Consolidate dqL/R_prim_dx/dy/dz_n triplets into a single derived type argument across m_rhs and m_riemann_solvers

[sbryngelson]

Background

PR #1432 consolidated several module-level directional triplets into derived types. Two triplets remain that are among the highest-leverage targets left: the primitive-variable gradient arrays that appear as three separate arguments in every viscous and diffusion-related subroutine call.

Current state

m_rhs.fpp declares six module-level allocatable triplets:

type(vector_field), allocatable, dimension(:) :: dqL_prim_dx_n, dqL_prim_dy_n, dqL_prim_dz_n
type(vector_field), allocatable, dimension(:) :: dqR_prim_dx_n, dqR_prim_dy_n, dqR_prim_dz_n

Each is a type(vector_field) array indexed by the Riemann direction index n. All three appear together in every allocation, deallocation, GPU declaration, and subroutine call — they are never used independently.

They propagate as three separate arguments through approximately 7 subroutines spanning m_rhs.fpp and m_riemann_solvers.fpp, including the cross-module interface at the Riemann solver call site.

Proposed change

Introduce a module-private wrapper type (or add to m_derived_types.fpp if reusable):

type, private :: vf_dir_t
    type(vector_field), allocatable, dimension(:) :: x, y, z
end type vf_dir_t

type(vf_dir_t) :: dqL_prim_dn, dqR_prim_dn

Update GPU_DECLARE, allocation, deallocation, and all call sites to use dqL_prim_dn%x/y/z.

The cross-module call in m_rhs.fpp into m_riemann_solvers.fpp means both modules must be updated together — this is a single atomic change.

What this enables

Signature reduction. Every subroutine currently taking dqL_dx, dqL_dy, dqL_dz, dqR_dx, dqR_dy, dqR_dz (6 arguments) takes dqL, dqR (2 arguments) after consolidation. With 7 subroutines affected, this removes ~42 argument slots from interfaces.

Single allocation/deallocation site. The current allocation block is:

@:ALLOCATE(dqL_prim_dx_n(1:num_dims))
@:ALLOCATE(dqL_prim_dy_n(1:num_dims))
@:ALLOCATE(dqL_prim_dz_n(1:num_dims))
@:ALLOCATE(dqR_prim_dx_n(1:num_dims))
@:ALLOCATE(dqR_prim_dy_n(1:num_dims))
@:ALLOCATE(dqR_prim_dz_n(1:num_dims))

This becomes 2 lines. Same for deallocation and GPU_DECLARE.

Foundation for directional sweep unification. Once the gradient triplets are in a struct, the directional dispatch loops in m_rhs.fpp that select dqL_prim_dx_n vs dqL_prim_dy_n vs dqL_prim_dz_n based on norm_dir can be collapsed to a single block using dqL_prim_dn%x/y/z as Fypp loop variables (see #1440).

Scope

Files affected: m_rhs.fpp, m_riemann_solvers.fpp (interface boundary).

The change is mechanical but touches a cross-module interface, so a GPU build and the full viscous/diffusion test suite should be run before merging.

Related

• Builds on PR refactor: consolidate grid, stretch, and body force params into derived types #1432 (struct consolidation groundwork)
• Pairs with Replace Fypp directional #:for blocks with runtime direction dispatch via struct components #1440 (directional sweep loop unification)
• Coordinate with Refactor m_riemann_solvers: reduce duplication, improve structure, add schemes #1426 (Riemann solver refactor — shares the m_riemann_solvers interface)