shoney.arickathil
2a18860260
Design-first deliverable for iteration 7b (no runtime/compiler code yet). docs/examples/gc-cycle explains, by example, how pointers flow through the heap and the collector's mark-sweep logic: - types.wo: Node (self-referential ?Node -> inferred `gc`/traced) vs Segment (acyclic -> `owned`, deterministically dropped) - main.wo: ring_demo builds a->b->c->a and abandons it; owned_demo shows the drop path with no collector - README.md: the model (ownership frees the 99%, tracing only the cyclic/ aliased residue, inference decides), the 16-byte header rewrite (retire rc+borrow -> 8-byte sweep-list link, colors in flag bits), where a traced pointer lives (root via pc gc-mask / GCREF field / container), and the tri-color incremental algorithm with the Yuasa deletion barrier. Two mermaid step diagrams (heap+roots, collector cycle) + the owned contrast. Grounded in the approved spec (2026-08-11-inferred-gc-mark-sweep-design.md) and the real runtime structures (obj.h/wob.h: wo_hdr, WO_K_GCREF, arena, wo_obj_size). Run status: honest — the sample does NOT build today; woc reports WO-E301 (use-after-move at the ring-closing store), which is exactly the aliasing that "traced classes alias freely" unblocks under 7b. README records this. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
214 lines
10 KiB
Markdown
214 lines
10 KiB
Markdown
# gc-cycle — inferred GC + incremental mark-sweep, by example
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The smallest program that needs a tracing collector (`ring_demo`) and the
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smallest that does not (`owned_demo`). This README defines **how pointers/
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addresses flow through the heap** and **the collector's mark-sweep logic**, the
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way iteration 7b specs it
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([`2026-08-11-inferred-gc-mark-sweep-design.md`](../../superpowers/specs/2026-08-11-inferred-gc-mark-sweep-design.md)).
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The one-line model: **ownership frees everything it can; the collector traces
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only the residue ownership cannot free — cycles and long-lived aliases — and
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the compiler infers which types those are.**
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---
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## 1. Inference — the compiler decides `owned` vs `gc`
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A pass between `types` and `owner` (`compiler/src/gcinfer.ml`) builds a
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**class-reference graph** — an edge `A → B` whenever `A` has a field of type
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`B`, `?B`, `multi B`, `map<B,_>`, or `map<_,B>`. **`ref B` creates no edge** (it
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is a row id, `Copy`). Tarjan's SCC over that graph: any class in a non-trivial
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SCC, or with a self-loop, is **traced (`gc`)**. Everything else is **`owned`**.
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For this sample, `woc --dump-gc` would print:
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```
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Node gc (cycle Node -> Node)
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Segment owned
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```
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`Node.next: ?Node` is the self-loop → `Node` is traced, and its `next` field
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gets kind `WO_K_GCREF`. `Segment` has no class-typed field → no edge → `owned`,
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freed deterministically. A second, **demand** half promotes a type when the
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ownership pass would otherwise report a long-lived-alias escape (the
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`PriceCache` shape) — not exercised here; every promotion is reported, never
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silent.
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---
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## 2. Address flow — where a pointer lives, and the object's shape
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Every heap object is a 16-byte `wo_hdr` followed by `field_cnt` 8-byte slots
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(`wo_fields(o)`; `wo_obj_size = 16 + field_cnt*8`). Allocation is the arena:
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bump + 16-byte size-class free lists (16…1024), larger falls to `malloc`. **The
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arena keeps no size headers and cannot enumerate objects** — so traced objects
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thread an intrusive **sweep list**.
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The header is where iteration 7b pays for the collector with **zero growth**:
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```
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today (RC) under 7b (tracing)
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┌──────────────────────────┐ ┌──────────────────────────┐
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│ class_id (4 bytes) │ │ class_id (4 bytes) │
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│ flags (1) + pad │ │ flags (1) ── 2 color bits (white/grey/black), + pad
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│ borrow (4 bytes) │ │ sweep-list link (8 bytes)│ ← reclaimed from
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│ rc (4 bytes) │ │ (next traced object) │ borrow + rc
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└──────────────────────────┘ └──────────────────────────┘
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16 bytes 16 bytes (unchanged)
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```
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`rc` is retired (no reference counting); traced objects are exempt from borrow
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rules so `borrow` is dead too — the two adjacent 4-byte words become one 64-bit
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list link. Color lives in 2 existing flag bits (`WO_F_COLOR`).
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A traced pointer (the address of a `wo_hdr`) is only ever held in one of three
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places, and these are exactly what the collector reads:
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| Holder | How the collector sees it |
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| --- | --- |
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| a **VM value-stack / frame slot** (a local like `a`) | a **root**, via the per-pc **gc-mask** the emitter already emits |
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| a **`GCREF` field slot** of another object (`a.next`) | followed during **mark** |
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| a **container** item (`multi`/`map`) whose element kind is `GCREF` | followed during mark |
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Here is the sample's heap after `ring_demo` builds the ring, while `a`/`b`/`c`
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are still live roots:
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```mermaid
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flowchart LR
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subgraph STACK["VM value stack (roots this pc, from the gc-mask)"]
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A["a"]:::root
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B["b"]:::root
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C["c"]:::root
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end
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subgraph HEAP["arena (one mmap region)"]
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NA["Node a<br/>hdr, label(TEXT), next(GCREF)"]
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NB["Node b<br/>hdr, label(TEXT), next(GCREF)"]
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NC["Node c<br/>hdr, label(TEXT), next(GCREF)"]
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end
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A --> NA
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B --> NB
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C --> NC
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NA -->|next| NB
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NB -->|next| NC
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NC -->|next, closes cycle| NA
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NA -.->|sweep link| NB
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NB -.->|sweep link| NC
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NC -.->|sweep link| NULL(("nil"))
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classDef root fill:#2b6,stroke:#083,color:#fff;
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```
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Solid arrows are `GCREF` pointers the mark phase follows; the dotted chain is
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the per-shard **traced list** the sweep phase walks (independent of
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reachability). When `a`/`b`/`c` leave scope, the three solid *root* arrows
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vanish — the ring still points to itself, but nothing points *in*, so it is
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unreachable yet un-freed. Ownership cannot help: `b` cannot be owned by both
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`let b` and `a.next`. That is the collector's entire job.
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---
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## 3. Mark-sweep logic — tri-color, incremental, with a deletion barrier
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Marking runs in **budgeted slices** (`WO_GC_BUDGET` objects per slice), so the
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pause is bounded regardless of heap size. Colors: **white** = unproven (candidate
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to free), **grey** = reachable but children not yet scanned, **black** = reachable
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and scanned.
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```mermaid
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flowchart TD
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ALLOC["allocate traced object<br/>color = WHITE, link into traced list"] --> LIVE
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LIVE["mutator runs<br/>(program executes)"] --> TRIG{"traced bytes since last cycle<br/>past heap goal?"}
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TRIG -- no --> LIVE
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TRIG -- yes --> ROOTS["START CYCLE<br/>shade every root GREY<br/>(value/frame slots via pc gc-mask)"]
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ROOTS --> SLICE
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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?"}
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GREY -- "no (budget hit)" --> SAFE["yield at next safepoint<br/>(loop back-edge / call)"]
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SAFE --> LIVE2["mutator resumes<br/>(barrier active)"]
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LIVE2 --> SLICE
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GREY -- yes --> SWEEP["SWEEP: walk traced list —<br/>WHITE: free (wo_obj_size) and unlink;<br/>BLACK: repaint WHITE, keep"]
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SWEEP --> DONE["cycle done"] --> LIVE
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```
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**Roots.** The value stack and frame stack, read through the per-pc gc-mask the
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emitter already produces (the drop-table's gc bits, reinterpreted from "rc_dec
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these on unwind" to "these registers are GC roots at this pc"). No stack
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scanning, no native frames — a bytecode VM with an explicit value stack has the
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map for free.
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**Owned objects are traversed, never freed.** An `owned` object can hold a
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`GCREF` field, so mark must walk through owned subtrees to reach traced
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objects — but it never frees an owned object (drop owns those). A precomputed
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class-table bit, **"transitively contains a gcref"**, lets mark skip any owned
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subtree that can reach no traced object at all.
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**The barrier — why incremental is safe.** Between slices the mutator keeps
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running and can hide a live object from a half-finished mark: store a white
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object into an already-**black** object, then drop the original grey/white
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reference to it. A **Yuasa deletion barrier** closes this: on any store into a
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`GCREF` slot **while marking is active**, shade the slot's **old** value grey
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before overwriting it. In the sample, `a.next = b` (and the ring-closing
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`c.next = a`) go through the store paths `SETF`/`map_set`/`push` where the
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barrier lives — no new opcode, because `SETF` already resolves the field kind
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from the class table. Owned stores, scalars, and Text pay nothing. Reading a
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shard's roots fresh from its masks each slice is what removes Go's Dijkstra
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insertion-half and the stack rescan.
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**Trigger & budget.** A cycle starts when the shard's traced bytes since the
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last cycle cross a heap goal; each slice marks at most `WO_GC_BUDGET` objects;
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`WO_GC_TRACE` prints freed/marked counts per slice. Collection is **per-shard**
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— ownership moves mean no traced object spans shards, so there is no global
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stop-the-world and no cross-shard tracing.
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---
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## 4. The owned path, for contrast (no collector at all)
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`owned_demo` builds a `Segment`. At the closing `}` the drop table lists its
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register in the **owned mask**; the VM frees the object and its `Text` field
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deterministically and immediately. No color, no list link, no barrier, no
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slice. This is the common case, and log-watcher proves it scales: 35 classes,
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**zero** traced, arena + deterministic drops end to end.
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```mermaid
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flowchart LR
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NEW["let s = Segment (from, len)"] --> USE["use s"] --> SCOPE["scope end"]
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SCOPE --> DROP["DROP (owned mask):<br/>free s.from (Text), free s"]
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DROP --> GONE["reclaimed — collector never involved"]
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```
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So a `.wo` program has **two** reclamation systems working together: ownership
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(deterministic, free, the 99%) and tracing (only the cyclic/aliased residue,
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inferred). The developer writes no memory annotations for either.
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---
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## Run status
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**This is a target sample for iteration 7b, which is not yet implemented.** It
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does **not** build on today's toolchain — and the compile error is precisely
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the motivation. `woc --emit docs/examples/gc-cycle` today reports:
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```
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main.wo: error WO-E301: use of `a` after it was moved
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c.next = a; <- `a` moved here
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```
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Under the current model a class instance is `owned` and single-owner, so storing
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`b` into `a.next` **moves** it and closing the ring with `c.next = a` re-uses a
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moved value. Iteration 7b classifies `Node` as **traced** — and *traced classes
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alias freely* (spec §3 rule 5), so the ring becomes legal and the collector,
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not ownership, reclaims it. The pieces still to build: the inference pass
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(`gcinfer.ml`), `--dump-gc`, the `.wob` opcode-27/28 retirement, the sweep list,
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and the incremental collector. Today's runtime still uses RC + a Bacon–Rajan
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cycle collector behind an explicit `@gc` annotation (`runtime/src/gc.c`).
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When 7b lands, the acceptance is:
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- `woc --dump-gc docs/examples/gc-cycle` classifies `Node gc (cycle …)` /
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`Segment owned`.
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- `ring_demo` prints `ring a -> b -> c -> a`, and after the roots die the ring
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is collected within budgeted slices (observable via `WO_GC_TRACE`), ASan-clean
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after repeated cycles.
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- The adversarial barrier fixture (hide a node between slices) frees the node
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when the barrier is compiled out and keeps it when it is in.
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Files: [`types.wo`](types.wo) (the two shapes), [`main.wo`](main.wo) (the two
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demos), [`wo.toml`](wo.toml).
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