Lesson 23 · Copying a reachable object - Build a Garbage Collector

Copying a reachable object

Collection in a copying collector means moving the survivors into the empty space. Today you build the single-object copy - duplicate a reachable object into the next to-space slot - the primitive the whole collection is made of.

The goal

Copy one object from from-space into the next free to-space slot and return its new id.

Start here - the target

TO DO

Scenario: A reachable object is copied into to-space

Givena from-space with a = New(1) at 0 and c = New(0) at 1, with a.field0 = c (so a.field0 = 1)

Whencopy(a) is called with the to-space cursor at 0

Thenit copies a into to-space slot 0, returns 0, and the copy's field0 is still 1 - the fields are duplicated verbatim, not yet updated

Anda following copy(c) lands in to-space slot 1, so copies are packed contiguously from 0

Background

A copying collector does not free objects one at a time; it evacuates the live ones out of from-space into the fresh to-space and abandons everything left behind. The atom of that process is copying a single object: allocate the next to-space slot, put a duplicate of the object there, and hand back its new id. Because to-space fills by a bump cursor, the copies land contiguously from slot 0 - compaction falls out for free, with no holes.

There is a deliberate loose end today. The copy’s fields are duplicated verbatim, so they still hold the object’s old from-space ids - a’s copy still says field0 = 1, pointing into the space you are about to abandon. Those references are stale and must be rewritten to point at the copies. Fixing them is the Cheney scan two lessons from now. First, the next lesson handles the trap that makes naive copying go wrong: an object reachable by two paths must be copied only once.

Make it work

// toNext bumps through to-space as objects are copied in
func (h *CopyingHeap) copy(r Ref) Ref {
  dst := h.toNext
  src := h.from[r]
  h.to[dst] = &object{fields: append([]Ref(nil), src.fields...)} // duplicate fields as-is
  h.toNext++
  return dst
}

CheckpointDONE

You can copy a single reachable object into to-space. Commit and stop here.