Make LocalsHandler depend on GraphBuilder instead of SsaBuilder.

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

Index: pkg/compiler/lib/src/ssa/locals_handler.dart
diff --git a/pkg/compiler/lib/src/ssa/locals_handler.dart b/pkg/compiler/lib/src/ssa/locals_handler.dart
new file mode 100644
index 0000000000000000000000000000000000000000..dda5e1827e7e256082643017910d1b34d9629f37
--- /dev/null
+++ b/pkg/compiler/lib/src/ssa/locals_handler.dart
@@ -0,0 +1,687 @@
+// Copyright (c) 2016, 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 '../closure.dart';
+import '../common.dart';
+import '../compiler.dart' show Compiler;
+import '../dart_types.dart';
+import '../elements/elements.dart';
+import '../io/source_information.dart';
+import '../js/js.dart' as js;
+import '../js_backend/js_backend.dart';
+import '../native/native.dart' as native;
+import '../tree/tree.dart' as ast;
+import '../types/types.dart';
+import '../world.dart' show ClassWorld;
+import 'builder.dart' show SyntheticLocal;
+import 'graph_builder.dart';
+import 'nodes.dart';
+import 'types.dart';
+
+/**

[Siggi Cherem (dart-lang), 2016/08/25 19:12:53]

feel free to convert to /// comment style if you so desire... we can also do a
pass later to convert everything at once, I believe John had a script that did
that.

[Harry Terkelsen, 2016/08/25 20:12:57]

Done.

+ * Keeps track of locals (including parameters and phis) when building. The
+ * 'this' reference is treated as parameter and hence handled by this class,
+ * too.
+ */
+class LocalsHandler {
+  /**
+   * The values of locals that can be directly accessed (without redirections
+   * to boxes or closure-fields).
+   *
+   * [directLocals] is iterated, so it is "insertion ordered" to make the
+   * iteration order a function only of insertions and not a function of
+   * e.g. Element hash codes.  I'd prefer to use a SortedMap but some elements
+   * don't have source locations for [Elements.compareByPosition].
+   */
+  Map<Local, HInstruction> directLocals = new Map<Local, HInstruction>();
+  Map<Local, CapturedVariable> redirectionMapping =
+      new Map<Local, CapturedVariable>();
+  final GraphBuilder builder;
+  ClosureClassMap closureData;
+  Map<TypeVariableType, TypeVariableLocal> typeVariableLocals =
+      new Map<TypeVariableType, TypeVariableLocal>();
+  final ExecutableElement executableContext;
+
+  /// The class that defines the current type environment or null if no type
+  /// variables are in scope.
+  ClassElement get contextClass => executableContext.contextClass;
+
+  /// The type of the current instance, if concrete.
+  ///
+  /// This allows for handling fixed type argument in case of inlining. For
+  /// instance, checking `'foo'` against `String` instead of `T` in `main`:
+  ///
+  ///     class Foo<T> {
+  ///       T field;
+  ///       Foo(this.field);
+  ///     }
+  ///     main() {
+  ///       new Foo<String>('foo');
+  ///     }
+  ///
+  /// [instanceType] is not used if it contains type variables, since these
+  /// might not be in scope or from the current instance.
+  ///
+  final InterfaceType instanceType;
+
+  final Compiler compiler;
+
+  final SourceInformationBuilder sourceInformationBuilder;
+
+  LocalsHandler(this.builder, this.executableContext,
+      InterfaceType instanceType, this.compiler, this.sourceInformationBuilder)
+      : this.instanceType =
+            instanceType == null || instanceType.containsTypeVariables
+                ? null
+                : instanceType;
+
+  /// Substituted type variables occurring in [type] into the context of
+  /// [contextClass].
+  DartType substInContext(DartType type) {
+    if (contextClass != null) {
+      ClassElement typeContext = Types.getClassContext(type);
+      if (typeContext != null) {
+        type = type.substByContext(contextClass.asInstanceOf(typeContext));
+      }
+    }
+    if (instanceType != null) {
+      type = type.substByContext(instanceType);
+    }
+    return type;
+  }
+
+  get typesTask => compiler.typesTask;
+
+  /**
+   * Creates a new [LocalsHandler] based on [other]. We only need to
+   * copy the [directLocals], since the other fields can be shared
+   * throughout the AST visit.
+   */
+  LocalsHandler.from(LocalsHandler other)
+      : directLocals = new Map<Local, HInstruction>.from(other.directLocals),
+        redirectionMapping = other.redirectionMapping,
+        executableContext = other.executableContext,
+        instanceType = other.instanceType,
+        builder = other.builder,
+        closureData = other.closureData,
+        compiler = other.compiler,
+        sourceInformationBuilder = other.sourceInformationBuilder;
+
+  /**
+   * Redirects accesses from element [from] to element [to]. The [to] element
+   * must be a boxed variable or a variable that is stored in a closure-field.
+   */
+  void redirectElement(Local from, CapturedVariable to) {
+    assert(redirectionMapping[from] == null);
+    redirectionMapping[from] = to;
+    assert(isStoredInClosureField(from) || isBoxed(from));
+  }
+
+  HInstruction createBox() {
+    // TODO(floitsch): Clean up this hack. Should we create a box-object by
+    // just creating an empty object literal?
+    JavaScriptBackend backend = compiler.backend;
+    HInstruction box = new HForeignCode(
+        js.js.parseForeignJS('{}'), backend.nonNullType, <HInstruction>[],
+        nativeBehavior: native.NativeBehavior.PURE_ALLOCATION);
+    builder.add(box);
+    return box;
+  }
+
+  /**
+   * If the scope (function or loop) [node] has captured variables then this
+   * method creates a box and sets up the redirections.
+   */
+  void enterScope(ast.Node node, Element element) {

[Siggi Cherem (dart-lang), 2016/08/25 19:12:53]

one idea (feel free to defer for later): we could parameterize this class as
LocalsHandler<T> (probably closureData too), and replace T for ast.Node or
ir.Node depending on where we use it.

Or create a map [Node -> ScopeId] that is separate from these classes, and only
use ScopeIds here...

[Harry Terkelsen, 2016/08/25 20:12:57]

Acknowledged.

+    // See if any variable in the top-scope of the function is captured. If yes
+    // we need to create a box-object.
+    ClosureScope scopeData = closureData.capturingScopes[node];
+    if (scopeData == null) return;
+    HInstruction box;
+    // The scope has captured variables.
+    if (element != null && element.isGenerativeConstructorBody) {
+      // The box is passed as a parameter to a generative
+      // constructor body.
+      JavaScriptBackend backend = compiler.backend;
+      box = builder.addParameter(scopeData.boxElement, backend.nonNullType);
+    } else {
+      box = createBox();
+    }
+    // Add the box to the known locals.
+    directLocals[scopeData.boxElement] = box;
+    // Make sure that accesses to the boxed locals go into the box. We also
+    // need to make sure that parameters are copied into the box if necessary.
+    scopeData.forEachCapturedVariable(
+        (LocalVariableElement from, BoxFieldElement to) {
+      // The [from] can only be a parameter for function-scopes and not
+      // loop scopes.
+      if (from.isRegularParameter && !element.isGenerativeConstructorBody) {
+        // Now that the redirection is set up, the update to the local will
+        // write the parameter value into the box.
+        // Store the captured parameter in the box. Get the current value
+        // before we put the redirection in place.
+        // We don't need to update the local for a generative
+        // constructor body, because it receives a box that already
+        // contains the updates as the last parameter.
+        HInstruction instruction = readLocal(from);
+        redirectElement(from, to);
+        updateLocal(from, instruction);
+      } else {
+        redirectElement(from, to);
+      }
+    });
+  }
+
+  /**
+   * Replaces the current box with a new box and copies over the given list
+   * of elements from the old box into the new box.
+   */
+  void updateCaptureBox(
+      BoxLocal boxElement, List<LocalVariableElement> toBeCopiedElements) {
+    // Create a new box and copy over the values from the old box into the
+    // new one.
+    HInstruction oldBox = readLocal(boxElement);
+    HInstruction newBox = createBox();
+    for (LocalVariableElement boxedVariable in toBeCopiedElements) {
+      // [readLocal] uses the [boxElement] to find its box. By replacing it
+      // behind its back we can still get to the old values.
+      updateLocal(boxElement, oldBox);
+      HInstruction oldValue = readLocal(boxedVariable);
+      updateLocal(boxElement, newBox);
+      updateLocal(boxedVariable, oldValue);
+    }
+    updateLocal(boxElement, newBox);
+  }
+
+  /**
+   * Documentation wanted -- johnniwinther
+   *
+   * Invariant: [function] must be an implementation element.
+   */
+  void startFunction(AstElement element, ast.Node node) {
+    assert(invariant(element, element.isImplementation));
+    closureData = compiler.closureToClassMapper
+        .computeClosureToClassMapping(element.resolvedAst);
+
+    if (element is FunctionElement) {
+      FunctionElement functionElement = element;
+      FunctionSignature params = functionElement.functionSignature;
+      ClosureScope scopeData = closureData.capturingScopes[node];
+      params.orderedForEachParameter((ParameterElement parameterElement) {
+        if (element.isGenerativeConstructorBody) {
+          if (scopeData != null &&
+              scopeData.isCapturedVariable(parameterElement)) {
+            // The parameter will be a field in the box passed as the
+            // last parameter. So no need to have it.
+            return;
+          }
+        }
+        HInstruction parameter = builder.addParameter(parameterElement,
+            TypeMaskFactory.inferredTypeForElement(parameterElement, compiler));
+        builder.parameters[parameterElement] = parameter;
+        directLocals[parameterElement] = parameter;
+      });
+    }
+
+    enterScope(node, element);
+
+    // If the freeVariableMapping is not empty, then this function was a
+    // nested closure that captures variables. Redirect the captured
+    // variables to fields in the closure.
+    closureData.forEachFreeVariable((Local from, CapturedVariable to) {
+      redirectElement(from, to);
+    });
+    JavaScriptBackend backend = compiler.backend;
+    if (closureData.isClosure) {
+      // Inside closure redirect references to itself to [:this:].
+      HThis thisInstruction =
+          new HThis(closureData.thisLocal, backend.nonNullType);
+      builder.graph.thisInstruction = thisInstruction;
+      builder.graph.entry.addAtEntry(thisInstruction);
+      updateLocal(closureData.closureElement, thisInstruction);
+    } else if (element.isInstanceMember) {
+      // Once closures have been mapped to classes their instance members might
+      // not have any thisElement if the closure was created inside a static
+      // context.
+      HThis thisInstruction = new HThis(closureData.thisLocal, getTypeOfThis());
+      builder.graph.thisInstruction = thisInstruction;
+      builder.graph.entry.addAtEntry(thisInstruction);
+      directLocals[closureData.thisLocal] = thisInstruction;
+    }
+
+    // If this method is an intercepted method, add the extra
+    // parameter to it, that is the actual receiver for intercepted
+    // classes, or the same as [:this:] for non-intercepted classes.
+    ClassElement cls = element.enclosingClass;
+
+    // When the class extends a native class, the instance is pre-constructed
+    // and passed to the generative constructor factory function as a parameter.
+    // Instead of allocating and initializing the object, the constructor
+    // 'upgrades' the native subclass object by initializing the Dart fields.
+    bool isNativeUpgradeFactory =
+        element.isGenerativeConstructor && backend.isNativeOrExtendsNative(cls);
+    if (backend.isInterceptedMethod(element)) {
+      bool isInterceptorClass = backend.isInterceptorClass(cls.declaration);
+      String name = isInterceptorClass ? 'receiver' : '_';
+      SyntheticLocal parameter = new SyntheticLocal(name, executableContext);
+      HParameterValue value = new HParameterValue(parameter, getTypeOfThis());
+      builder.graph.explicitReceiverParameter = value;
+      builder.graph.entry.addAfter(directLocals[closureData.thisLocal], value);
+      if (builder.lastAddedParameter == null) {
+        // If this is the first parameter inserted, make sure it stays first.
+        builder.lastAddedParameter = value;
+      }
+      if (isInterceptorClass) {
+        // Only use the extra parameter in intercepted classes.
+        directLocals[closureData.thisLocal] = value;
+      }
+    } else if (isNativeUpgradeFactory) {
+      SyntheticLocal parameter =
+          new SyntheticLocal('receiver', executableContext);
+      // Unlike `this`, receiver is nullable since direct calls to generative
+      // constructor call the constructor with `null`.
+      ClassWorld classWorld = compiler.world;
+      HParameterValue value =
+          new HParameterValue(parameter, new TypeMask.exact(cls, classWorld));
+      builder.graph.explicitReceiverParameter = value;
+      builder.graph.entry.addAtEntry(value);
+    }
+  }
+
+  /**
+   * Returns true if the local can be accessed directly. Boxed variables or
+   * captured variables that are stored in the closure-field return [:false:].
+   */
+  bool isAccessedDirectly(Local local) {
+    assert(local != null);
+    return !redirectionMapping.containsKey(local) &&
+        !closureData.variablesUsedInTryOrGenerator.contains(local);
+  }
+
+  bool isStoredInClosureField(Local local) {
+    assert(local != null);
+    if (isAccessedDirectly(local)) return false;
+    CapturedVariable redirectTarget = redirectionMapping[local];
+    if (redirectTarget == null) return false;
+    return redirectTarget is ClosureFieldElement;
+  }
+
+  bool isBoxed(Local local) {
+    if (isAccessedDirectly(local)) return false;
+    if (isStoredInClosureField(local)) return false;
+    return redirectionMapping.containsKey(local);
+  }
+
+  bool isUsedInTryOrGenerator(Local local) {
+    return closureData.variablesUsedInTryOrGenerator.contains(local);
+  }
+
+  /**
+   * Returns an [HInstruction] for the given element. If the element is
+   * boxed or stored in a closure then the method generates code to retrieve
+   * the value.
+   */
+  HInstruction readLocal(Local local, {SourceInformation sourceInformation}) {
+    if (isAccessedDirectly(local)) {
+      if (directLocals[local] == null) {
+        if (local is TypeVariableElement) {
+          compiler.reporter.internalError(compiler.currentElement,
+              "Runtime type information not available for $local.");
+        } else {
+          compiler.reporter.internalError(
+              local, "Cannot find value $local in ${directLocals.keys}.");
+        }
+      }
+      HInstruction value = directLocals[local];
+      if (sourceInformation != null) {
+        value = new HRef(value, sourceInformation);
+        builder.add(value);
+      }
+      return value;
+    } else if (isStoredInClosureField(local)) {
+      ClosureFieldElement redirect = redirectionMapping[local];
+      HInstruction receiver = readLocal(closureData.closureElement);
+      TypeMask type = local is BoxLocal
+          ? (compiler.backend as JavaScriptBackend).nonNullType
+          : getTypeOfCapturedVariable(redirect);
+      HInstruction fieldGet = new HFieldGet(redirect, receiver, type);
+      builder.add(fieldGet);
+      return fieldGet..sourceInformation = sourceInformation;
+    } else if (isBoxed(local)) {
+      BoxFieldElement redirect = redirectionMapping[local];
+      // In the function that declares the captured variable the box is
+      // accessed as direct local. Inside the nested closure the box is
+      // accessed through a closure-field.
+      // Calling [readLocal] makes sure we generate the correct code to get
+      // the box.
+      HInstruction box = readLocal(redirect.box);
+      HInstruction lookup =
+          new HFieldGet(redirect, box, getTypeOfCapturedVariable(redirect));
+      builder.add(lookup);
+      return lookup..sourceInformation = sourceInformation;
+    } else {
+      assert(isUsedInTryOrGenerator(local));
+      HLocalValue localValue = getLocal(local);
+      HInstruction instruction = new HLocalGet(
+          local,
+          localValue,
+          (compiler.backend as JavaScriptBackend).dynamicType,
+          sourceInformation);
+      builder.add(instruction);
+      return instruction;
+    }
+  }
+
+  HInstruction readThis() {
+    HInstruction res = readLocal(closureData.thisLocal);
+    if (res.instructionType == null) {
+      res.instructionType = getTypeOfThis();
+    }
+    return res;
+  }
+
+  HLocalValue getLocal(Local local, {SourceInformation sourceInformation}) {
+    // If the element is a parameter, we already have a
+    // HParameterValue for it. We cannot create another one because
+    // it could then have another name than the real parameter. And
+    // the other one would not know it is just a copy of the real
+    // parameter.
+    if (local is ParameterElement) {
+      assert(invariant(local, builder.parameters.containsKey(local),
+          message: "No local value for parameter $local in "
+              "${builder.parameters}."));
+      return builder.parameters[local];
+    }
+
+    return activationVariables.putIfAbsent(local, () {
+      JavaScriptBackend backend = compiler.backend;
+      HLocalValue localValue = new HLocalValue(local, backend.nonNullType)
+        ..sourceInformation = sourceInformation;
+      builder.graph.entry.addAtExit(localValue);
+      return localValue;
+    });
+  }
+
+  Local getTypeVariableAsLocal(TypeVariableType type) {
+    return typeVariableLocals.putIfAbsent(type, () {
+      return new TypeVariableLocal(type, executableContext);
+    });
+  }
+
+  /**
+   * Sets the [element] to [value]. If the element is boxed or stored in a
+   * closure then the method generates code to set the value.
+   */
+  void updateLocal(Local local, HInstruction value,
+      {SourceInformation sourceInformation}) {
+    if (value is HRef) {
+      HRef ref = value;
+      value = ref.value;
+    }
+    assert(!isStoredInClosureField(local));
+    if (isAccessedDirectly(local)) {
+      directLocals[local] = value;
+    } else if (isBoxed(local)) {
+      BoxFieldElement redirect = redirectionMapping[local];
+      // The box itself could be captured, or be local. A local variable that
+      // is captured will be boxed, but the box itself will be a local.
+      // Inside the closure the box is stored in a closure-field and cannot
+      // be accessed directly.
+      HInstruction box = readLocal(redirect.box);
+      builder.add(new HFieldSet(redirect, box, value)
+        ..sourceInformation = sourceInformation);
+    } else {
+      assert(isUsedInTryOrGenerator(local));
+      HLocalValue localValue = getLocal(local);
+      builder.add(new HLocalSet(local, localValue, value)
+        ..sourceInformation = sourceInformation);
+    }
+  }
+
+  /**
+   * This function, startLoop, must be called before visiting any children of
+   * the loop. In particular it needs to be called before executing the
+   * initializers.
+   *
+   * The [LocalsHandler] will make the boxes and updates at the right moment.
+   * The builder just needs to call [enterLoopBody] and [enterLoopUpdates]
+   * (for [ast.For] loops) at the correct places. For phi-handling
+   * [beginLoopHeader] and [endLoop] must also be called.
+   *
+   * The correct place for the box depends on the given loop. In most cases
+   * the box will be created when entering the loop-body: while, do-while, and
+   * for-in (assuming the call to [:next:] is inside the body) can always be
+   * constructed this way.
+   *
+   * Things are slightly more complicated for [ast.For] loops. If no declared
+   * loop variable is boxed then the loop-body approach works here too. If a
+   * loop-variable is boxed we need to introduce a new box for the
+   * loop-variable before we enter the initializer so that the initializer
+   * writes the values into the box. In any case we need to create the box
+   * before the condition since the condition could box the variable.
+   * Since the first box is created outside the actual loop we have a second
+   * location where a box is created: just before the updates. This is
+   * necessary since updates are considered to be part of the next iteration
+   * (and can again capture variables).
+   *
+   * For example the following Dart code prints 1 3 -- 3 4.
+   *
+   *     var fs = [];
+   *     for (var i = 0; i < 3; (f() { fs.add(f); print(i); i++; })()) {
+   *       i++;
+   *     }
+   *     print("--");
+   *     for (var i = 0; i < 2; i++) fs[i]();
+   *
+   * We solve this by emitting the following code (only for [ast.For] loops):
+   *  <Create box>    <== move the first box creation outside the loop.
+   *  <initializer>;
+   *  loop-entry:
+   *    if (!<condition>) goto loop-exit;
+   *    <body>
+   *    <update box>  // create a new box and copy the captured loop-variables.
+   *    <updates>
+   *    goto loop-entry;
+   *  loop-exit:
+   */
+  void startLoop(ast.Node node) {
+    ClosureScope scopeData = closureData.capturingScopes[node];
+    if (scopeData == null) return;
+    if (scopeData.hasBoxedLoopVariables()) {
+      // If there are boxed loop variables then we set up the box and
+      // redirections already now. This way the initializer can write its
+      // values into the box.
+      // For other loops the box will be created when entering the body.
+      enterScope(node, null);
+    }
+  }
+
+  /**
+   * Create phis at the loop entry for local variables (ready for the values
+   * from the back edge).  Populate the phis with the current values.
+   */
+  void beginLoopHeader(HBasicBlock loopEntry) {
+    // Create a copy because we modify the map while iterating over it.
+    Map<Local, HInstruction> savedDirectLocals =
+        new Map<Local, HInstruction>.from(directLocals);
+
+    JavaScriptBackend backend = compiler.backend;
+    // Create phis for all elements in the definitions environment.
+    savedDirectLocals.forEach((Local local, HInstruction instruction) {
+      if (isAccessedDirectly(local)) {
+        // We know 'this' cannot be modified.
+        if (local != closureData.thisLocal) {
+          HPhi phi =
+              new HPhi.singleInput(local, instruction, backend.dynamicType);
+          loopEntry.addPhi(phi);
+          directLocals[local] = phi;
+        } else {
+          directLocals[local] = instruction;
+        }
+      }
+    });
+  }
+
+  void enterLoopBody(ast.Node node) {
+    ClosureScope scopeData = closureData.capturingScopes[node];
+    if (scopeData == null) return;
+    // If there are no declared boxed loop variables then we did not create the
+    // box before the initializer and we have to create the box now.
+    if (!scopeData.hasBoxedLoopVariables()) {
+      enterScope(node, null);
+    }
+  }
+
+  void enterLoopUpdates(ast.Node node) {
+    // If there are declared boxed loop variables then the updates might have
+    // access to the box and we must switch to a new box before executing the
+    // updates.
+    // In all other cases a new box will be created when entering the body of
+    // the next iteration.
+    ClosureScope scopeData = closureData.capturingScopes[node];
+    if (scopeData == null) return;
+    if (scopeData.hasBoxedLoopVariables()) {
+      updateCaptureBox(scopeData.boxElement, scopeData.boxedLoopVariables);
+    }
+  }
+
+  /**
+   * Goes through the phis created in beginLoopHeader entry and adds the
+   * input from the back edge (from the current value of directLocals) to them.
+   */
+  void endLoop(HBasicBlock loopEntry) {
+    // If the loop has an aborting body, we don't update the loop
+    // phis.
+    if (loopEntry.predecessors.length == 1) return;
+    loopEntry.forEachPhi((HPhi phi) {
+      Local element = phi.sourceElement;
+      HInstruction postLoopDefinition = directLocals[element];
+      phi.addInput(postLoopDefinition);
+    });
+  }
+
+  /**
+   * Merge [otherLocals] into this locals handler, creating phi-nodes when
+   * there is a conflict.
+   * If a phi node is necessary, it will use this handler's instruction as the
+   * first input, and the otherLocals instruction as the second.
+   */
+  void mergeWith(LocalsHandler otherLocals, HBasicBlock joinBlock) {
+    // If an element is in one map but not the other we can safely
+    // ignore it. It means that a variable was declared in the
+    // block. Since variable declarations are scoped the declared
+    // variable cannot be alive outside the block. Note: this is only
+    // true for nodes where we do joins.
+    Map<Local, HInstruction> joinedLocals = new Map<Local, HInstruction>();
+    JavaScriptBackend backend = compiler.backend;
+    otherLocals.directLocals.forEach((Local local, HInstruction instruction) {
+      // We know 'this' cannot be modified.
+      if (local == closureData.thisLocal) {
+        assert(directLocals[local] == instruction);
+        joinedLocals[local] = instruction;
+      } else {
+        HInstruction mine = directLocals[local];
+        if (mine == null) return;
+        if (identical(instruction, mine)) {
+          joinedLocals[local] = instruction;
+        } else {
+          HInstruction phi = new HPhi.manyInputs(
+              local, <HInstruction>[mine, instruction], backend.dynamicType);
+          joinBlock.addPhi(phi);
+          joinedLocals[local] = phi;
+        }
+      }
+    });
+    directLocals = joinedLocals;
+  }
+
+  /**
+   * When control flow merges, this method can be used to merge several
+   * localsHandlers into a new one using phis.  The new localsHandler is
+   * returned.  Unless it is also in the list, the current localsHandler is not
+   * used for its values, only for its declared variables. This is a way to
+   * exclude local values from the result when they are no longer in scope.
+   */
+  LocalsHandler mergeMultiple(
+      List<LocalsHandler> localsHandlers, HBasicBlock joinBlock) {
+    assert(localsHandlers.length > 0);
+    if (localsHandlers.length == 1) return localsHandlers[0];
+    Map<Local, HInstruction> joinedLocals = new Map<Local, HInstruction>();
+    HInstruction thisValue = null;
+    JavaScriptBackend backend = compiler.backend;
+    directLocals.forEach((Local local, HInstruction instruction) {
+      if (local != closureData.thisLocal) {
+        HPhi phi = new HPhi.noInputs(local, backend.dynamicType);
+        joinedLocals[local] = phi;
+        joinBlock.addPhi(phi);
+      } else {
+        // We know that "this" never changes, if it's there.
+        // Save it for later. While merging, there is no phi for "this",
+        // so we don't have to special case it in the merge loop.
+        thisValue = instruction;
+      }
+    });
+    for (LocalsHandler handler in localsHandlers) {
+      handler.directLocals.forEach((Local local, HInstruction instruction) {
+        HPhi phi = joinedLocals[local];
+        if (phi != null) {
+          phi.addInput(instruction);
+        }
+      });
+    }
+    if (thisValue != null) {
+      // If there was a "this" for the scope, add it to the new locals.
+      joinedLocals[closureData.thisLocal] = thisValue;
+    }
+
+    // Remove locals that are not in all handlers.
+    directLocals = new Map<Local, HInstruction>();
+    joinedLocals.forEach((Local local, HInstruction instruction) {
+      if (local != closureData.thisLocal &&
+          instruction.inputs.length != localsHandlers.length) {
+        joinBlock.removePhi(instruction);
+      } else {
+        directLocals[local] = instruction;
+      }
+    });
+    return this;
+  }
+
+  TypeMask cachedTypeOfThis;
+
+  TypeMask getTypeOfThis() {
+    TypeMask result = cachedTypeOfThis;
+    if (result == null) {
+      ThisLocal local = closureData.thisLocal;
+      ClassElement cls = local.enclosingClass;
+      ClassWorld classWorld = compiler.world;
+      if (classWorld.isUsedAsMixin(cls)) {
+        // If the enclosing class is used as a mixin, [:this:] can be
+        // of the class that mixins the enclosing class. These two
+        // classes do not have a subclass relationship, so, for
+        // simplicity, we mark the type as an interface type.
+        result = new TypeMask.nonNullSubtype(cls.declaration, compiler.world);
+      } else {
+        result = new TypeMask.nonNullSubclass(cls.declaration, compiler.world);
+      }
+      cachedTypeOfThis = result;
+    }
+    return result;
+  }
+
+  Map<Element, TypeMask> cachedTypesOfCapturedVariables =
+      new Map<Element, TypeMask>();
+
+  TypeMask getTypeOfCapturedVariable(Element element) {
+    assert(element.isField);
+    return cachedTypesOfCapturedVariables.putIfAbsent(element, () {
+      return TypeMaskFactory.inferredTypeForElement(element, compiler);
+    });
+  }
+
+  /// Variables stored in the current activation. These variables are
+  /// being updated in try/catch blocks, and should be
+  /// accessed indirectly through [HLocalGet] and [HLocalSet].
+  Map<Local, HLocalValue> activationVariables = <Local, HLocalValue>{};
+}