assemblyscript/std/assembly/collector

Garbage collector interface

A garbage collector for AssemblyScript must implement the common and either of the tracing or reference counting interfaces.

Common

• __ref_collect()
  Triggers a full garbage collection cycle.

Tracing

• __ref_register(ref: usize)
  Sets up a new reference.

• __ref_link(ref: usize, parentRef: usize)
  Links a reference to a parent that is now referencing it.

• __ref_unlink(ref: usize, parentRef: usize)
  Unlinks a reference from a parent that was referencing it.

Reference counting

• __ref_retain(ref: usize)
  Retains a reference, usually incrementing RC.

• __ref_release(ref: usize)
  Releases a reference, usually decrementing RC.

Reference counting may also implement __ref_register if necessary.

Typical patterns

Standard library components make use of the interface where managed references are stored or deleted. Common patterns are:

General

/// <reference path="./collector/index.d.ts" />

if (isManaged<T>()) {
  // compiled only if T is a managed reference
  ... pattern ...
}

Insertion

if (isNullable<T>()) {
  if (ref) {
    if (isDefined(__ref_link)) __ref_link(ref, parentRef);
    else if (isDefined(__ref_retain)) __ref_retain(ref);
    else assert(false);
  }
} else {
  if (isDefined(__ref_link)) __ref_link(ref, parentRef);
  else if (isDefined(__ref_retain)) __ref_retain(ref);
  else assert(false);
}

Replacement

if (ref !== oldRef) {
  if (isNullable<T>()) {
    if (isDefined(__ref_link)) {
      if (oldRef) __ref_unlink(oldRef, parentRef);
      if (ref) __ref_link(ref, parentRef);
    } else if (isDefined(__ref_retain)) {
      if (oldRef) __ref_release(oldRef);
      if (ref) __ref_retain(ref);
    } else assert(false);
  } else {
    if (isDefined(__ref_link)) {
      __ref_unlink(oldRef, parentRef);
      __ref_link(ref, parentRef);
    } else if (isDefined(__ref_retain)) {
      __ref_release(oldRef);
      __ref_retain(ref);
    } else assert(false);
  }
}

Deletion

if (isNullable<T>()) {
  if (ref) {
    if (isDefined(__ref_link)) __ref_unlink(ref, parentRef);
    else if (isDefined(__ref_retain)) __ref_release(ref);
    else assert(false);
  }
} else {
  if (isDefined(__ref_link)) __ref_unlink(ref, parentRef);
  else if (isDefined(__ref_retain)) __ref_release(ref);
  else assert(false);
}

Note that some data structures may contain null values even though the value type isn't nullable. May be the case when appending a new element to an array for example.