam Layout policy for strides and layout ordering
+ * @tparam Accessor Accessor policy for the input and output
+ * @param mds raft::mdspan object
+ * @return raft::mdspan
+ */
+template <class ElementType, class Extents, class Layout, class Accessor>
+auto make_const_mdspan(mdspan<ElementType, Extents, Layout, Accessor> mds)
+{
+  auto acc_c = accessor_of_const(mds.accessor());
+  return mdspan<std::add_const_t<ElementType>, Extents, Layout, decltype(acc_c)>{
+    mds.data_handle(), mds.mapping(), acc_c};
+}
+
 }  // namespace raft

cpp/test/cluster/kmeans.cu

@@ -112,8 +112,7 @@ class KmeansTest : public ::testing::TestWithParam<KmeansInputs<T>> {
     rmm::device_uvector<char> workspace(0, stream);
     rmm::device_uvector<T> L2NormBuf_OR_DistBuf(0, stream);
     rmm::device_uvector<T> inRankCp(0, stream);
-    auto X_view =
-      raft::make_device_matrix_view<const T, int>(X.data_handle(), X.extent(0), X.extent(1));
+    auto X_view = raft::make_const_mdspan(X.view());
     auto centroids_view =
       raft::make_device_matrix_view<T, int>(d_centroids.data(), params.n_clusters, n_features);
     auto miniX = raft::make_device_matrix<T, int>(handle, n_samples / 4, n_features);
@@ -126,12 +125,8 @@ class KmeansTest : public ::testing::TestWithParam<KmeansInputs<T>> {
       miniX.extent(0),
       params.rng_state.seed);
 
-    raft::cluster::kmeans::init_plus_plus(handle,
-                                          params,
-                                          raft::make_device_matrix_view<const T, int>(
-                                            miniX.data_handle(), miniX.extent(0), miniX.extent(1)),
-                                          centroids_view,
-                                          workspace);
+    raft::cluster::kmeans::init_plus_plus(
+      handle, params, raft::make_const_mdspan(miniX.view()), centroids_view, workspace);
 
     auto minClusterDistance = raft::make_device_vector<T, int>(handle, n_samples);
     auto minClusterAndDistance =
@@ -285,10 +280,9 @@ class KmeansTest : public ::testing::TestWithParam<KmeansInputs<T>> {
 
     raft::copy(d_labels_ref.data(), labels.data_handle(), n_samples, stream);
 
-    T inertia  = 0;
-    int n_iter = 0;
-    auto X_view =
-      raft::make_device_matrix_view<const T, int>(X.data_handle(), X.extent(0), X.extent(1));
+    T inertia   = 0;
+    int n_iter  = 0;
+    auto X_view = raft::make_const_mdspan(X.view());
 
     raft::cluster::kmeans_fit_predict<T, int>(
       handle,

cpp/test/core/mdspan_utils.cu

@@ -214,4 +214,31 @@ void test_reshape()
 
 TEST(MDArray, Reshape) { test_reshape(); }
 
+void test_const_mdspan()
+{
+  // 3d host array
+  {
+    using two_d_extents = extents<int, 5, 5>;
+    using two_d_mdarray = host_mdarray<float, two_d_extents>;
+
+    typename two_d_mdarray::mapping_type layout{two_d_extents{}};
+    typename two_d_mdarray::container_policy_type policy;
+    two_d_mdarray mda{layout, policy};
+
+    auto const_mda = make_const_mdspan(mda.view());
+
+    static_assert(std::is_same_v<const float, typename decltype(const_mda)::element_type>,
+                  "elements not the same");
+    static_assert(std::is_same_v<typename decltype(mda)::extents_type,
+                                 typename decltype(const_mda)::extents_type>,
+                  "extents not the same");
+    static_assert(std::is_same_v<typename decltype(mda)::layout_type,
+                                 typename decltype(const_mda)::layout_type>,
+                  "layouts not the same");
+    ASSERT_EQ(mda.size(), const_mda.size());
+  }
+}
+
+TEST(MDSpan, ConstMDSpan) { test_const_mdspan(); }
+
 }  // namespace raft

docs/source/cpp_api/mdspan_mdspan.rst

@@ -22,6 +22,9 @@ mdspan: Multi-dimensional Non-owning View
 .. doxygenfunction:: raft::unravel_index
     :project: RAFT
 
+.. doxygenfunction:: raft::make_const_mdspan(mdspan_type mds)
+    :project: RAFT
+
 
 Device Vocabulary
 -----------------