gather_scatter.f90

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module gather_scatter
  use neko_config, only : neko_bcknd_device, neko_bcknd_sx, neko_bcknd_hip, &
       neko_bcknd_cuda, neko_bcknd_opencl, neko_bcknd_metal, neko_device_mpi
  use gs_bcknd, only : gs_bcknd_t, gs_bcknd_cpu, gs_bcknd_sx, gs_bcknd_dev
  use gs_device, only : gs_device_t
  use gs_sx, only : gs_sx_t
  use gs_cpu, only : gs_cpu_t
  use gs_ops, only : gs_op_add, gs_op_max, gs_op_min, gs_op_mul
  use gs_comm, only : gs_comm_t, gs_comm_mpi, gs_comm_mpigpu, gs_comm_nccl, &
       gs_comm_nvshmem, gs_comm_openshmem, gs_comm_caf, gs_comm_neighbour, &
       gs_comm_utofu, gs_vec_nc
  use gs_mpi, only : gs_mpi_t
  use gs_neighbour, only : gs_neighbour_t
  ! Only the backend types are needed here; what tells whether a backend can
  ! run at all is used by the autotuning, and imported by the gs_tune
  ! submodule instead
  use gs_shmem, only : gs_shmem_t
  use gs_caf, only : gs_caf_t
  use gs_utofu, only : gs_utofu_t
  use gs_device_mpi, only : gs_device_mpi_t
  use gs_device_nccl, only : gs_device_nccl_t
  use gs_device_shmem, only : gs_device_shmem_t
  use mesh, only : mesh_t
  use comm, only : pe_rank, pe_size, global_pe_size, neko_comm
  use mpi_f08, only : mpi_reduce, mpi_allreduce, mpi_barrier, mpi_in_place, &
       mpi_wtime, mpi_sum, mpi_min, mpi_integer, mpi_integer8, &
       mpi_double_precision
  use dofmap, only : dofmap_t
  use field, only : field_t
  use num_types, only : rp, dp, i2, i8, c_rp
  use htable, only : htable_i8_t, htable_iter_i8_t
  use stack, only : stack_i4_t, stack_i8_t
  use crystal_router, only : crystal_router_transfer, crystal_router_pack
  use math, only : sort
  use utils, only : neko_error, linear_index
  use logger, only : neko_log, log_size
  use profiler, only : profiler_start_region, profiler_end_region
  use device, only : device_memcpy, host_to_device, device_to_device, &
       device_sync, device_map, device_unmap, device_get_ptr, &
       device_event_record, device_deassociate
  use, intrinsic :: iso_c_binding, only : c_ptr, c_null_ptr, c_intptr_t, &
       c_sizeof, c_associated, c_size_t
  !$ use omp_lib, only : omp_get_thread_num
  implicit none
  private
 
  type, public :: gs_t
     real(kind=rp), allocatable :: local_gs(:) 
     integer, allocatable :: local_dof_gs(:)
     integer, allocatable :: local_gs_dof(:)
     integer, allocatable :: local_blk_len(:)
     integer, allocatable :: local_blk_off(:)
     real(kind=rp), allocatable :: shared_gs(:) 
     real(kind=rp), allocatable :: shared_gs_v(:)
     type(c_ptr) :: shared_gs_v_d = c_null_ptr 
     integer, allocatable :: shared_dof_gs(:)
     integer, allocatable :: shared_gs_dof(:)
     integer, allocatable :: shared_blk_len(:)
     integer, allocatable :: shared_blk_off(:)
     type(dofmap_t), pointer ::dofmap
     type(htable_i8_t) :: shared_dofs
     integer :: nlocal
     integer :: nshared
     integer :: nlocal_blks
     integer :: nshared_blks
     integer :: local_facet_offset
     integer :: shared_facet_offset
     class(gs_bcknd_t), allocatable :: bcknd
     class(gs_comm_t), allocatable :: comm
   contains
     procedure, private, pass(gs) :: gs_op_fld
     procedure, private, pass(gs) :: gs_op_r4
     procedure, pass(gs) :: gs_op_vector
     procedure, pass(gs) :: gs_op_r3
     procedure, pass(gs) :: gs_op_vector3
     procedure, pass(gs) :: init => gs_init
     procedure, pass(gs) :: free => gs_free
     generic :: op => gs_op_fld, gs_op_r4, gs_op_vector, gs_op_r3, &
          gs_op_vector3
  end type gs_t
 
  ! Expose available gather-scatter operation
  public :: gs_op_add, gs_op_mul, gs_op_min, gs_op_max
 
  ! Expose available gather-scatter backends
  public :: gs_bcknd_cpu, gs_bcknd_sx, gs_bcknd_dev
 
  ! Expose available gather-scatter comm. backends
  public :: gs_comm_mpi, gs_comm_mpigpu, gs_comm_nccl, gs_comm_nvshmem, &
       gs_comm_openshmem, gs_comm_caf, gs_comm_neighbour, gs_comm_utofu
 
  ! These routines (used by the gs_tune submodule) have to be public
  ! since gfortran gives a private module procedure internal linkage
  public :: gs_comm_t, gs_comm_alloc, gs_comm_name
 
  integer, parameter :: gs_tune_ntrials = 100
  integer, parameter :: gs_tune_nwarmup = 2
 
  interface
 
     module function gs_time_ops(gs, u, n, op, ntrials) result(t)
       type(gs_t), intent(inout) :: gs
       integer, intent(in) :: n
       real(kind=rp), dimension(n), intent(inout) :: u
       integer, intent(in) :: op, ntrials
       real(kind=dp) :: t
     end function gs_time_ops
 
     module subroutine gs_tune_comm(gs, n, comm_bcknd)
       type(gs_t), intent(inout) :: gs
       integer, intent(in) :: n
       integer, intent(in) :: comm_bcknd
     end subroutine gs_tune_comm
  end interface
 
contains
 
  subroutine gs_init(gs, dofmap, bcknd, comm_bcknd)
    class(gs_t), intent(inout) :: gs
    type(dofmap_t), target, intent(inout) :: dofmap
    character(len=LOG_SIZE) :: log_buf
    character(len=20) :: bcknd_str
    integer, optional :: bcknd, comm_bcknd
    integer :: i, ierr, bcknd_, comm_bcknd_
    integer(i8) :: glb_nshared, glb_nlocal
    logical :: use_device_mpi, use_device_nccl, use_device_shmem, use_host_mpi
    logical :: use_host_shmem
    logical :: use_caf
    logical :: use_neighbour
    logical :: use_utofu
    logical :: tune_comm
    real(kind=rp), allocatable :: tmp(:)
    type(c_ptr) :: tmp_d = c_null_ptr
    integer :: strtgy(4) = [int(b'00'), int(b'01'), int(b'10'), int(b'11')]
    integer :: avg_strtgy, env_len
    character(len=255) :: env_strtgy, env_gscomm
    real(kind=dp) :: strtgy_time(4)
 
    call gs%free()
 
    call neko_log%section('Gather-Scatter')
    ! Currently this uses the dofmap which also contains geometric information
    ! Only connectivity/numbering of points is technically necessary for gs
    gs%dofmap => dofmap
 
    use_device_mpi = .false.
    use_device_nccl = .false.
    use_device_shmem = .false.
    use_host_mpi = .false.
    use_host_shmem = .false.
    use_caf = .false.
    use_neighbour = .false.
    use_utofu = .false.
    tune_comm = .false.
 
    ! Check if a comm-backend is requested via env. variables
    call get_environment_variable("NEKO_GS_COMM", env_gscomm, env_len)
    if (env_len .gt. 0) then
       if (env_gscomm(1:env_len) .eq. "MPI") then
          use_host_mpi = .true.
       else if (env_gscomm(1:env_len) .eq. "MPIGPU") then
          use_device_mpi = .true.
       else if (env_gscomm(1:env_len) .eq. "NCCL") then
          use_device_nccl = .true.
       else if (env_gscomm(1:env_len) .eq. "SHMEM") then
          if (neko_bcknd_device .eq. 1) then
             use_device_shmem = .true.
          else
             use_host_shmem = .true.
          end if
       else if (env_gscomm(1:env_len) .eq. "CAF") then
          use_caf = .true.
       else if (env_gscomm(1:env_len) .eq. "NEIGHBOUR" .or. &
            env_gscomm(1:env_len) .eq. "NEIGHBOR") then
          use_neighbour = .true.
       else if (env_gscomm(1:env_len) .eq. "UTOFU") then
          use_utofu = .true.
       else
          call neko_error('Unknown Gather-scatter comm. backend')
       end if
    end if
 
 
    if (present(comm_bcknd)) then
       comm_bcknd_ = comm_bcknd
    else if (use_host_mpi) then
       comm_bcknd_ = gs_comm_mpi
    else if (use_device_mpi) then
       comm_bcknd_ = gs_comm_mpigpu
    else if (use_device_nccl) then
       comm_bcknd_ = gs_comm_nccl
    else if (use_device_shmem) then
       comm_bcknd_ = gs_comm_nvshmem
    else if (use_host_shmem) then
       comm_bcknd_ = gs_comm_openshmem
    else if (use_caf) then
       comm_bcknd_ = gs_comm_caf
    else if (use_neighbour) then
       comm_bcknd_ = gs_comm_neighbour
    else if (use_utofu) then
       comm_bcknd_ = gs_comm_utofu
    else
       if (neko_device_mpi) then
          comm_bcknd_ = gs_comm_mpigpu
          use_device_mpi = .true.
       else
          ! No backend requested, benchmark the host backends once the
          ! schedule is known and keep the fastest one (see gs_tune_comm)
          comm_bcknd_ = gs_comm_mpi
          tune_comm = (pe_size .gt. 1)
       end if
    end if
 
    call gs_comm_alloc(gs%comm, comm_bcknd_)
 
    if (tune_comm) then
       call neko_log%message('Comm         :         auto')
    else
       call neko_log%message('Comm         : ' // gs_comm_name(comm_bcknd_))
    end if
    ! Initialize a stack for each rank containing which dofs to send/recv at
    ! that rank
    call gs%comm%init_dofs()
    ! Initialize mapping between local ids and gather-scatter ids
    ! based on the global numbering in dofmap
    call gs_init_mapping(gs)
    ! Setup buffers and which ranks to send/recv data from based on mapping
    ! and initializes gs%comm (sets up gs%comm%send_dof and gs%comm%recv_dof and
    ! recv_pe/send_pe)
    call gs_schedule(gs)
    ! Global number of points not needing to be sent over mpi for gs operations
    ! "Internal points"
    glb_nlocal = int(gs%nlocal, i8)
    ! Global number of points needing to be communicated with other pes/ranks
    ! "external points"
    glb_nshared = int(gs%nshared, i8)
    ! Can be thought of a measure of the volume of this rank (glb_nlocal) and
    ! the surface area (glb_nshared) that is shared with other ranks
    ! Lots of internal volume compared to surface that needs communication is
    ! good
 
    if (pe_rank .eq. 0) then
       call mpi_reduce(mpi_in_place, glb_nlocal, 1, &
            mpi_integer8, mpi_sum, 0, neko_comm, ierr)
 
       call mpi_reduce(mpi_in_place, glb_nshared, 1, &
            mpi_integer8, mpi_sum, 0, neko_comm, ierr)
    else
       call mpi_reduce(glb_nlocal, glb_nlocal, 1, &
            mpi_integer8, mpi_sum, 0, neko_comm, ierr)
 
       call mpi_reduce(glb_nshared, glb_nshared, 1, &
            mpi_integer8, mpi_sum, 0, neko_comm, ierr)
    end if
 
    write(log_buf, '(A,I12)') 'Avg. internal: ', glb_nlocal/pe_size
    call neko_log%message(log_buf)
    write(log_buf, '(A,I12)') 'Avg. external: ', glb_nshared/pe_size
    call neko_log%message(log_buf)
 
    if (present(bcknd)) then
       bcknd_ = bcknd
    else
       if (neko_bcknd_sx .eq. 1) then
          bcknd_ = gs_bcknd_sx
       else if (neko_bcknd_device .eq. 1) then
          bcknd_ = gs_bcknd_dev
       else
          bcknd_ = gs_bcknd_cpu
       end if
    end if
 
    ! Setup Gather-scatter backend
    select case (bcknd_)
    case (gs_bcknd_cpu)
       allocate(gs_cpu_t::gs%bcknd)
       bcknd_str = '         std'
    case (gs_bcknd_dev)
       allocate(gs_device_t::gs%bcknd)
       if (neko_bcknd_hip .eq. 1) then
          bcknd_str = '         hip'
       else if (neko_bcknd_cuda .eq. 1) then
          bcknd_str = '        cuda'
       else if (neko_bcknd_opencl .eq. 1) then
          bcknd_str = '      opencl'
       else if (neko_bcknd_metal .eq. 1) then
          bcknd_str = '       metal'
       end if
    case (gs_bcknd_sx)
       allocate(gs_sx_t::gs%bcknd)
       bcknd_str = '          sx'
    case default
       call neko_error('Unknown Gather-scatter backend')
    end select
 
    write(log_buf, '(A)') 'Backend      : ' // trim(bcknd_str)
    call neko_log%message(log_buf)
 
 
    call gs%bcknd%init(gs%nlocal, gs%nshared, gs%nlocal_blks, gs%nshared_blks)
 
    ! Plain base-type assignment; setting this through a
    ! select type (gs_device_t) miscompiles with CCE 21 at -O2/-O3,
    ! silently leaving shared points on the host so that the scatter
    ! overwrites the unpacked halo data with the stale host buffer
    if (use_device_mpi .or. use_device_nccl .or. use_device_shmem) then
       gs%bcknd%shared_on_host = .false.
    end if
 
    if (use_device_mpi) then
       if (pe_size .gt. 1) then
          ! Select fastest device MPI strategy at runtime
          select type (c => gs%comm)
          type is (gs_device_mpi_t)
             call get_environment_variable("NEKO_GS_STRTGY", env_strtgy, &
                  env_len)
             if (env_len .eq. 0) then
                allocate(tmp(dofmap%size()))
                call device_map(tmp, tmp_d, dofmap%size())
                tmp = 1.0_rp
                call device_memcpy(tmp, tmp_d, dofmap%size(), &
                     host_to_device, sync = .false.)
 
                do i = 1, size(strtgy)
                   c%nb_strtgy = strtgy(i)
                   strtgy_time(i) = gs_time_ops(gs, tmp, dofmap%size(), &
                        gs_op_add, gs_tune_ntrials)
                end do
 
                call device_unmap(tmp, tmp_d)
                deallocate(tmp)
 
                c%nb_strtgy = strtgy(minloc(strtgy_time, 1))
 
                avg_strtgy = minloc(strtgy_time, 1)
                call mpi_allreduce(mpi_in_place, avg_strtgy, 1, &
                     mpi_integer, mpi_sum, neko_comm)
                avg_strtgy = avg_strtgy / pe_size
 
                write(log_buf, '(A,B0.2,A)') 'Avg. strtgy  :         [', &
                     strtgy(avg_strtgy), ']'
 
             else
                read(env_strtgy(1:env_len), *) i
 
                if (i .lt. 1 .or. i .gt. 4) then
                   call neko_error('Invalid gs sync strtgy')
                end if
 
                c%nb_strtgy = strtgy(i)
                avg_strtgy = i
 
                write(log_buf, '(A,B0.2,A)') 'Env. strtgy  :         [', &
                     strtgy(avg_strtgy), ']'
             end if
 
             call neko_log%message(log_buf)
 
          end select
       end if
    end if
 
    ! Select the fastest host comm. backend at runtime
    if (tune_comm) then
       call gs_tune_comm(gs, dofmap%size(), comm_bcknd_)
    end if
 
    call neko_log%end_section()
 
  end subroutine gs_init
 
  subroutine gs_comm_alloc(comm, comm_bcknd)
    class(gs_comm_t), allocatable, intent(out) :: comm
    integer, intent(in) :: comm_bcknd
 
    select case (comm_bcknd)
    case (gs_comm_mpi)
       allocate(gs_mpi_t::comm)
    case (gs_comm_mpigpu)
       allocate(gs_device_mpi_t::comm)
    case (gs_comm_nccl)
       allocate(gs_device_nccl_t::comm)
    case (gs_comm_nvshmem)
       allocate(gs_device_shmem_t::comm)
    case (gs_comm_openshmem)
       allocate(gs_shmem_t::comm)
    case (gs_comm_caf)
       allocate(gs_caf_t::comm)
    case (gs_comm_neighbour)
       allocate(gs_neighbour_t::comm)
    case (gs_comm_utofu)
       allocate(gs_utofu_t::comm)
    case default
       call neko_error('Unknown Gather-scatter comm. backend')
    end select
 
  end subroutine gs_comm_alloc
 
  function gs_comm_name(comm_bcknd) result(name)
    integer, intent(in) :: comm_bcknd
    character(len=12) :: name
 
    select case (comm_bcknd)
    case (gs_comm_mpi)
       name = '         MPI'
    case (gs_comm_mpigpu)
       name = '  Device MPI'
    case (gs_comm_nccl)
       name = '        NCCL'
    case (gs_comm_nvshmem)
       name = '     NVSHMEM'
    case (gs_comm_openshmem)
       name = '   OpenSHMEM'
    case (gs_comm_caf)
       name = '         CAF'
    case (gs_comm_neighbour)
       name = '  MPI neigh.'
    case (gs_comm_utofu)
       name = '       uTofu'
    case default
       name = '     unknown'
       call neko_error('Unknown Gather-scatter comm. backend')
    end select
 
  end function gs_comm_name
 
  subroutine gs_free(gs)
    class(gs_t), intent(inout) :: gs
 
    nullify(gs%dofmap)
 
    ! The device backend lazily maps these arrays in gather/scatter,
    ! and its free only releases the device side; remove any stale
    ! address table entries before deallocating the host side
    if (allocated(gs%local_gs)) then
       if (neko_bcknd_device .eq. 1) then
          call device_deassociate(gs%local_gs)
       end if
       deallocate(gs%local_gs)
    end if
 
    if (allocated(gs%local_dof_gs)) then
       if (neko_bcknd_device .eq. 1) then
          call device_deassociate(gs%local_dof_gs)
       end if
       deallocate(gs%local_dof_gs)
    end if
 
    if (allocated(gs%local_gs_dof)) then
       if (neko_bcknd_device .eq. 1) then
          call device_deassociate(gs%local_gs_dof)
       end if
       deallocate(gs%local_gs_dof)
    end if
 
    if (allocated(gs%local_blk_len)) then
       if (neko_bcknd_device .eq. 1) then
          call device_deassociate(gs%local_blk_len)
       end if
       deallocate(gs%local_blk_len)
    end if
 
    if (allocated(gs%local_blk_off)) then
       if (neko_bcknd_device .eq. 1) then
          call device_deassociate(gs%local_blk_off)
       end if
       deallocate(gs%local_blk_off)
    end if
 
    if (allocated(gs%shared_gs)) then
       if (neko_bcknd_device .eq. 1) then
          call device_deassociate(gs%shared_gs)
       end if
       deallocate(gs%shared_gs)
    end if
 
    if (allocated(gs%shared_gs_v)) then
       if (neko_bcknd_device .eq. 1 .and. c_associated(gs%shared_gs_v_d)) then
          call device_unmap(gs%shared_gs_v, gs%shared_gs_v_d)
       end if
       deallocate(gs%shared_gs_v)
    end if
 
    if (allocated(gs%shared_dof_gs)) then
       if (neko_bcknd_device .eq. 1) then
          call device_deassociate(gs%shared_dof_gs)
       end if
       deallocate(gs%shared_dof_gs)
    end if
 
    if (allocated(gs%shared_gs_dof)) then
       if (neko_bcknd_device .eq. 1) then
          call device_deassociate(gs%shared_gs_dof)
       end if
       deallocate(gs%shared_gs_dof)
    end if
 
    if (allocated(gs%shared_blk_len)) then
       if (neko_bcknd_device .eq. 1) then
          call device_deassociate(gs%shared_blk_len)
       end if
       deallocate(gs%shared_blk_len)
    end if
 
    if (allocated(gs%shared_blk_off)) then
       if (neko_bcknd_device .eq. 1) then
          call device_deassociate(gs%shared_blk_off)
       end if
       deallocate(gs%shared_blk_off)
    end if
 
    gs%nlocal = 0
    gs%nshared = 0
    gs%nlocal_blks = 0
    gs%nshared_blks = 0
 
    call gs%shared_dofs%free()
 
    if (allocated(gs%bcknd)) then
       call gs%bcknd%free()
       deallocate(gs%bcknd)
    end if
 
    if (allocated(gs%comm)) then
       call gs%comm%free()
       deallocate(gs%comm)
    end if
 
  end subroutine gs_free
 
  subroutine gs_init_mapping(gs)
    type(gs_t), target, intent(inout) :: gs
    type(mesh_t), pointer :: msh
    type(dofmap_t), pointer :: dofmap
    type(stack_i4_t), target :: local_dof, dof_local, shared_dof, dof_shared
    type(stack_i4_t), target :: local_face_dof, face_dof_local
    type(stack_i4_t), target :: shared_face_dof, face_dof_shared
    integer :: i, j, k, l, lx, ly, lz, max_id, max_sid, id, lid, dm_size
    type(htable_i8_t) :: dm
    type(htable_i8_t), pointer :: sdm
 
    dofmap => gs%dofmap
    msh => dofmap%msh
    sdm => gs%shared_dofs
 
    lx = dofmap%Xh%lx
    ly = dofmap%Xh%ly
    lz = dofmap%Xh%lz
    dm_size = dofmap%size()/lx
 
    call dm%init(dm_size, i)
    call sdm%init(dofmap%size(), i)
 
 
    call local_dof%init()
    call dof_local%init()
 
    call local_face_dof%init()
    call face_dof_local%init()
 
    call shared_dof%init()
    call dof_shared%init()
 
    call shared_face_dof%init()
    call face_dof_shared%init()
 
    !
    ! Setup mapping for dofs points
    !
 
    max_id = 0
    max_sid = 0
    do i = 1, msh%nelv
       ! Local id of vertices
       lid = linear_index(1, 1, 1, i, lx, ly, lz)
       ! Check if this dof is shared among ranks or not
       if (dofmap%shared_dof(1, 1, 1, i)) then
          id = gs_mapping_add_dof(sdm, dofmap%dof(1, 1, 1, i), max_sid)
          !If add unique gather-scatter id to shared_dof stack
          call shared_dof%push(id)
          !If add local id to dof_shared stack
          call dof_shared%push(lid)
          !Now we have the mapping of local id <-> gather scatter id!
       else
          ! Same here, only here we know the point is local
          ! It will as such not need to be sent to other ranks later
          id = gs_mapping_add_dof(dm, dofmap%dof(1, 1, 1, i), max_id)
          call local_dof%push(id)
          call dof_local%push(lid)
       end if
       ! This procedure is then repeated for all vertices and edges
       ! Facets can be treated a little bit differently since they only have one
       ! neighbor
 
       lid = linear_index(lx, 1, 1, i, lx, ly, lz)
       if (dofmap%shared_dof(lx, 1, 1, i)) then
          id = gs_mapping_add_dof(sdm, dofmap%dof(lx, 1, 1, i), max_sid)
          call shared_dof%push(id)
          call dof_shared%push(lid)
       else
          id = gs_mapping_add_dof(dm, dofmap%dof(lx, 1, 1, i), max_id)
          call local_dof%push(id)
          call dof_local%push(lid)
       end if
 
       lid = linear_index(1, ly, 1, i, lx, ly, lz)
       if (dofmap%shared_dof(1, ly, 1, i)) then
          id = gs_mapping_add_dof(sdm, dofmap%dof(1, ly, 1, i), max_sid)
          call shared_dof%push(id)
          call dof_shared%push(lid)
       else
          id = gs_mapping_add_dof(dm, dofmap%dof(1, ly, 1, i), max_id)
          call local_dof%push(id)
          call dof_local%push(lid)
       end if
 
       lid = linear_index(lx, ly, 1, i, lx, ly, lz)
       if (dofmap%shared_dof(lx, ly, 1, i)) then
          id = gs_mapping_add_dof(sdm, dofmap%dof(lx, ly, 1, i), max_sid)
          call shared_dof%push(id)
          call dof_shared%push(lid)
       else
          id = gs_mapping_add_dof(dm, dofmap%dof(lx, ly, 1, i), max_id)
          call local_dof%push(id)
          call dof_local%push(lid)
       end if
       if (lz .gt. 1) then
          lid = linear_index(1, 1, lz, i, lx, ly, lz)
          if (dofmap%shared_dof(1, 1, lz, i)) then
             id = gs_mapping_add_dof(sdm, dofmap%dof(1, 1, lz, i), max_sid)
             call shared_dof%push(id)
             call dof_shared%push(lid)
          else
             id = gs_mapping_add_dof(dm, dofmap%dof(1, 1, lz, i), max_id)
             call local_dof%push(id)
             call dof_local%push(lid)
          end if
 
          lid = linear_index(lx, 1, lz, i, lx, ly, lz)
          if (dofmap%shared_dof(lx, 1, lz, i)) then
             id = gs_mapping_add_dof(sdm, dofmap%dof(lx, 1, lz, i), max_sid)
             call shared_dof%push(id)
             call dof_shared%push(lid)
          else
             id = gs_mapping_add_dof(dm, dofmap%dof(lx, 1, lz, i), max_id)
             call local_dof%push(id)
             call dof_local%push(lid)
          end if
 
          lid = linear_index(1, ly, lz, i, lx, ly, lz)
          if (dofmap%shared_dof(1, ly, lz, i)) then
             id = gs_mapping_add_dof(sdm, dofmap%dof(1, ly, lz, i), max_sid)
             call shared_dof%push(id)
             call dof_shared%push(lid)
          else
             id = gs_mapping_add_dof(dm, dofmap%dof(1, ly, lz, i), max_id)
             call local_dof%push(id)
             call dof_local%push(lid)
          end if
 
          lid = linear_index(lx, ly, lz, i, lx, ly, lz)
          if (dofmap%shared_dof(lx, ly, lz, i)) then
             id = gs_mapping_add_dof(sdm, dofmap%dof(lx, ly, lz, i), max_sid)
             call shared_dof%push(id)
             call dof_shared%push(lid)
          else
             id = gs_mapping_add_dof(dm, dofmap%dof(lx, ly, lz, i), max_id)
             call local_dof%push(id)
             call dof_local%push(lid)
          end if
       end if
    end do
 
    ! Clear local dofmap table
    call dm%clear()
    ! Get gather scatter ids and local ids of edges
    if (lz .gt. 1) then
       !
       ! Setup mapping for dofs on edges
       !
       do i = 1, msh%nelv
 
          !
          ! dofs on edges in x-direction
          !
          if (dofmap%shared_dof(2, 1, 1, i)) then
             do j = 2, lx - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(j, 1, 1, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(j, 1, 1, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do j = 2, lx - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(j, 1, 1, i), max_id)
                call local_dof%push(id)
                id = linear_index(j, 1, 1, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
          if (dofmap%shared_dof(2, 1, lz, i)) then
             do j = 2, lx - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(j, 1, lz, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(j, 1, lz, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do j = 2, lx - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(j, 1, lz, i), max_id)
                call local_dof%push(id)
                id = linear_index(j, 1, lz, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          if (dofmap%shared_dof(2, ly, 1, i)) then
             do j = 2, lx - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(j, ly, 1, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(j, ly, 1, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
 
          else
             do j = 2, lx - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(j, ly, 1, i), max_id)
                call local_dof%push(id)
                id = linear_index(j, ly, 1, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
          if (dofmap%shared_dof(2, ly, lz, i)) then
             do j = 2, lx - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(j, ly, lz, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(j, ly, lz, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do j = 2, lx - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(j, ly, lz, i), max_id)
                call local_dof%push(id)
                id = linear_index(j, ly, lz, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          !
          ! dofs on edges in y-direction
          !
          if (dofmap%shared_dof(1, 2, 1, i)) then
             do k = 2, ly - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(1, k, 1, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(1, k, 1, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do k = 2, ly - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(1, k, 1, i), max_id)
                call local_dof%push(id)
                id = linear_index(1, k, 1, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
          if (dofmap%shared_dof(1, 2, lz, i)) then
             do k = 2, ly - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(1, k, lz, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(1, k, lz, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do k = 2, ly - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(1, k, lz, i), max_id)
                call local_dof%push(id)
                id = linear_index(1, k, lz, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          if (dofmap%shared_dof(lx, 2, 1, i)) then
             do k = 2, ly - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(lx, k, 1, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(lx, k, 1, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do k = 2, ly - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(lx, k, 1, i), max_id)
                call local_dof%push(id)
                id = linear_index(lx, k, 1, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
          if (dofmap%shared_dof(lx, 2, lz, i)) then
             do k = 2, ly - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(lx, k, lz, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(lx, k, lz, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do k = 2, ly - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(lx, k, lz, i), max_id)
                call local_dof%push(id)
                id = linear_index(lx, k, lz, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
          !
          ! dofs on edges in z-direction
          !
          if (dofmap%shared_dof(1, 1, 2, i)) then
             do l = 2, lz - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(1, 1, l, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(1, 1, l, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do l = 2, lz - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(1, 1, l, i), max_id)
                call local_dof%push(id)
                id = linear_index(1, 1, l, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          if (dofmap%shared_dof(lx, 1, 2, i)) then
             do l = 2, lz - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(lx, 1, l, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(lx, 1, l, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do l = 2, lz - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(lx, 1, l, i), max_id)
                call local_dof%push(id)
                id = linear_index(lx, 1, l, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          if (dofmap%shared_dof(1, ly, 2, i)) then
             do l = 2, lz - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(1, ly, l, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(1, ly, l, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do l = 2, lz - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(1, ly, l, i), max_id)
                call local_dof%push(id)
                id = linear_index(1, ly, l, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
 
          if (dofmap%shared_dof(lx, ly, 2, i)) then
             do l = 2, lz - 1
                id = gs_mapping_add_dof(sdm, dofmap%dof(lx, ly, l, i), max_sid)
                call shared_dof%push(id)
                id = linear_index(lx, ly, l, i, lx, ly, lz)
                call dof_shared%push(id)
             end do
          else
             do l = 2, lz - 1
                id = gs_mapping_add_dof(dm, dofmap%dof(lx, ly, l, i), max_id)
                call local_dof%push(id)
                id = linear_index(lx, ly, l, i, lx, ly, lz)
                call dof_local%push(id)
             end do
          end if
       end do
    end if
 
    ! Clear local dofmap table
    call dm%clear()
 
    !
    ! Setup mapping for dofs on facets
    !
    ! This is for 2d
    if (lz .eq. 1) then
       do i = 1, msh%nelv
 
          !
          ! dofs on edges in x-direction
          !
          if (msh%facet_neigh(3, i) .ne. 0) then
             if (dofmap%shared_dof(2, 1, 1, i)) then
                do j = 2, lx - 1
                   id = gs_mapping_add_dof(sdm, dofmap%dof(j, 1, 1, i), max_sid)
                   call shared_face_dof%push(id)
                   id = linear_index(j, 1, 1, i, lx, ly, lz)
                   call face_dof_shared%push(id)
                end do
             else
                do j = 2, lx - 1
                   id = gs_mapping_add_dof(dm, dofmap%dof(j, 1, 1, i), max_id)
                   call local_face_dof%push(id)
                   id = linear_index(j, 1, 1, i, lx, ly, lz)
                   call face_dof_local%push(id)
                end do
             end if
          end if
 
          if (msh%facet_neigh(4, i) .ne. 0) then
             if (dofmap%shared_dof(2, ly, 1, i)) then
                do j = 2, lx - 1
                   id = gs_mapping_add_dof(sdm, dofmap%dof(j, ly, 1, i), &
                        max_sid)
                   call shared_face_dof%push(id)
                   id = linear_index(j, ly, 1, i, lx, ly, lz)
                   call face_dof_shared%push(id)
                end do
 
             else
                do j = 2, lx - 1
                   id = gs_mapping_add_dof(dm, dofmap%dof(j, ly, 1, i), &
                        max_id)
                   call local_face_dof%push(id)
                   id = linear_index(j, ly, 1, i, lx, ly, lz)
                   call face_dof_local%push(id)
                end do
             end if
          end if
 
          !
          ! dofs on edges in y-direction
          !
          if (msh%facet_neigh(1, i) .ne. 0) then
             if (dofmap%shared_dof(1, 2, 1, i)) then
                do k = 2, ly - 1
                   id = gs_mapping_add_dof(sdm, dofmap%dof(1, k, 1, i), max_sid)
                   call shared_face_dof%push(id)
                   id = linear_index(1, k, 1, i, lx, ly, lz)
                   call face_dof_shared%push(id)
                end do
             else
                do k = 2, ly - 1
                   id = gs_mapping_add_dof(dm, dofmap%dof(1, k, 1, i), max_id)
                   call local_face_dof%push(id)
                   id = linear_index(1, k, 1, i, lx, ly, lz)
                   call face_dof_local%push(id)
                end do
             end if
          end if
 
          if (msh%facet_neigh(2, i) .ne. 0) then
             if (dofmap%shared_dof(lx, 2, 1, i)) then
                do k = 2, ly - 1
                   id = gs_mapping_add_dof(sdm, dofmap%dof(lx, k, 1, i), &
                        max_sid)
                   call shared_face_dof%push(id)
                   id = linear_index(lx, k, 1, i, lx, ly, lz)
                   call face_dof_shared%push(id)
                end do
             else
                do k = 2, ly - 1
                   id = gs_mapping_add_dof(dm, dofmap%dof(lx, k, 1, i), &
                        max_id)
                   call local_face_dof%push(id)
                   id = linear_index(lx, k, 1, i, lx, ly, lz)
                   call face_dof_local%push(id)
                end do
             end if
          end if
       end do
    else
       do i = 1, msh%nelv
 
          ! Facets in x-direction (s, t)-plane
          if (msh%facet_neigh(1, i) .ne. 0) then
             if (dofmap%shared_dof(1, 2, 2, i)) then
                do l = 2, lz - 1
                   do k = 2, ly - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(1, k, l, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(1, k, l, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do l = 2, lz - 1
                   do k = 2, ly - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(1, k, l, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(1, k, l, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
 
          if (msh%facet_neigh(2, i) .ne. 0) then
             if (dofmap%shared_dof(lx, 2, 2, i)) then
                do l = 2, lz - 1
                   do k = 2, ly - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(lx, k, l, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(lx, k, l, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do l = 2, lz - 1
                   do k = 2, ly - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(lx, k, l, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(lx, k, l, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
 
          ! Facets in y-direction (r, t)-plane
          if (msh%facet_neigh(3, i) .ne. 0) then
             if (dofmap%shared_dof(2, 1, 2, i)) then
                do l = 2, lz - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(j, 1, l, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(j, 1, l, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do l = 2, lz - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(j, 1, l, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(j, 1, l, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
 
          if (msh%facet_neigh(4, i) .ne. 0) then
             if (dofmap%shared_dof(2, ly, 2, i)) then
                do l = 2, lz - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(j, ly, l, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(j, ly, l, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do l = 2, lz - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(j, ly, l, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(j, ly, l, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
 
          ! Facets in z-direction (r, s)-plane
          if (msh%facet_neigh(5, i) .ne. 0) then
             if (dofmap%shared_dof(2, 2, 1, i)) then
                do k = 2, ly - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(j, k, 1, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(j, k, 1, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do k = 2, ly - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(j, k, 1, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(j, k, 1, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
 
          if (msh%facet_neigh(6, i) .ne. 0) then
             if (dofmap%shared_dof(2, 2, lz, i)) then
                do k = 2, ly - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(sdm, dofmap%dof(j, k, lz, i), &
                           max_sid)
                      call shared_face_dof%push(id)
                      id = linear_index(j, k, lz, i, lx, ly, lz)
                      call face_dof_shared%push(id)
                   end do
                end do
             else
                do k = 2, ly - 1
                   do j = 2, lx - 1
                      id = gs_mapping_add_dof(dm, dofmap%dof(j, k, lz, i), &
                           max_id)
                      call local_face_dof%push(id)
                      id = linear_index(j, k, lz, i, lx, ly, lz)
                      call face_dof_local%push(id)
                   end do
                end do
             end if
          end if
       end do
    end if
 
 
    call dm%free()
 
    gs%nlocal = local_dof%size() + local_face_dof%size()
    gs%local_facet_offset = local_dof%size() + 1
 
    ! Finalize local dof to gather-scatter index
    allocate(gs%local_dof_gs(gs%nlocal))
 
    ! Add dofs on points and edges
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => local_dof%data)
    type is (integer)
       j = local_dof%size()
       do i = 1, j
          gs%local_dof_gs(i) = dof_array(i)
       end do
    end select
    call local_dof%free()
 
    ! Add dofs on faces
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => local_face_dof%data)
    type is (integer)
       do i = 1, local_face_dof%size()
          gs%local_dof_gs(i + j) = dof_array(i)
       end do
    end select
    call local_face_dof%free()
 
    ! Finalize local gather-scatter index to dof
    allocate(gs%local_gs_dof(gs%nlocal))
 
    ! Add gather-scatter index on points and edges
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => dof_local%data)
    type is (integer)
       j = dof_local%size()
       do i = 1, j
          gs%local_gs_dof(i) = dof_array(i)
       end do
    end select
    call dof_local%free()
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => face_dof_local%data)
    type is (integer)
       do i = 1, face_dof_local%size()
          gs%local_gs_dof(i+j) = dof_array(i)
       end do
    end select
    call face_dof_local%free()
 
    call gs_qsort_dofmap(gs%local_dof_gs, gs%local_gs_dof, &
         gs%nlocal, 1, gs%nlocal)
 
    call gs_find_blks(gs%local_dof_gs, gs%local_blk_len, &
         gs%local_blk_off, gs%nlocal_blks, gs%nlocal, gs%local_facet_offset)
 
    ! Allocate buffer for local gs-ops
    allocate(gs%local_gs(gs%nlocal))
 
    gs%nshared = shared_dof%size() + shared_face_dof%size()
    gs%shared_facet_offset = shared_dof%size() + 1
 
    ! Finalize shared dof to gather-scatter index
    allocate(gs%shared_dof_gs(gs%nshared))
 
    ! Add shared dofs on points and edges
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => shared_dof%data)
    type is (integer)
       j = shared_dof%size()
       do i = 1, j
          gs%shared_dof_gs(i) = dof_array(i)
       end do
    end select
    call shared_dof%free()
 
    ! Add shared dofs on faces
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => shared_face_dof%data)
    type is (integer)
       do i = 1, shared_face_dof%size()
          gs%shared_dof_gs(i + j) = dof_array(i)
       end do
    end select
    call shared_face_dof%free()
 
    ! Finalize shared gather-scatter index to dof
    allocate(gs%shared_gs_dof(gs%nshared))
 
    ! Add dofs on points and edges
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => dof_shared%data)
    type is (integer)
       j = dof_shared%size()
       do i = 1, j
          gs%shared_gs_dof(i) = dof_array(i)
       end do
    end select
    call dof_shared%free()
 
    ! We should use the %array() procedure, which works great for
    ! GNU, Intel and NEC, but it breaks horribly on Cray when using
    ! certain data types
    select type (dof_array => face_dof_shared%data)
    type is (integer)
       do i = 1, face_dof_shared%size()
          gs%shared_gs_dof(i + j) = dof_array(i)
       end do
    end select
    call face_dof_shared%free()
 
    ! Allocate buffer for shared gs-ops
    allocate(gs%shared_gs(gs%nshared))
 
    ! Compact multi-component shared buffer for the fused vector gs. On the
    ! device it is mapped so the fused exchange can use its device pointer.
    allocate(gs%shared_gs_v(max(1, gs_vec_nc * gs%nshared)))
    if (neko_bcknd_device .eq. 1) then
       call device_map(gs%shared_gs_v, gs%shared_gs_v_d, &
            max(1, gs_vec_nc * gs%nshared))
    end if
 
    if (gs%nshared .gt. 0) then
       call gs_qsort_dofmap(gs%shared_dof_gs, gs%shared_gs_dof, &
            gs%nshared, 1, gs%nshared)
 
       call gs_find_blks(gs%shared_dof_gs, gs%shared_blk_len, &
            gs%shared_blk_off, gs%nshared_blks, gs%nshared, &
            gs%shared_facet_offset)
    end if
 
  contains
 
    function gs_mapping_add_dof(map_, dof, max_id) result(id)
      type(htable_i8_t), intent(inout) :: map_
      integer(kind=i8), intent(inout) :: dof
      integer, intent(inout) :: max_id
      integer :: id
 
      if (map_%get(dof, id) .gt. 0) then
         max_id = max_id + 1
         call map_%set(dof, max_id)
         id = max_id
      end if
 
    end function gs_mapping_add_dof
 
    recursive subroutine gs_qsort_dofmap(dg, gd, n, lo, hi)
      integer, intent(inout) :: n
      integer, dimension(n), intent(inout) :: dg
      integer, dimension(n), intent(inout) :: gd
      integer :: lo, hi
      integer :: tmp, i, j, pivot
 
      i = lo - 1
      j = hi + 1
      pivot = dg((lo + hi) / 2)
      do
         do
            i = i + 1
            if (dg(i) .ge. pivot) exit
         end do
 
         do
            j = j - 1
            if (dg(j) .le. pivot) exit
         end do
 
         if (i .lt. j) then
            tmp = dg(i)
            dg(i) = dg(j)
            dg(j) = tmp
 
            tmp = gd(i)
            gd(i) = gd(j)
            gd(j) = tmp
         else if (i .eq. j) then
            i = i + 1
            exit
         else
            exit
         end if
      end do
      if (lo .lt. j) call gs_qsort_dofmap(dg, gd, n, lo, j)
      if (i .lt. hi) call gs_qsort_dofmap(dg, gd, n, i, hi)
 
    end subroutine gs_qsort_dofmap
 
    subroutine gs_find_blks(dg, blk_len, blk_off, nblks, n, m)
      integer, intent(in) :: n
      integer, intent(in) :: m
      integer, dimension(n), intent(inout) :: dg
      integer, allocatable, intent(inout) :: blk_len(:)
      integer, allocatable, intent(inout) :: blk_off(:)
      integer, intent(inout) :: nblks
      integer :: i, j
      integer :: id, count
      type(stack_i4_t), target :: blks
 
      call blks%init()
      i = 1
      do while (i .lt. m)
         id = dg(i)
         count = 1
         j = i
         do while ( j+1 .le. n .and. dg(j+1) .eq. id)
            j = j + 1
            count = count + 1
         end do
         call blks%push(count)
         i = j + 1
      end do
 
      select type (blk_array => blks%data)
      type is (integer)
         nblks = blks%size()
         allocate(blk_len(nblks))
         do i = 1, nblks
            blk_len(i) = blk_array(i)
         end do
         allocate(blk_off(nblks))
         blk_off(1) = 0
         do i = 2, nblks
            blk_off(i) = blk_off(i - 1) + blk_len(i - 1)
         end do
      end select
      call blks%free()
 
    end subroutine gs_find_blks
 
  end subroutine gs_init_mapping
 
  subroutine gs_schedule(gs)
    type(gs_t), target, intent(inout) :: gs
    type(htable_iter_i8_t) :: it
    type(stack_i4_t) :: send_pe, recv_pe
    type(stack_i8_t) :: cr_buf
    integer(i8), allocatable :: buf(:)
    integer(i8), pointer :: cr_data(:)
    integer(i8), allocatable :: rgid(:), gtmp(:)
    integer, allocatable :: rpeer(:), rgsid(:), rperm(:), gperm(:)
    integer(i8) :: gid
    integer :: i, j, n, owner, nrec, peer, shared_gs_id, tmp
    integer :: a, b, cnt, t
 
    call send_pe%init()
    call recv_pe%init()
 
    !
    ! Phase 1: route every local shared dof to its canonical owner.
    !   record = [dest=owner, len=2, gid, origin]
    !
    call cr_buf%init(max(gs%shared_dofs%num_entries(), 1) * 4)
    call it%init(gs%shared_dofs)
    do while (it%next())
       gid = it%key()
       owner = int(modulo(gid, int(pe_size, i8)))
       call crystal_router_pack(cr_buf, owner, [gid, int(pe_rank, i8)])
    end do
 
    n = cr_buf%size()
    allocate(buf(max(n, 1)))
    if (n .gt. 0) then
       cr_data => cr_buf%array()
       buf(1:n) = cr_data(1:n)
    end if
    call cr_buf%free()
 
    call crystal_router_transfer(buf, n)
 
    !
    ! Phase 2: at the owner, group holders by gid and reflect, to each holder,
    !   every other holder of the same dof.
    !   reply = [dest=holder, len=2, gid, peer]
    !
    nrec = n / 4 ! every record here has the fixed form [me, 2, gid, origin]
    allocate(rgid(max(nrec, 1)), rgsid(max(nrec, 1)), gperm(max(nrec, 1)))
    do i = 1, nrec
       rgid(i) = buf((i - 1) * 4 + 3) ! gid
       rgsid(i) = int(buf((i - 1) * 4 + 4)) ! origin rank (reuse array)
    end do
    if (nrec .gt. 0) call gs_sort_i8(rgid, gperm, nrec)
 
    call cr_buf%init(max(n, 1))
    i = 1
    do while (i .le. nrec)
       j = i
       do while (j .le. nrec)
          if (rgid(j) .ne. rgid(i)) exit
          j = j + 1
       end do
       ! Reflect, to each holder, every other holder of this dof.
       if (j - i .gt. 1) then
          do a = i, j - 1 ! recipient holder
             do b = i, j - 1 ! the other holder
                if (a .eq. b) cycle
                call crystal_router_pack(cr_buf, rgsid(gperm(a)), &
                     [rgid(i), int(rgsid(gperm(b)), i8)])
             end do
          end do
       end if
       i = j
    end do
    deallocate(rgid, rgsid, gperm)
 
    n = cr_buf%size()
    if (allocated(buf)) deallocate(buf)
    allocate(buf(max(n, 1)))
    if (n .gt. 0) then
       cr_data => cr_buf%array()
       buf(1:n) = cr_data(1:n)
    end if
    call cr_buf%free()
 
    call crystal_router_transfer(buf, n)
 
    !
    ! Phase 3: register each (dof, peer) for both send and receive. Order each
    !   peer's dof list by gid so both ranks of a pair agree on the order.
    !
    nrec = n / 4 ! replies are [me, 2, gid, peer]
    allocate(rgid(max(nrec, 1)), rpeer(max(nrec, 1)), rgsid(max(nrec, 1)), &
         rperm(max(nrec, 1)))
    do i = 1, nrec
       gid = buf((i - 1) * 4 + 3)
       rgid(i) = gid
       rpeer(i) = int(buf((i - 1) * 4 + 4))
       tmp = gs%shared_dofs%get(gid, shared_gs_id)
       rgsid(i) = shared_gs_id
    end do
 
    ! Sort by peer; within each peer run, sort by gid and register in that order.
    if (nrec .gt. 0) call sort(rpeer, rperm, nrec)
    a = 1
    do while (a .le. nrec)
       b = a
       do while (b .le. nrec)
          if (rpeer(b) .ne. rpeer(a)) exit
          b = b + 1
       end do
       peer = rpeer(a)
       cnt = b - a
       allocate(gtmp(cnt), gperm(cnt))
       do t = 1, cnt
          gtmp(t) = rgid(rperm(a + t - 1))
       end do
       call gs_sort_i8(gtmp, gperm, cnt)
       do t = 1, cnt
          shared_gs_id = rgsid(rperm(a + gperm(t) - 1))
          call gs%comm%send_dof(peer)%push(shared_gs_id)
          call gs%comm%recv_dof(peer)%push(shared_gs_id)
       end do
       deallocate(gtmp, gperm)
       call send_pe%push(peer)
       call recv_pe%push(peer)
       a = b
    end do
    deallocate(rgid, rpeer, rgsid, rperm)
    if (allocated(buf)) deallocate(buf)
 
    call gs%comm%init(send_pe, recv_pe)
 
    call send_pe%free()
    call recv_pe%free()
 
    !This arrays seems to take massive amounts of memory...
    call gs%shared_dofs%free()
 
  end subroutine gs_schedule
 
  subroutine gs_sort_i8(a, ind, n)
    integer, intent(in) :: n
    integer(i8), intent(inout) :: a(n)
    integer, intent(out) :: ind(n)
    integer(i8) :: aa
    integer :: j, ir, i, ii, l
 
    do j = 1, n
       ind(j) = j
    end do
 
    if (n .le. 1) return
 
    l = n/2 + 1
    ir = n
    do while (.true.)
       if (l .gt. 1) then
          l = l - 1
          aa = a(l)
          ii = ind(l)
       else
          aa = a(ir)
          ii = ind(ir)
          a(ir) = a(1)
          ind(ir) = ind(1)
          ir = ir - 1
          if (ir .eq. 1) then
             a(1) = aa
             ind(1) = ii
             return
          end if
       end if
       i = l
       j = l + l
       do while (j .le. ir)
          if (j .lt. ir) then
             if (a(j) .lt. a(j + 1)) j = j + 1
          end if
          if (aa .lt. a(j)) then
             a(i) = a(j)
             ind(i) = ind(j)
             i = j
             j = j + j
          else
             j = ir + 1
          end if
       end do
       a(i) = aa
       ind(i) = ii
    end do
  end subroutine gs_sort_i8
 
  subroutine gs_op_fld(gs, u, op, event)
    class(gs_t), intent(inout) :: gs
    type(field_t), intent(inout) :: u
    type(c_ptr), optional, intent(inout) :: event
    integer :: n, op
 
    n = u%msh%nelv * u%Xh%lx * u%Xh%ly * u%Xh%lz
    if (present(event)) then
       call gs_op_vector(gs, u%x, n, op, event)
    else
       call gs_op_vector(gs, u%x, n, op)
    end if
 
  end subroutine gs_op_fld
 
  subroutine gs_op_r4(gs, u, n, op, event)
    class(gs_t), intent(inout) :: gs
    integer, intent(in) :: n
    real(kind=rp), contiguous, dimension(:,:,:,:), intent(inout) :: u
    type(c_ptr), optional, intent(inout) :: event
    integer :: op
 
    if (present(event)) then
       call gs_op_vector(gs, u, n, op, event)
    else
       call gs_op_vector(gs, u, n, op)
    end if
 
  end subroutine gs_op_r4
 
  subroutine gs_op_vector(gs, u, n, op, event)
    class(gs_t), intent(inout) :: gs
    integer, intent(in) :: n
    real(kind=rp), dimension(n), intent(inout) :: u
    type(c_ptr), optional, intent(inout) :: event
    integer :: m, l, op, lo, so, tid
    type(c_ptr) :: scatter_event
 
    lo = gs%local_facet_offset
    so = -gs%shared_facet_offset
    m = gs%nlocal
    l = gs%nshared
 
    ! Capture the calling thread id before opening any parallel region; it
    ! is used as the MPI tag so concurrent gs ops driven from different
    ! threads (device path) don't collide.
    tid = 0
    !$ tid = omp_get_thread_num()
 
    ! Resolve the optional event into a non-optional local before opening
    ! the parallel region. An absent optional dummy must not be captured by
    ! the region's data-sharing, otherwise the outlined region prologue
    ! dereferences a null descriptor (segfaults on CCE).
    scatter_event = c_null_ptr
    if (present(event)) scatter_event = event
 
    !$omp parallel if (NEKO_BCKND_DEVICE .eq. 0)
    call profiler_start_region("gather_scatter", 5)
    ! Gather shared dofs
    if (pe_size .gt. 1 .and. n .gt. 0) then
       call profiler_start_region("gs_nbrecv", 13)
       call gs%comm%nbrecv(tid)
       call profiler_end_region("gs_nbrecv", 13)
       call profiler_start_region("gs_gather_shared", 14)
       call gs%bcknd%gather(gs%shared_gs, l, so, gs%shared_dof_gs, u, n, &
            gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
            gs%shared_blk_off, op, .true.)
       call profiler_end_region("gs_gather_shared", 14)
       call profiler_start_region("gs_nbsend", 6)
       call gs%comm%nbsend(gs%shared_gs, l, tid, &
            gs%bcknd%gather_event, gs%bcknd%gs_stream)
       call profiler_end_region("gs_nbsend", 6)
 
    end if
 
    ! Gather-scatter local dofs
    call profiler_start_region("gs_local", 12)
    call gs%bcknd%gather(gs%local_gs, m, lo, gs%local_dof_gs, u, n, &
         gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, gs%local_blk_off, &
         op, .false.)
    call gs%bcknd%scatter(gs%local_gs, m, gs%local_dof_gs, u, n, &
         gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, gs%local_blk_off, &
         .false., c_null_ptr)
    call profiler_end_region("gs_local", 12)
    ! Scatter shared dofs
    if (pe_size .gt. 1 .and. n .gt. 0) then
       call profiler_start_region("gs_nbwait", 7)
       call gs%comm%nbwait(gs%shared_gs, l, op, gs%bcknd%gs_stream)
       call profiler_end_region("gs_nbwait", 7)
       call profiler_start_region("gs_scatter_shared", 15)
       call gs%bcknd%scatter(gs%shared_gs, l,&
            gs%shared_dof_gs, u, n, &
            gs%shared_gs_dof, gs%nshared_blks, &
            gs%shared_blk_len, gs%shared_blk_off, .true., scatter_event)
       call profiler_end_region("gs_scatter_shared", 15)
    end if
 
    call profiler_end_region("gather_scatter", 5)
    !$omp end parallel
  end subroutine gs_op_vector
 
  subroutine gs_op_r3(gs, u1, u2, u3, n, op, event)
    class(gs_t), intent(inout) :: gs
    integer, intent(in) :: n
    real(kind=rp), contiguous, dimension(:,:,:,:), intent(inout) :: u1, u2, u3
    type(c_ptr), optional, intent(inout) :: event
    integer :: op
 
    if (present(event)) then
       call gs_op_vector3(gs, u1, u2, u3, n, op, event)
    else
       call gs_op_vector3(gs, u1, u2, u3, n, op)
    end if
 
  end subroutine gs_op_r3
 
  subroutine gs_op_vector3(gs, u1, u2, u3, n, op, event)
    class(gs_t), intent(inout) :: gs
    integer, intent(in) :: n
    real(kind=rp), dimension(n), intent(inout) :: u1, u2, u3
    type(c_ptr), optional, intent(inout) :: event
    integer :: m, l, op, lo, so, tid
    integer, parameter :: nc = 3
    type(c_ptr) :: scatter_event
 
    ! Fall back to nc independent scalar exchanges when the comm backend has
    ! no fused vector path.
    if (.not. gs%comm%vec_supported) then
       if (present(event)) then
          call gs_op_vector(gs, u1, n, op, event)
          call gs_op_vector(gs, u2, n, op, event)
          call gs_op_vector(gs, u3, n, op, event)
       else
          call gs_op_vector(gs, u1, n, op)
          call gs_op_vector(gs, u2, n, op)
          call gs_op_vector(gs, u3, n, op)
       end if
       return
    end if
 
    lo = gs%local_facet_offset
    so = -gs%shared_facet_offset
    m = gs%nlocal
    l = gs%nshared
 
    tid = 0
    !$ tid = omp_get_thread_num()
 
    scatter_event = c_null_ptr
    if (present(event)) scatter_event = event
 
    if (neko_bcknd_device .eq. 0) then
 
       !$omp parallel
       call profiler_start_region("gather_scatter", 5)
 
       ! Gather each component's shared dofs directly into its column of
       ! shared_gs_v (the host backends write the actual argument), then
       ! launch ONE fused exchange covering all nc components.
       if (pe_size .gt. 1 .and. n .gt. 0) then
          call gs%comm%nbrecv_vec(tid, nc)
          call gs%bcknd%gather(gs%shared_gs_v(1), l, so, gs%shared_dof_gs, &
               u1, n, gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
               gs%shared_blk_off, op, .true.)
          call gs%bcknd%gather(gs%shared_gs_v(l + 1), l, so, &
               gs%shared_dof_gs, u2, n, gs%shared_gs_dof, gs%nshared_blks, &
               gs%shared_blk_len, gs%shared_blk_off, op, .true.)
          call gs%bcknd%gather(gs%shared_gs_v(2*l + 1), l, so, &
               gs%shared_dof_gs, u3, n, gs%shared_gs_dof, gs%nshared_blks, &
               gs%shared_blk_len, gs%shared_blk_off, op, .true.)
          call gs%comm%nbsend_vec(gs%shared_gs_v, l, nc, tid, &
               gs%bcknd%gather_event, gs%bcknd%gs_stream)
       end if
 
       ! Local gather-scatter, one scalar pass per component (reuses local_gs;
       ! the internal barriers make the sequential reuse safe).
       call gs%bcknd%gather(gs%local_gs, m, lo, gs%local_dof_gs, u1, n, &
            gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, &
            gs%local_blk_off, op, .false.)
       call gs%bcknd%scatter(gs%local_gs, m, gs%local_dof_gs, u1, n, &
            gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, &
            gs%local_blk_off, .false., c_null_ptr)
       call gs%bcknd%gather(gs%local_gs, m, lo, gs%local_dof_gs, u2, n, &
            gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, &
            gs%local_blk_off, op, .false.)
       call gs%bcknd%scatter(gs%local_gs, m, gs%local_dof_gs, u2, n, &
            gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, &
            gs%local_blk_off, .false., c_null_ptr)
       call gs%bcknd%gather(gs%local_gs, m, lo, gs%local_dof_gs, u3, n, &
            gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, &
            gs%local_blk_off, op, .false.)
       call gs%bcknd%scatter(gs%local_gs, m, gs%local_dof_gs, u3, n, &
            gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, &
            gs%local_blk_off, .false., c_null_ptr)
 
       ! Wait for the fused exchange and scatter each component back.
       if (pe_size .gt. 1 .and. n .gt. 0) then
          call gs%comm%nbwait_vec(gs%shared_gs_v, l, nc, op, &
               gs%bcknd%gs_stream)
          call gs%bcknd%scatter(gs%shared_gs_v(1), l, gs%shared_dof_gs, u1, &
               n, gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
               gs%shared_blk_off, .true., scatter_event)
          call gs%bcknd%scatter(gs%shared_gs_v(l + 1), l, gs%shared_dof_gs, &
               u2, n, gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
               gs%shared_blk_off, .true., scatter_event)
          call gs%bcknd%scatter(gs%shared_gs_v(2*l + 1), l, &
               gs%shared_dof_gs, u3, n, gs%shared_gs_dof, gs%nshared_blks, &
               gs%shared_blk_len, gs%shared_blk_off, .true., scatter_event)
       end if
 
       call profiler_end_region("gather_scatter", 5)
       !$omp end parallel
 
    else
 
       call gs_op_r3_device(gs, u1, u2, u3, n, op, nc, lo, so, m, l, tid, &
            scatter_event)
 
    end if
 
  end subroutine gs_op_vector3
 
  subroutine gs_op_r3_device(gs, u1, u2, u3, n, op, nc, lo, so, m, l, tid, &
       scatter_event)
    class(gs_t), intent(inout) :: gs
    integer, intent(in) :: n, op, nc, lo, so, m, l, tid
    real(kind=rp), dimension(n), intent(inout) :: u1, u2, u3
    type(c_ptr), intent(inout) :: scatter_event
    type(c_ptr) :: sgs_d, col_d, col_event
    integer(c_intptr_t) :: sv_addr, off_bytes
    integer(c_size_t) :: colbytes
    real(c_rp) :: rp_dummy
    logical :: on_host
 
    on_host = .true.
    sgs_d = c_null_ptr
    select type (b => gs%bcknd)
    type is (gs_device_t)
       on_host = b%shared_on_host
       sgs_d = b%shared_gs_d
    end select
 
    sv_addr = transfer(gs%shared_gs_v_d, sv_addr)
    colbytes = c_sizeof(rp_dummy) * int(l, c_size_t)
    off_bytes = int(l, c_intptr_t) * int(c_sizeof(rp_dummy), c_intptr_t)
 
    ! With a host-mirrored shared buffer, each scatter below issues an
    ! asynchronous host-to-device copy of shared_gs; a null event makes the
    ! scatter sync so the next column may safely overwrite the host buffer.
    ! Device-resident staging is stream-ordered and carries the caller's
    ! event.
    if (on_host) then
       col_event = c_null_ptr
    else
       col_event = scatter_event
    end if
 
    if (pe_size .gt. 1 .and. n .gt. 0) then
       call gs%comm%nbrecv_vec(tid, nc)
 
       ! Gather each component into the backend's shared buffer, then stage
       ! it into its column of shared_gs_v.
       call gs%bcknd%gather(gs%shared_gs, l, so, gs%shared_dof_gs, u1, n, &
            gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
            gs%shared_blk_off, op, .true.)
       if (on_host) then
          ! The gather mirrored the shared buffer to the host (synchronous).
          gs%shared_gs_v(1:l) = gs%shared_gs(1:l)
       else
          ! shared_gs_d is created lazily on the first gather.
          if (.not. c_associated(sgs_d)) then
             select type (b => gs%bcknd)
             type is (gs_device_t)
                sgs_d = b%shared_gs_d
             end select
          end if
          col_d = transfer(sv_addr, col_d)
          call device_memcpy(col_d, sgs_d, colbytes, device_to_device, &
               sync = .false., strm = gs%bcknd%gs_stream)
       end if
 
       call gs%bcknd%gather(gs%shared_gs, l, so, gs%shared_dof_gs, u2, n, &
            gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
            gs%shared_blk_off, op, .true.)
       if (on_host) then
          gs%shared_gs_v(l + 1:2*l) = gs%shared_gs(1:l)
       else
          col_d = transfer(sv_addr + off_bytes, col_d)
          call device_memcpy(col_d, sgs_d, colbytes, device_to_device, &
               sync = .false., strm = gs%bcknd%gs_stream)
       end if
 
       call gs%bcknd%gather(gs%shared_gs, l, so, gs%shared_dof_gs, u3, n, &
            gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
            gs%shared_blk_off, op, .true.)
       if (on_host) then
          gs%shared_gs_v(2*l + 1:3*l) = gs%shared_gs(1:l)
       else
          col_d = transfer(sv_addr + 2_c_intptr_t*off_bytes, col_d)
          call device_memcpy(col_d, sgs_d, colbytes, device_to_device, &
               sync = .false., strm = gs%bcknd%gs_stream)
          ! Re-record the gather event so it covers the column copies above.
          ! Comm backends that order their per-peer packing streams on this
          ! event (NCCL, NVSHMEM) would otherwise race with the copies; the
          ! device MPI backend packs on gs_stream itself and is unaffected.
          call device_event_record(gs%bcknd%gather_event, gs%bcknd%gs_stream)
       end if
 
       call gs%comm%nbsend_vec(gs%shared_gs_v, l, nc, tid, &
            gs%bcknd%gather_event, gs%bcknd%gs_stream)
    end if
 
    ! Local gather-scatter per component.
    call gs%bcknd%gather(gs%local_gs, m, lo, gs%local_dof_gs, u1, n, &
         gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, gs%local_blk_off, &
         op, .false.)
    call gs%bcknd%scatter(gs%local_gs, m, gs%local_dof_gs, u1, n, &
         gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, gs%local_blk_off, &
         .false., c_null_ptr)
    call gs%bcknd%gather(gs%local_gs, m, lo, gs%local_dof_gs, u2, n, &
         gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, gs%local_blk_off, &
         op, .false.)
    call gs%bcknd%scatter(gs%local_gs, m, gs%local_dof_gs, u2, n, &
         gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, gs%local_blk_off, &
         .false., c_null_ptr)
    call gs%bcknd%gather(gs%local_gs, m, lo, gs%local_dof_gs, u3, n, &
         gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, gs%local_blk_off, &
         op, .false.)
    call gs%bcknd%scatter(gs%local_gs, m, gs%local_dof_gs, u3, n, &
         gs%local_gs_dof, gs%nlocal_blks, gs%local_blk_len, gs%local_blk_off, &
         .false., c_null_ptr)
 
    ! Wait for the fused exchange (reduces into shared_gs_v), then stage each
    ! column back into the shared buffer and scatter.
    if (pe_size .gt. 1 .and. n .gt. 0) then
       call gs%comm%nbwait_vec(gs%shared_gs_v, l, nc, op, gs%bcknd%gs_stream)
 
       if (on_host) then
          gs%shared_gs(1:l) = gs%shared_gs_v(1:l)
       else
          col_d = transfer(sv_addr, col_d)
          call device_memcpy(sgs_d, col_d, colbytes, device_to_device, &
               sync = .false., strm = gs%bcknd%gs_stream)
       end if
       call gs%bcknd%scatter(gs%shared_gs, l, gs%shared_dof_gs, u1, n, &
            gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
            gs%shared_blk_off, .true., col_event)
 
       if (on_host) then
          gs%shared_gs(1:l) = gs%shared_gs_v(l + 1:2*l)
       else
          col_d = transfer(sv_addr + off_bytes, col_d)
          call device_memcpy(sgs_d, col_d, colbytes, device_to_device, &
               sync = .false., strm = gs%bcknd%gs_stream)
       end if
       call gs%bcknd%scatter(gs%shared_gs, l, gs%shared_dof_gs, u2, n, &
            gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
            gs%shared_blk_off, .true., col_event)
 
       if (on_host) then
          gs%shared_gs(1:l) = gs%shared_gs_v(2*l + 1:3*l)
       else
          col_d = transfer(sv_addr + 2_c_intptr_t*off_bytes, col_d)
          call device_memcpy(sgs_d, col_d, colbytes, device_to_device, &
               sync = .false., strm = gs%bcknd%gs_stream)
       end if
       call gs%bcknd%scatter(gs%shared_gs, l, gs%shared_dof_gs, u3, n, &
            gs%shared_gs_dof, gs%nshared_blks, gs%shared_blk_len, &
            gs%shared_blk_off, .true., col_event)
    end if
 
  end subroutine gs_op_r3_device
 
end module gather_scatter