writeonce/docs/examples/gc-cycle

gc-cycle — inferred GC + incremental mark-sweep, by example

The smallest program that needs a tracing collector (ring_demo) and the smallest that does not (owned_demo). This README defines how pointers/ addresses flow through the heap and the collector's mark-sweep logic, the way iteration 7b specs it (2026-08-11-inferred-gc-mark-sweep-design.md).

The one-line model: ownership frees everything it can; the collector traces only the residue ownership cannot free — cycles and long-lived aliases — and the compiler infers which types those are.

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1. Inference — the compiler decides owned vs gc

A pass between types and owner (compiler/src/gcinfer.ml) builds a class-reference graph — an edge A → B whenever A has a field of type B, ?B, multi B, map<B,_>, or map<_,B>. ref B creates no edge (it is a row id, Copy). Tarjan's SCC over that graph: any class in a non-trivial SCC, or with a self-loop, is traced (gc). Everything else is owned.

For this sample, woc --dump-gc would print:

Node       gc      (cycle Node -> Node)
Segment    owned

Node.next: ?Node is the self-loop → Node is traced, and its next field gets kind WO_K_GCREF. Segment has no class-typed field → no edge → owned, freed deterministically. A second, demand half promotes a type when the ownership pass would otherwise report a long-lived-alias escape (the PriceCache shape) — not exercised here; every promotion is reported, never silent.

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2. Address flow — where a pointer lives, and the object's shape

Every heap object is a 16-byte wo_hdr followed by field_cnt 8-byte slots (wo_fields(o); wo_obj_size = 16 + field_cnt*8). Allocation is the arena: bump + 16-byte size-class free lists (16…1024), larger falls to malloc. The arena keeps no size headers and cannot enumerate objects — so traced objects thread an intrusive sweep list.

The header is where iteration 7b pays for the collector with zero growth:

        today (RC)                        under 7b (tracing)
  ┌──────────────────────────┐      ┌──────────────────────────┐
  │ class_id      (4 bytes)  │      │ class_id      (4 bytes)  │
  │ flags         (1) + pad  │      │ flags (1)  ── 2 color bits (white/grey/black), + pad
  │ borrow        (4 bytes)  │      │ sweep-list link (8 bytes)│  ← reclaimed from
  │ rc            (4 bytes)  │      │  (next traced object)    │     borrow + rc
  └──────────────────────────┘      └──────────────────────────┘
        16 bytes                           16 bytes  (unchanged)

rc is retired (no reference counting); traced objects are exempt from borrow rules so borrow is dead too — the two adjacent 4-byte words become one 64-bit list link. Color lives in 2 existing flag bits (WO_F_COLOR).

A traced pointer (the address of a wo_hdr) is only ever held in one of three places, and these are exactly what the collector reads:

Holder	How the collector sees it
a VM value-stack / frame slot (a local like a)	a root, via the per-pc gc-mask the emitter already emits
a GCREF field slot of another object (a.next)	followed during mark
a container item (multi/map) whose element kind is GCREF	followed during mark

Here is the sample's heap after ring_demo builds the ring, while a/b/c are still live roots:

flowchart LR
  subgraph STACK["VM value stack (roots this pc, from the gc-mask)"]
    A["a"]:::root
    B["b"]:::root
    C["c"]:::root
  end
  subgraph HEAP["arena (one mmap region)"]
    NA["Node a<br/>hdr, label(TEXT), next(GCREF)"]
    NB["Node b<br/>hdr, label(TEXT), next(GCREF)"]
    NC["Node c<br/>hdr, label(TEXT), next(GCREF)"]
  end
  A --> NA
  B --> NB
  C --> NC
  NA -->|next| NB
  NB -->|next| NC
  NC -->|next, closes cycle| NA
  NA -.->|sweep link| NB
  NB -.->|sweep link| NC
  NC -.->|sweep link| NULL(("nil"))
  classDef root fill:#2b6,stroke:#083,color:#fff;

Solid arrows are GCREF pointers the mark phase follows; the dotted chain is the per-shard traced list the sweep phase walks (independent of reachability). When a/b/c leave scope, the three solid root arrows vanish — the ring still points to itself, but nothing points in, so it is unreachable yet un-freed. Ownership cannot help: b cannot be owned by both let b and a.next. That is the collector's entire job.

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3. Mark-sweep logic — tri-color, incremental, with a deletion barrier

Marking runs in budgeted slices (WO_GC_BUDGET objects per slice), so the pause is bounded regardless of heap size. Colors: white = unproven (candidate to free), grey = reachable but children not yet scanned, black = reachable and scanned.

flowchart TD
  ALLOC["allocate traced object<br/>color = WHITE, link into traced list"] --> LIVE
  LIVE["mutator runs<br/>(program executes)"] --> TRIG{"traced bytes since last cycle<br/>past heap goal?"}
  TRIG -- no --> LIVE
  TRIG -- yes --> ROOTS["START CYCLE<br/>shade every root GREY<br/>(value/frame slots via pc gc-mask)"]
  ROOTS --> SLICE
  SLICE["MARK SLICE (budgeted)<br/>pop a GREY object,<br/>scan its GCREF fields +<br/>owned subtrees that may reach a gcref,<br/>shade each WHITE child GREY,<br/>then paint this object BLACK"] --> GREY{"grey set empty?"}
  GREY -- "no (budget hit)" --> SAFE["yield at next safepoint<br/>(loop back-edge / call)"]
  SAFE --> LIVE2["mutator resumes<br/>(barrier active)"]
  LIVE2 --> SLICE
  GREY -- yes --> SWEEP["SWEEP: walk traced list —<br/>WHITE: free (wo_obj_size) and unlink;<br/>BLACK: repaint WHITE, keep"]
  SWEEP --> DONE["cycle done"] --> LIVE

Roots. The value stack and frame stack, read through the per-pc gc-mask the emitter already produces (the drop-table's gc bits, reinterpreted from "rc_dec these on unwind" to "these registers are GC roots at this pc"). No stack scanning, no native frames — a bytecode VM with an explicit value stack has the map for free.

Owned objects are traversed, never freed. An owned object can hold a GCREF field, so mark must walk through owned subtrees to reach traced objects — but it never frees an owned object (drop owns those). A precomputed class-table bit, "transitively contains a gcref", lets mark skip any owned subtree that can reach no traced object at all.

The barrier — why incremental is safe. Between slices the mutator keeps running and can hide a live object from a half-finished mark: store a white object into an already-black object, then drop the original grey/white reference to it. A Yuasa deletion barrier closes this: on any store into a GCREF slot while marking is active, shade the slot's old value grey before overwriting it. In the sample, a.next = b (and the ring-closing c.next = a) go through the store paths SETF/map_set/push where the barrier lives — no new opcode, because SETF already resolves the field kind from the class table. Owned stores, scalars, and Text pay nothing. Reading a shard's roots fresh from its masks each slice is what removes Go's Dijkstra insertion-half and the stack rescan.

Trigger & budget. A cycle starts when the shard's traced bytes since the last cycle cross a heap goal; each slice marks at most WO_GC_BUDGET objects; WO_GC_TRACE prints freed/marked counts per slice. Collection is per-shard — ownership moves mean no traced object spans shards, so there is no global stop-the-world and no cross-shard tracing.

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4. The owned path, for contrast (no collector at all)

owned_demo builds a Segment. At the closing } the drop table lists its register in the owned mask; the VM frees the object and its Text field deterministically and immediately. No color, no list link, no barrier, no slice. This is the common case, and log-watcher proves it scales: 35 classes, zero traced, arena + deterministic drops end to end.

flowchart LR
  NEW["let s = Segment (from, len)"] --> USE["use s"] --> SCOPE["scope end"]
  SCOPE --> DROP["DROP (owned mask):<br/>free s.from (Text), free s"]
  DROP --> GONE["reclaimed — collector never involved"]

So a .wo program has two reclamation systems working together: ownership (deterministic, free, the 99%) and tracing (only the cyclic/aliased residue, inferred). The developer writes no memory annotations for either.

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Run status

This is a target sample for iteration 7b, which is not yet implemented. It does not build on today's toolchain — and the compile error is precisely the motivation. woc --emit docs/examples/gc-cycle today reports:

main.wo: error WO-E301: use of `a` after it was moved
  c.next = a;   <- `a` moved here

Under the current model a class instance is owned and single-owner, so storing b into a.next moves it and closing the ring with c.next = a re-uses a moved value. Iteration 7b classifies Node as traced — and traced classes alias freely (spec §3 rule 5), so the ring becomes legal and the collector, not ownership, reclaims it. The pieces still to build: the inference pass (gcinfer.ml), --dump-gc, the .wob opcode-27/28 retirement, the sweep list, and the incremental collector. Today's runtime still uses RC + a Bacon–Rajan cycle collector behind an explicit @gc annotation (runtime/src/gc.c).

When 7b lands, the acceptance is:

• woc --dump-gc docs/examples/gc-cycle classifies Node gc (cycle …) / Segment owned.
• ring_demo prints ring a -> b -> c -> a, and after the roots die the ring is collected within budgeted slices (observable via WO_GC_TRACE), ASan-clean after repeated cycles.
• The adversarial barrier fixture (hide a node between slices) frees the node when the barrier is compiled out and keeps it when it is in.

Files: types.wo (the two shapes), main.wo (the two demos), wo.toml.