Use raft::copy in the inner loop of the host-side raft::gather

cpp/include/raft/matrix/detail/gather.cuh

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <raft/common/nvtx.hpp>
+#include <raft/core/copy.hpp>
 #include <raft/core/device_mdarray.hpp>
 #include <raft/core/device_mdspan.hpp>
 #include <raft/core/host_mdarray.hpp>
@@ -350,20 +351,23 @@ void gather_if(const InputIteratorT in,
  * Helper function to gather a set of vectors from a (host) dataset.
  */
 template <typename T, typename IdxT, typename MatIdxT = int64_t>
-void gather_buff(host_matrix_view<const T, MatIdxT> dataset,
+void gather_buff(raft::resources const& res,
+                 host_matrix_view<const T, MatIdxT> dataset,
                  host_vector_view<const IdxT, MatIdxT> indices,
                  MatIdxT offset,
                  pinned_matrix_view<T, MatIdxT> buff)
 {
-  raft::common::nvtx::range<common::nvtx::domain::raft> fun_scope("gather_host_buff");
+  auto dim        = buff.extent(1);
   IdxT batch_size = std::min<IdxT>(buff.extent(0), indices.extent(0) - offset);
+  raft::common::nvtx::range<common::nvtx::domain::raft> fun_scope(
+    "gather_host_buff(%zu, %zu)", uint64_t(batch_size), uint64_t(dim));
 
 #pragma omp for
   for (IdxT i = 0; i < batch_size; i++) {
     IdxT in_idx = indices(offset + i);
-    for (IdxT k = 0; k < buff.extent(1); k++) {
-      buff(i, k) = dataset(in_idx, k);
-    }
+    raft::copy(res,
+               raft::make_pinned_vector_view<T, MatIdxT>(&buff(i, 0), dim),
+               raft::make_host_vector_view<const T, MatIdxT>(&dataset(in_idx, 0), dim));
   }
 }
 
@@ -373,9 +377,10 @@ void gather(raft::resources const& res,
             device_vector_view<const IdxT, MatIdxT> indices,
             raft::device_matrix_view<T, MatIdxT> output)
 {
-  raft::common::nvtx::range<common::nvtx::domain::raft> fun_scope("gather");
-  IdxT n_dim        = output.extent(1);
-  IdxT n_train      = output.extent(0);
+  IdxT n_dim   = output.extent(1);
+  IdxT n_train = output.extent(0);
+  raft::common::nvtx::range<common::nvtx::domain::raft> fun_scope(
+    "gather(%zu, %zu)", uint64_t(n_train), uint64_t(n_dim));
   auto indices_host = raft::make_host_vector<IdxT, MatIdxT>(n_train);
   raft::copy(
     indices_host.data_handle(), indices.data_handle(), n_train, resource::get_cuda_stream(res));
@@ -400,7 +405,7 @@ void gather(raft::resources const& res,
   {
     auto view1 = out_tmp1.view();
     auto view2 = out_tmp2.view();
-    gather_buff(dataset, make_const_mdspan(indices_host.view()), (MatIdxT)0, view1);
+    gather_buff(res, dataset, make_const_mdspan(indices_host.view()), (MatIdxT)0, view1);
     for (MatIdxT device_offset = 0; device_offset < n_train; device_offset += max_batch_size) {
       MatIdxT batch_size = std::min<IdxT>(max_batch_size, n_train - device_offset);
 
@@ -413,7 +418,7 @@ void gather(raft::resources const& res,
       MatIdxT host_offset = device_offset + batch_size;
       batch_size          = std::min<IdxT>(max_batch_size, n_train - host_offset);
       if (batch_size > 0) {
-        gather_buff(dataset, make_const_mdspan(indices_host.view()), host_offset, view2);
+        gather_buff(res, dataset, make_const_mdspan(indices_host.view()), host_offset, view2);
       }
 #pragma omp master
       resource::sync_stream(res);