Dead Store Elimination (DSE)

Dead Store Elimination removes stores to signals that are never read, producing faster JIT code. It is especially effective for large designs where most internal signals are unobserved during testing.

Policies

Policy	Behavior
"off" (default)	No elimination — all stores preserved
"preserveTopPorts"	Eliminate except stores to top-module ports
"preserveAllPorts"	Eliminate except stores to ports of all instances

Usage

With optLevel: "O2"

The simplest way to enable DSE is to use optimization level O2, which defaults to preserveTopPorts:

const sim = Simulator.fromSource(SOURCE, "Top", {
  optLevel: "O2",
});

With explicit deadStorePolicy

You can also set the policy directly (overrides the optLevel default):

const sim = Simulator.fromSource(SOURCE, "Top", {
  deadStorePolicy: "preserveTopPorts",
});

With Vite Plugin

Use the ?dse= query parameter on the import path:

import { Top } from "../src/Top.veryl?dse=preserveAllPorts";

const sim = Simulator.create(Top);

The policy is baked into the ModuleDefinition as defaultOptions.deadStorePolicy. You can override it per-call:

// Module has ?dse=preserveAllPorts, but this call uses preserveTopPorts
const sim = Simulator.create(Top, {
  deadStorePolicy: "preserveTopPorts",
});

?dse without a value defaults to "preserveAllPorts" (the safe choice).

Choosing a Policy

preserveTopPorts

Best for performance-focused benchmarks where you only need the top-module I/O. Sub-instance signals are eliminated, so sim.dut.u_sub will be undefined:

const sim = Simulator.fromSource(SOURCE, "Top", {
  deadStorePolicy: "preserveTopPorts",
});

sim.dut.top_in = 0xABn;
sim.tick();
expect(sim.dut.top_out).toBe(0xABn);

// Child instance is NOT available
expect((sim.dut as any).u_sub).toBeUndefined();

preserveAllPorts

Best for general testing — gives you DSE performance benefits while keeping all instance ports accessible for debugging:

const sim = Simulator.fromSource(SOURCE, "Top", {
  deadStorePolicy: "preserveAllPorts",
});

sim.dut.top_in = 0x42n;
sim.tick();
expect(sim.dut.top_out).toBe(0x42n);

// Child instance ports remain accessible
expect(sim.dut.u_sub.o_data).toBe(0x42n);

Internal variables

With both DSE policies, internal variables (var declarations that are not ports) may be eliminated if no execution unit reads them. Only port signals are guaranteed to survive.

How It Works

1. The Rust optimizer analyzes which stores are loaded by execution units.
2. Stores that are never loaded are candidates for elimination.
3. The policy adds externally live signals to the protected set:
   • PreserveTopPorts — top-module port addresses only.
   • PreserveAllPorts — port addresses from every instance in the design.
4. On the TypeScript side, filterHierarchyForDse() adjusts the DUT hierarchy so that eliminated sub-instances are not exposed as broken accessors.

Further Reading

• Child Instance Access — How the DUT hierarchy works.
• Vite Plugin — Plugin configuration and query parameters.
• Writing Tests — Simulator and Simulation patterns.