Update manual

README.md

@@ -26,6 +26,8 @@ See the [documentation](./doc/README.md).
  - Inline allocation / write barrier fast paths (supporting JIT)
  - One unified API with no-overhead abstraction: switch collectors when
    you like
+ - Three policies for sizing heaps: fixed, proportional to live size, and
+   [MemBalancer](http://marisa.moe/balancer.html)
 
 ## Source repository structure
 
@@ -42,9 +44,9 @@ See the [documentation](./doc/README.md).
 
 ## Status and roadmap
 
-As of September 2024, Whippet is almost feature-complete.  The main
-missing feature is dynamic heap growth and shrinkage
-(https://github.com/wingo/whippet/issues/5), which should land soon.
+As of September 2024, Whippet is almost feature-complete.  We need to
+land a per-object pinning API, and an API for cooperative safepoints for
+use by threads that are looping without allocating.
 
 After that, the next phase on the roadmap is support for tracing, and
 some performance noodling.

doc/collector-bdw.md

@@ -15,6 +15,12 @@ finalizers), and both ephemerons and finalizers only approximate the
 Whippet behavior, because they are implemented in terms of what BDW-GC
 provides.
 
+`bdw` supports the `fixed` and `growable` heap-sizing policies, but not
+`adaptive`, as BDW-GC can't reliably return memory to the OS.  Also,
+[`growable` has an effective limit of a 3x heap
+multiplier](https://github.com/wingo/whippet/blob/main/src/bdw.c#L478).
+Oh well!
+
 It's a bit of an oddball from a Whippet perspective, but useful as a
 migration path if you have an embedder that is already using BDW-GC.
 And, it is a useful performance comparison.

doc/manual.md

@@ -77,13 +77,15 @@ visitor function on all outgoing edges in an object.  It also includes a
 of an object.  `trace_edge` and `size` may be `NULL`, in which case no
 tracing or size computation should be performed.
 
-### Tracing ephemerons
+### Tracing ephemerons and finalizers
 
 Most kinds of GC-managed object are defined by the program, but the GC
-itself has support for a specific object kind: ephemerons.  If the
-program allocates ephemerons, it should trace them in the
-`gc_trace_object` function by calling `gc_trace_ephemeron` from
-[`gc-ephemerons.h`](../api/gc-ephemerons.h).
+itself has support for two specific object kind: ephemerons and
+finalizers.  If the program allocates ephemerons, it should trace them
+in the `gc_trace_object` function by calling `gc_trace_ephemeron` from
+[`gc-ephemerons.h`](../api/gc-ephemerons.h).  Likewise if the program
+allocates finalizers, it should trace them by calling
+`gc_trace_finalizer` from [`gc-finalizer.h`](../api/gc-finalizer.h).
 
 ### Remembered-set bits
 
@@ -299,6 +301,12 @@ We do this by including the `gc-embedder-api.h` implementation, via
 $(COMPILE) -include foo-embedder.h -o gc-ephemeron.o -c gc-ephemeron.c
 ```
 
+As for ephemerons, finalizers also have their own compilation unit.
+
+```
+$(COMPILE) -include foo-embedder.h -o gc-finalizer.o -c gc-finalizer.c
+```
+
 #### Compile-time options
 
 There are a number of pre-processor definitions that can parameterize
@@ -469,7 +477,7 @@ defined for all collectors:
 
 You can set these options via `gc_option_set_int` and so on; see
 [`gc-options.h`](../api/gc-options.h).  Or, you can parse options from
-trings: `heap-size-policy`, `heap-size`, `maximum-heap-size`, and so
+strings: `heap-size-policy`, `heap-size`, `maximum-heap-size`, and so
 on.  Use `gc_option_from_string` to determine if a string is really an
 option.  Use `gc_option_parse_and_set` to parse a value for an option.
 Use `gc_options_parse_and_set_many` to parse a number of comma-delimited
@@ -669,4 +677,49 @@ An ephemeron association can be removed via `gc_ephemeron_mark_dead`.
 
 ### Finalizers
 
-Not yet implemented!
+A finalizer allows the embedder to be notified when an object becomes
+unreachable.
+
+A finalizer has a priority.  When the heap is created, the embedder
+should declare how many priorities there are.  Lower-numbered priorities
+take precedence; if an object has a priority-0 finalizer outstanding,
+that will prevent any finalizer at level 1 (or 2, ...)  from firing
+until no priority-0 finalizer remains.
+
+Call `gc_attach_finalizer`, from `gc-finalizer.h`, to attach a finalizer
+to an object.
+
+A finalizer also references an associated GC-managed closure object.
+A finalizer's reference to the closure object is strong:  if a
+finalizer's closure closure references its finalizable object,
+directly or indirectly, the finalizer will never fire.
+
+When an object with a finalizer becomes unreachable, it is added to a
+queue.  The embedder can call `gc_pop_finalizable` to get the next
+finalizable object and its associated closure.  At that point the
+embedder can do anything with the object, including keeping it alive.
+Ephemeron associations will still be present while the finalizable
+object is live.  Note however that any objects referenced by the
+finalizable object may themselves be already finalized; finalizers are
+enqueued for objects when they become unreachable, which can concern
+whole subgraphs of objects at once.
+
+The usual way for an embedder to know when the queue of finalizable
+object is non-empty is to call `gc_set_finalizer_callback` to
+provide a function that will be invoked when there are pending
+finalizers.
+
+Arranging to call `gc_pop_finalizable` and doing something with the
+finalizable object and closure is the responsibility of the embedder.
+The embedder's finalization action can end up invoking arbitrary code,
+so unless the embedder imposes some kind of restriction on what
+finalizers can do, generally speaking finalizers should be run in a
+dedicated thread instead of recursively from within whatever mutator
+thread caused GC.  Setting up such a thread is the responsibility of the
+mutator.  `gc_pop_finalizable` is thread-safe, allowing multiple
+finalization threads if that is appropriate.
+
+`gc_allocate_finalizer` returns a finalizer, which is a fresh GC-managed
+heap object.  The mutator should then directly attach it to an object
+using `gc_finalizer_attach`.  When the finalizer is fired, it becomes
+available to the mutator via `gc_pop_finalizable`.