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Child Instance Access ​

When a Veryl module instantiates submodules, their ports are accessible through the DUT accessor using the instance name.

Basic Usage ​

Given a design with an instantiated submodule:

veryl
module Sub (
    clk:    input  clock,
    i_data: input  logic<8>,
    o_data: output logic<8>,
) {
    always_comb {
        o_data = i_data;
    }
}

module Top (
    clk:     input  clock,
    rst:     input  reset,
    top_in:  input  logic<8>,
    top_out: output logic<8>,
) {
    inst u_sub: Sub (
        clk,
        i_data: top_in,
        o_data: top_out,
    );
}

You can observe u_sub's ports through sim.dut.u_sub:

typescript
const sim = Simulator.fromSource(SOURCE, "Top");

sim.dut.top_in = 0xABn;
sim.tick();

// Top-level output
expect(sim.dut.top_out).toBe(0xABn);

// Same value visible through child instance accessor
expect((sim.dut as any).u_sub.o_data).toBe(0xABn);
expect((sim.dut as any).u_sub.i_data).toBe(0xABn);

sim.dispose();

Child instance accessors are useful for observing internal signals during debugging, without needing to add extra top-level output ports.

Type-Safe Access with the Vite Plugin ​

When using the Vite plugin, the generated type definition includes child instances as nested objects:

typescript
export interface TopPorts {
  top_in: bigint;
  readonly top_out: bigint;
  readonly u_sub: {
    i_data: bigint;
    readonly o_data: bigint;
  };
}

So you can write:

typescript
import { Top } from "../src/Top.veryl";
const sim = Simulator.create(Top);

expect(sim.dut.u_sub.o_data).toBe(0xABn); // fully typed, no cast needed

Type-Safe Access with fromSource ​

When using fromSource, define the interface yourself:

typescript
interface TopPorts {
  top_in: bigint;
  readonly top_out: bigint;
  readonly u_sub: {
    i_data: bigint;
    readonly o_data: bigint;
  };
}

const sim = Simulator.fromSource<TopPorts>(SOURCE, "Top");
expect(sim.dut.u_sub.o_data).toBe(0xABn);

Nested Hierarchies ​

Child accessors are recursive — deeply nested instances work the same way:

typescript
sim.dut.u_mid.u_inner.some_port

DSE and Child Instances ​

When Dead Store Elimination is enabled, child instance accessors may not be available:

  • deadStorePolicy: "preserveTopPorts" — child instances are stripped from the DUT; sim.dut.u_sub will be undefined.
  • deadStorePolicy: "preserveAllPorts" — all instance ports are preserved; child accessors work normally.

If you need to observe sub-instance signals with DSE enabled, use "preserveAllPorts".

Further Reading ​