Use indexing for type variable access when the receiver type constrains the substitution(s)

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.
 
 import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
@@ skipping 2840 matching lines @@
   }
 
   void visitTypeInfoReadRaw(HTypeInfoReadRaw node) {
     use(node.inputs[0]);
     js.Expression receiver = pop();
     push(js.js(r'#.#', [receiver, backend.namer.rtiFieldName]));
   }
 
   void visitTypeInfoReadVariable(HTypeInfoReadVariable node) {
     TypeVariableElement element = node.variable.element;
-    ClassElement context = element.enclosingClass;
 
     int index = element.index;
-    use(node.inputs[0]);
+    HInstruction object = node.object;
+    use(object);
     js.Expression receiver = pop();
 
-    if (needsSubstitutionForTypeVariableAccess(context)) {
+    if (typeVariableAccessNeedsSubstitution(element, object.instructionType)) {
       js.Expression typeName =
-          js.quoteName(backend.namer.runtimeTypeName(context));
+          js.quoteName(backend.namer.runtimeTypeName(element.enclosingClass));
       Element helperElement = helpers.getRuntimeTypeArgument;
       registry.registerStaticUse(
           new StaticUse.staticInvoke(helperElement, CallStructure.THREE_ARGS));
       js.Expression helper =
           backend.emitter.staticFunctionAccess(helperElement);
       push(js.js(
           r'#(#, #, #)', [helper, receiver, typeName, js.js.number(index)]));
     } else {
       Element helperElement = helpers.getTypeArgumentByIndex;
       registry.registerStaticUse(
@@ skipping 23 matching lines @@
       case TypeInfoExpressionKind.INSTANCE:
         // We expect only flat types for the INSTANCE representation.
         assert(
             node.dartType == (node.dartType.element as ClassElement).thisType);
         registry.registerInstantiatedClass(coreClasses.listClass);
         push(new js.ArrayInitializer(arguments)
             .withSourceInformation(node.sourceInformation));
     }
   }
 
-  bool needsSubstitutionForTypeVariableAccess(ClassElement cls) {
+  bool typeVariableAccessNeedsSubstitution(
+      TypeVariableElement element, TypeMask receiverMask) {
+    ClassElement cls = element.enclosingClass;
     ClassWorld classWorld = compiler.closedWorld;
+
+    // See if the receiver type narrows the set of classes to ones that can be
+    // indexed.
+    // TODO(sra): Currently the only convenient query is [singleClass]. We
+    // should iterate over all the concrete classes in [receiverMask].
+    ClassElement receiverClass = receiverMask.singleClass(classWorld);
+    if (receiverClass != null) {
+      if (backend.rti.isTrivialSubstitution(receiverClass, cls)) {
+        return false;
+      }
+    }
+
     if (classWorld.isUsedAsMixin(cls)) return true;
 
-    return compiler.closedWorld.anyStrictSubclassOf(cls,
-        (ClassElement subclass) {
+    return classWorld.anyStrictSubclassOf(cls, (ClassElement subclass) {
       return !backend.rti.isTrivialSubstitution(subclass, cls);
     });
   }
 
   void visitReadTypeVariable(HReadTypeVariable node) {
     TypeVariableElement element = node.dartType.element;
     Element helperElement = helpers.convertRtiToRuntimeType;
     registry.registerStaticUse(
         new StaticUse.staticInvoke(helperElement, CallStructure.ONE_ARG));
 
@@ skipping 57 matching lines @@
     registry.registerStaticUse(new StaticUse.staticInvoke(
         helper, new CallStructure.unnamed(argumentCount)));
     return backend.emitter.staticFunctionAccess(helper);
   }
 
   @override
   void visitRef(HRef node) {
     visit(node.value);
   }
 }