perf(digest): Use linked list for watchers

Performance

This makes it a lot faster to add and remove watchers and it doesn't seem to slow down the digest loop.

dart2js (new: linked watchers)

noop: => 153,147,355,556 ops/sec (0 us) stdev(415.89679)
empty scope $digest(): => 567,981 ops/sec (2 us) stdev(0.01002)
adding/removing 4000 watchers: => 406 ops/sec (2,461 us) stdev(0)
4000 dummy watchers on scope: => 7,680 ops/sec (130 us) stdev(0.0002)
3000 watchers on scope: => 201 ops/sec (4,965 us) stdev(0.00001)

dart2js (old: non-linked watchers)

noop: => 153,222,244,444 ops/sec (0 us) stdev(150.92445)
empty scope $digest(): => 537,580 ops/sec (2 us) stdev(0.01016)
adding/removing 4000 watchers: => 1 ops/sec (1,327,937 us) stdev(0)
4000 dummy watchers on scope: => 4,091 ops/sec (244 us) stdev(0.00015)
3000 watchers on scope: => 177 ops/sec (5,638 us) stdev(0.00001)

lib/core/scope.dart

@@ -45,7 +45,7 @@ class Scope implements Map {
   final NgZone _zone;
   final num _ttl;
   final Map<String, Object> _properties = {};
-  final List<_Watch> _watchers = [];
+  final _WatchList _watchers = new _WatchList();
   final Map<String, List<Function>> _listeners = {};
   final bool _isolate;
   final bool _lazy;
@@ -208,11 +208,7 @@ class Scope implements Map {
     }
     var watcher = new _Watch(_compileToFn(listener), _initWatchVal,
         _compileToFn(watchExpression), watchStr);
-
-    // we use unshift since we use a while loop in $digest for speed.
-    // the while loop reads in reverse order.
-    _watchers.insert(0, watcher);
-
+    _watchers.addLast(watcher);
     return () => _watchers.remove(watcher);
   }
 
@@ -404,12 +400,10 @@ class Scope implements Map {
 
   $digest() {
     var innerAsyncQueue = _innerAsyncQueue;
-    int length;
     _Watch lastDirtyWatch = null;
     _Watch lastLoopLastDirtyWatch;
     int _ttlLeft = _ttl;
     List<List<String>> watchLog = [];
-    List<_Watch> watchers;
     _Watch watch;
     Scope next, current, target = this;
 
@@ -443,30 +437,27 @@ class Scope implements Map {
         digestLoop:
         do { // "traverse the scopes" loop
           scopeCount++;
-          if ((watchers = current._watchers) != null) {
-            // process our watches
-            length = watchers.length;
-            watcherCount += length;
-            while (length-- > 0) {
-              try {
-                watch = watchers[length];
-                if (identical(lastLoopLastDirtyWatch, watch)) {
-                  break digestLoop;
-                }
-                var value = watch.get(current);
-                var last = watch.last;
-                if (!_identical(value, last)) {
-                  lastDirtyWatch = watch;
-                  lastLoopLastDirtyWatch = null;
-                  watch.last = value;
-                  var fireTimer;
-                  assert((fireTimer = _perf.startTimer('ng.fire', watch.exp)) != false);
-                  watch.fn(value, ((last == _initWatchVal) ? value : last), current);
-                  assert(_perf.stopTimer(fireTimer) != false);
-                }
-              } catch (e, s) {
-                _exceptionHandler(e, s);
+          // process our watches
+          _WatchList watchers = current._watchers;
+          watcherCount += watchers.length;
+          for (_Watch watch = watchers.head; watch != null; watch = watch.next) {
+            try {
+              if (identical(lastLoopLastDirtyWatch, watch)) {
+                break digestLoop;
               }
+              var value = watch.get(current);
+              var last = watch.last;
+              if (!_identical(value, last)) {
+                lastDirtyWatch = watch;
+                lastLoopLastDirtyWatch = null;
+                watch.last = value;
+                var fireTimer;
+                assert((fireTimer = _perf.startTimer('ng.fire', watch.exp)) != false);
+                watch.fn(value, ((last == _initWatchVal) ? value : last), current);
+                assert(_perf.stopTimer(fireTimer) != false);
+              }
+            } catch (e, s) {
+              _exceptionHandler(e, s);
             }
           }
 
@@ -708,17 +699,58 @@ class Scope implements Map {
   }
 }
 
-var _initWatchVal = new Object();
+class _InitWatchVal { const _InitWatchVal(); }
+const _initWatchVal = const _InitWatchVal();
 
 class _Watch {
-  Function fn;
-  dynamic last;
-  Function get;
-  String exp;
-
-  _Watch(fn, this.last, getFn, this.exp) {
-    this.fn = relaxFnArgs3(fn);
-    this.get = relaxFnArgs1(getFn);
+  final Function fn;
+  final Function get;
+  final String exp;
+  var last;
+
+  _Watch previous;
+  _Watch next;
+
+  _Watch(fn, this.last, getFn, this.exp)
+      : this.fn  = relaxFnArgs3(fn)
+      , this.get = relaxFnArgs1(getFn);
+}
+
+class _WatchList {
+  int length = 0;
+  _Watch head;
+  _Watch tail;
+
+  void addLast(_Watch watch) {
+    assert(watch.previous == null);
+    assert(watch.next == null);
+    if (tail == null) {
+      tail = head = watch;
+    } else {
+      watch.previous = tail;
+      tail.next = watch;
+      tail = watch;
+    }
+    length++;
+  }
+
+  void remove(_Watch watch) {
+    if (watch == head) {
+      _Watch next = watch.next;
+      if (next == null) tail = null;
+      else next.previous = null;
+      head = next;
+    } else if (watch == tail) {
+      _Watch previous = watch.previous;
+      previous.next = null;
+      tail = previous;
+    } else {
+      _Watch next = watch.next;
+      _Watch previous = watch.previous;
+      previous.next = next;
+      next.previous = previous;
+    }
+    length--;
   }
 }
 

perf/_perf.dart

@@ -51,7 +51,8 @@ measure(b) {
     }
   }
   stopwatch.stop();
-  return new Sample(count, stopwatch.elapsedMicroseconds);
+  int elapsed = max(1, stopwatch.elapsedMicroseconds);
+  return new Sample(count, elapsed);
 }
 
 main() {}

perf/scope_perf.dart

@@ -33,6 +33,16 @@ main() {
 
   scope.a = new A();
 
+  List watchFns = new List.generate(4000, (i) => () => i);
+  time('adding/removing 4000 watchers', () {
+    List watchers = watchFns.map(scope.$watch).toList();
+    watchers.forEach((e) => e());
+  });
+
+  List watchers = watchFns.map(scope.$watch).toList();
+  time('4000 dummy watchers on scope', () => scope.$digest());
+  watchers.forEach((e) => e());
+
   for(var i = 0; i < 1000; i++ ) {
     scope.$watch('a.number', () => null);
     scope.$watch('a.str', () => null);