writeonce/docs/stories/language-runtime-database/07b-inferred-gc-mark-sweep.md

Iteration 7b — inferred GC + incremental mark-sweep

Format: product/story-iteration-template. Part of Story — one language, one runtime, one database, one binary.

Inserted 2026-08-11, after the plan was first drawn — hence 7b rather than a renumber. It sits here because the log-watcher critical path (iterations 3–7) must not be delayed, and because the collector should be settled before iteration 8 multiplies shards.

Status (2026-08-14): not on the driving workload's path, measured: docs/examples/log-watcher declares no @gc class — 35 classes in its image, none with the gc flag, and 0 RC_INC / 0 RC_DEC instructions against 78 DROPs. Its memory story is arena + deterministic drops end to end, so this iteration (and iteration 4's open gc/held-cycle leak, and set's @gc retention gap) cannot affect whether log-watcher runs. It stays queued for workloads that build cycles; the gc/ corpus fixtures remain its only users.

Goals

• The developer stops deciding which types are garbage collected. @gc disappears from the language; the compiler infers GC-ness and reports every decision with its reason.
• Reference counting is replaced by an incremental per-shard tri-color mark-sweep collector, so the compiler no longer has to emit a balanced acquire/release at every alias site — the source of every recorded @gc defect.
• Milestone 1's acceptance criterion 3 (ASan-clean across the corpus) closes, because the leak blocking it is one of the defects this deletes.

Acceptance Criteria

• What to achieve?
  • Given a class whose declaration can form a reference cycle, and a separate class that is only ever shared through a long-lived alias,
  • when the program is compiled,
  • then both are classified GC-managed without any annotation, and --dump-gc names the reason for each — a cycle path for the first, the escaping alias site for the second.
• What to achieve?
  • Given any .wo source containing @gc,
  • when it is compiled,
  • then it is a diagnostic pointing at inference and --dump-gc, not a silently accepted no-op.
• What to achieve?
  • Given a program that hides a traced object from the collector — storing it into an already-blackened object between marking slices and dropping the original reference,
  • when the collector completes,
  • then the object is still alive; and the same fixture fails loudly if the write barrier is compiled out.
• What to achieve?
  • Given an abandoned cycle and a cycle still rooted from a live frame,
  • when collection runs,
  • then the abandoned one is freed within budgeted slices with no slice exceeding the configured budget, and the rooted one survives.
• What to achieve?
  • Given the whole conformance corpus, run repeatedly so several collection cycles occur,
  • when it runs under ASan,
  • then zero leaks and zero errors — the clause that currently fails.
• What to achieve?
  • Given any emitted .wob image,
  • when it is disassembled,
  • then no RC_INC or RC_DEC appears, and the format doc records opcodes 27–28 as reserved behind a version bump.

Out Of Scope

• Cross-shard tracing — ownership moves mean no traced object spans shards.
• Generational collection and compaction. Non-moving is load-bearing: no forwarding pointers, no read barrier. Go's collector is not generational either.
• Scheduler-integrated pacing beyond the heap-goal trigger; that stays iteration 8's concern, which is part of why this lands first.
• ref T semantics, unchanged — it is an id, not a pointer, and creates no edge in the inference graph.

Info

• Governing spec: docs/superpowers/specs/2026-08-11-inferred-gc-mark-sweep-design.md.
• Gated by the benchmark (2026-08-15): this is the "implement garbage collection" lever of the performance arc — tri-color mark-sweep replacing RC changes the write path's tail latency, so landing it means re-running iteration 9e and recording the delta (does tracing help or hurt p99 under write load?).
• Constraint added by the database track (2026-08-15): a GC-managed value in a @table field is a compile error (the engine/heap bulkhead — 9b design, section 6). Once GC-ness is inferred rather than annotated, the inference pass must classify every class before table-field validation, and the diagnostic must name the inference reason ("class X is garbage-collected via Y and cannot be stored in a table field") — otherwise the error becomes unactionable exactly when it stops being self-evident.
• Why the annotation was insufficient, not merely inconvenient: the OOP spec's own example, @gc class PriceCache { entries: map<SKU, Money> }, is acyclic. It needs GC because it is shared, and second-class borrows cannot be stored or returned. So the developer was being asked to reason about type shape and whole-program aliasing at once — hence the hybrid rule (structural SCC plus reported demand promotion).
• Why tracing rather than better reference counting: all four recorded @gc defects are RC bookkeeping failures — push missing an increment, set still missing one, the mut-@gc clobber, and the held-cycle leak. Inferring GC-ness would widen that population and so widen that bug class. Tracing emits no per-alias bookkeeping at all.
• Most of what tracing needs already exists. The emitter already produces precise per-pc pointer masks (the drop table's gc mask) and the class table already carries per-field kinds — the two pieces Go gets from stack maps and type maps. Go's dependence on OS threads is incidental; the algorithm needs only per-frame PC→map lookup and the ability to suspend one stack.
• The one real runtime addition: the arena cannot enumerate objects — bump allocation plus size-class free lists, with large objects on bare malloc and no size headers anywhere. Sweep needs its own list. Retiring rc, plus the borrow word that traced objects never use, frees exactly eight contiguous bytes for an intrusive link, so the 16-byte header survives.
• This iteration supersedes part of iteration 2's memory model (§4 of the OOP spec) and closes iteration 4's open gate clause. Neither is renumbered; both carry pointers here.

Proposed Solution

• Write the implementation plan from the approved spec, then execute it: the gcinfer.ml pass (Tarjan SCC over the class-reference graph, then demand promotion to a fixpoint, with a note per decision), the @gc removal and its diagnostic, retiring RC_INC/RC_DEC and the rc machinery from owner.ml/emit.ml, the per-shard traced list and sweep, incremental tri-color marking with roots read from the existing pc masks, the Yuasa deletion barrier inside the VM's store paths, and the doc/golden migration the spec's §8 table enumerates.