Automatic Reference Counting (ARC) Garbage Collector

reference-counting-gc.js

/**
 * Automatic Reference Counting (ARC) Garbage Collection technique.
 *
 * This diff describes Reference counting GC technique.
 * See also previous lesson on Mark and Sweep GC:
 * https://gist.github.com/4391763
 *
 * Covered topics:
 *
 * 1. Reference value in ECMAScript (JS specific)
 * 2. ARC algorimths from generic computer science.
 *
 * *** NOTE: here is simplified version of this diff, which
 * doesn't use ECMAScript-specific Reference type:
 * https://gist.github.com/4646658
 * 
 * by Dmitry Soshnikov <[email protected]>
 * MIT Style License
 */

// ******************************************************************
// I. Preamble: heap and references
// ******************************************************************

// ------------------------------------------------------------------
// 1. Heap
// ------------------------------------------------------------------

// For simplicity, we represent the "heap" (our memory where all objects
// are stored) as a JS array. Address at which an object is stored is
// the array index.
var heap = [];

// ------------------------------------------------------------------
// 2. References
// ------------------------------------------------------------------

// To *refer* objects on the heap, we need special values which
// store *addresses* of the objects. These are *pointers*, or in our
// cases -- *references*.
// 
// For example, if "x" reference stores value 1, it means that at index
// 1 of the heap array there is an object that "x" points to.
//
// For global variables we could store addresses directly in the variables
// (like var x = 1), however, some properties of objects may be the references
// themselves and refer another objects.
// 
// To handle both cases in the same way, we introduce small
// helper class, the "Reference" class.
//
// In JS world, we achieve it through two components of a reference object:
// the "base" object and the "offset" in borders of this object.
// (The "offset" is a property name of this object in JS)
// 
// Examples:
//
// Global variable (reference): the "base" is the global object,
// the "offset" (the property name) is "a":
//
// var a = {name: 'a'}; // Reference(global, "a")
//
// A reference within another object, the "base" is "a" object,
// the offset (property name) is "x": 
//
// a.x = {name: 'x'}; // Reference(a, "x");
// 
// The value stored at these coordinates (base:offset) is exactly
// the *address* on the heap (the index of the heap array). Having
// the address, we then can get the value stored at it.
function Reference(base, offset) {
  this._base = base;
  this._offset = offset;
}

Reference.prototype = {
  // gets the reference value (the address)
  getAddress: function () {
    return this._base[this._offset];
  },
  // Gets the actual contents stored by this address.
  // This operation is known as *dereferencing* of
  // a reference (of a pointer).
  getContents: function () {
    return heap[this.getAddress()];
  },
  // Rebinds the reference to another address.
  setAddress: function (address) {
    this._base[this._offset] = address;
  }
};

// (BTW, exactly this implementation of references is used and specified
// in ECMAScript specification).

// ******************************************************************
// II. Reference counting GC
// ******************************************************************

// Automatic reference counting (ARC) is a garbage collection technique
// with a very simple algorithm: every object stores a special value (reference
// count value) which is increased every time a new reference is assigned the
// address of this object, and decreased each time the address is removed from
// some reference to this object. The whole "magic" happens at assignment to
// a reference, rebinding the reference to another address and removing
// objects from the heap. 

// (Note, JS's GC is not specified. However, in practice usually can be found
// Mark and Sweep GC (since ARC has some disadvantages described bellow).
// ARC is used in some languages though, like Objective-C).

// ------------------------------------------------------------------
// 3. Objects allocation
// ------------------------------------------------------------------

// To manage this reference counting special value, we introduce a handy
// "allocate" function which initializes reference counting flag to zero,
// and also provides actual allocation of this object on the heap (at first
// available free address). As a result, the function returns the address
// at which object was allocated.

function allocate(obj) {
  var allocAddress = heap.length;
  obj.__rc__ = 0;
  heap.push(obj);
  return allocAddress;
}

// ------------------------------------------------------------------
// 4. Assignment to references, ARC algorithm
// ------------------------------------------------------------------

// That's said, the main algorithm actually happens at assignment operation.
//
// 1. We get the *previous (current) object* to which the assigning reference
//    points to and *decrease* its reference counting.
//
// 2. Then we get the object stored at *new address* and *increase*
//    its reference count (but, only if the new address is not null:
//    in this case we just reset this reference at first step).
//
// 3. Now, if reference count flag of the *previously referred object*
//    is zero, we immediately remove the object from the heap.
//
// 4. Finally, we do actual assignment to the reference, i.e. set it
//    to the new passed address.

function assign(ref, address) {
  // Previously (currently) stored object
  // at the address of this reference.
  var valueOfRef = ref.getContents();
  
  // Object at new address.
  var valueAtAddress = heap[address];
  
  // Decrement the reference count of the previous object
  // (if its reference was not null-reference of course)
  valueOfRef && valueOfRef.__rc__--;
  
  // And if the new address is not null, increase the
  // reference count of the new object.
  if (address != null) {
    valueAtAddress.__rc__++;
  }
  
  // Check and remove old object from the heap, if there no reference to
  // it anymore (i.e. its reference count value is zero).
  recycleObjectAtAddressIfNoRef(ref.getAddress());
  
  // Finally, assign the reference to the new address.
  ref.setAddress(address);
  
  // And return the reference back.
  return ref;
}

// ------------------------------------------------------------------
// 5. Recycling non-referenced objects, ARC algorithm
// ------------------------------------------------------------------

// As we said, if reference counting value becomes zero,
// the object may be removed from the heap (since no one refers
// it anymore). However, at destructuring the object, we should
// deallocate all its properties too. And if some of its properties
// are reference to objects too, we should decrease reference counting
// value of these sub-objects as well, and potentially check them
// for recycling.  

function recycleObjectAtAddressIfNoRef(address) {
  // Object stored at address.
  var valueAtAddress = heap[address];
  // If it's not null and no one refers it anymore, it's time
  // to remove it from the heap.
  if (valueAtAddress && valueAtAddress.__rc__ == 0) {
    // But, before recycling the object itself, first all its reference
    // sub-properties, decreasing reference count of object they refer.
    for (var k in valueAtAddress) if (k != '__rc__' && isAddress(valueAtAddress[k])) {
      // The address on the heap of this sub-object.
      var subObjectAddress = valueAtAddress[k];
      // Actual object stored by this address.
      var subObject = heap[subObjectAddress];
      // Decrease its reference count.
      subObject.__rc__--;
      // And also check for possible releasing from the heap.
      recycleObjectAtAddressIfNoRef(subObjectAddress);
    }
    // Now we can remove our object itself from the heap.
    heap.splice(address, 1);
  }
}

// For simplicity, use this helper funciton checking whether
// a property of some object is also the reference to another
// object, i.e. stores *address* (just the numed, the heap's array
// indecies).
function isAddress(value) {
  return typeof value == 'number';
}

// ------------------------------------------------------------------
// 6. Testing
// ------------------------------------------------------------------

// global variables (references) have the global
// object as their "base" part.
var global = this;

// -- 6.1 One reference to an object: -------------------------------

// Allocate "a" object and assign it to "aRef" reference
// (this is rougly equivalent to var aRef = {name: 'a'} in JS).
// aRef ---> {name: 'a'}
var aRef = assign(new Reference(global, 'a'), allocate({name: 'a'}));

// The same with "b" object, which is assigned to "bRef" reference,
// (var bRef = {name: 'b'} in JS).
// bRef ---> {name: 'b'}
var bRef = assign(new Reference(global, 'b'), allocate({name: 'b'}));

// Check the heap state: currently "a" and "b" objects
// are referenced by one reference per each object
// (i.e. __rc__ flag of both objects is 1).

// [{name: 'a', __rc__: 1}, {name: 'b', __rc__: 1}]
console.log(heap); // "a" and "b" objects

// -- 6.2 Two references points to the same object: -----------------

// Now set the "bRef" reference to the *same address* as "aRef" has. 
assign(bRef, aRef.getAddress());

// This causes decreasing reference count of "b" object on the heap and
// increasing reference count of the "a" object. Now "b" object doesn't
// have any reference to it, which makes it automatically garbage collected:

// aRef ---> {name: 'a', __rc__: 2} <--- bRef
//           {name: 'b', __rc__: 0}

// [{name: 'a', __rc__: 2}]
console.log(heap); // only "a" object is left and "b" object is removed.

// -- 6.3 Reset references (set to null): ---------------------------

// As we implemented in "assign", if the assigning address is null,
// it means the __rc__ of the object the reference points to is
// just decreased:

assign(aRef, null); // reset "aRef" reference
console.log(heap); // [{name: 'a', __rc__: 1}]

// Still there is one reference to "a" object via "bRef",
// reset it as well. It automatically removes "a" object
// from the heap, since __rc__ becomes 0.
assign(bRef, null);
console.log(heap); // [], empty.

// -- 6.4 Issue: reference cycles -----------------------------------

// There is one problem with ARC which cannot be handled in obvious way
// (although, there are some workarounds) -- this is *reference cycles*.
//
// Consider a use case (in JS-code example):
//
// var xRef = {name: 'x'};
// x.next = {name: 'x:next'};
// x.next.backToX = x;
//
// In our implementation, it's represented as follows:

// Allocate "x" object, x ---> {name: 'x'}
var xRef = assign(new Reference(global, 'x'), allocate({name: 'x'}));

// Allocate "next" object which is referenced as x.next:
// x ---> {name: 'x', next: ---> {name: 'x:next'}}
var xNextRef = assign(new Reference(xRef.getContents(), 'next'), allocate({name: 'x:next'}));

// And now property "backToX" of the "next" object refers back to the
// "x" object. Set "x.next.backToX" to the same address as "x" reference has.
var backToXRef = assign(new Reference(xNextRef.getContents(), 'backToX'), xRef.getAddress());

// Check the heap state, it should be 2 objects: "x" is referred twice (from the
// global "xRef" and from the "backToX", and "x:next" is referred once from "x.next").
// Notice how "next" and "backToX" properties stores addresses (heaps array indices).
//
// [{name: 'x', next: 1, __rc__: 2}, {name: 'x:next', backToX: 0, __rc__: 1}]
console.log(heap); // "x" and "x:next"

// Now the actual problem happens if we reset global "xRef" reference setting it to null:
assign(xRef, null);

// This decreased __rc__ of the "x" object by one and from the user code we cannot reach
// this object anymore. But it *cannot be garbage collected* by ARC, since its __rc__ is
// still not zero! -- there is still one reference to it from "x.next.backToX" property!

// [{name: 'x', next: 1, __rc__: 1}, {name: 'x:next', backToX: 0, __rc__: 1}] 
console.log(heap);

// To avoid this situation, *before setting xRef to null* we should set *manually* reset
// "x.next.backToX" reference. I.e. set it manually to null too.

// (Restore "xRef" for our experiment, from the "backToRef" now)
assign(xRef, backToXRef.getAddress());

// Check that "x" object is again referenced by two pointers:
// [{name: 'x', next: 1, __rc__: 2}, {name: 'x:next', backToX: 0, __rc__: 1}]
console.log(heap); // "x" and "x:next"

// Now do it right, and reset first "backToRef":
assign(backToXRef, null);

// Check the heap, see "x" is correctly referenced by one reference:
// [{name: 'x', next: 1, __rc__: 1}, {name: 'x:next', backToX: 0, __rc__: 1}] 
console.log(heap);

// And now we can safely reset the global "xRef" pointer, which automatically
// will garbage collect both "x" object and "x.next" as one of its sub-objects:
assign(xRef, null);

console.log(heap); // [], empty.

// ------------------------------------------------------------------
// 7. Conclusion
// ------------------------------------------------------------------

// ARC GC technique has its own advantages and disadvantages as we saw:
//
// Advantages:
//
// 1. Simple at implementation
// 2. No pauses for GC cycles as e.g. in Mark and Sweep GC.
//
// Disadvantages:
//
// 1. Reference cycles which cannot be handled in obvious way.
//    In the example above we used manual resetting sub-objects
//    to null. Alternatively, a runtime may use both ARC and
//    Mark and Sweep GC (first removes objects immediately, the second
//    cleans up objects which ARC couldn't handle).
//
// 2. Performance issues. The assignment operation besides actual assignment
//    should additionally and every time handle __rc__ value of two objects.