transformer.dart

// Copyright (c) 2017, 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.

/// Transformations based on type flow analysis.
library vm.transformations.type_flow.transformer;

import 'dart:core' hide Type;

import 'package:kernel/target/targets.dart';
import 'package:kernel/ast.dart' hide Statement, StatementVisitor;
import 'package:kernel/core_types.dart' show CoreTypes;
import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
import 'package:kernel/library_index.dart' show LibraryIndex;
import 'package:kernel/type_environment.dart';

import 'analysis.dart';
import 'calls.dart';
import 'signature_shaking.dart';
import 'protobuf_handler.dart' show ProtobufHandler;
import 'summary.dart';
import 'table_selector_assigner.dart';
import 'types.dart';
import 'unboxing_info.dart';
import 'utils.dart';
import '../pragma.dart';
import '../devirtualization.dart' show Devirtualization;
import '../../metadata/direct_call.dart';
import '../../metadata/inferred_type.dart';
import '../../metadata/procedure_attributes.dart';
import '../../metadata/table_selector.dart';
import '../../metadata/unboxing_info.dart';
import '../../metadata/unreachable.dart';

const bool kDumpClassHierarchy =
    const bool.fromEnvironment('global.type.flow.dump.class.hierarchy');

/// Whole-program type flow analysis and transformation.
/// Assumes strong mode and closed world.
Component transformComponent(
    Target target, CoreTypes coreTypes, Component component,
    {PragmaAnnotationParser matcher,
    bool treeShakeSignatures: true,
    bool treeShakeWriteOnlyFields: true,
    bool treeShakeProtobufs: false}) {
  void ignoreAmbiguousSupertypes(Class cls, Supertype a, Supertype b) {}
  final hierarchy = new ClassHierarchy(component, coreTypes,
      onAmbiguousSupertypes: ignoreAmbiguousSupertypes);
  final types = new TypeEnvironment(coreTypes, hierarchy);
  final libraryIndex = new LibraryIndex.all(component);
  final genericInterfacesInfo = new GenericInterfacesInfoImpl(hierarchy);
  final protobufHandler = treeShakeProtobufs
      ? ProtobufHandler.forComponent(component, coreTypes)
      : null;

  Statistics.reset();
  final analysisStopWatch = new Stopwatch()..start();

  final typeFlowAnalysis = new TypeFlowAnalysis(
      target,
      component,
      coreTypes,
      hierarchy,
      genericInterfacesInfo,
      types,
      libraryIndex,
      protobufHandler,
      matcher);

  Procedure main = component.mainMethod;

  // `main` can be null, roots can also come from @pragma("vm:entry-point").
  if (main != null) {
    final Selector mainSelector = new DirectSelector(main);
    typeFlowAnalysis.addRawCall(mainSelector);
  }

  typeFlowAnalysis.process();

  analysisStopWatch.stop();

  if (kDumpClassHierarchy) {
    debugPrint(typeFlowAnalysis.hierarchyCache);
  }

  final transformsStopWatch = new Stopwatch()..start();

  final treeShaker = new TreeShaker(component, typeFlowAnalysis,
      treeShakeWriteOnlyFields: treeShakeWriteOnlyFields);
  treeShaker.transformComponent(component);

  new TFADevirtualization(
          component, typeFlowAnalysis, hierarchy, treeShaker.fieldMorpher)
      .visitComponent(component);

  final tableSelectorAssigner = new TableSelectorAssigner(component);

  if (treeShakeSignatures) {
    final signatureShaker =
        new SignatureShaker(typeFlowAnalysis, tableSelectorAssigner);
    signatureShaker.transformComponent(component);
  }

  final unboxingInfo = new UnboxingInfoManager(typeFlowAnalysis)
    ..analyzeComponent(component, typeFlowAnalysis, tableSelectorAssigner);

  new AnnotateKernel(component, typeFlowAnalysis, treeShaker.fieldMorpher,
          tableSelectorAssigner, unboxingInfo)
      .visitComponent(component);

  transformsStopWatch.stop();

  statPrint("TF analysis took ${analysisStopWatch.elapsedMilliseconds}ms");
  statPrint("TF transforms took ${transformsStopWatch.elapsedMilliseconds}ms");

  Statistics.print("TFA statistics");

  return component;
}

/// Devirtualization based on results of type flow analysis.
class TFADevirtualization extends Devirtualization {
  final TypeFlowAnalysis _typeFlowAnalysis;
  final FieldMorpher fieldMorpher;

  TFADevirtualization(Component component, this._typeFlowAnalysis,
      ClassHierarchy hierarchy, this.fieldMorpher)
      : super(_typeFlowAnalysis.environment.coreTypes, component, hierarchy);

  @override
  DirectCallMetadata getDirectCall(TreeNode node, Member interfaceTarget,
      {bool setter = false}) {
    final callSite = _typeFlowAnalysis.callSite(node);
    if (callSite != null) {
      final Member singleTarget = fieldMorpher
          .getMorphedMember(callSite.monomorphicTarget, isSetter: setter);
      if (singleTarget != null) {
        return new DirectCallMetadata(
            singleTarget, callSite.isNullableReceiver);
      }
    }
    return null;
  }
}

/// Annotates kernel AST with metadata using results of type flow analysis.
class AnnotateKernel extends RecursiveVisitor {
  final TypeFlowAnalysis _typeFlowAnalysis;
  final FieldMorpher fieldMorpher;
  final DirectCallMetadataRepository _directCallMetadataRepository;
  final InferredTypeMetadataRepository _inferredTypeMetadata;
  final UnreachableNodeMetadataRepository _unreachableNodeMetadata;
  final ProcedureAttributesMetadataRepository _procedureAttributesMetadata;
  final TableSelectorMetadataRepository _tableSelectorMetadata;
  final TableSelectorAssigner _tableSelectorAssigner;
  final UnboxingInfoMetadataRepository _unboxingInfoMetadata;
  final UnboxingInfoManager _unboxingInfo;
  final Class _intClass;
  Constant _nullConstant;

  AnnotateKernel(Component component, this._typeFlowAnalysis, this.fieldMorpher,
      this._tableSelectorAssigner, this._unboxingInfo)
      : _directCallMetadataRepository =
            component.metadata[DirectCallMetadataRepository.repositoryTag],
        _inferredTypeMetadata = new InferredTypeMetadataRepository(),
        _unreachableNodeMetadata = new UnreachableNodeMetadataRepository(),
        _procedureAttributesMetadata =
            new ProcedureAttributesMetadataRepository(),
        _tableSelectorMetadata = new TableSelectorMetadataRepository(),
        _unboxingInfoMetadata = new UnboxingInfoMetadataRepository(),
        _intClass = _typeFlowAnalysis.environment.coreTypes.intClass {
    component.addMetadataRepository(_inferredTypeMetadata);
    component.addMetadataRepository(_unreachableNodeMetadata);
    component.addMetadataRepository(_procedureAttributesMetadata);
    component.addMetadataRepository(_tableSelectorMetadata);
    component.addMetadataRepository(_unboxingInfoMetadata);
  }

  // Query whether a call site was marked as a direct call by the analysis.
  bool _callSiteUsesDirectCall(TreeNode node) {
    return _directCallMetadataRepository.mapping.containsKey(node);
  }

  InferredType _convertType(Type type,
      {bool skipCheck: false, bool receiverNotInt: false}) {
    assert(type != null);

    Class concreteClass;
    Constant constantValue;
    bool isInt = false;

    final nullable = type is NullableType;
    if (nullable) {
      type = (type as NullableType).baseType;
    }

    if (nullable && type == const EmptyType()) {
      concreteClass =
          _typeFlowAnalysis.environment.coreTypes.deprecatedNullClass;
      constantValue = _nullConstant ??= new NullConstant();
    } else {
      concreteClass = type.getConcreteClass(_typeFlowAnalysis.hierarchyCache);

      if (concreteClass == null) {
        isInt = type.isSubtypeOf(_typeFlowAnalysis.hierarchyCache, _intClass);
      }

      if (type is ConcreteType && !nullable) {
        constantValue = type.constant;
      }
    }

    List<DartType> typeArgs;
    if (type is ConcreteType && type.typeArgs != null) {
      typeArgs = type.typeArgs
          .take(type.numImmediateTypeArgs)
          .map((t) =>
              t is UnknownType ? null : (t as RuntimeType).representedType)
          .toList();
    }

    if (concreteClass != null ||
        !nullable ||
        isInt ||
        constantValue != null ||
        skipCheck ||
        receiverNotInt) {
      return new InferredType(concreteClass, nullable, isInt, constantValue,
          exactTypeArguments: typeArgs,
          skipCheck: skipCheck,
          receiverNotInt: receiverNotInt);
    }

    return null;
  }

  void _setInferredType(TreeNode node, Type type,
      {bool skipCheck: false, bool receiverNotInt: false}) {
    final inferredType = _convertType(type,
        skipCheck: skipCheck, receiverNotInt: receiverNotInt);
    if (inferredType != null) {
      _inferredTypeMetadata.mapping[node] = inferredType;
    }
  }

  void _setUnreachable(TreeNode node) {
    _unreachableNodeMetadata.mapping[node] = const UnreachableNode();
  }

  void _annotateCallSite(TreeNode node, Member interfaceTarget) {
    final callSite = _typeFlowAnalysis.callSite(node);
    if (callSite == null) return;
    if (!callSite.isReachable) {
      _setUnreachable(node);
      return;
    }

    final bool markSkipCheck = !callSite.useCheckedEntry &&
        (node is MethodInvocation || node is PropertySet);

    bool markReceiverNotInt = false;

    if (!callSite.receiverMayBeInt) {
      // No information is needed for static calls.
      if (node is! StaticInvocation &&
          node is! StaticSet &&
          node is! StaticGet) {
        // The compiler uses another heuristic in addition to the call-site
        // annotation: if the call-site is non-dynamic and the interface target does
        // not exist in the parent chain of _Smi (int is used as an approxmiation
        // here), then the receiver cannot be _Smi. This heuristic covers most
        // cases, so we skip these to avoid showering the AST with annotations.
        if (interfaceTarget == null ||
            _typeFlowAnalysis.hierarchyCache.hierarchy.isSubtypeOf(
                _typeFlowAnalysis.hierarchyCache.coreTypes.intClass,
                interfaceTarget.enclosingClass)) {
          markReceiverNotInt = true;
        }
      }
    }

    // If the call is not marked as 'isResultUsed', the 'resultType' will
    // not be observed (i.e., it will always be EmptyType). This is the
    // case even if the result actually might be used but is not used by
    // the summary, e.g. if the result is an argument to a closure call.
    // Therefore, we need to pass in 'NullableType(AnyType)' as the
    // inferred result type here (since we don't know what it actually
    // is).
    final Type resultType = callSite.isResultUsed
        ? callSite.resultType
        : NullableType(const AnyType());

    if (markSkipCheck || markReceiverNotInt || callSite.isResultUsed) {
      _setInferredType(node, resultType,
          skipCheck: markSkipCheck, receiverNotInt: markReceiverNotInt);
    }

    // Tell the table selector assigner about the callsite.
    final Selector selector = callSite.selector;
    if (selector is InterfaceSelector && !_callSiteUsesDirectCall(node)) {
      if (node is PropertyGet) {
        _tableSelectorAssigner.registerGetterCall(
            selector.member, callSite.isNullableReceiver);
      } else {
        assert(node is MethodInvocation || node is PropertySet);
        _tableSelectorAssigner.registerMethodOrSetterCall(
            selector.member, callSite.isNullableReceiver);
      }
    }
  }

  void _annotateMember(Member member) {
    if (_typeFlowAnalysis.isMemberUsed(member)) {
      if (member is Field) {
        _setInferredType(member, _typeFlowAnalysis.fieldType(member));
      } else {
        Args<Type> argTypes = _typeFlowAnalysis.argumentTypes(member);
        final uncheckedParameters =
            _typeFlowAnalysis.uncheckedParameters(member);
        assert(argTypes != null);

        final int firstParamIndex =
            numTypeParams(member) + (hasReceiverArg(member) ? 1 : 0);

        final positionalParams = member.function.positionalParameters;
        assert(argTypes.positionalCount ==
            firstParamIndex + positionalParams.length);

        for (int i = 0; i < positionalParams.length; i++) {
          _setInferredType(
              positionalParams[i], argTypes.values[firstParamIndex + i],
              skipCheck: uncheckedParameters.contains(positionalParams[i]));
        }

        // TODO(dartbug.com/32292): make sure parameters are sorted in kernel
        // AST and iterate parameters in parallel, without lookup.
        final names = argTypes.names;
        for (int i = 0; i < names.length; i++) {
          final param = findNamedParameter(member.function, names[i]);
          assert(param != null);
          _setInferredType(param,
              argTypes.values[firstParamIndex + positionalParams.length + i],
              skipCheck: uncheckedParameters.contains(param));
        }

        // TODO(alexmarkov): figure out how to pass receiver type.
      }

      final unboxingInfoMetadata =
          _unboxingInfo.getUnboxingInfoOfMember(member);
      if (unboxingInfoMetadata != null && !unboxingInfoMetadata.isFullyBoxed) {
        _unboxingInfoMetadata.mapping[member] = unboxingInfoMetadata;
      }
    } else {
      if (!member.isAbstract &&
          !fieldMorpher.isExtraMemberWithReachableBody(member)) {
        _setUnreachable(member);
      }

      if (member is! Field) {
        final unboxingInfoMetadata =
            _unboxingInfo.getUnboxingInfoOfMember(member);
        if (unboxingInfoMetadata != null) {
          // Check for partitions that only have abstract methods should be marked as boxed.
          if (unboxingInfoMetadata.returnInfo ==
              UnboxingInfoMetadata.kUnboxingCandidate) {
            unboxingInfoMetadata.returnInfo = UnboxingInfoMetadata.kBoxed;
          }
          for (int i = 0;
              i < unboxingInfoMetadata.unboxedArgsInfo.length;
              i++) {
            if (unboxingInfoMetadata.unboxedArgsInfo[i] ==
                UnboxingInfoMetadata.kUnboxingCandidate) {
              unboxingInfoMetadata.unboxedArgsInfo[i] =
                  UnboxingInfoMetadata.kBoxed;
            }
          }
          if (!unboxingInfoMetadata.isFullyBoxed) {
            _unboxingInfoMetadata.mapping[member] = unboxingInfoMetadata;
          }
        }
      }
    }

    // We need to attach ProcedureAttributesMetadata to all members, even
    // unreachable ones, since an unreachable member could still be used as an
    // interface target, and table dispatch calls need selector IDs for all
    // interface targets.
    if (member.isInstanceMember) {
      final original = fieldMorpher.getOriginalMember(member);
      final attrs = new ProcedureAttributesMetadata(
          methodOrSetterCalledDynamically:
              _typeFlowAnalysis.isCalledDynamically(original),
          getterCalledDynamically:
              _typeFlowAnalysis.isGetterCalledDynamically(original),
          hasThisUses: _typeFlowAnalysis.isCalledViaThis(original),
          hasNonThisUses: _typeFlowAnalysis.isCalledNotViaThis(original),
          hasTearOffUses: _typeFlowAnalysis.isTearOffTaken(original),
          methodOrSetterSelectorId:
              _tableSelectorAssigner.methodOrSetterSelectorId(member),
          getterSelectorId: _tableSelectorAssigner.getterSelectorId(member));
      _procedureAttributesMetadata.mapping[member] = attrs;
    }
  }

  @override
  visitConstructor(Constructor node) {
    _annotateMember(node);
    super.visitConstructor(node);
  }

  @override
  visitProcedure(Procedure node) {
    _annotateMember(node);
    super.visitProcedure(node);
  }

  @override
  visitField(Field node) {
    _annotateMember(node);
    super.visitField(node);
  }

  @override
  visitMethodInvocation(MethodInvocation node) {
    _annotateCallSite(node, node.interfaceTarget);
    super.visitMethodInvocation(node);
  }

  @override
  visitPropertyGet(PropertyGet node) {
    _annotateCallSite(node, node.interfaceTarget);
    super.visitPropertyGet(node);
  }

  @override
  visitPropertySet(PropertySet node) {
    _annotateCallSite(node, node.interfaceTarget);
    super.visitPropertySet(node);
  }

  @override
  visitSuperMethodInvocation(SuperMethodInvocation node) {
    _annotateCallSite(node, node.interfaceTarget);
    super.visitSuperMethodInvocation(node);
  }

  @override
  visitSuperPropertyGet(SuperPropertyGet node) {
    _annotateCallSite(node, node.interfaceTarget);
    super.visitSuperPropertyGet(node);
  }

  @override
  visitSuperPropertySet(SuperPropertySet node) {
    _annotateCallSite(node, node.interfaceTarget);
    super.visitSuperPropertySet(node);
  }

  @override
  visitStaticInvocation(StaticInvocation node) {
    _annotateCallSite(node, node.target);
    super.visitStaticInvocation(node);
  }

  @override
  visitStaticGet(StaticGet node) {
    _annotateCallSite(node, node.target);
    super.visitStaticGet(node);
  }

  @override
  visitStaticSet(StaticSet node) {
    _annotateCallSite(node, node.target);
    super.visitStaticSet(node);
  }

  @override
  visitComponent(Component node) {
    super.visitComponent(node);
    _tableSelectorMetadata.mapping[node] = _tableSelectorAssigner.metadata;
  }
}

/// Tree shaking based on results of type flow analysis (TFA).
///
/// TFA provides information about allocated classes and reachable member
/// bodies. However, it is not enough to perform tree shaking in one pass:
/// we need to figure out which classes, members and typedefs are used
/// in types, interface targets and annotations.
///
/// So, tree shaking is performed in 2 passes:
///
/// * Pass 1 visits declarations of classes and members, and dives deep into
///   bodies of reachable members. It collects sets of used classes, members
///   and typedefs. Also, while visiting bodies of reachable members, it
///   transforms unreachable calls into 'throw' expressions.
///
/// * Pass 2 removes unused classes and members, and replaces bodies of
///   used but unreachable members.
///
class TreeShaker {
  final TypeFlowAnalysis typeFlowAnalysis;
  final bool treeShakeWriteOnlyFields;
  final Set<Class> _usedClasses = new Set<Class>();
  final Set<Class> _classesUsedInType = new Set<Class>();
  final Set<Member> _usedMembers = new Set<Member>();
  final Set<Extension> _usedExtensions = new Set<Extension>();
  final Set<Typedef> _usedTypedefs = new Set<Typedef>();
  FieldMorpher fieldMorpher;
  _TreeShakerTypeVisitor typeVisitor;
  _TreeShakerConstantVisitor constantVisitor;
  _TreeShakerPass1 _pass1;
  _TreeShakerPass2 _pass2;

  TreeShaker(Component component, this.typeFlowAnalysis,
      {this.treeShakeWriteOnlyFields: true}) {
    fieldMorpher = new FieldMorpher(this);
    typeVisitor = new _TreeShakerTypeVisitor(this);
    constantVisitor = new _TreeShakerConstantVisitor(this, typeVisitor);
    _pass1 = new _TreeShakerPass1(this);
    _pass2 = new _TreeShakerPass2(this);
  }

  transformComponent(Component component) {
    _pass1.transform(component);
    _pass2.transform(component);
  }

  bool isClassReferencedFromNativeCode(Class c) =>
      typeFlowAnalysis.nativeCodeOracle.isClassReferencedFromNativeCode(c);
  bool isClassUsed(Class c) => _usedClasses.contains(c);
  bool isClassUsedInType(Class c) => _classesUsedInType.contains(c);
  bool isClassAllocated(Class c) => typeFlowAnalysis.isClassAllocated(c);
  bool isMemberUsed(Member m) => _usedMembers.contains(m);
  bool isExtensionUsed(Extension e) => _usedExtensions.contains(e);
  bool isMemberBodyReachable(Member m) =>
      typeFlowAnalysis.isMemberUsed(m) ||
      fieldMorpher.isExtraMemberWithReachableBody(m);
  bool isFieldInitializerReachable(Field f) =>
      typeFlowAnalysis.isFieldInitializerUsed(f);
  bool isFieldGetterReachable(Field f) => typeFlowAnalysis.isFieldGetterUsed(f);
  bool isFieldSetterReachable(Field f) => typeFlowAnalysis.isFieldSetterUsed(f);
  bool isMemberReferencedFromNativeCode(Member m) =>
      typeFlowAnalysis.nativeCodeOracle.isMemberReferencedFromNativeCode(m);
  bool isTypedefUsed(Typedef t) => _usedTypedefs.contains(t);

  bool retainField(Field f) =>
      isMemberBodyReachable(f) &&
          (!treeShakeWriteOnlyFields ||
              isFieldGetterReachable(f) ||
              (!f.isStatic &&
                  f.initializer != null &&
                  isFieldInitializerReachable(f) &&
                  mayHaveSideEffects(f.initializer)) ||
              (f.isLate && f.isFinal)) ||
      isMemberReferencedFromNativeCode(f);

  void addClassUsedInType(Class c) {
    if (_classesUsedInType.add(c)) {
      if (kPrintDebug) {
        debugPrint('Class ${c.name} used in type');
      }
      _usedClasses.add(c);
      visitIterable(c.supers, typeVisitor);
      transformList(c.typeParameters, _pass1, c);
      transformList(c.annotations, _pass1, c);
      // Preserve NSM forwarders. They are overlooked by TFA / tree shaker
      // as they are abstract and don't have a body.
      for (Procedure p in c.procedures) {
        if (p.isAbstract && p.isNoSuchMethodForwarder) {
          addUsedMember(p);
        }
      }
    }
  }

  void addUsedMember(Member m) {
    if (_usedMembers.add(m)) {
      final enclosingClass = m.enclosingClass;
      if (enclosingClass != null) {
        if (kPrintDebug) {
          debugPrint('Member $m from class ${enclosingClass.name} is used');
        }
        _usedClasses.add(enclosingClass);
      }

      FunctionNode func = null;
      if (m is Field) {
        m.type.accept(typeVisitor);
      } else if (m is Procedure) {
        func = m.function;
        if (m.concreteForwardingStubTarget != null) {
          m.stubTarget = fieldMorpher.adjustInstanceCallTarget(
              m.concreteForwardingStubTarget,
              isSetter: m.isSetter);
          addUsedMember(m.concreteForwardingStubTarget);
        }
        if (m.abstractForwardingStubTarget != null) {
          m.stubTarget = fieldMorpher.adjustInstanceCallTarget(
              m.abstractForwardingStubTarget,
              isSetter: m.isSetter);
          addUsedMember(m.abstractForwardingStubTarget);
        }
        if (m.memberSignatureOrigin != null) {
          m.stubTarget = fieldMorpher.adjustInstanceCallTarget(
              m.memberSignatureOrigin,
              isSetter: m.isSetter);
          addUsedMember(m.memberSignatureOrigin);
        }
      } else if (m is Constructor) {
        func = m.function;
      } else {
        throw 'Unexpected member ${m.runtimeType}: $m';
      }

      if (func != null) {
        transformList(func.typeParameters, _pass1, func);
        transformList(func.positionalParameters, _pass1, func);
        transformList(func.namedParameters, _pass1, func);
        func.returnType.accept(typeVisitor);
      }

      transformList(m.annotations, _pass1, m);

      // If the member is kept alive we need to keep the extension alive.
      if (m.isExtensionMember) {
        // The AST should have exactly one [Extension] for [m].
        final extension = m.enclosingLibrary.extensions.firstWhere((extension) {
          return extension.members
              .any((descriptor) => descriptor.member.asMember == m);
        }, orElse: () => null);
        assert(extension != null);

        // Ensure we retain the [Extension] itself (though members might be
        // shaken)
        addUsedExtension(extension);
      }
    }
  }

  void addUsedExtension(Extension node) {
    if (_usedExtensions.add(node)) {
      transformList(node.typeParameters, _pass1, node);
      node.onType?.accept(typeVisitor);
    }
  }

  void addUsedTypedef(Typedef typedef) {
    if (_usedTypedefs.add(typedef)) {
      transformList(typedef.annotations, _pass1, typedef);
      transformList(typedef.typeParameters, _pass1, typedef);
      transformList(typedef.typeParametersOfFunctionType, _pass1, typedef);
      transformList(typedef.positionalParameters, _pass1, typedef);
      transformList(typedef.namedParameters, _pass1, typedef);
      typedef.type?.accept(typeVisitor);
    }
  }
}

class FieldMorpher {
  final TreeShaker shaker;
  final Set<Member> _extraMembersWithReachableBody = <Member>{};
  final Map<Field, Member> _gettersForRemovedFields = <Field, Member>{};
  final Map<Field, Member> _settersForRemovedFields = <Field, Member>{};
  final Map<Member, Field> _removedFields = <Member, Field>{};

  FieldMorpher(this.shaker);

  Member _createAccessorForRemovedField(Field field, bool isSetter) {
    assert(!field.isStatic);
    assert(!shaker.retainField(field));
    Procedure accessor;
    if (isSetter) {
      final isAbstract = !shaker.isFieldSetterReachable(field);
      final parameter = new VariableDeclaration('value', type: field.type)
        ..isCovariant = field.isCovariant
        ..isGenericCovariantImpl = field.isGenericCovariantImpl
        ..fileOffset = field.fileOffset;
      accessor = new Procedure(
          field.name,
          ProcedureKind.Setter,
          new FunctionNode(null,
              positionalParameters: [parameter], returnType: const VoidType())
            ..fileOffset = field.fileOffset,
          isAbstract: isAbstract,
          fileUri: field.fileUri);
      if (!isAbstract) {
        _extraMembersWithReachableBody.add(accessor);
      }
    } else {
      accessor = new Procedure(field.name, ProcedureKind.Getter,
          new FunctionNode(null, returnType: field.type),
          isAbstract: true, fileUri: field.fileUri);
    }
    accessor.fileOffset = field.fileOffset;
    field.enclosingClass.addProcedure(accessor);
    _removedFields[accessor] = field;
    shaker.addUsedMember(accessor);
    return accessor;
  }

  /// Return a replacement for an instance call target.
  /// If necessary, creates a getter or setter as a replacement if target is a
  /// field which is going to be removed by the tree shaker.
  /// This method is used during tree shaker pass 1.
  Member adjustInstanceCallTarget(Member target, {bool isSetter = false}) {
    if (target is Field && !shaker.retainField(target)) {
      final targets =
          isSetter ? _settersForRemovedFields : _gettersForRemovedFields;
      return targets[target] ??=
          _createAccessorForRemovedField(target, isSetter);
    }
    return target;
  }

  bool isExtraMemberWithReachableBody(Member member) =>
      _extraMembersWithReachableBody.contains(member);

  /// Return a member which replaced [target] in instance calls.
  /// This method can be used after tree shaking to discover replacement.
  Member getMorphedMember(Member target, {bool isSetter = false}) {
    if (target == null) {
      return null;
    }
    final targets =
        isSetter ? _settersForRemovedFields : _gettersForRemovedFields;
    return targets[target] ?? target;
  }

  /// Return original member which was replaced by [target] in instance calls.
  /// This method can be used after tree shaking.
  Member getOriginalMember(Member target) {
    if (target == null) {
      return null;
    }
    return _removedFields[target] ?? target;
  }
}

/// Visits Dart types and collects all classes and typedefs used in types.
/// This visitor is used during pass 1 of tree shaking. It is a separate
/// visitor because [Transformer] does not provide a way to traverse types.
class _TreeShakerTypeVisitor extends RecursiveVisitor {
  final TreeShaker shaker;

  _TreeShakerTypeVisitor(this.shaker);

  @override
  visitInterfaceType(InterfaceType node) {
    shaker.addClassUsedInType(node.classNode);
    node.visitChildren(this);
  }

  @override
  visitSupertype(Supertype node) {
    shaker.addClassUsedInType(node.classNode);
    node.visitChildren(this);
  }

  @override
  visitTypedefType(TypedefType node) {
    shaker.addUsedTypedef(node.typedefNode);
    node.visitChildren(this);
  }

  @override
  visitFunctionType(FunctionType node) {
    node.visitChildren(this);
    final typedef = node.typedef;
    if (typedef != null) {
      shaker.addUsedTypedef(typedef);
    }
  }

  @override
  visitTypeParameterType(TypeParameterType node) {
    final parent = node.parameter.parent;
    if (parent is Class) {
      shaker.addClassUsedInType(parent);
    }
  }
}

/// The first pass of [TreeShaker].
/// Visits all classes, members and bodies of reachable members.
/// Collects all used classes, members and types, and
/// transforms unreachable calls into 'throw' expressions.
class _TreeShakerPass1 extends Transformer {
  final TreeShaker shaker;
  final FieldMorpher fieldMorpher;
  final TypeEnvironment environment;
  Procedure _unsafeCast;

  StaticTypeContext _staticTypeContext;
  Member _currentMember;

  StaticTypeContext get staticTypeContext =>
      _staticTypeContext ??= StaticTypeContext(currentMember, environment);

  Member get currentMember => _currentMember;
  set currentMember(Member m) {
    _currentMember = m;
    _staticTypeContext = null;
  }

  _TreeShakerPass1(this.shaker)
      : fieldMorpher = shaker.fieldMorpher,
        environment = shaker.typeFlowAnalysis.environment;

  void transform(Component component) {
    component.transformChildren(this);
  }

  bool _isUnreachable(TreeNode node) {
    final callSite = shaker.typeFlowAnalysis.callSite(node);
    return (callSite != null) && !callSite.isReachable;
  }

  List<Expression> _flattenArguments(Arguments arguments,
      {Expression receiver}) {
    final args = <Expression>[];
    if (receiver != null) {
      args.add(receiver);
    }
    args.addAll(arguments.positional);
    args.addAll(arguments.named.map((a) => a.value));
    return args;
  }

  bool _isThrowExpression(Expression expr) {
    while (expr is Let) {
      expr = (expr as Let).body;
    }
    return expr is Throw;
  }

  TreeNode _evaluateArguments(List<Expression> args, Expression result) {
    Expression node = result;
    for (var arg in args.reversed) {
      if (mayHaveSideEffects(arg)) {
        node = Let(VariableDeclaration(null, initializer: arg), node);
      }
    }
    return node;
  }

  TreeNode _makeUnreachableCall(List<Expression> args) {
    Expression node;
    final int last = args.indexWhere(_isThrowExpression);
    if (last >= 0) {
      // One of the arguments is a Throw expression.
      // Ignore the rest of the arguments.
      node = args[last];
      args = args.sublist(0, last);
      Statistics.throwExpressionsPruned++;
    } else {
      node = Throw(StringLiteral(
          'Attempt to execute code removed by Dart AOT compiler (TFA)'));
    }
    Statistics.callsDropped++;
    return _evaluateArguments(args, node);
  }

  TreeNode _makeUnreachableInitializer(List<Expression> args) {
    return new LocalInitializer(
        new VariableDeclaration(null, initializer: _makeUnreachableCall(args)));
  }

  NarrowNotNull _getNullTest(TreeNode node) =>
      shaker.typeFlowAnalysis.nullTest(node);

  TreeNode _visitAssertNode(TreeNode node) {
    if (kRemoveAsserts) {
      return null;
    } else {
      node.transformChildren(this);
      return node;
    }
  }

  @override
  DartType visitDartType(DartType node) {
    node.accept(shaker.typeVisitor);
    return node;
  }

  @override
  Supertype visitSupertype(Supertype node) {
    node.accept(shaker.typeVisitor);
    return node;
  }

  @override
  TreeNode visitTypedef(Typedef node) {
    return node; // Do not go deeper.
  }

  @override
  Extension visitExtension(Extension node) {
    // The extension can be considered a weak node, we'll only retain it if
    // normal code references any of it's members.
    return node;
  }

  @override
  TreeNode visitClass(Class node) {
    if (shaker.isClassAllocated(node) ||
        shaker.isClassReferencedFromNativeCode(node)) {
      shaker.addClassUsedInType(node);
    }
    transformList(node.constructors, this, node);
    transformList(node.procedures, this, node);
    transformList(node.fields, this, node);
    transformList(node.redirectingFactoryConstructors, this, node);
    return node;
  }

  @override
  TreeNode defaultMember(Member node) {
    currentMember = node;
    if (shaker.isMemberBodyReachable(node)) {
      if (kPrintTrace) {
        tracePrint("Visiting $node");
      }
      shaker.addUsedMember(node);
      node.transformChildren(this);
    } else if (shaker.isMemberReferencedFromNativeCode(node)) {
      // Preserve members referenced from native code to satisfy lookups, even
      // if they are not reachable. An instance member could be added via
      // native code entry point but still unreachable if no instances of
      // its enclosing class are allocated.
      shaker.addUsedMember(node);
    }
    currentMember = null;
    return node;
  }

  @override
  TreeNode visitField(Field node) {
    currentMember = node;
    if (shaker.retainField(node)) {
      if (kPrintTrace) {
        tracePrint("Visiting $node");
      }
      shaker.addUsedMember(node);
      if (node.initializer != null) {
        if (shaker.isFieldInitializerReachable(node)) {
          node.transformChildren(this);
        } else {
          node.initializer = _makeUnreachableCall([])..parent = node;
        }
      }
    } else if (shaker.isFieldSetterReachable(node) && !node.isStatic) {
      // Make sure setter is created to replace the field even if field is not
      // used as an instance call target.
      fieldMorpher.adjustInstanceCallTarget(node, isSetter: true);
    }
    currentMember = null;
    return node;
  }

  @override
  TreeNode visitMethodInvocation(MethodInvocation node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableCall(
          _flattenArguments(node.arguments, receiver: node.receiver));
    }
    if (isComparisonWithNull(node)) {
      final nullTest = _getNullTest(node);
      if (nullTest.isAlwaysNull || nullTest.isAlwaysNotNull) {
        return _evaluateArguments(
            _flattenArguments(node.arguments, receiver: node.receiver),
            BoolLiteral(nullTest.isAlwaysNull));
      }
    }
    node.interfaceTarget =
        fieldMorpher.adjustInstanceCallTarget(node.interfaceTarget);
    if (node.interfaceTarget != null) {
      shaker.addUsedMember(node.interfaceTarget);
    }
    return node;
  }

  @override
  TreeNode visitPropertyGet(PropertyGet node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableCall([node.receiver]);
    } else {
      node.interfaceTarget =
          fieldMorpher.adjustInstanceCallTarget(node.interfaceTarget);
      if (node.interfaceTarget != null) {
        shaker.addUsedMember(node.interfaceTarget);
      }
      return node;
    }
  }

  @override
  TreeNode visitPropertySet(PropertySet node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableCall([node.receiver, node.value]);
    } else {
      node.interfaceTarget = fieldMorpher
          .adjustInstanceCallTarget(node.interfaceTarget, isSetter: true);
      if (node.interfaceTarget != null) {
        shaker.addUsedMember(node.interfaceTarget);
      }
      return node;
    }
  }

  @override
  TreeNode visitSuperMethodInvocation(SuperMethodInvocation node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableCall(_flattenArguments(node.arguments));
    } else {
      node.interfaceTarget =
          fieldMorpher.adjustInstanceCallTarget(node.interfaceTarget);
      if (node.interfaceTarget != null) {
        shaker.addUsedMember(node.interfaceTarget);
      }
      return node;
    }
  }

  @override
  TreeNode visitSuperPropertyGet(SuperPropertyGet node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableCall([]);
    } else {
      node.interfaceTarget =
          fieldMorpher.adjustInstanceCallTarget(node.interfaceTarget);
      if (node.interfaceTarget != null) {
        shaker.addUsedMember(node.interfaceTarget);
      }
      return node;
    }
  }

  @override
  TreeNode visitSuperPropertySet(SuperPropertySet node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableCall([node.value]);
    } else {
      node.interfaceTarget = fieldMorpher
          .adjustInstanceCallTarget(node.interfaceTarget, isSetter: true);
      if (node.interfaceTarget != null) {
        shaker.addUsedMember(node.interfaceTarget);
      }
      return node;
    }
  }

  @override
  TreeNode visitStaticInvocation(StaticInvocation node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableCall(_flattenArguments(node.arguments));
    }

    final target = node.target;
    assert(shaker.isMemberBodyReachable(target),
        "Member body is not reachable: $target");

    return node;
  }

  @override
  TreeNode visitStaticGet(StaticGet node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableCall([]);
    } else {
      if (!shaker.isMemberBodyReachable(node.target)) {
        // Annotations could contain references to constant fields.
        assert((node.target is Field) && (node.target as Field).isConst);
        shaker.addUsedMember(node.target);
      }
      return node;
    }
  }

  @override
  Constant visitConstant(Constant node) {
    shaker.constantVisitor.analyzeConstant(node);
    return node;
  }

  @override
  TreeNode visitStaticSet(StaticSet node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableCall([node.value]);
    } else {
      final target = node.target;
      assert(shaker.isMemberBodyReachable(target),
          "Target should be reachable: $node");
      if (target is Field && !shaker.retainField(target)) {
        return node.value;
      }
      return node;
    }
  }

  @override
  TreeNode visitConstructorInvocation(ConstructorInvocation node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableCall(_flattenArguments(node.arguments));
    } else {
      if (!shaker.isMemberBodyReachable(node.target)) {
        // Annotations could contain references to const constructors.
        assert(node.isConst);
        shaker.addUsedMember(node.target);
      }
      return node;
    }
  }

  @override
  TreeNode visitRedirectingInitializer(RedirectingInitializer node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableInitializer(_flattenArguments(node.arguments));
    } else {
      assert(shaker.isMemberBodyReachable(node.target),
          "Target should be reachable: ${node.target}");
      return node;
    }
  }

  @override
  TreeNode visitSuperInitializer(SuperInitializer node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableInitializer(_flattenArguments(node.arguments));
    } else {
      // Can't assert that node.target is used due to partial mixin resolution.
      return node;
    }
  }

  @override
  TreeNode visitFieldInitializer(FieldInitializer node) {
    node.transformChildren(this);
    if (_isUnreachable(node)) {
      return _makeUnreachableInitializer([node.value]);
    } else {
      assert(shaker.isMemberBodyReachable(node.field),
          "Field should be reachable: ${node.field}");
      if (!shaker.retainField(node.field)) {
        if (mayHaveSideEffects(node.value)) {
          return LocalInitializer(
              VariableDeclaration(null, initializer: node.value));
        } else {
          return null;
        }
      }
      return node;
    }
  }

  @override
  TreeNode visitAssertStatement(AssertStatement node) {
    return _visitAssertNode(node);
  }

  @override
  TreeNode visitAssertBlock(AssertBlock node) {
    return _visitAssertNode(node);
  }

  @override
  TreeNode visitAssertInitializer(AssertInitializer node) {
    return _visitAssertNode(node);
  }

  @override
  TreeNode visitAsExpression(AsExpression node) {
    node.transformChildren(this);
    TypeCheck check = shaker.typeFlowAnalysis.explicitCast(node);
    if (check != null && check.canAlwaysSkip) {
      return StaticInvocation(
          unsafeCast, Arguments([node.operand], types: [node.type]))
        ..fileOffset = node.fileOffset;
    }
    return node;
  }

  @override
  TreeNode visitNullCheck(NullCheck node) {
    node.transformChildren(this);
    final nullTest = _getNullTest(node);
    if (nullTest.isAlwaysNotNull) {
      return StaticInvocation(
          unsafeCast,
          Arguments([node.operand],
              types: [node.getStaticType(staticTypeContext)]))
        ..fileOffset = node.fileOffset;
    }
    return node;
  }

  Procedure get unsafeCast {
    _unsafeCast ??= shaker.typeFlowAnalysis.environment.coreTypes.index
        .getTopLevelMember('dart:_internal', 'unsafeCast');
    assert(_unsafeCast != null);
    return _unsafeCast;
  }
}

/// The second pass of [TreeShaker]. It is called after set of used
/// classes, members and typedefs is determined during the first pass.
/// This pass visits classes and members and removes unused classes and members.
/// Bodies of unreachable but used members are replaced with 'throw'
/// expressions. This pass does not dive deeper than member level.
class _TreeShakerPass2 extends Transformer {
  final TreeShaker shaker;

  _TreeShakerPass2(this.shaker);

  void transform(Component component) {
    component.transformChildren(this);
    for (Source source in component.uriToSource.values) {
      source?.constantCoverageConstructors?.removeWhere((Reference reference) {
        Member node = reference.asMember;
        return !shaker.isMemberUsed(node) && !_preserveSpecialMember(node);
      });
    }
  }

  @override
  TreeNode visitLibrary(Library node) {
    node.transformChildren(this);
    // The transformer API does not iterate over `Library.additionalExports`,
    // so we manually delete the references to shaken nodes.
    node.additionalExports.removeWhere((Reference reference) {
      final node = reference.node;
      if (node is Class) {
        return !shaker.isClassUsed(node);
      } else if (node is Typedef) {
        return !shaker.isTypedefUsed(node);
      } else if (node is Extension) {
        return !shaker.isExtensionUsed(node);
      } else {
        return !shaker.isMemberUsed(node as Member);
      }
    });
    return node;
  }

  @override
  Typedef visitTypedef(Typedef node) {
    return shaker.isTypedefUsed(node) ? node : null;
  }

  @override
  Class visitClass(Class node) {
    if (!shaker.isClassUsed(node)) {
      debugPrint('Dropped class ${node.name}');
      // Ensure that kernel file writer will not be able to
      // write a dangling reference to the deleted class.
      node.reference.canonicalName = null;
      Statistics.classesDropped++;
      return null; // Remove the class.
    }

    if (!shaker.isClassUsedInType(node)) {
      debugPrint('Dropped supers from class ${node.name}');
      // The class is only a namespace for static members.  Remove its
      // hierarchy information.   This is mandatory, since these references
      // might otherwise become dangling.
      node.supertype = shaker
          .typeFlowAnalysis.environment.coreTypes.objectClass.asRawSupertype;
      node.implementedTypes.clear();
      node.typeParameters.clear();
      node.isAbstract = true;
      // Mixin applications cannot have static members.
      assert(node.mixedInType == null);
      node.annotations = const <Expression>[];
    }

    if (!shaker.isClassAllocated(node)) {
      debugPrint('Class ${node.name} converted to abstract');
      node.isAbstract = true;
    }

    node.transformChildren(this);

    return node;
  }

  /// Preserve instance fields of enums as VM relies on their existence.
  bool _preserveSpecialMember(Member node) =>
      node is Field &&
      !node.isStatic &&
      node.enclosingClass != null &&
      node.enclosingClass.isEnum;

  @override
  Member defaultMember(Member node) {
    if (!shaker.isMemberUsed(node) && !_preserveSpecialMember(node)) {
      // Ensure that kernel file writer will not be able to
      // write a dangling reference to the deleted member.
      node.reference.canonicalName = null;
      Statistics.membersDropped++;
      return null;
    }

    if (!shaker.isMemberBodyReachable(node)) {
      if (node is Procedure) {
        // Remove body of unused member.
        if (!node.isStatic && node.enclosingClass.isAbstract) {
          node.isAbstract = true;
          node.function.body = null;
        } else {
          // If the enclosing class is not abstract, the method should still
          // have a body even if it can never be called.
          _makeUnreachableBody(node.function);
        }
        node.function.asyncMarker = AsyncMarker.Sync;
        switch (node.stubKind) {
          case ProcedureStubKind.Regular:
          case ProcedureStubKind.NoSuchMethodForwarder:
            break;
          case ProcedureStubKind.MemberSignature:
          case ProcedureStubKind.AbstractForwardingStub:
          case ProcedureStubKind.ConcreteForwardingStub:
          case ProcedureStubKind.AbstractMixinStub:
          case ProcedureStubKind.ConcreteMixinStub:
            // Make the stub look like a regular procedure so the stub target
            // isn't expected to be non-null, for instance by the verifier.
            node.stubKind = ProcedureStubKind.Regular;
            node.stubTarget = null;
            break;
        }
        Statistics.methodBodiesDropped++;
      } else if (node is Field) {
        node.initializer = null;
        Statistics.fieldInitializersDropped++;
      } else if (node is Constructor) {
        _makeUnreachableBody(node.function);
        node.initializers = const <Initializer>[];
        Statistics.constructorBodiesDropped++;
      } else {
        throw 'Unexpected member ${node.runtimeType}: $node';
      }
    }

    return node;
  }

  @override
  Extension visitExtension(Extension node) {
    if (shaker.isExtensionUsed(node)) {
      int writeIndex = 0;
      for (int i = 0; i < node.members.length; ++i) {
        final ExtensionMemberDescriptor descriptor = node.members[i];

        // To avoid depending on the order in which members and extensions are
        // visited during the transformation, we handle both cases: either the
        // member was already removed or it will be removed later.
        final Reference memberReference = descriptor.member;
        final bool isBound = memberReference.node != null;
        if (isBound && shaker.isMemberUsed(memberReference.node)) {
          node.members[writeIndex++] = descriptor;
        }
      }
      node.members.length = writeIndex;

      // We only retain the extension if at least one member is retained.
      assert(node.members.length > 0);
      return node;
    }
    return null;
  }

  void _makeUnreachableBody(FunctionNode function) {
    if (function.body != null) {
      function.body = new ExpressionStatement(new Throw(new StringLiteral(
          "Attempt to execute method removed by Dart AOT compiler (TFA)")))
        ..parent = function;
    }
  }

  @override
  TreeNode defaultTreeNode(TreeNode node) {
    return node; // Do not traverse into other nodes.
  }
}

class _TreeShakerConstantVisitor extends ConstantVisitor<Null> {
  final TreeShaker shaker;
  final _TreeShakerTypeVisitor typeVisitor;
  final Set<Constant> constants = new Set<Constant>();
  final Set<InstanceConstant> instanceConstants = new Set<InstanceConstant>();

  _TreeShakerConstantVisitor(this.shaker, this.typeVisitor);

  analyzeConstant(Constant constant) {
    if (constants.add(constant)) {
      constant.accept(this);
    }
  }

  @override
  defaultConstant(Constant constant) {
    throw 'There is no support for constant "$constant" in TFA yet!';
  }

  @override
  visitNullConstant(NullConstant constant) {}

  @override
  visitBoolConstant(BoolConstant constant) {}

  @override
  visitIntConstant(IntConstant constant) {}

  @override
  visitDoubleConstant(DoubleConstant constant) {}

  @override
  visitStringConstant(StringConstant constant) {}

  @override
  visitSymbolConstant(SymbolConstant constant) {
    // The Symbol class and it's _name field are always retained.
  }

  @override
  visitMapConstant(MapConstant node) {
    throw 'The kernel2kernel constants transformation desugars const maps!';
  }

  @override
  visitListConstant(ListConstant constant) {
    for (final Constant entry in constant.entries) {
      analyzeConstant(entry);
    }
  }

  @override
  visitInstanceConstant(InstanceConstant constant) {
    instanceConstants.add(constant);
    shaker.addClassUsedInType(constant.classNode);
    visitList(constant.typeArguments, typeVisitor);
    constant.fieldValues.forEach((Reference fieldRef, Constant value) {
      shaker.addUsedMember(fieldRef.asField);
      analyzeConstant(value);
    });
  }

  @override
  visitTearOffConstant(TearOffConstant constant) {
    shaker.addUsedMember(constant.procedure);
  }

  @override
  visitPartialInstantiationConstant(PartialInstantiationConstant constant) {
    analyzeConstant(constant.tearOffConstant);
  }

  @override
  visitTypeLiteralConstant(TypeLiteralConstant constant) {
    constant.type.accept(typeVisitor);
  }
}