defparam_examples

defparam_examples — hierarchical parameter override (Enhancement-58)

Demonstrates defparam — the legacy Verilog-2001 compile-time parameter override (LRM 2.6) — using the committed openvaf-r and ngspice-46.

What was broken

defparam did not parse at all: it was a reserved word with no grammar rule, so defparam u1.r = 2e3; produced the misleading error 'defparam' was not found in the current scope. (The idiomatic instance overrides #(.r(2e3)) and #(2e3) already worked; defparam is the deprecated hierarchical-override form.)

What now works

• DEFPARAM_KW is a keyword token with a parser rule producing a DEFPARAM syntax node — deliberately not a typed ModuleItem, so the later compiler stages (hir_def onward) never see it; it is consumed entirely by the E-5 elaboration pass.
• Each defparam target is resolved through the same instance-chain rewrite E-49 uses for hierarchical references: u1.r → u1__r, u1.u2.r → u1__u2__r — exactly the flattened name the target parameter is given when its instance is inlined — and the flattened parameter's default is rewritten.
• defparam has higher precedence than an instance #(...) override (LRM 2.6): dp_leaf #(.r(5e3)) up(...) with defparam up.r = 2e3 yields 2k, not 5k.
• Multi-assignment (defparam up.r = 2e3, up.g = 1e-3;) and override expressions that reference the enclosing module's parameters (defparam u1.r = 2.0*scale;) both work.
• An unresolved target is a hard error naming the original source path (defparam target(s) did not resolve to any parameter: u1.typo).

Run

python3 verify_defparam.py

Checks (4, ALL PASS, exact op-point conductances): a basic instance override (1k → 2k → 0.5 mA); a two-level target plus a multi-assignment defparam overriding an instance #() (1.75 mA total); an expression-valued override referencing a parent parameter (6k → 1/6 mA); and the unresolved-target compile error.

Note

defparam targeting a same-module parameter in a module that has no instantiation at all is not rewritten (the elaboration pass only runs when the file contains instantiations) — use the parameter's declared default there. The hierarchical instance-override use case, which always involves instantiation, is fully supported.