# PR-3.5 — Implement Mark Phase (Reachability Traversal)

### Briefing

This PR introduces the mark phase of a simple mark-sweep collector. It must traverse from roots and mark reachable objects.

### Target

* Implement reachability marking from root set.
* Set mark bits on visited objects.

### Work items

* Implement a `mark()` function in the heap.
* Traverse roots using the root iterator.
* For each `HeapRef`, mark the object.
* Recursively traverse references inside objects.
* Ensure no infinite loops on cycles.

### Acceptance checklist

* [ ] Mark phase visits all reachable objects.
* [ ] Cycles are handled safely.
* [ ] Unreachable objects remain unmarked.
* [ ] `cargo test` passes.

### Tests

* Add tests:

  * Reachable object stays marked.
  * Unreachable object remains unmarked.
  * Cyclic references do not crash.

### Junie instructions

**You MAY:**

* Implement a simple DFS or stack-based marking.

**You MUST NOT:**

* Add generational, incremental, or parallel GC.
* Change object layout.

**If unclear:**

* Ask before deciding traversal structure.

---

# PR-3.6 — Implement Sweep Phase (Reclaim Unmarked Objects)

### Briefing

After marking, the GC must reclaim unreachable objects. This PR implements the sweep phase.

### Target

* Remove or reclaim unmarked objects.
* Reset mark bits for the next cycle.

### Work items

* Iterate over heap storage.
* For each object:

  * If unmarked, reclaim it.
  * If marked, clear the mark bit.
* Ensure handles to reclaimed objects become invalid or reused safely.

### Acceptance checklist

* [ ] Unreachable objects are reclaimed.
* [ ] Reachable objects remain intact.
* [ ] Mark bits are cleared after sweep.
* [ ] `cargo test` passes.

### Tests

* Add tests:

  * Allocate objects, drop references, run sweep, confirm removal.
  * Confirm live objects survive.

### Junie instructions

**You MAY:**

* Implement a simple sweep over the heap vector.

**You MUST NOT:**

* Implement compaction or handle relocation.
* Introduce advanced memory strategies.

**If unclear:**

* Ask before choosing handle invalidation strategy.

---

# PR-3.7 — Integrate GC Cycle at Safepoint (`FRAME_SYNC`)

### Briefing

The GC must only run at safepoints. This PR connects the mark-sweep collector to the VM’s safepoint logic, primarily at `FRAME_SYNC`.

### Target

* Trigger GC cycles only at safepoints.
* Keep execution deterministic.

### Work items

* Identify safepoint handling code in the VM.
* Add logic:

  * If allocation threshold exceeded, run GC at `FRAME_SYNC`.
* Ensure GC does not run in arbitrary instruction contexts.

### Acceptance checklist

* [ ] GC runs only at safepoints.
* [ ] No GC during arbitrary instruction execution.
* [ ] VM remains deterministic.
* [ ] `cargo test` passes.

### Tests

* Add a test where allocations trigger GC only at `FRAME_SYNC`.

### Junie instructions

**You MAY:**

* Hook GC invocation into safepoint handling.

**You MUST NOT:**

* Trigger GC at random points.
* Add background or concurrent GC.

**If unclear:**

* Ask before modifying safepoint semantics.

---

# PR-3.8 — GC Smoke and Stress Tests

### Briefing

We need confidence that the GC behaves correctly under simple and stressed conditions.

### Target

* Add deterministic smoke and stress tests for the GC.

### Work items

* Add tests:

  * Simple allocation and collection cycle.
  * Many short-lived objects.
  * Cyclic references.
* Ensure tests are deterministic.

### Acceptance checklist

* [ ] Smoke tests pass.
* [ ] Stress tests pass.
* [ ] No nondeterministic failures.
* [ ] `cargo test` passes.

### Tests

* New GC-specific tests.

### Junie instructions

**You MAY:**

* Add deterministic tests.

**You MUST NOT:**

* Introduce random or timing-dependent tests.
* Modify GC semantics to satisfy tests.

**If unclear:**

* Ask before changing test scenarios.