// 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;

abstract class OptimizationPhase {
  String get name;
  void visitGraph(HGraph graph);
}

class SsaOptimizerTask extends CompilerTask {
  final JavaScriptBackend backend;
  SsaOptimizerTask(JavaScriptBackend backend)
    : this.backend = backend,
      super(backend.compiler);
  String get name => 'SSA optimizer';
  Compiler get compiler => backend.compiler;

  void runPhases(HGraph graph, List<OptimizationPhase> phases) {
    for (OptimizationPhase phase in phases) {
      runPhase(graph, phase);
    }
  }

  void runPhase(HGraph graph, OptimizationPhase phase) {
    phase.visitGraph(graph);
    compiler.tracer.traceGraph(phase.name, graph);
    assert(graph.isValid());
  }

  void optimize(CodegenWorkItem work, HGraph graph, bool speculative) {
    ConstantSystem constantSystem = compiler.backend.constantSystem;
    JavaScriptItemCompilationContext context = work.compilationContext;
    measure(() {
      List<OptimizationPhase> phases = <OptimizationPhase>[
          // Run trivial constant folding first to optimize
          // some patterns useful for type conversion.
          new SsaConstantFolder(constantSystem, backend, work),
          new SsaTypeConversionInserter(compiler),
          new SsaNonSpeculativeTypePropagator(compiler),
          new SsaConstantFolder(constantSystem, backend, work),
          // The constant folder affects the types of instructions, so
          // we run the type propagator again. Note that this would
          // not be necessary if types were directly stored on
          // instructions.
          new SsaNonSpeculativeTypePropagator(compiler),
          new SsaCheckInserter(backend, work, context.boundsChecked),
          new SsaRedundantPhiEliminator(),
          new SsaDeadPhiEliminator(),
          new SsaConstantFolder(constantSystem, backend, work),
          new SsaNonSpeculativeTypePropagator(compiler),
          new SsaReceiverSpecialization(compiler),
          new SsaGlobalValueNumberer(compiler),
          new SsaCodeMotion(),
          new SsaValueRangeAnalyzer(constantSystem, work),
          // Previous optimizations may have generated new
          // opportunities for constant folding.
          new SsaConstantFolder(constantSystem, backend, work),
          new SsaSimplifyInterceptors(constantSystem),
          new SsaDeadCodeEliminator()];
      runPhases(graph, phases);
      if (!speculative) {
        runPhase(graph, new SsaConstructionFieldTypes(backend, work));
      }
    });
  }

  bool trySpeculativeOptimizations(CodegenWorkItem work, HGraph graph) {
    if (work.element.isField()) {
      // Lazy initializers may not have bailout methods.
      return false;
    }
    JavaScriptItemCompilationContext context = work.compilationContext;
    return measure(() {
      // Run the phases that will generate type guards.
      List<OptimizationPhase> phases = <OptimizationPhase>[
          new SsaTypeGuardInserter(compiler, work),
          new SsaEnvironmentBuilder(compiler),
          // Then run the [SsaCheckInserter] because the type propagator also
          // propagated types non-speculatively. For example, it might have
          // propagated the type array for a call to the List constructor.
          new SsaCheckInserter(backend, work, context.boundsChecked)];
      runPhases(graph, phases);
      return !work.guards.isEmpty;
    });
  }

  void prepareForSpeculativeOptimizations(CodegenWorkItem work, HGraph graph) {
    JavaScriptItemCompilationContext context = work.compilationContext;
    measure(() {
      // In order to generate correct code for the bailout version, we did not
      // propagate types from the instruction to the type guard. We do it
      // now to be able to optimize further.
      work.guards.forEach((HTypeGuard guard) {
        guard.bailoutTarget.disable();
        guard.enable();
      });
      // We also need to insert range and integer checks for the type
      // guards. Now that they claim to have a certain type, some
      // depending instructions might become builtin (like native array
      // accesses) and need to be checked.
      // Also run the type propagator, to please the codegen in case
      // no other optimization is run.
      runPhases(graph, <OptimizationPhase>[
          new SsaCheckInserter(backend, work, context.boundsChecked),
          new SsaNonSpeculativeTypePropagator(compiler)]);
    });
  }
}

/**
 * If both inputs to known operations are available execute the operation at
 * compile-time.
 */
class SsaConstantFolder extends HBaseVisitor implements OptimizationPhase {
  final String name = "SsaConstantFolder";
  final JavaScriptBackend backend;
  final CodegenWorkItem work;
  final ConstantSystem constantSystem;
  HGraph graph;
  Compiler get compiler => backend.compiler;

  SsaConstantFolder(this.constantSystem, this.backend, this.work);

  void visitGraph(HGraph visitee) {
    graph = visitee;
    visitDominatorTree(visitee);
  }

  visitBasicBlock(HBasicBlock block) {
    HInstruction instruction = block.first;
    while (instruction != null) {
      HInstruction next = instruction.next;
      HInstruction replacement = instruction.accept(this);
      if (replacement != instruction) {
        block.rewrite(instruction, replacement);

        // If we can replace [instruction] with [replacement], then
        // [replacement]'s type can be narrowed.
        HType newType = replacement.instructionType.intersection(
            instruction.instructionType, compiler);
        if (!newType.isConflicting()) {
          // [HType.intersection] may give up when doing the
          // intersection of two types is too complicated, and return
          // [HType.CONFLICTING]. We do not want instructions to have
          // [HType.CONFLICTING], so we only update the type if the
          // intersection did not give up.
          replacement.instructionType = newType;
        }

        // If the replacement instruction does not know its
        // source element, use the source element of the
        // instruction.
        if (replacement.sourceElement == null) {
          replacement.sourceElement = instruction.sourceElement;
        }
        if (replacement.sourcePosition == null) {
          replacement.sourcePosition = instruction.sourcePosition;
        }
        if (!replacement.isInBasicBlock()) {
          // The constant folding can return an instruction that is already
          // part of the graph (like an input), so we only add the replacement
          // if necessary.
          block.addAfter(instruction, replacement);
          // Visit the replacement as the next instruction in case it
          // can also be constant folded away.
          next = replacement;
        }
        block.remove(instruction);
      }
      instruction = next;
    }
  }

  HInstruction visitInstruction(HInstruction node) {
    return node;
  }

  HInstruction visitBoolify(HBoolify node) {
    List<HInstruction> inputs = node.inputs;
    assert(inputs.length == 1);
    HInstruction input = inputs[0];
    HType type = input.instructionType;
    if (type.isBoolean()) return input;
    // All values !== true are boolified to false.
    if (!type.isBooleanOrNull() && !type.isUnknown()) {
      return graph.addConstantBool(false, constantSystem);
    }
    return node;
  }

  HInstruction visitNot(HNot node) {
    List<HInstruction> inputs = node.inputs;
    assert(inputs.length == 1);
    HInstruction input = inputs[0];
    if (input is HConstant) {
      HConstant constant = input;
      bool isTrue = constant.constant.isTrue();
      return graph.addConstantBool(!isTrue, constantSystem);
    } else if (input is HNot) {
      return input.inputs[0];
    }
    return node;
  }

  HInstruction visitInvokeUnary(HInvokeUnary node) {
    HInstruction folded =
        foldUnary(node.operation(constantSystem), node.operand);
    return folded != null ? folded : node;
  }

  HInstruction foldUnary(UnaryOperation operation, HInstruction operand) {
    if (operand is HConstant) {
      HConstant receiver = operand;
      Constant folded = operation.fold(receiver.constant);
      if (folded != null) return graph.addConstant(folded);
    }
    return null;
  }

  HInstruction tryOptimizeLengthInterceptedGetter(HInvokeDynamic node) {
    HInstruction actualReceiver = node.inputs[1];
    if (actualReceiver.isIndexablePrimitive()) {
      if (actualReceiver.isConstantString()) {
        HConstant constantInput = actualReceiver;
        StringConstant constant = constantInput.constant;
        return graph.addConstantInt(constant.length, constantSystem);
      } else if (actualReceiver.isConstantList()) {
        HConstant constantInput = actualReceiver;
        ListConstant constant = constantInput.constant;
        return graph.addConstantInt(constant.length, constantSystem);
      }
      Element element;
      bool isAssignable;
      if (actualReceiver.isString()) {
        element = backend.jsStringLength;
        isAssignable = false;
      } else {
        element = backend.jsArrayLength;
        isAssignable = !actualReceiver.isFixedArray();
      }
      HFieldGet result = new HFieldGet(
          element, actualReceiver, isAssignable: isAssignable);
      result.instructionType = HType.INTEGER;
      return result;
    } else if (actualReceiver.isConstantMap()) {
      HConstant constantInput = actualReceiver;
      MapConstant constant = constantInput.constant;
      return graph.addConstantInt(constant.length, constantSystem);
    }
    return null;
  }

  HInstruction handleInterceptorCall(HInvokeDynamic node) {
    // Try constant folding the instruction.
    Operation operation = node.specializer.operation(constantSystem);
    if (operation != null) {
      HInstruction instruction = node.inputs.length == 2
          ? foldUnary(operation, node.inputs[1])
          : foldBinary(operation, node.inputs[1], node.inputs[2]);
      if (instruction != null) return instruction;
    }

    // Try converting the instruction to a builtin instruction.
    HInstruction instruction =
        node.specializer.tryConvertToBuiltin(node);
    if (instruction != null) return instruction;

    Selector selector = node.selector;
    var interceptor = node.inputs[0];
    HInstruction input = node.inputs[1];

    if (selector.isCall()) {
      Element target;
      if (input.isExtendableArray()) {
        if (selector.applies(backend.jsArrayRemoveLast, compiler)) {
          target = backend.jsArrayRemoveLast;
        } else if (selector.applies(backend.jsArrayAdd, compiler)) {
          // The codegen special cases array calls, but does not
          // inline argument type checks.
          if (!compiler.enableTypeAssertions) {
            target = backend.jsArrayAdd;
          }
        }
      } else if (input.isString()) {
        if (selector.applies(backend.jsStringSplit, compiler)) {
          if (node.inputs[2].isString()) {
            target = backend.jsStringSplit;
          }
        } else if (selector.applies(backend.jsStringConcat, compiler)) {
          if (node.inputs[2].isString()) {
            target = backend.jsStringConcat;
          }
        } else if (selector.applies(backend.jsStringToString, compiler)) {
          return input;
        }
      }
      if (target != null) {
        // TODO(ngeoffray): There is a strong dependency between codegen
        // and this optimization that the dynamic invoke does not need an
        // interceptor. We currently need to keep a
        // HInvokeDynamicMethod and not create a HForeign because
        // HForeign is too opaque for the SsaCheckInserter (that adds a
        // bounds check on removeLast). Once we start inlining, the
        // bounds check will become explicit, so we won't need this
        // optimization.
        HInvokeDynamicMethod result = new HInvokeDynamicMethod(
            node.selector, node.inputs.getRange(1, node.inputs.length - 1));
        result.element = target;
        return result;
      }
    } else if (selector.isGetter()) {
      if (selector.applies(backend.jsArrayLength, compiler)) {
        HInstruction optimized = tryOptimizeLengthInterceptedGetter(node);
        if (optimized != null) return optimized;
      }
    }

    // If the intercepted call is through a constant interceptor, we
    // know which element to call.
    if (node is !HOneShotInterceptor
        && interceptor.isConstant()
        && selector.isCall()) {
      DartType type = interceptor.instructionType.computeType(compiler);
      ClassElement cls = type.element;
      Element target = cls.lookupSelector(selector);
      if (target != null && selector.applies(target, compiler)) {
        node.element = target;
      }
    }
    return node;
  }

  bool isFixedSizeListConstructor(HInvokeStatic node) {
    Element element = node.target.element;
    if (backend.fixedLengthListConstructor == null) {
      backend.fixedLengthListConstructor =
        compiler.listClass.lookupConstructor(
            new Selector.callConstructor(const SourceString(""),
                                         compiler.listClass.getLibrary()));
    }
    // TODO(ngeoffray): checking if the second input is an integer
    // should not be necessary but it currently makes it easier for
    // other optimizations to reason on a fixed length constructor
    // that we know takes an int.
    return element == backend.fixedLengthListConstructor
        && node.inputs.length == 2
        && node.inputs[1].isInteger();
  }

  HInstruction visitInvokeStatic(HInvokeStatic node) {
    if (isFixedSizeListConstructor(node)) {
      node.instructionType = HType.FIXED_ARRAY;
    }
    return node;
  }

  HInstruction visitInvokeDynamicMethod(HInvokeDynamicMethod node) {
    if (node.isCallOnInterceptor) return handleInterceptorCall(node);
    HType receiverType = node.receiver.instructionType;
    Selector selector = receiverType.refine(node.selector, compiler);
    Element element = compiler.world.locateSingleElement(selector);
    // TODO(ngeoffray): Also fold if it's a getter or variable.
    if (element != null && element.isFunction()) {
      FunctionElement method = element;
      FunctionSignature parameters = method.computeSignature(compiler);
      // TODO(ngeoffray): If the method has optional parameters,
      // we should pass the default values.
      if (parameters.optionalParameterCount == 0
          || parameters.parameterCount == node.selector.argumentCount) {
        node.element = element;
      }
    }
    return node;
  }

  HInstruction visitIntegerCheck(HIntegerCheck node) {
    HInstruction value = node.value;
    if (value.isInteger()) return value;
    if (value.isConstant()) {
      HConstant constantInstruction = value;
      assert(!constantInstruction.constant.isInt());
      if (!constantSystem.isInt(constantInstruction.constant)) {
        // -0.0 is a double but will pass the runtime integer check.
        node.alwaysFalse = true;
      }
    }
    return node;
  }

  HInstruction foldBinary(BinaryOperation operation,
                          HInstruction left,
                          HInstruction right) {
    if (left is HConstant && right is HConstant) {
      HConstant op1 = left;
      HConstant op2 = right;
      Constant folded = operation.fold(op1.constant, op2.constant);
      if (folded != null) return graph.addConstant(folded);
    }
    return null;
  }

  HInstruction visitInvokeBinary(HInvokeBinary node) {
    HInstruction left = node.left;
    HInstruction right = node.right;
    BinaryOperation operation = node.operation(constantSystem);
    HConstant folded = foldBinary(operation, left, right);
    if (folded != null) return folded;
    return node;
  }

  bool allUsersAreBoolifies(HInstruction instruction) {
    List<HInstruction> users = instruction.usedBy;
    int length = users.length;
    for (int i = 0; i < length; i++) {
      if (users[i] is! HBoolify) return false;
    }
    return true;
  }

  HInstruction visitRelational(HRelational node) {
    if (allUsersAreBoolifies(node)) {
      // TODO(ngeoffray): Call a boolified selector.
      // This node stays the same, but the Boolify node will go away.
    }
    // Note that we still have to call [super] to make sure that we end up
    // in the remaining optimizations.
    return super.visitRelational(node);
  }

  HInstruction handleIdentityCheck(HRelational node) {
    HInstruction left = node.left;
    HInstruction right = node.right;
    HType leftType = left.instructionType;
    HType rightType = right.instructionType;

    // We don't optimize on numbers to preserve the runtime semantics.
    if (!(left.isNumberOrNull() && right.isNumberOrNull()) &&
        leftType.intersection(rightType, compiler).isConflicting()) {
      return graph.addConstantBool(false, constantSystem);
    }

    if (left.isConstantBoolean() && right.isBoolean()) {
      HConstant constant = left;
      if (constant.constant.isTrue()) {
        return right;
      } else {
        return new HNot(right);
      }
    }

    if (right.isConstantBoolean() && left.isBoolean()) {
      HConstant constant = right;
      if (constant.constant.isTrue()) {
        return left;
      } else {
        return new HNot(left);
      }
    }

    return null;
  }

  HInstruction visitIdentity(HIdentity node) {
    HInstruction newInstruction = handleIdentityCheck(node);
    return newInstruction == null ? super.visitIdentity(node) : newInstruction;
  }

  HInstruction visitTypeGuard(HTypeGuard node) {
    HInstruction value = node.guarded;
    // If the intersection of the types is still the incoming type then
    // the incoming type was a subtype of the guarded type, and no check
    // is required.
    HType combinedType =
        value.instructionType.intersection(node.guardedType, compiler);
    return (combinedType == value.instructionType) ? value : node;
  }

  HInstruction visitIs(HIs node) {
    DartType type = node.typeExpression;
    Element element = type.element;
    if (element.isTypeVariable()) {
      compiler.unimplemented("visitIs for type variables");
    } if (element.isTypedef()) {
      return node;
    }

    HType expressionType = node.expression.instructionType;
    if (identical(element, compiler.objectClass)
        || identical(element, compiler.dynamicClass)) {
      return graph.addConstantBool(true, constantSystem);
    } else if (expressionType.isInteger()) {
      if (identical(element, compiler.intClass)
          || identical(element, compiler.numClass)
          || Elements.isNumberOrStringSupertype(element, compiler)) {
        return graph.addConstantBool(true, constantSystem);
      } else if (identical(element, compiler.doubleClass)) {
        // We let the JS semantics decide for that check. Currently
        // the code we emit will always return true.
        return node;
      } else {
        return graph.addConstantBool(false, constantSystem);
      }
    } else if (expressionType.isDouble()) {
      if (identical(element, compiler.doubleClass)
          || identical(element, compiler.numClass)
          || Elements.isNumberOrStringSupertype(element, compiler)) {
        return graph.addConstantBool(true, constantSystem);
      } else if (identical(element, compiler.intClass)) {
        // We let the JS semantics decide for that check. Currently
        // the code we emit will return true for a double that can be
        // represented as a 31-bit integer and for -0.0.
        return node;
      } else {
        return graph.addConstantBool(false, constantSystem);
      }
    } else if (expressionType.isNumber()) {
      if (identical(element, compiler.numClass)) {
        return graph.addConstantBool(true, constantSystem);
      }
      // We cannot just return false, because the expression may be of
      // type int or double.
    } else if (expressionType.isString()) {
      if (identical(element, compiler.stringClass)
               || Elements.isStringOnlySupertype(element, compiler)
               || Elements.isNumberOrStringSupertype(element, compiler)) {
        return graph.addConstantBool(true, constantSystem);
      } else {
        return graph.addConstantBool(false, constantSystem);
      }
    } else if (expressionType.isArray()) {
      if (identical(element, compiler.listClass)
          || Elements.isListSupertype(element, compiler)) {
        return graph.addConstantBool(true, constantSystem);
      } else {
        return graph.addConstantBool(false, constantSystem);
      }
    // TODO(karlklose): remove the hasTypeArguments check.
    } else if (expressionType.isUseful()
               && !expressionType.canBeNull()
               && !RuntimeTypeInformation.hasTypeArguments(type)) {
      DartType receiverType = expressionType.computeType(compiler);
      if (receiverType != null) {
        if (!receiverType.isMalformed &&
            !type.isMalformed &&
            compiler.types.isSubtype(receiverType, type)) {
          return graph.addConstantBool(true, constantSystem);
        } else if (expressionType.isExact()) {
          return graph.addConstantBool(false, constantSystem);
        }
      }
    }
    return node;
  }

  HInstruction visitTypeConversion(HTypeConversion node) {
    HInstruction value = node.inputs[0];
    DartType type = node.instructionType.computeType(compiler);
    if (identical(type.element, compiler.dynamicClass)
        || identical(type.element, compiler.objectClass)) {
      return value;
    }
    if (value.instructionType.canBeNull() && node.isBooleanConversionCheck) {
      return node;
    }
    HType combinedType =
        value.instructionType.intersection(node.instructionType, compiler);
    return (combinedType == value.instructionType) ? value : node;
  }

  Element findConcreteFieldForDynamicAccess(HInstruction receiver,
                                            Selector selector) {
    HType receiverType = receiver.instructionType;
    return compiler.world.locateSingleField(
        receiverType.refine(selector, compiler));
  }

  HInstruction visitFieldGet(HFieldGet node) {
    if (node.element == backend.jsArrayLength) {
      if (node.receiver is HInvokeStatic) {
        // Try to recognize the length getter with input
        // [:new List(int):].
        HInvokeStatic call = node.receiver;
        if (isFixedSizeListConstructor(call)) {
          return call.inputs[1];
        }
      }
    }
    return node;
  }

  HInstruction visitInvokeDynamicGetter(HInvokeDynamicGetter node) {
    if (node.isCallOnInterceptor) return handleInterceptorCall(node);

    Element field =
        findConcreteFieldForDynamicAccess(node.receiver, node.selector);
    if (field == null) return node;

    Modifiers modifiers = field.modifiers;
    bool isFinalOrConst = modifiers.isFinal() || modifiers.isConst();
    if (!compiler.resolverWorld.hasInvokedSetter(field, compiler)) {
      // If no setter is ever used for this field it is only initialized in the
      // initializer list.
      isFinalOrConst = true;
    }
    HFieldGet result = new HFieldGet(
        field, node.inputs[0], isAssignable: !isFinalOrConst);

    // Some native fields are views of data that may be changed by operations.
    // E.g. node.firstChild depends on parentNode.removeBefore(n1, n2).
    // TODO(sra): Refine the effect classification so that native effects are
    // distinct from ordinary Dart effects.
    if (field.isNative()) {
      result.setDependsOnSomething();
    }

    if (field.getEnclosingClass().isNative()) {
      result.instructionType =
          new HType.subtype(field.computeType(compiler), compiler);
    } else {
      HType type = new HType.inferredForElement(field, compiler);
      if (type.isUnknown()) {
        type = backend.optimisticFieldType(field);
        if (type != null) {
          backend.registerFieldTypesOptimization(
              work.element, field, result.instructionType);
        }
      }
      if (type != null) {
        result.instructionType = type;
      }
    }
    return result;
  }

  HInstruction visitInvokeDynamicSetter(HInvokeDynamicSetter node) {
    if (node.isCallOnInterceptor) return handleInterceptorCall(node);

    Element field =
        findConcreteFieldForDynamicAccess(node.receiver, node.selector);
    if (field == null || !field.isAssignable()) return node;
    // Use [:node.inputs.last:] in case the call follows the
    // interceptor calling convention, but is not a call on an
    // interceptor.
    HInstruction value = node.inputs.last;
    if (compiler.enableTypeAssertions) {
      HInstruction other = value.convertType(
          compiler,
          field.computeType(compiler),
          HTypeConversion.CHECKED_MODE_CHECK);
      if (other != value) {
        node.block.addBefore(node, other);
        value = other;
      }
    }
    return new HFieldSet(field, node.inputs[0], value);
  }

  HInstruction visitStringConcat(HStringConcat node) {
    DartString folded = const LiteralDartString("");
    for (int i = 0; i < node.inputs.length; i++) {
      HInstruction part = node.inputs[i];
      if (!part.isConstant()) return node;
      HConstant constant = part;
      if (!constant.constant.isPrimitive()) return node;
      PrimitiveConstant primitive = constant.constant;
      folded = new DartString.concat(folded, primitive.toDartString());
    }
    return graph.addConstant(constantSystem.createString(folded, node.node));
  }

  HInstruction visitInterceptor(HInterceptor node) {
    if (node.isConstant()) return node;
    // If the intercepted object does not need to be intercepted, just
    // return the object (the [:getInterceptor:] method would have
    // returned the object).
    HType type = node.receiver.instructionType;
    if (!type.canBePrimitive(compiler)) {
      if (!(type.canBeNull()
            && node.interceptedClasses.contains(compiler.objectClass))) {
        return node.receiver;
      }
    }
    HInstruction constant = tryComputeConstantInterceptor(
        node.inputs[0], node.interceptedClasses);
    if (constant == null) return node;

    return constant;
  }

  HInstruction tryComputeConstantInterceptor(HInstruction input,
                                             Set<ClassElement> intercepted) {
    HType type = input.instructionType;
    ClassElement constantInterceptor;
    if (type.isInteger()) {
      constantInterceptor = backend.jsIntClass;
    } else if (type.isDouble()) {
      constantInterceptor = backend.jsDoubleClass;
    } else if (type.isBoolean()) {
      constantInterceptor = backend.jsBoolClass;
    } else if (type.isString()) {
      constantInterceptor = backend.jsStringClass;
    } else if (type.isArray()) {
      constantInterceptor = backend.jsArrayClass;
    } else if (type.isNull()) {
      constantInterceptor = backend.jsNullClass;
    } else if (type.isNumber()) {
      // If the method being intercepted is not defined in [int] or
      // [double] we can safely use the number interceptor.
      if (!intercepted.contains(compiler.intClass)
          && !intercepted.contains(compiler.doubleClass)) {
        constantInterceptor = backend.jsNumberClass;
      }
    }

    if (constantInterceptor == null) return null;
    if (constantInterceptor == work.element.getEnclosingClass()) {
      return graph.thisInstruction;
    }

    Constant constant = new ConstructedConstant(
        constantInterceptor.computeType(compiler), <Constant>[]);
    return graph.addConstant(constant);
  }

  HInstruction visitOneShotInterceptor(HOneShotInterceptor node) {
    HInstruction newInstruction = handleInterceptorCall(node);
    if (newInstruction != node) return newInstruction;

    HInstruction constant = tryComputeConstantInterceptor(
        node.inputs[1], node.interceptedClasses);

    if (constant == null) return node;

    Selector selector = node.selector;
    // TODO(ngeoffray): make one shot interceptors know whether
    // they have side effects.
    if (selector.isGetter()) {
      HInstruction res = new HInvokeDynamicGetter(
          selector, node.element, constant, false);
      res.inputs.add(node.inputs[1]);
      return res;
    } else if (node.selector.isSetter()) {
      HInstruction res = new HInvokeDynamicSetter(
          selector, node.element, constant, node.inputs[1], false);
      res.inputs.add(node.inputs[2]);
      return res;
    } else {
      List<HInstruction> inputs = new List<HInstruction>.from(node.inputs);
      inputs[0] = constant;
      return new HInvokeDynamicMethod(selector, inputs, true);
    }
  }
}

class SsaCheckInserter extends HBaseVisitor implements OptimizationPhase {
  final Set<HInstruction> boundsChecked;
  final CodegenWorkItem work;
  final JavaScriptBackend backend;
  final String name = "SsaCheckInserter";
  HGraph graph;

  SsaCheckInserter(this.backend,
                   this.work,
                   this.boundsChecked);

  void visitGraph(HGraph graph) {
    this.graph = graph;
    visitDominatorTree(graph);
  }

  void visitBasicBlock(HBasicBlock block) {
    HInstruction instruction = block.first;
    while (instruction != null) {
      HInstruction next = instruction.next;
      instruction = instruction.accept(this);
      instruction = next;
    }
  }

  HBoundsCheck insertBoundsCheck(HInstruction node,
                                 HInstruction receiver,
                                 HInstruction index) {
    bool isAssignable = !receiver.isFixedArray();
    HFieldGet length = new HFieldGet(
        backend.jsArrayLength, receiver, isAssignable: isAssignable);
    length.instructionType = HType.INTEGER;
    length.instructionType = HType.INTEGER;
    node.block.addBefore(node, length);

    HBoundsCheck check = new HBoundsCheck(index, length);
    node.block.addBefore(node, check);
    boundsChecked.add(node);
    return check;
  }

  HIntegerCheck insertIntegerCheck(HInstruction node, HInstruction value) {
    HIntegerCheck check = new HIntegerCheck(value);
    node.block.addBefore(node, check);
    Set<HInstruction> dominatedUsers = value.dominatedUsers(node);
    for (HInstruction user in dominatedUsers) {
      user.changeUse(value, check);
    }
    return check;
  }

  void visitIndex(HIndex node) {
    if (boundsChecked.contains(node)) return;
    HInstruction index = node.index;
    if (!node.index.isInteger()) {
      index = insertIntegerCheck(node, index);
    }
    index = insertBoundsCheck(node, node.receiver, index);
    node.changeUse(node.index, index);
  }

  void visitIndexAssign(HIndexAssign node) {
    if (!node.receiver.isMutableArray()) return;
    if (boundsChecked.contains(node)) return;
    HInstruction index = node.index;
    if (!node.index.isInteger()) {
      index = insertIntegerCheck(node, index);
    }
    index = insertBoundsCheck(node, node.receiver, index);
    node.changeUse(node.index, index);
  }

  void visitInvokeDynamicMethod(HInvokeDynamicMethod node) {
    Element element = node.element;
    if (node.isInterceptorCall) return;
    if (element != backend.jsArrayRemoveLast) return;
    if (boundsChecked.contains(node)) return;
    insertBoundsCheck(
        node, node.receiver, graph.addConstantInt(0, backend.constantSystem));
  }
}

class SsaDeadCodeEliminator extends HGraphVisitor implements OptimizationPhase {
  final String name = "SsaDeadCodeEliminator";

  SsaDeadCodeEliminator();

  bool isDeadCode(HInstruction instruction) {
    return !instruction.hasSideEffects()
           && !instruction.canThrow()
           && instruction.usedBy.isEmpty
           && instruction is !HTypeGuard
           && instruction is !HParameterValue
           && instruction is !HLocalSet
           && !instruction.isControlFlow();
  }

  void visitGraph(HGraph graph) {
    visitPostDominatorTree(graph);
  }

  void visitBasicBlock(HBasicBlock block) {
    HInstruction instruction = block.last;
    while (instruction != null) {
      var previous = instruction.previous;
      if (isDeadCode(instruction)) block.remove(instruction);
      instruction = previous;
    }
  }
}

class SsaDeadPhiEliminator implements OptimizationPhase {
  final String name = "SsaDeadPhiEliminator";

  void visitGraph(HGraph graph) {
    final List<HPhi> worklist = <HPhi>[];
    // A set to keep track of the live phis that we found.
    final Set<HPhi> livePhis = new Set<HPhi>();

    // Add to the worklist all live phis: phis referenced by non-phi
    // instructions.
    for (final block in graph.blocks) {
      block.forEachPhi((HPhi phi) {
        for (final user in phi.usedBy) {
          if (user is !HPhi) {
            worklist.add(phi);
            livePhis.add(phi);
            break;
          }
        }
      });
    }

    // Process the worklist by propagating liveness to phi inputs.
    while (!worklist.isEmpty) {
      HPhi phi = worklist.removeLast();
      for (final input in phi.inputs) {
        if (input is HPhi && !livePhis.contains(input)) {
          worklist.add(input);
          livePhis.add(input);
        }
      }
    }

    // Remove phis that are not live.
    // Traverse in reverse order to remove phis with no uses before the
    // phis that they might use.
    // NOTICE: Doesn't handle circular references, but we don't currently
    // create any.
    List<HBasicBlock> blocks = graph.blocks;
    for (int i = blocks.length - 1; i >= 0; i--) {
      HBasicBlock block = blocks[i];
      HPhi current = block.phis.first;
      HPhi next = null;
      while (current != null) {
        next = current.next;
        if (!livePhis.contains(current)
            // TODO(ahe): Not sure the following is correct.
            && current.usedBy.isEmpty) {
          block.removePhi(current);
        }
        current = next;
      }
    }
  }
}

class SsaRedundantPhiEliminator implements OptimizationPhase {
  final String name = "SsaRedundantPhiEliminator";

  void visitGraph(HGraph graph) {
    final List<HPhi> worklist = <HPhi>[];

    // Add all phis in the worklist.
    for (final block in graph.blocks) {
      block.forEachPhi((HPhi phi) => worklist.add(phi));
    }

    while (!worklist.isEmpty) {
      HPhi phi = worklist.removeLast();

      // If the phi has already been processed, continue.
      if (!phi.isInBasicBlock()) continue;

      // Find if the inputs of the phi are the same instruction.
      // The builder ensures that phi.inputs[0] cannot be the phi
      // itself.
      assert(!identical(phi.inputs[0], phi));
      HInstruction candidate = phi.inputs[0];
      for (int i = 1; i < phi.inputs.length; i++) {
        HInstruction input = phi.inputs[i];
        // If the input is the phi, the phi is still candidate for
        // elimination.
        if (!identical(input, candidate) && !identical(input, phi)) {
          candidate = null;
          break;
        }
      }

      // If the inputs are not the same, continue.
      if (candidate == null) continue;

      // Because we're updating the users of this phi, we may have new
      // phis candidate for elimination. Add phis that used this phi
      // to the worklist.
      for (final user in phi.usedBy) {
        if (user is HPhi) worklist.add(user);
      }
      phi.block.rewrite(phi, candidate);
      phi.block.removePhi(phi);
    }
  }
}

class SsaGlobalValueNumberer implements OptimizationPhase {
  final String name = "SsaGlobalValueNumberer";
  final Compiler compiler;
  final Set<int> visited;

  List<int> blockChangesFlags;
  List<int> loopChangesFlags;

  SsaGlobalValueNumberer(this.compiler) : visited = new Set<int>();

  void visitGraph(HGraph graph) {
    computeChangesFlags(graph);
    moveLoopInvariantCode(graph);
    visitBasicBlock(graph.entry, new ValueSet());
  }

  void moveLoopInvariantCode(HGraph graph) {
    for (int i = graph.blocks.length - 1; i >= 0; i--) {
      HBasicBlock block = graph.blocks[i];
      if (block.isLoopHeader()) {
        int changesFlags = loopChangesFlags[block.id];
        HLoopInformation info = block.loopInformation;
        // Iterate over all blocks of this loop. Note that blocks in
        // inner loops are not visited here, but we know they
        // were visited before because we are iterating in post-order.
        // So instructions that are GVN'ed in an inner loop are in their
        // loop entry, and [info.blocks] contains this loop entry.
        for (HBasicBlock other in info.blocks) {
          moveLoopInvariantCodeFromBlock(other, block, changesFlags);
        }
      }
    }
  }

  void moveLoopInvariantCodeFromBlock(HBasicBlock block,
                                      HBasicBlock loopHeader,
                                      int changesFlags) {
    assert(block.parentLoopHeader == loopHeader);
    HBasicBlock preheader = loopHeader.predecessors[0];
    int dependsFlags = HInstruction.computeDependsOnFlags(changesFlags);
    HInstruction instruction = block.first;
    while (instruction != null) {
      HInstruction next = instruction.next;
      if (instruction.useGvn()
          && (instruction is !HCheck)
          && (instruction.flags & dependsFlags) == 0) {
        bool loopInvariantInputs = true;
        List<HInstruction> inputs = instruction.inputs;
        for (int i = 0, length = inputs.length; i < length; i++) {
          if (isInputDefinedAfterDominator(inputs[i], preheader)) {
            loopInvariantInputs = false;
            break;
          }
        }

        // If the inputs are loop invariant, we can move the
        // instruction from the current block to the pre-header block.
        if (loopInvariantInputs) {
          block.detach(instruction);
          preheader.moveAtExit(instruction);
        }
      }
      int oldChangesFlags = changesFlags;
      changesFlags |= instruction.getChangesFlags();
      if (oldChangesFlags != changesFlags) {
        dependsFlags = HInstruction.computeDependsOnFlags(changesFlags);
      }
      instruction = next;
    }
  }

  bool isInputDefinedAfterDominator(HInstruction input,
                                    HBasicBlock dominator) {
    return input.block.id > dominator.id;
  }

  void visitBasicBlock(HBasicBlock block, ValueSet values) {
    HInstruction instruction = block.first;
    if (block.isLoopHeader()) {
      int flags = loopChangesFlags[block.id];
      values.kill(flags);
    }
    while (instruction != null) {
      HInstruction next = instruction.next;
      int flags = instruction.getChangesFlags();
      assert(flags == 0 || !instruction.useGvn());
      values.kill(flags);
      if (instruction.useGvn()) {
        HInstruction other = values.lookup(instruction);
        if (other != null) {
          assert(other.gvnEquals(instruction) && instruction.gvnEquals(other));
          block.rewriteWithBetterUser(instruction, other);
          block.remove(instruction);
        } else {
          values.add(instruction);
        }
      }
      instruction = next;
    }

    List<HBasicBlock> dominatedBlocks = block.dominatedBlocks;
    for (int i = 0, length = dominatedBlocks.length; i < length; i++) {
      HBasicBlock dominated = dominatedBlocks[i];
      // No need to copy the value set for the last child.
      ValueSet successorValues = (i == length - 1) ? values : values.copy();
      // If we have no values in our set, we do not have to kill
      // anything. Also, if the range of block ids from the current
      // block to the dominated block is empty, there is no blocks on
      // any path from the current block to the dominated block so we
      // don't have to do anything either.
      assert(block.id < dominated.id);
      if (!successorValues.isEmpty && block.id + 1 < dominated.id) {
        visited.clear();
        int changesFlags = getChangesFlagsForDominatedBlock(block, dominated);
        successorValues.kill(changesFlags);
      }
      visitBasicBlock(dominated, successorValues);
    }
  }

  void computeChangesFlags(HGraph graph) {
    // Create the changes flags lists. Make sure to initialize the
    // loop changes flags list to zero so we can use bitwise or when
    // propagating loop changes upwards.
    final int length = graph.blocks.length;
    blockChangesFlags = new List<int>(length);
    loopChangesFlags = new List<int>(length);
    for (int i = 0; i < length; i++) loopChangesFlags[i] = 0;

    // Run through all the basic blocks in the graph and fill in the
    // changes flags lists.
    for (int i = length - 1; i >= 0; i--) {
      final HBasicBlock block = graph.blocks[i];
      final int id = block.id;

      // Compute block changes flags for the block.
      int changesFlags = 0;
      HInstruction instruction = block.first;
      while (instruction != null) {
        changesFlags |= instruction.getChangesFlags();
        instruction = instruction.next;
      }
      assert(blockChangesFlags[id] == null);
      blockChangesFlags[id] = changesFlags;

      // Loop headers are part of their loop, so update the loop
      // changes flags accordingly.
      if (block.isLoopHeader()) {
        loopChangesFlags[id] |= changesFlags;
      }

      // Propagate loop changes flags upwards.
      HBasicBlock parentLoopHeader = block.parentLoopHeader;
      if (parentLoopHeader != null) {
        loopChangesFlags[parentLoopHeader.id] |= (block.isLoopHeader())
            ? loopChangesFlags[id]
            : changesFlags;
      }
    }
  }

  int getChangesFlagsForDominatedBlock(HBasicBlock dominator,
                                       HBasicBlock dominated) {
    int changesFlags = 0;
    List<HBasicBlock> predecessors = dominated.predecessors;
    for (int i = 0, length = predecessors.length; i < length; i++) {
      HBasicBlock block = predecessors[i];
      int id = block.id;
      // If the current predecessor block is on the path from the
      // dominator to the dominated, it must have an id that is in the
      // range from the dominator to the dominated.
      if (dominator.id < id && id < dominated.id && !visited.contains(id)) {
        visited.add(id);
        changesFlags |= blockChangesFlags[id];
        // Loop bodies might not be on the path from dominator to dominated,
        // but they can invalidate values.
        changesFlags |= loopChangesFlags[id];
        changesFlags |= getChangesFlagsForDominatedBlock(dominator, block);
      }
    }
    return changesFlags;
  }
}

// This phase merges equivalent instructions on different paths into
// one instruction in a dominator block. It runs through the graph
// post dominator order and computes a ValueSet for each block of
// instructions that can be moved to a dominator block. These
// instructions are the ones that:
// 1) can be used for GVN, and
// 2) do not use definitions of their own block.
//
// A basic block looks at its sucessors and finds the intersection of
// these computed ValueSet. It moves all instructions of the
// intersection into its own list of instructions.
class SsaCodeMotion extends HBaseVisitor implements OptimizationPhase {
  final String name = "SsaCodeMotion";

  List<ValueSet> values;

  void visitGraph(HGraph graph) {
    values = new List<ValueSet>(graph.blocks.length);
    for (int i = 0; i < graph.blocks.length; i++) {
      values[graph.blocks[i].id] = new ValueSet();
    }
    visitPostDominatorTree(graph);
  }

  void visitBasicBlock(HBasicBlock block) {
    List<HBasicBlock> successors = block.successors;

    // Phase 1: get the ValueSet of all successors (if there are more than one),
    // compute the intersection and move the instructions of the intersection
    // into this block.
    if (successors.length > 1) {
      ValueSet instructions = values[successors[0].id];
      for (int i = 1; i < successors.length; i++) {
        ValueSet other = values[successors[i].id];
        instructions = instructions.intersection(other);
      }

      if (!instructions.isEmpty) {
        List<HInstruction> list = instructions.toList();
        for (HInstruction instruction in list) {
          // Move the instruction to the current block.
          instruction.block.detach(instruction);
          block.moveAtExit(instruction);
          // Go through all successors and rewrite their instruction
          // to the shared one.
          for (final successor in successors) {
            HInstruction toRewrite = values[successor.id].lookup(instruction);
            if (toRewrite != instruction) {
              successor.rewriteWithBetterUser(toRewrite, instruction);
              successor.remove(toRewrite);
            }
          }
        }
      }
    }

    // Don't try to merge instructions to a dominator if we have
    // multiple predecessors.
    if (block.predecessors.length != 1) return;

    // Phase 2: Go through all instructions of this block and find
    // which instructions can be moved to a dominator block.
    ValueSet set_ = values[block.id];
    HInstruction instruction = block.first;
    int flags = 0;
    while (instruction != null) {
      int dependsFlags = HInstruction.computeDependsOnFlags(flags);
      flags |= instruction.getChangesFlags();

      HInstruction current = instruction;
      instruction = instruction.next;

      // TODO(ngeoffray): this check is needed because we currently do
      // not have flags to express 'Gvn'able', but not movable.
      if (current is HCheck) continue;
      if (!current.useGvn()) continue;
      if ((current.flags & dependsFlags) != 0) continue;

      bool canBeMoved = true;
      for (final HInstruction input in current.inputs) {
        if (input.block == block) {
          canBeMoved = false;
          break;
        }
      }
      if (!canBeMoved) continue;

      // This is safe because we are running after GVN.
      // TODO(ngeoffray): ensure GVN has been run.
      set_.add(current);
    }
  }
}

class SsaTypeConversionInserter extends HBaseVisitor
    implements OptimizationPhase {
  final String name = "SsaTypeconversionInserter";
  final Compiler compiler;

  SsaTypeConversionInserter(this.compiler);

  void visitGraph(HGraph graph) {
    visitDominatorTree(graph);
  }


  // Update users of [input] that are dominated by [:dominator.first:]
  // to use [newInput] instead.
  void changeUsesDominatedBy(HBasicBlock dominator,
                             HInstruction input,
                             HType convertedType) {
    Set<HInstruction> dominatedUsers = input.dominatedUsers(dominator.first);
    if (dominatedUsers.isEmpty) return;

    HTypeConversion newInput = new HTypeConversion(convertedType, input);
    dominator.addBefore(dominator.first, newInput);
    dominatedUsers.forEach((HInstruction user) {
      user.changeUse(input, newInput);
    });
  }

  void visitIs(HIs instruction) {
    HInstruction input = instruction.expression;
    HType convertedType = new HType.nonNullSubtype(
        instruction.typeExpression, compiler);

    List<HInstruction> ifUsers = <HInstruction>[];
    List<HInstruction> notIfUsers = <HInstruction>[];

    for (HInstruction user in instruction.usedBy) {
      if (user is HIf) {
        ifUsers.add(user);
      } else if (user is HNot) {
        for (HInstruction notUser in user.usedBy) {
          if (notUser is HIf) notIfUsers.add(notUser);
        }
      }
    }

    if (ifUsers.isEmpty && notIfUsers.isEmpty) return;

    for (HIf ifUser in ifUsers) {
      changeUsesDominatedBy(ifUser.thenBlock, input, convertedType);
      // TODO(ngeoffray): Also change uses for the else block on a HType
      // that knows it is not of a specific Type.
    }

    for (HIf ifUser in notIfUsers) {
      changeUsesDominatedBy(ifUser.elseBlock, input, convertedType);
      // TODO(ngeoffray): Also change uses for the then block on a HType
      // that knows it is not of a specific Type.
    }
  }
}


// Analyze the constructors to see if some fields will always have a specific
// type after construction. If this is the case we can ignore the type given
// by the field initializer. This is especially useful when the field
// initializer is initializing the field to null.
class SsaConstructionFieldTypes
    extends HBaseVisitor implements OptimizationPhase {
  final JavaScriptBackend backend;
  final CodegenWorkItem work;
  final String name = "SsaConstructionFieldTypes";
  final Set<HInstruction> thisUsers;
  final Set<Element> allSetters;
  final Map<HBasicBlock, Map<Element, HType>> blockFieldSetters;
  bool thisExposed = false;
  HGraph currentGraph;
  Map<Element, HType> currentFieldSetters;

  SsaConstructionFieldTypes(this.backend, this.work)
      : thisUsers = new Set<HInstruction>(),
        allSetters = new Set<Element>(),
        blockFieldSetters = new Map<HBasicBlock, Map<Element, HType>>();

  void visitGraph(HGraph graph) {
    currentGraph = graph;
    if (!work.element.isGenerativeConstructorBody() &&
        !work.element.isGenerativeConstructor()) return;
    visitDominatorTree(graph);
    if (work.element.isGenerativeConstructor()) {
      backend.registerConstructor(work.element);
    }
  }

  visitBasicBlock(HBasicBlock block) {
    if (block.predecessors.length == 0) {
      // Create a new empty map for the first block.
      currentFieldSetters = new Map<Element, HType>();
    } else {
      // Build a map which intersects the fields from all predecessors. For
      // each field in this intersection it unions the types.
      currentFieldSetters =
          new Map.from(blockFieldSetters[block.predecessors[0]]);
      // Loop headers are the only nodes with back edges.
      if (!block.isLoopHeader()) {
        for (int i = 1; i < block.predecessors.length; i++) {
          Map<Element, HType> predecessorsFieldSetters =
              blockFieldSetters[block.predecessors[i]];
          Map<Element, HType> newFieldSetters = new Map<Element, HType>();
          predecessorsFieldSetters.forEach((Element element, HType type) {
            HType currentType = currentFieldSetters[element];
            if (currentType != null) {
              newFieldSetters[element] =
                  currentType.union(type, backend.compiler);
            }
          });
          currentFieldSetters = newFieldSetters;
        }
      } else {
        assert(block.predecessors.length <= 2);
      }
    }
    block.forEachPhi((HPhi phi) => phi.accept(this));
    block.forEachInstruction(
        (HInstruction instruction) => instruction.accept(this));
    assert(currentFieldSetters != null);
    blockFieldSetters[block] = currentFieldSetters;
  }

  visitInstruction(HInstruction instruction) {
    // All instructions not explicitly handled below will flag the this
    // exposure if using this.
    thisExposed = thisExposed || thisUsers.contains(instruction);
  }

  visitPhi(HPhi phi) {
    if (thisUsers.contains(phi)) {
      thisUsers.addAll(phi.usedBy);
    }
  }

  visitThis(HThis instruction) {
    // Collect all users of this in a set to make the this exposed check simple
    // and cheap.
    thisUsers.addAll(instruction.usedBy);
  }

  visitFieldGet(HInstruction _) {
    // The field get instruction is allowed to use this.
  }

  visitForeignNew(HForeignNew node) {
    // The HForeignNew instruction is used in the generative constructor to
    // initialize all fields in newly created objects. The fields are
    // initialized to the value present in the initializer list or set to null
    // if not otherwise initialized.
    // Here we handle members in superclasses as well, as the handling of
    // the generative constructor bodies will ensure, that the initializer
    // type will not be used if the field is in any of these.
    int j = 0;
    node.element.forEachInstanceField(
        (ClassElement enclosingClass, Element element) {
          backend.registerFieldInitializer(
              element, node.inputs[j].instructionType);
          j++;
        },
        includeBackendMembers: false,
        includeSuperMembers: true);
  }

  visitFieldSet(HFieldSet node) {
    Element field = node.element;
    HInstruction value = node.value;
    HType type = value.instructionType;
    // [HFieldSet] is also used for variables in try/catch.
    if (field.isField()) allSetters.add(field);
    // Don't handle fields defined in superclasses. Given that the field is
    // always added to the [allSetters] set, setting a field defined in a
    // superclass will get an inferred type of UNKNOWN.
    if (identical(work.element.getEnclosingClass(), field.getEnclosingClass()) &&
        !value.instructionType.isUnknown()) {
      currentFieldSetters[field] = type;
    }
  }

  visitExit(HExit node) {
    // If this has been exposed then we cannot say anything about types after
    // construction.
    if (!thisExposed) {
      // Register the known field types.
      currentFieldSetters.forEach((Element element, HType type) {
        backend.registerFieldConstructor(element, type);
        allSetters.remove(element);
      });
    }

    // For other fields having setters in the generative constructor body, set
    // the type to UNKNOWN to avoid relying on the type set in the initializer
    // list.
    allSetters.forEach((Element element) {
      backend.registerFieldConstructor(element, HType.UNKNOWN);
    });
  }
}

/**
 * This phase specializes dominated uses of a call, where the call
 * can give us some type information of what the receiver might be.
 * For example, after a call to [:a.foo():], if [:foo:] is only
 * in class [:A:], a can be of type [:A:].
 */
class SsaReceiverSpecialization extends HBaseVisitor
    implements OptimizationPhase {
  final String name = "SsaReceiverSpecialization";
  final Compiler compiler;

  SsaReceiverSpecialization(this.compiler);

  void visitGraph(HGraph graph) {
    visitDominatorTree(graph);
  }

  void visitInterceptor(HInterceptor interceptor) {
    HInstruction receiver = interceptor.receiver;
    JavaScriptBackend backend = compiler.backend;
    for (var user in receiver.usedBy) {
      if (user is HInterceptor && interceptor.dominates(user)) {
        Set<ClassElement> otherIntercepted = user.interceptedClasses;
        // If the dominated interceptor intercepts the int class or
        // the double class, we make sure these classes are also being
        // intercepted by the dominating interceptor. Otherwise, the
        // dominating interceptor could just intercept the number
        // class and therefore not implement the methods in the int or
        // double class.
        if (otherIntercepted.contains(backend.jsIntClass)
            || otherIntercepted.contains(backend.jsDoubleClass)) {
          // We cannot modify the intercepted classes set without
          // copying because it may be shared by multiple interceptors
          // because of caching in the backend.
          Set<ClassElement> copy = interceptor.interceptedClasses.toSet();
          copy.addAll(user.interceptedClasses);
          interceptor.interceptedClasses = copy;
        }
        user.interceptedClasses = interceptor.interceptedClasses;
      }
    }
  }

  // TODO(ngeoffray): Also implement it for non-intercepted calls.
}

/**
 * This phase replaces all interceptors that are used only once with
 * one-shot interceptors. It saves code size and makes the receiver of
 * an intercepted call a candidate for being generated at use site.
 */
class SsaSimplifyInterceptors extends HBaseVisitor
    implements OptimizationPhase {
  final String name = "SsaSimplifyInterceptors";
  final ConstantSystem constantSystem;
  HGraph graph;

  SsaSimplifyInterceptors(this.constantSystem);

  void visitGraph(HGraph graph) {
    this.graph = graph;
    visitDominatorTree(graph);
  }

  void visitInterceptor(HInterceptor node) {
    if (node.usedBy.length != 1) return;
    // [HBailoutTarget] instructions might have the interceptor as
    // input. In such situation we let the dead code analyzer find out
    // the interceptor is not needed.
    if (node.usedBy[0] is !HInvokeDynamic) return;

    HInvokeDynamic user = node.usedBy[0];

    // If [node] was loop hoisted, we keep the interceptor.
    if (!user.hasSameLoopHeaderAs(node)) return;

    // Replace the user with a [HOneShotInterceptor].
    HConstant nullConstant = graph.addConstantNull(constantSystem);
    List<HInstruction> inputs = new List<HInstruction>.from(user.inputs);
    inputs[0] = nullConstant;
    HOneShotInterceptor interceptor = new HOneShotInterceptor(
        user.selector, inputs, node.interceptedClasses);
    interceptor.sourcePosition = user.sourcePosition;
    interceptor.sourceElement = user.sourceElement;
    interceptor.instructionType = user.instructionType;

    HBasicBlock block = user.block;
    block.addAfter(user, interceptor);
    block.rewrite(user, interceptor);
    block.remove(user);

    // The interceptor will be removed in the dead code elimination
    // phase. Note that removing it here would not work because of how
    // the [visitBasicBlock] is implemented.
  }
}