0003-Declare-constants-at-the-native-precision-of-MPAS.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Kuan-Chih Wang <kuanchihw@ucar.edu>
Date: Wed, 13 Nov 2024 17:19:10 -0700
Subject: [PATCH] Declare constants at the native precision of MPAS

When building MPAS as a dycore, certain constants in the `mpas_constants` module are
imported from the `physconst` module, which is a part of CAM/CAM-SIMA. However,
multiple issues arise if the precision of those constants differs from MPAS.

For example, building MPAS in single precision mode with CAM-SIMA fails due to
multiple occurrences of type mismatch between actual and dummy arguments.

mpas_geometry_utils.F:885:157:

  885 |          call mpas_log_write('$r', MPAS_LOG_ERR, realArgs=(/mpas_triangle_signed_area_sphere(a,b,c,sphereRadius) - pii/2.0_RKIND*sphereRadius*sphereRadius/))
      |                                                                                                                                                             1
Error: Type mismatch in argument 'realargs' at (1); passed REAL(8) to REAL(4)

Here, `pii` is declared by CAM-SIMA to be double precision, and it causes unintended
floating-point promotion in the expression.

The solution is to ensure that constants in the `mpas_constants` module are declared
at the native precision of MPAS.

This downstream patch is maintained by CAM-SIMA for its particular use case of MPAS.
---
 src/framework/mpas_constants.F | 44 ++++++++++++++++++++++++++--------
 1 file changed, 34 insertions(+), 10 deletions(-)

diff --git a/src/framework/mpas_constants.F b/src/framework/mpas_constants.F
index c98cb810..68164bab 100644
--- a/src/framework/mpas_constants.F
+++ b/src/framework/mpas_constants.F
@@ -23,16 +23,30 @@ module mpas_constants
    use mpas_kind_types
 
 #ifdef MPAS_CAM_DYCORE
-   use physconst, only : pii => pi
-   use physconst, only : gravity => gravit
-   use physconst, only : omega
-   use physconst, only : a => rearth
-   use physconst, only : cp => cpair
-   use physconst, only : rgas => rair
-   use physconst, only : rv => rh2o
-   real (kind=RKIND) :: rvord = huge(1.0_RKIND)                       ! Derived in mpas_constants_compute_derived
-   real (kind=RKIND) :: cv = huge(1.0_RKIND)                          ! Derived in mpas_constants_compute_derived
-   real (kind=RKIND) :: cvpm = huge(1.0_RKIND)                        ! Derived in mpas_constants_compute_derived
+   use physconst, only: external_pii => pi
+   use physconst, only: external_a => rearth
+   use physconst, only: external_omega => omega
+   use physconst, only: external_gravity => gravit
+   use physconst, only: external_rgas => rair
+   use physconst, only: external_rv => rh2o
+   use physconst, only: external_cp => cpair
+   private :: external_pii
+   private :: external_a
+   private :: external_omega
+   private :: external_gravity
+   private :: external_rgas
+   private :: external_rv
+   private :: external_cp
+   real (kind=RKIND), protected :: pii = huge(1.0_RKIND)              ! Derived in mpas_constants_compute_derived
+   real (kind=RKIND), protected :: a = huge(1.0_RKIND)                ! Derived in mpas_constants_compute_derived
+   real (kind=RKIND), protected :: omega = huge(1.0_RKIND)            ! Derived in mpas_constants_compute_derived
+   real (kind=RKIND), protected :: gravity = huge(1.0_RKIND)          ! Derived in mpas_constants_compute_derived
+   real (kind=RKIND), protected :: rgas = huge(1.0_RKIND)             ! Derived in mpas_constants_compute_derived
+   real (kind=RKIND), protected :: rv = huge(1.0_RKIND)               ! Derived in mpas_constants_compute_derived
+   real (kind=RKIND), protected :: cp = huge(1.0_RKIND)               ! Derived in mpas_constants_compute_derived
+   real (kind=RKIND), protected :: rvord = huge(1.0_RKIND)            ! Derived in mpas_constants_compute_derived
+   real (kind=RKIND), protected :: cv = huge(1.0_RKIND)               ! Derived in mpas_constants_compute_derived
+   real (kind=RKIND), protected :: cvpm = huge(1.0_RKIND)             ! Derived in mpas_constants_compute_derived
 #else
    real (kind=RKIND), parameter :: pii     = 3.141592653589793_RKIND  !< Constant: Pi
    real (kind=RKIND), parameter :: a       = 6371229.0_RKIND          !< Constant: Spherical Earth radius [m]
@@ -77,6 +91,16 @@ module mpas_constants
        implicit none
 
 #ifdef MPAS_CAM_DYCORE
+       ! Convert external constants to the native precision of MPAS (i.e., `RKIND`).
+
+       pii = real(external_pii, RKIND)
+       a = real(external_a, RKIND)
+       omega = real(external_omega, RKIND)
+       gravity = real(external_gravity, RKIND)
+       rgas = real(external_rgas, RKIND)
+       rv = real(external_rv, RKIND)
+       cp = real(external_cp, RKIND)
+
        !
        ! In the case of CAM-MPAS, rgas may depend on a CAM namelist option,
        ! so physical constants that depend on rgas must be computed here after
-- 
2.43.0