Ice-shelf FMAs (#729)

* Fixes to preserve rotational symmetry when using Fused-multiply-adds

Tested in ice-ocean mode and ice-shelf-only mode using ISOMIP and
MISOMIP+, respectively

* Added more parentheses to preserve rotational symmetry when using FMAs.

Note that the changes here are to the MOM_ice_shelf_dynamics.F90 function 'quad_area', which is only called by the subroutine 'bilinear_shape_functions'; this subroutine is currently unused so that rotationally-consistent use of FMAs in both 'quad_area' and 'bilinear_shape_functions' has not been tested

Author: alex-huth

src/ice_shelf/MOM_ice_shelf.F90

@@ -450,11 +450,11 @@ subroutine shelf_calc_flux(sfc_state_in, fluxes_in, Time, time_step_in, CS)
     I_au = 0.0 ; if (asu1 + asu2 > 0.0) I_au = 1.0 / (asu1 + asu2)
     I_av = 0.0 ; if (asv1 + asv2 > 0.0) I_av = 1.0 / (asv1 + asv2)
     if (allocated(sfc_state%taux_shelf) .and. allocated(sfc_state%tauy_shelf)) then
-      taux2 = (asu1 * sfc_state%taux_shelf(I-1,j)**2 + asu2 * sfc_state%taux_shelf(I,j)**2  ) * I_au
-      tauy2 = (asv1 * sfc_state%tauy_shelf(i,J-1)**2 + asv2 * sfc_state%tauy_shelf(i,J)**2  ) * I_av
+      taux2 = (((asu1 * (sfc_state%taux_shelf(I-1,j)**2)) + (asu2 * (sfc_state%taux_shelf(I,j)**2))  ) * I_au)
+      tauy2 = (((asv1 * (sfc_state%tauy_shelf(i,J-1)**2)) + (asv2 * (sfc_state%tauy_shelf(i,J)**2))  ) * I_av)
     endif
-    u2_av = (asu1 * sfc_state%u(I-1,j)**2 + asu2 * sfc_state%u(I,j)**2) * I_au
-    v2_av = (asv1 * sfc_state%v(i,J-1)**2 + asu2 * sfc_state%v(i,J)**2) * I_av
+    u2_av = (((asu1 * (sfc_state%u(I-1,j)**2)) + (asu2 * sfc_state%u(I,j)**2)) * I_au)
+    v2_av = (((asv1 * (sfc_state%v(i,J-1)**2)) + (asu2 * sfc_state%v(i,J)**2)) * I_av)
 
     if ((taux2 + tauy2 > 0.0) .and. .not.CS%ustar_shelf_from_vel) then
       if (CS%ustar_max >= 0.0) then