Good day, hackfolk. Today's note tries to extend our semi-space
collector with support for ephemerons. Spoiler alert: we fail in a
subtle and interesting way. See if you can spot it before the end 🙂

Recall that, as we concluded in an earlier
article,
a memory manager needs to incorporate ephemerons as a core part of the
tracing algorithm. Ephemerons are not macro-expressible in terms of
object trace functions.

Instead, to support ephemerons we need to augment our core trace
routine. When we see an ephemeron E, we need to check if the key K
is already visited (and therefore live); if so, we trace the value V
directly, and we're done. Otherwise, we add E to a global table of
pending ephemerons T, indexed under K. Finally whenever we trace a
new object O, ephemerons included, we look up O in T, to trace any
pending ephemerons for O.

So, taking our semi-space
collector
as a workbench, let's start by defining what an ephemeron is.

struct gc_ephemeron {
  struct gc_obj header;
  int dead;
  struct gc_obj *key;
  struct gc_obj *value;
};

enum gc_obj_kind { ..., EPHEMERON, ... };

static struct gc_ephemeron* as_ephemeron(struct gc_obj *obj) {
  uintptr_t ephemeron_tag = NOT_FORWARDED_BIT | (EPHEMERON << 1);
  if (obj->tag == ephemeron_tag)
    return (struct gc_ephemeron*)obj;
  return NULL;
}

First we need to allow the GC to know when an object is an ephemeron or
not. This is somewhat annoying, as you would like to make this concern
entirely the responsibility of the user, and let the GC be indifferent
to the kinds of objects it's dealing with, but it seems to be
unavoidable.

The heap will need some kind of data structure to track pending
ephemerons:

struct gc_pending_ephemeron_table;

struct gc_heap {
  ...
  struct gc_pending_ephemeron_table *pending_ephemerons;
}

struct gc_ephemeron *
pop_pending_ephemeron(struct gc_pending_ephemeron_table*,
                      struct gc_obj*);
struct gc_ephemeron *
add_pending_ephemeron(struct gc_pending_ephemeron_table*,
                      struct gc_obj*, struct gc_ephemeron*);
struct gc_ephemeron *
pop_any_pending_ephemeron(struct gc_pending_ephemeron_table*);

Now let's define a function to handle ephemeron shenanigans:

void visit_ephemerons(struct gc_heap *heap, struct gc_obj *obj) {
  // We are visiting OBJ for the first time.
  // OBJ is the old address, but it is already forwarded.
  ASSERT(is_forwarded(obj));

  // First, visit any pending ephemeron for OBJ.
  struct gc_ephemeron *ephemeron;
  while ((ephemeron =
            pop_pending_ephemeron(heap->pending_ephemerons, obj))) {
    ASSERT(obj == ephemeron->key);
    ephemeron->key = forwarded(obj);
    visit_field(&ephemeron->value, heap);
  }

  // Then if OBJ is itself an ephemeron, trace it.
  if ((ephemeron = as_ephemeron(forwarded(obj)))
      && !ephemeron->dead) {
    if (is_forwarded(ephemeron->key)) {
      ephemeron->key = forwarded(ephemeron->key);
      visit_field(&ephemeron->value, heap);
    } else {
      add_pending_ephemeron(heap->pending_ephemerons,
                            ephemeron->key, ephemeron);
    }
  }
}

struct gc_obj* copy(struct gc_heap *heap, struct gc_obj *obj) {
  ...
  visit_ephemerons(heap, obj); // *
  return new_obj;
}

We wire it into the copy routine, as that's the bit of the collector
that is called only once per object and which has access to the old
address. We actually can't process ephemerons during the Cheney field
scan, as there we don't have old object addresses.

Then at the end of collection, we kill any ephemeron whose key hasn't
been traced:

void kill_pending_ephemerons(struct gc_heap *heap) {
  struct gc_ephemeron *ephemeron;
  while ((ephemeron =
            pop_any_pending_ephemeron(heap->pending_ephemerons)))
    ephemeron->dead = 1;    
}

void collect(struct gc_heap *heap) {
  // ...
  kill_pending_ephemerons(heap);
}

First observation: Gosh, this is quite a mess. It's more code than the
core collector, and it's gnarly. There's a hash table, for goodness'
sake. Goodbye, elegant algorithm!

Second observation: Well, at least it works.

Third observation: Oh. It works in the same way as tracing in the
copy
routine
works: well enough for shallow graphs, but catastrophically for
arbitrary graphs. Calling visit_field from within copy introduces
unbounded recursion, as tracing one value can cause more ephemerons to
resolve, ad infinitum.

Well. We seem to have reached a dead-end, for now. Will our hero wrest
victory from the jaws of defeat? Tune in next time for find out: same
garbage time (unpredictable), same garbage channel (my wordhoard). Happy hacking!