scratch_3d_buffer_enter_data_impl Subroutine

   subroutine scratch_3d_buffer_enter_data_impl(this)
      !! Attach the scratch payload to the device holding the HOST
      !! payload: zero from `init` (`allocate(..., source = 0.0_wp)`), or
      !! whatever a warm-restart read restored into it before the map.
      !! The zero half is a contract, not an implementation detail: a
      !! consumer whose producer was SKIPPED this step is entitled to read
      !! zero on a cold start, and it must read zero on BOTH toolchains
      !! (see the inline note below for the `pred_corr` predictor that
      !! does exactly that).  `type(...)` (not `class`) dummy on purpose:
      !! a by-reference non-polymorphic dummy aliases the heap object, so
      !! `copyin(this%data)` attaches against a heap base — no polymorphic
      !! stack box for AMD to reject.
      !!
      !! No-op when the payload was never allocated.  Slots may GATE a
      !! buffer's `init` on a runtime knob (see
      !! `ocean_pressure_force_t%scratch_gated`) and still call this
      !! unconditionally from their `enter_data` walk; mapping an
      !! unallocated allocatable is not defined behaviour, so the guard
      !! lives here rather than at each call site.
      type(scratch_3d_buffer_t), intent(inout) :: this
      if (.not. allocated(this%data)) return
      ! `copyin`, not `create`: `create` attaches UNINITIALISED device
      ! memory, so the zero `init` put in the host allocation never
      ! crosses.  Most consumers write every element before reading, but
      ! a producer is allowed to be SKIPPED for a step and leave its
      ! buffer to be read as "what the previous step produced" — MOM6's
      ! predictor does exactly that with `diffu` (`split_scheme =
      ! "pred_corr"` skips the viscous recompute in stage 1), and on step
      ! 1 there is no previous producer.  On `-gpu=...,mem:separate` a
      ! bare `create` yielded whatever the allocator handed back, which is
      ! why a uniform, perfectly balanced periodic jet once acquired an
      ! O(0.25 m/s²) viscous tendency out of nothing.  The fix for that
      ! was a device-side zero-fill after the `create`; it is `copyin` now
      ! because those buffers are also RESTART state (`hvisc_du_visc`/
      ! `hvisc_dv_visc` in `ocean_state_build_restart_registry`), and the
      ! restart read lands on the host BEFORE the map — a zero-fill wiped
      ! the restored tendency on both toolchains (host builds run the
      ! `do concurrent` on the host array itself), so the first
      ! post-restart predictor ran with no lateral viscosity in any column.
      ! One H2D of each buffer at setup; nothing per step.
      !$acc enter data copyin(this%data)
   end subroutine scratch_3d_buffer_enter_data_impl