Offload functions with optional attribute and other offloaded function calls

[sergisiso]

In NEMO we need to bring to the GPU the functions like:

  elemental function q_sat_sclr(pta, ppa, l_ice)
    real(kind=wp), intent(in) :: pta
    real(kind=wp), intent(in) :: ppa
    LOGICAL, INTENT(IN), OPTIONAL :: l_ice
    real(kind=wp) :: q_sat_sclr
    real(kind=wp) :: ze_s
    logical :: lice

    lice = .false.
    if (PRESENT(l_ice)) then
      lice = l_ice
    end if
    if (lice) then
      ze_s = e_sat_ice(pta)
    else
      ze_s = e_sat(pta)
    end if
    q_sat_sclr = reps0 * ze_s / (ppa - (1._wp - reps0) * ze_s)

  end function q_sat_sclr

(from sbc_phy.f90)

We typically do this with "declare target" or "inline" but we can not do that for this case because of the OPTIONAL attribute.
For inlining there is #2801. But for declare target we could also create a copy of the function with the attribute being mandatory.

Another problem is that it is not transitive, so:

  elemental function dq_sat_dt_ice_sclr(pta, ppa)
    real(kind=wp), intent(in) :: pta
    real(kind=wp), intent(in) :: ppa
    real(kind=wp) :: dq_sat_dt_ice_sclr
    real(kind=wp) :: ze_s
    real(kind=wp) :: zde_s_dt
    real(kind=wp) :: ztmp
    ze_s = e_sat_ice_sclr(pta)
    zde_s_dt = de_sat_dt_ice(pta)
    ztmp = (reps0 - 1._wp) * ze_s + ppa
    dq_sat_dt_ice_sclr = reps0 * ppa * zde_s_dt / (ztmp * ztmp)

  end function dq_sat_dt_ice_sclr

Is not on the GPU because it calls 'e_sat_ice_sclr' and 'de_sat_dt_ice', but this could be "declare target" in turn.

[arporter]

Martin has a PR ready for review that adds support for optional arguments.