Tpetra_Details_getEntryOnHost.hpp

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#ifndef TPETRA_DETAILS_GETENTRYONHOST_HPP
#define TPETRA_DETAILS_GETENTRYONHOST_HPP


#include "TpetraCore_config.h"
#include "Kokkos_Core.hpp"

namespace Tpetra {
namespace Details {
namespace Impl {

template<class ViewType,
         class IndexType = typename ViewType::size_type,
         const bool isHostSpace =
           std::is_same<typename ViewType::memory_space,
                        Kokkos::HostSpace>::value>
struct GetEntryOnHost {
  static typename ViewType::non_const_value_type
  getEntryOnHost (const ViewType& x,
                  const IndexType ind);
};

template<class ViewType,
         class IndexType>
struct GetEntryOnHost<ViewType, IndexType, true> {
  static typename ViewType::non_const_value_type
  getEntryOnHost (const ViewType& x,
                  const IndexType ind)
  {
    static_assert (ViewType::Rank == 1, "x must be a rank-1 Kokkos::View.");
    return x(ind);
  }
};

template<class ViewType,
         class IndexType>
struct GetEntryOnHost<ViewType, IndexType, false> {
  static typename ViewType::non_const_value_type
  getEntryOnHost (const ViewType& x,
                  const IndexType ind)
  {
    // Don't assume UVM.  Carefully get a 0-D subview of the entry of
    // the array, and copy to device.  Do not use host mirror, because
    // that could just be a UVM View if using UVM.
    static_assert (ViewType::Rank == 1, "x must be a rank-1 Kokkos::View.");
#ifdef KOKKOS_ENABLE_CUDA
    // Do not use Kokkos::create_mirror_view, because that could just
    // be a UVM View if using UVM.
    typedef typename ViewType::device_type device_type;
    // Hide this in ifdef, to avoid unused typedef warning.
    typedef typename device_type::execution_space dev_exec_space;
#endif // KOKKOS_ENABLE_CUDA
    typedef typename ViewType::HostMirror::execution_space host_exec_space;
    typedef Kokkos::Device<host_exec_space, Kokkos::HostSpace> host_device_type;
    typedef typename ViewType::non_const_value_type value_type;

    value_type val;
    Kokkos::View<value_type, host_device_type> view_h (&val);
    auto view_d = Kokkos::subview (x, ind); // 0-D View
#ifdef KOKKOS_ENABLE_CUDA
    typedef typename device_type::memory_space dev_memory_space;
    if (std::is_same<dev_memory_space, Kokkos::CudaUVMSpace>::value) {
      dev_exec_space::fence (); // for UVM's sake.
    }
#endif // KOKKOS_ENABLE_CUDA
    Kokkos::deep_copy (view_h, view_d);
#ifdef KOKKOS_ENABLE_CUDA
    if (std::is_same<dev_memory_space, Kokkos::CudaUVMSpace>::value) {
      dev_exec_space::fence (); // for UVM's sake.
    }
#endif // KOKKOS_ENABLE_CUDA
    return val;
  }
};

} // namespace Impl

template<class ViewType,
         class IndexType>
typename ViewType::non_const_value_type
getEntryOnHost (const ViewType& x,
                const IndexType ind)
{
  return Impl::GetEntryOnHost<ViewType, IndexType>::getEntryOnHost (x, ind);
}

} // namespace Details
} // namespace Tpetra

#endif // TPETRA_DETAILS_GETENTRYONHOST_HPP