Split Layer 2 into language-agnostic compile() + C++ render step (unlocks non-C++ backends) #18

Summary

ConstitutiveModel::emit_compute_function() (include/numsim_codegen/recipe.h:209-241) returns rendered C++ source. The CSE state, the typed expression DAG, and the per-output RHS exist in structured form internally, but the entry point hard-commits to C++ rendering.

This locks out every non-C++ backend:

• FEniCS/UFL wants the typed DAG (UFL operator forms, not auto t5 = lam * t3;).
• JAX/Python wants Python with jnp.einsum.
• MLIR wants SSA values.
• LaTeX/SymPy export (useful for verification) wants the AST.

None of these can consume the rendered C++ string. Without this split, the multi-backend ambition stated in Target::target_name() cannot extend beyond C++ frameworks (MOOSE, Abaqus, Ansys, LS-DYNA per existing issues #2/#3/#4).

Where

• The string-rendering step is buried inside recipe.h:render_compute_function() (recipe.h:287-336).
• The structured form already exists: CodeGenContext::statements() returns std::vector<Statement>; per-output RHS strings are separated before being concatenated.

Proposed design

Split Layer 2 into two stages:

struct CompiledModel {                       // language-agnostic
    struct ArgSpec {
        std::string name;
        SymbolDecl::Kind kind;
        std::size_t dim;
        std::size_t rank;
        Role role;
        bool is_output;
    };
    struct OutputBinding {
        std::string name;
        // Reference to a leaf node in the DAG OR the final rendered RHS;
        // see "decision points" below.
    };

    std::vector<Statement> body;             // CSE-ordered
    std::vector<OutputBinding> outputs;
    std::vector<ArgSpec> arguments;
    std::string function_name;
};

class ConstitutiveModel {
public:
    [[nodiscard]] CompiledModel compile() const;          // CSE + validate, no render
    [[nodiscard]] std::string emit_compute_function() const {
        return cxx_render(compile());                     // existing behavior preserved
    }
    // ...
};

// C++ rendering moves into a free function (or a small renderer class):
std::string cxx_render(CompiledModel const &);

Non-C++ backends consume compile() directly and supply their own renderer (ufl_render, python_render, mlir_render, ...).

Decision points

1. Statement::rhs is still a C++ string today. For full non-C++ support, the DAG node should travel alongside the rhs so backends can re-walk it. Either keep Statement carrying the typed node and let backends re-render, or commit to one IR.
2. ArgSpec ordering. This issue also fixes MOOSE call-site argument order diverges from compute function signature #6 (call-site arg order mismatch) — argument_order becomes the single source of truth.
3. Where does CSE live? Probably still in compile() since CSE is language-agnostic, but a backend that wants a different CSE strategy (e.g. fewer temporaries for inlined GPU code) might want a hook.

Scope

This is the biggest single refactor in the backlog. Touches recipe.h, all three emitters, both shipped targets, and probably the tests. Multi-day work. Don't attempt before:

• Operation registry (the new flexibility issue, see issue list) — registry-dispatched emitters are easier to make work over an IR than the current virtual-dispatch visitors.
• MOOSE call-site argument order diverges from compute function signature #6 (arg-order fix) — gets simpler when ArgSpec is the source of truth.

Defer until backend #3 (Abaqus / Ansys / LS-DYNA, #2/#3/#4) lands or a non-C++ backend is actively planned. Until then, file this and don't ship more C++-only assumptions.

Refs

• Promoted from the Code review follow-up: Med & Low findings (umbrella) #14 umbrella (was ARCH-C1)
• Related: MOOSE call-site argument order diverges from compute function signature #6 (arg-order), the operation-registry issue
• Severity: High for the multi-backend ambition; Med if the only planned backends are C++ FEM frameworks