pkg/compiler/lib/src/kernel/transformations/modular/late_lowering.dart

// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:kernel/ast.dart';
import 'package:kernel/core_types.dart';
import 'package:kernel/type_algebra.dart';

import '../../../options.dart';

class _Reader {
  final Procedure _procedure;
  final FunctionType _type;

  _Reader(this._procedure) : _type = _procedure.getterType as FunctionType;
}

const _lateInstanceFieldPrefix = '_#';
const _lateFinalUninitializedSuffix = '#F';
const _lateAssignableUninitializedSuffix = '#A';
const _lateFinalInitializedSuffix = '#FI';
const _lateAssignableInitializedSuffix = '#AI';

bool _hasFinalSuffix(String name) {
  return name.endsWith(_lateFinalUninitializedSuffix) ||
      name.endsWith(_lateFinalInitializedSuffix);
}

bool _hasAssignableSuffix(String name) {
  return name.endsWith(_lateAssignableUninitializedSuffix) ||
      name.endsWith(_lateAssignableInitializedSuffix);
}

bool isNameOfLateInstanceBackingField(String? name) {
  return name != null &&
      name.startsWith(_lateInstanceFieldPrefix) &&
      (_hasFinalSuffix(name) || _hasAssignableSuffix(name));
}

bool isBackingFieldForLateInstanceField(Field field) {
  assert(!field.isStatic);
  if (!field.isInternalImplementation) return false;
  return isNameOfLateInstanceBackingField(field.name.text);
}

bool isBackingFieldForLateFinalInstanceField(Field field) {
  if (!field.isInternalImplementation) return false;
  final name = field.name.text;
  return name.startsWith(_lateInstanceFieldPrefix) && _hasFinalSuffix(name);
}

class LateLowering {
  final CoreTypes _coreTypes;

  final bool _omitLateNames;

  final _Reader _readLocal;
  final _Reader _readField;
  final _Reader _readInitialized;
  final _Reader _readInitializedFinal;

  // Each map contains the mapping from late local variables to cells for a
  // given function scope. All late local variables are lowered to cells.
  final List<Map<VariableDeclaration, VariableDeclaration>?> _variableCells =
      [];

  // Uninitialized late static fields are lowered to cells.
  final Map<Field, Field> _fieldCells = {};

  // Late instance fields are lowered to a backing field (plus a getter/setter
  // pair).
  final Map<Field, Field> _backingInstanceFields = {};

  Member? _contextMember;

  LateLowering(this._coreTypes, CompilerOptions? _options)
    : _omitLateNames = _options?.omitLateNames ?? false,
      _readLocal = _Reader(_coreTypes.cellReadLocal),
      _readField = _Reader(_coreTypes.cellReadField),
      _readInitialized = _Reader(_coreTypes.initializedCellRead),
      _readInitializedFinal = _Reader(_coreTypes.initializedCellReadFinal);

  Nullability get nonNullable => _contextMember!.enclosingLibrary.nonNullable;

  bool _shouldLowerVariable(VariableDeclaration variable) => variable.isLate;

  bool _shouldLowerUninitializedVariable(VariableDeclaration variable) =>
      _shouldLowerVariable(variable) && variable.initializer == null;

  bool _shouldLowerInitializedVariable(VariableDeclaration variable) =>
      _shouldLowerVariable(variable) && variable.initializer != null;

  bool _shouldLowerStaticField(Field field) =>
      field.isLate && field.isStatic && field.initializer == null;

  bool _shouldLowerInstanceField(Field field) =>
      field.isLate && !field.isStatic;

  Name _mangleFieldCellName(Field field) {
    assert(_shouldLowerStaticField(field));
    return Name('_#${field.name.text}', field.enclosingLibrary);
  }

  Name _mangleFieldName(Field field) {
    assert(_shouldLowerInstanceField(field));
    final prefix = _lateInstanceFieldPrefix;
    final suffix =
        field.initializer == null
            ? field.isFinal
                ? _lateFinalUninitializedSuffix
                : _lateAssignableUninitializedSuffix
            : field.isFinal
            ? _lateFinalInitializedSuffix
            : _lateAssignableInitializedSuffix;

    Class cls = field.enclosingClass!;
    return Name(
      '$prefix${cls.name}#${field.name.text}$suffix',
      field.enclosingLibrary,
    );
  }

  ConstructorInvocation _callCellConstructor(Expression name, int fileOffset) =>
      _omitLateNames
          ? _callCellUnnamedConstructor(fileOffset)
          : _callCellNamedConstructor(name, fileOffset);

  ConstructorInvocation _callCellUnnamedConstructor(int fileOffset) =>
      ConstructorInvocation(
        _coreTypes.cellConstructor,
        Arguments.empty()..fileOffset = fileOffset,
      )..fileOffset = fileOffset;

  ConstructorInvocation _callCellNamedConstructor(
    Expression name,
    int fileOffset,
  ) => ConstructorInvocation(
    _coreTypes.cellNamedConstructor,
    Arguments([name])..fileOffset = fileOffset,
  )..fileOffset = fileOffset;

  ConstructorInvocation _callInitializedCellConstructor(
    Expression name,
    Expression initializer,
    int fileOffset,
  ) =>
      _omitLateNames
          ? _callInitializedCellUnnamedConstructor(initializer, fileOffset)
          : _callInitializedCellNamedConstructor(name, initializer, fileOffset);

  ConstructorInvocation _callInitializedCellUnnamedConstructor(
    Expression initializer,
    int fileOffset,
  ) => ConstructorInvocation(
    _coreTypes.initializedCellConstructor,
    Arguments([initializer])..fileOffset = fileOffset,
  )..fileOffset = fileOffset;

  ConstructorInvocation _callInitializedCellNamedConstructor(
    Expression name,
    Expression initializer,
    int fileOffset,
  ) => ConstructorInvocation(
    _coreTypes.initializedCellNamedConstructor,
    Arguments([name, initializer])..fileOffset = fileOffset,
  )..fileOffset = fileOffset;

  StringLiteral _nameLiteral(String? name, int fileOffset) =>
      StringLiteral(name ?? '')..fileOffset = fileOffset;

  InstanceInvocation _callReader(
    _Reader reader,
    Expression receiver,
    DartType type,
    int fileOffset,
  ) {
    Procedure procedure = reader._procedure;
    List<DartType> typeArguments = [type];
    return InstanceInvocation(
      InstanceAccessKind.Instance,
      receiver,
      procedure.name,
      Arguments(const [], types: typeArguments)..fileOffset = fileOffset,
      interfaceTarget: procedure,
      functionType: FunctionTypeInstantiator.instantiate(
        reader._type,
        typeArguments,
      ),
    )..fileOffset = fileOffset;
  }

  InstanceSet _callSetter(
    Procedure setter,
    Expression receiver,
    Expression value,
    int fileOffset,
  ) => InstanceSet(
    InstanceAccessKind.Instance,
    receiver,
    setter.name,
    value,
    interfaceTarget: setter,
  )..fileOffset = fileOffset;

  StaticInvocation _callIsSentinel(Expression value, int fileOffset) =>
      StaticInvocation(
        _coreTypes.isSentinelMethod,
        Arguments([value])..fileOffset = fileOffset,
      )..fileOffset = fileOffset;

  void exitLibrary() {
    assert(_variableCells.isEmpty);
    _fieldCells.clear();
    _backingInstanceFields.clear();
  }

  void enterScope() {
    _variableCells.add(null);
  }

  void exitScope() {
    _variableCells.removeLast();
  }

  VariableDeclaration? _lookupVariableCell(VariableDeclaration variable) {
    assert(_shouldLowerVariable(variable));
    for (final scope in _variableCells) {
      if (scope == null) continue;
      final cell = scope[variable];
      if (cell != null) return cell;
    }
    return null;
  }

  VariableDeclaration _addToCurrentScope(
    VariableDeclaration variable,
    VariableDeclaration cell,
  ) {
    assert(_shouldLowerVariable(variable));
    assert(_lookupVariableCell(variable) == null);
    return (_variableCells.last ??= {})[variable] = cell;
  }

  VariableDeclaration _variableCell(VariableDeclaration variable) {
    assert(_shouldLowerVariable(variable));
    final cell = _lookupVariableCell(variable);
    if (cell != null) return cell;
    return variable.initializer == null
        ? _uninitializedVariableCell(variable)
        : _initializedVariableCell(variable);
  }

  VariableDeclaration _uninitializedVariableCell(VariableDeclaration variable) {
    assert(_shouldLowerUninitializedVariable(variable));
    int fileOffset = variable.fileOffset;
    String? name = variable.name;
    final cell = VariableDeclaration(
      name,
      initializer: _callCellConstructor(
        _nameLiteral(name, fileOffset),
        fileOffset,
      ),
      type: InterfaceType(_coreTypes.cellClass, nonNullable),
      isFinal: true,
      isSynthesized: true,
    )..fileOffset = fileOffset;

    return _addToCurrentScope(variable, cell);
  }

  FunctionExpression _initializerClosure(
    Expression initializer,
    DartType type,
  ) {
    int fileOffset = initializer.fileOffset;
    ReturnStatement body = ReturnStatement(initializer)
      ..fileOffset = fileOffset;
    FunctionNode closure = FunctionNode(body, returnType: type)
      ..fileOffset = fileOffset;
    return FunctionExpression(closure)..fileOffset = fileOffset;
  }

  VariableDeclaration _initializedVariableCell(VariableDeclaration variable) {
    assert(_shouldLowerInitializedVariable(variable));
    int fileOffset = variable.fileOffset;
    String? name = variable.name;
    final cell = VariableDeclaration(
      name,
      initializer: _callInitializedCellConstructor(
        _nameLiteral(name, fileOffset),
        _initializerClosure(variable.initializer!, variable.type),
        fileOffset,
      ),
      type: InterfaceType(_coreTypes.initializedCellClass, nonNullable),
      isFinal: true,
      isSynthesized: true,
    )..fileOffset = fileOffset;
    return _addToCurrentScope(variable, cell);
  }

  TreeNode transformVariableDeclaration(
    VariableDeclaration variable,
    Member? contextMember,
  ) {
    _contextMember = contextMember;

    if (!_shouldLowerVariable(variable)) return variable;

    // A [VariableDeclaration] being used as a statement must be a direct child
    // of a [Block].
    if (variable.parent is! Block) return variable;

    return _variableCell(variable);
  }

  VariableGet _variableCellRead(VariableDeclaration variable, int fileOffset) {
    assert(_shouldLowerVariable(variable));
    return VariableGet(_variableCell(variable))..fileOffset = fileOffset;
  }

  TreeNode transformVariableGet(VariableGet node, Member contextMember) {
    _contextMember = contextMember;

    VariableDeclaration variable = node.variable;
    if (!_shouldLowerVariable(variable)) return node;

    int fileOffset = node.fileOffset;
    VariableGet cell = _variableCellRead(variable, fileOffset);
    _Reader reader =
        variable.initializer == null
            ? _readLocal
            : (variable.isFinal ? _readInitializedFinal : _readInitialized);
    return _callReader(
      reader,
      cell,
      node.promotedType ?? variable.type,
      fileOffset,
    );
  }

  TreeNode transformVariableSet(VariableSet node, Member contextMember) {
    _contextMember = contextMember;

    VariableDeclaration variable = node.variable;
    if (!_shouldLowerVariable(variable)) return node;

    int fileOffset = node.fileOffset;
    VariableGet cell = _variableCellRead(variable, fileOffset);
    Procedure setter =
        variable.initializer == null
            ? (variable.isFinal
                ? _coreTypes.cellFinalLocalValueSetter
                : _coreTypes.cellValueSetter)
            : (variable.isFinal
                ? _coreTypes.initializedCellFinalValueSetter
                : _coreTypes.initializedCellValueSetter);
    return _callSetter(setter, cell, node.value, fileOffset);
  }

  Field _fieldCell(Field field) {
    assert(_shouldLowerStaticField(field));
    return _fieldCells.putIfAbsent(field, () {
      int fileOffset = field.fileOffset;
      Name name = field.name;
      Uri fileUri = field.fileUri;
      DartType type = field.type;
      // We need to unbind the canonical name since we reuse the reference but
      // change the name.
      field.fieldReference.canonicalName?.unbind();
      Field fieldCell =
          Field.immutable(
              _mangleFieldCellName(field),
              type: InterfaceType(_coreTypes.cellClass, nonNullable),
              initializer: _callCellConstructor(
                _nameLiteral(name.text, fileOffset),
                fileOffset,
              ),
              isFinal: true,
              isStatic: true,
              fileUri: fileUri,
              fieldReference: field.fieldReference,
            )
            ..fileOffset = fileOffset
            // TODO(fishythefish,srujzs,johnniwinther): Also mark the getter/setter
            //  as extension/extension type members.
            ..isExtensionMember = field.isExtensionMember
            ..isExtensionTypeMember = field.isExtensionTypeMember;
      StaticGet fieldCellAccess() =>
          StaticGet(fieldCell)..fileOffset = fileOffset;

      Procedure getter = Procedure(
        name,
        ProcedureKind.Getter,
        FunctionNode(
          ReturnStatement(
            _callReader(_readField, fieldCellAccess(), type, fileOffset),
          )..fileOffset = fileOffset,
          returnType: type,
        )..fileOffset = fileOffset,
        isStatic: true,
        fileUri: fileUri,
        reference: field.getterReference,
      )..fileOffset = fileOffset;

      VariableDeclaration setterValue = VariableDeclaration(
        'value',
        type: type,
        isSynthesized: true,
      )..fileOffset = fileOffset;
      VariableGet setterValueRead() =>
          VariableGet(setterValue)..fileOffset = fileOffset;

      Procedure setter = Procedure(
        name,
        ProcedureKind.Setter,
        FunctionNode(
          ReturnStatement(
            _callSetter(
              field.isFinal
                  ? _coreTypes.cellFinalFieldValueSetter
                  : _coreTypes.cellValueSetter,
              fieldCellAccess(),
              setterValueRead(),
              fileOffset,
            ),
          )..fileOffset = fileOffset,
          positionalParameters: [setterValue],
          returnType: VoidType(),
        )..fileOffset = fileOffset,
        isStatic: true,
        fileUri: fileUri,
        reference: field.setterReference,
      )..fileOffset = fileOffset;

      TreeNode parent = field.parent!;
      if (parent is Class) {
        parent.addProcedure(getter);
        parent.addProcedure(setter);
      } else if (parent is Library) {
        parent.addProcedure(getter);
        parent.addProcedure(setter);
      }

      return fieldCell;
    });
  }

  Field _backingInstanceField(Field field) {
    assert(_shouldLowerInstanceField(field));
    return _backingInstanceFields[field] ??= _computeBackingInstanceField(
      field,
    );
  }

  Field _computeBackingInstanceField(Field field) {
    assert(_shouldLowerInstanceField(field));
    assert(!_backingInstanceFields.containsKey(field));
    int fileOffset = field.fileOffset;
    Uri fileUri = field.fileUri;
    Name name = field.name;
    String nameText = name.text;
    Name mangledName = _mangleFieldName(field);
    DartType type = field.type;
    Expression? initializer = field.initializer;
    Class enclosingClass = field.enclosingClass!;

    // We need to unbind the canonical name since we reuse the reference but
    // change the name.
    field.fieldReference.canonicalName?.unbind();
    Field backingField =
        Field.mutable(
            mangledName,
            type: type,
            initializer: StaticInvocation(
              _coreTypes.createSentinelMethod,
              Arguments(const [], types: [type])..fileOffset = fileOffset,
            )..fileOffset = fileOffset,
            fileUri: fileUri,
            fieldReference: field.fieldReference,
          )
          ..fileOffset = fileOffset
          ..isInternalImplementation = true;
    InstanceGet fieldRead() => InstanceGet(
      InstanceAccessKind.Instance,
      ThisExpression()..fileOffset = fileOffset,
      mangledName,
      interfaceTarget: backingField,
      resultType: type,
    )..fileOffset = fileOffset;
    InstanceSet fieldWrite(Expression value) => InstanceSet(
      InstanceAccessKind.Instance,
      ThisExpression()..fileOffset = fileOffset,
      mangledName,
      value,
      interfaceTarget: backingField,
    )..fileOffset = fileOffset;

    Statement getterBody() {
      if (initializer == null) {
        // The lowered getter for `late T field;` and `late final T field;` is
        //
        // T get field => _lateReadCheck<T>(this._#field, "field");
        return ReturnStatement(
          StaticInvocation(
            _coreTypes.lateReadCheck,
            Arguments(
              [fieldRead(), _nameLiteral(nameText, fileOffset)],
              types: [type],
            )..fileOffset = fileOffset,
          )..fileOffset = fileOffset,
        )..fileOffset = fileOffset;
      } else if (field.isFinal) {
        // The lowered getter for `late final T field = e;` is
        //
        // T get field {
        //   var value = this._#field;
        //   if (isSentinel(value)) {
        //     final result = e;
        //     _lateInitializeOnceCheck(this._#field, "field");
        //     value = this._#field = result;
        //   }
        //   return value;
        // }
        VariableDeclaration value = VariableDeclaration(
          'value',
          initializer: fieldRead(),
          type: type,
          isSynthesized: true,
        )..fileOffset = fileOffset;
        VariableGet valueRead() => VariableGet(value)..fileOffset = fileOffset;
        VariableDeclaration result = VariableDeclaration(
          'result',
          initializer: initializer,
          type: type,
          isFinal: true,
          isSynthesized: true,
        )..fileOffset = fileOffset;
        VariableGet resultRead() =>
            VariableGet(result)..fileOffset = fileOffset;
        return Block([
          value,
          IfStatement(
            _callIsSentinel(valueRead(), fileOffset),
            Block([
              result,
              ExpressionStatement(
                StaticInvocation(
                  _coreTypes.lateInitializeOnceCheck,
                  Arguments([fieldRead(), _nameLiteral(nameText, fileOffset)])
                    ..fileOffset = fileOffset,
                )..fileOffset = fileOffset,
              )..fileOffset = fileOffset,
              ExpressionStatement(
                VariableSet(value, fieldWrite(resultRead()))
                  ..fileOffset = fileOffset,
              )..fileOffset = fileOffset,
            ])..fileOffset = fileOffset,
            null,
          )..fileOffset = fileOffset,
          ReturnStatement(valueRead())..fileOffset = fileOffset,
        ])..fileOffset = fileOffset;
      } else {
        // The lowered getter for `late T field = e;` is
        //
        // T get field {
        //   var value = this._#field;
        //   if (isSentinel(value)) {
        //     value = this._#field = e;
        //   }
        //   return value;
        // }
        //
        // The following lowering is also possible but currently worse:
        //
        // T get field {
        //   var value = this._#field;
        //   return isSentinel(value) ? this._#field = e : value;
        // }
        //
        // This lowering avoids generating an extra narrowing node in inference,
        // but the generated code is worse due to poor register allocation.
        VariableDeclaration value = VariableDeclaration(
          'value',
          initializer: fieldRead(),
          type: type,
          isSynthesized: true,
        )..fileOffset = fileOffset;
        VariableGet valueRead() => VariableGet(value)..fileOffset = fileOffset;
        return Block([
          value,
          IfStatement(
            _callIsSentinel(valueRead(), fileOffset),
            ExpressionStatement(
              VariableSet(value, fieldWrite(initializer))
                ..fileOffset = fileOffset,
            )..fileOffset = fileOffset,
            null,
          )..fileOffset = fileOffset,
          ReturnStatement(valueRead())..fileOffset = fileOffset,
        ])..fileOffset = fileOffset;
      }
    }

    Procedure getter = Procedure(
      name,
      ProcedureKind.Getter,
      FunctionNode(getterBody(), returnType: type)..fileOffset = fileOffset,
      fileUri: fileUri,
      reference: field.getterReference,
    )..fileOffset = fileOffset;
    // The initializer is copied from [field] to [getter] so we copy the
    // transformer flags to reflect whether the getter contains super calls.
    getter.transformerFlags = field.transformerFlags;
    _copyAnnotations(getter, field);
    enclosingClass.addProcedure(getter);

    VariableDeclaration setterValue = VariableDeclaration(
      'value',
      type: type,
      isSynthesized: true,
    )..fileOffset = fileOffset;
    VariableGet setterValueRead() =>
        VariableGet(setterValue)..fileOffset = fileOffset;

    Statement? setterBody() {
      if (!field.isFinal) {
        // The lowered setter for `late T field;` and `late T field = e;` is
        //
        // set field(T value) {
        //   this._#field = value;
        // }
        return ExpressionStatement(fieldWrite(setterValueRead()))
          ..fileOffset = fileOffset;
      } else if (initializer == null) {
        // The lowered setter for `late final T field;` is
        //
        // set field(T value) {
        //   _lateWriteOnceCheck(this._#field, "field");
        //   this._#field = value;
        // }
        return Block([
          ExpressionStatement(
            StaticInvocation(
              _coreTypes.lateWriteOnceCheck,
              Arguments([fieldRead(), _nameLiteral(nameText, fileOffset)])
                ..fileOffset = fileOffset,
            )..fileOffset = fileOffset,
          )..fileOffset = fileOffset,
          ExpressionStatement(fieldWrite(setterValueRead()))
            ..fileOffset = fileOffset,
        ])..fileOffset = fileOffset;
      } else {
        // There is no setter for `late final T field = e;`.
        return null;
      }
    }

    Statement? body = setterBody();
    if (body != null) {
      Procedure setter = Procedure(
        name,
        ProcedureKind.Setter,
        FunctionNode(
          body,
          positionalParameters: [setterValue],
          returnType: VoidType(),
        )..fileOffset = fileOffset,
        fileUri: fileUri,
        reference: field.setterReference,
      )..fileOffset = fileOffset;
      _copyAnnotations(setter, field);
      enclosingClass.addProcedure(setter);
    }

    return backingField;
  }

  void _copyAnnotations(Member target, Member source) {
    for (final annotation in source.annotations) {
      if (annotation is ConstantExpression) {
        target.addAnnotation(
          ConstantExpression(annotation.constant, annotation.type)
            ..fileOffset = annotation.fileOffset,
        );
      } else {
        throw StateError('Non-constant annotation on $source');
      }
    }
  }

  TreeNode transformField(Field field, Member contextMember) {
    _contextMember = contextMember;

    if (_shouldLowerStaticField(field)) return _fieldCell(field);
    if (_shouldLowerInstanceField(field)) return _backingInstanceField(field);

    return field;
  }
}