Rewrite for-in loop as indexing loop for JavaScript indexable arrays

pkg/compiler/lib/src/ssa/codegen.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of ssa;
 
 class SsaCodeGeneratorTask extends CompilerTask {
 
   final JavaScriptBackend backend;
 
@@ skipping 1606 matching lines @@
                          backend.namer.invocationName(call),
                          visitArguments(node.inputs)),
          node);
     registry.registerDynamicInvocation(call);
   }
 
   visitInvokeStatic(HInvokeStatic node) {
     Element element = node.element;
     List<DartType> instantiatedTypes = node.instantiatedTypes;
 
-    registry.registerStaticInvocation(element);
-
     if (instantiatedTypes != null && !instantiatedTypes.isEmpty) {
       instantiatedTypes.forEach((type) {
         registry.registerInstantiatedType(type);
       });
     }
 
-    push(backend.emitter.staticFunctionAccess(node.element));
-    push(new js.Call(pop(), visitArguments(node.inputs, start: 0)), node);
+    List<js.Expression> arguments = visitArguments(node.inputs, start: 0);
+
+    if (element == backend.getCheckConcurrentModificationError()) {
+      // Manually inline the [checkConcurrentModificationError] function.  This
+      // function is only called from a for-loop update.  Ideally we would just
+      // generate the conditionalcontrol flow in the builder but it adds basic
+      // blocks in the loop update that interfere with other optimizations and
+      // confuses loop recognition.
+
+      assert(arguments.length == 2);
+      Element throwFunction = backend.getThrowConcurrentModificationError();
+      registry.registerStaticInvocation(throwFunction);
+
+      // Calling using `(0, #)(#)` instead of `#(#)` separates the property load
+      // of the static function access from the call.  For some reason this
+      // helps V8 see that the call never happens so V8 makes the call a
+      // deoptimization. This removes the call from the optimized loop, making
+      // more optimizations available to the loop.  This form is 50% faster on
+      // some small loop, almost as fast as loops with no concurrent
+      // modification check.
+      push(js.js('# || (0, #)(#)',[
+          arguments[0],
+          backend.emitter.staticFunctionAccess(throwFunction),
+          arguments[1]]));
+    } else {
+      registry.registerStaticInvocation(element);
+      push(backend.emitter.staticFunctionAccess(element));
+      push(new js.Call(pop(), arguments), node);
+    }
+
   }
 
   visitInvokeSuper(HInvokeSuper node) {
     Element superMethod = node.element;
     registry.registerSuperInvocation(superMethod);
     ClassElement superClass = superMethod.enclosingClass;
     if (superMethod.kind == ElementKind.FIELD) {
       String fieldName = backend.namer.instanceFieldPropertyName(superMethod);
       use(node.inputs[0]);
       js.PropertyAccess access =
@@ skipping 1031 matching lines @@
   js.PropertyAccess accessHelper(String name) {
     Element helper = backend.findHelper(name);
     if (helper == null) {
       // For mocked-up tests.
       return js.js('(void 0).$name');
     }
     registry.registerStaticUse(helper);
     return backend.emitter.staticFunctionAccess(helper);
   }
 }