source: branches/CodeEditor/HeuristicLab.ExtLibs/HeuristicLab.NRefactory/5.5.0/NRefactory.CSharp-5.5.0/Parser/mcs/iterators.cs @ 11700

//
// iterators.cs: Support for implementing iterators
//
// Author:
//   Miguel de Icaza (miguel@ximian.com)
//   Marek Safar (marek.safar@gmail.com)
//
// Dual licensed under the terms of the MIT X11 or GNU GPL
// Copyright 2003 Ximian, Inc.
// Copyright 2003-2008 Novell, Inc.
// Copyright 2011 Xamarin Inc.
//

using System;
using System.Collections.Generic;
using Mono.CompilerServices.SymbolWriter;

#if STATIC
using IKVM.Reflection.Emit;
#else
using System.Reflection.Emit;
#endif

namespace Mono.CSharp
{
  public abstract class YieldStatement<T> : ResumableStatement where T : StateMachineInitializer
  {
    protected Expression expr;
    protected bool unwind_protect;
    protected T machine_initializer;
    int resume_pc;
    ExceptionStatement inside_try_block;

    protected YieldStatement (Expression expr, Location l)
    {
      this.expr = expr;
      loc = l;
    }

    public Expression Expr {
      get { return this.expr; }
    }
    
    protected override void CloneTo (CloneContext clonectx, Statement t)
    {
      var target = (YieldStatement<T>) t;
      target.expr = expr.Clone (clonectx);
    }

    protected override void DoEmit (EmitContext ec)
    {
      machine_initializer.InjectYield (ec, expr, resume_pc, unwind_protect, resume_point);
    }

    protected override bool DoFlowAnalysis (FlowAnalysisContext fc)
    {
      expr.FlowAnalysis (fc);

      RegisterResumePoint ();

      return false;
    }

    public override bool Resolve (BlockContext bc)
    {
      expr = expr.Resolve (bc);
      if (expr == null)
        return false;

      machine_initializer = bc.CurrentAnonymousMethod as T;
      inside_try_block = bc.CurrentTryBlock;
      return true;
    }

    public void RegisterResumePoint ()
    {
      if (resume_pc != 0)
        return;

      if (inside_try_block == null) {
        resume_pc = machine_initializer.AddResumePoint (this);
      } else {
        resume_pc = inside_try_block.AddResumePoint (this, resume_pc, machine_initializer);
        unwind_protect = true;
        inside_try_block = null;
      }
    }
  }

  public class Yield : YieldStatement<Iterator>
  {
    public Yield (Expression expr, Location loc)
      : base (expr, loc)
    {
    }

    public static bool CheckContext (BlockContext bc, Location loc)
    {
      if (!bc.CurrentAnonymousMethod.IsIterator) {
        bc.Report.Error (1621, loc,
          "The yield statement cannot be used inside anonymous method blocks");
        return false;
      }

      if (bc.HasSet (ResolveContext.Options.FinallyScope)) {
        bc.Report.Error (1625, loc, "Cannot yield in the body of a finally clause");
        return false;
      }

      return true;
    }

    public override bool Resolve (BlockContext bc)
    {
      if (!CheckContext (bc, loc))
        return false;

      if (bc.HasAny (ResolveContext.Options.TryWithCatchScope)) {
        bc.Report.Error (1626, loc, "Cannot yield a value in the body of a try block with a catch clause");
      }

      if (bc.HasSet (ResolveContext.Options.CatchScope)) {
        bc.Report.Error (1631, loc, "Cannot yield a value in the body of a catch clause");
      }

      if (!base.Resolve (bc))
        return false;

      var otype = bc.CurrentIterator.OriginalIteratorType;
      if (expr.Type != otype) {
        expr = Convert.ImplicitConversionRequired (bc, expr, otype, loc);
        if (expr == null)
          return false;
      }

      return true;
    }
    
    public override object Accept (StructuralVisitor visitor)
    {
      return visitor.Visit (this);
    }
  }

  public class YieldBreak : ExitStatement
  {
    Iterator iterator;

    public YieldBreak (Location l)
    {
      loc = l;
    }

    protected override bool IsLocalExit {
      get {
        return false;
      }
    }

    protected override void CloneTo (CloneContext clonectx, Statement target)
    {
      throw new NotSupportedException ();
    }

    protected override bool DoResolve (BlockContext bc)
    {
      iterator = bc.CurrentIterator;
      return Yield.CheckContext (bc, loc);
    }

    protected override void DoEmit (EmitContext ec)
    {
      iterator.EmitYieldBreak (ec, unwind_protect);
    }

    protected override bool DoFlowAnalysis (FlowAnalysisContext fc)
    {
      return true;
    }

    public override Reachability MarkReachable (Reachability rc)
    {
      base.MarkReachable (rc);
      return Reachability.CreateUnreachable ();
    }
    
    public override object Accept (StructuralVisitor visitor)
    {
      return visitor.Visit (this);
    }
  }

  public abstract class StateMachine : AnonymousMethodStorey
  {
    public enum State
    {
      Running = -3, // Used only in CurrentPC, never stored into $PC
      Uninitialized = -2,
      After = -1,
      Start = 0
    }

    Field pc_field;
    StateMachineMethod method;
    int local_name_idx;

    protected StateMachine (ParametersBlock block, TypeDefinition parent, MemberBase host, TypeParameters tparams, string name, MemberKind kind)
      : base (block, parent, host, tparams, name, kind)
    {
      OriginalTypeParameters = tparams;
    }

    #region Properties

    public TypeParameters OriginalTypeParameters { get; private set; }

    public StateMachineMethod StateMachineMethod {
      get {
        return method;
      }
    }

    public Field PC {
      get {
        return pc_field;
      }
    }

    #endregion

    public void AddEntryMethod (StateMachineMethod method)
    {
      if (this.method != null)
        throw new InternalErrorException ();

      this.method = method;
      Members.Add (method);
    }

    protected override bool DoDefineMembers ()
    {
      pc_field = AddCompilerGeneratedField ("$PC", new TypeExpression (Compiler.BuiltinTypes.Int, Location));

      return base.DoDefineMembers ();
    }

    protected override string GetVariableMangledName (LocalVariable local_info)
    {
      if (local_info.IsCompilerGenerated)
        return base.GetVariableMangledName (local_info);

      return "<" + local_info.Name + ">__" + local_name_idx++.ToString ("X");
    }
  }

  class IteratorStorey : StateMachine
  {
    class GetEnumeratorMethod : StateMachineMethod
    {
      sealed class GetEnumeratorStatement : Statement
      {
        readonly IteratorStorey host;
        readonly StateMachineMethod host_method;

        Expression new_storey;

        public GetEnumeratorStatement (IteratorStorey host, StateMachineMethod host_method)
        {
          this.host = host;
          this.host_method = host_method;
          loc = host_method.Location;
        }

        protected override void CloneTo (CloneContext clonectx, Statement target)
        {
          throw new NotSupportedException ();
        }

        public override bool Resolve (BlockContext ec)
        {
          TypeExpression storey_type_expr = new TypeExpression (host.Definition, loc);
          List<Expression> init = null;
          if (host.hoisted_this != null) {
            init = new List<Expression> (host.hoisted_params == null ? 1 : host.HoistedParameters.Count + 1);
            HoistedThis ht = host.hoisted_this;
            FieldExpr from = new FieldExpr (ht.Field, loc);
            from.InstanceExpression = new CompilerGeneratedThis (ec.CurrentType, loc);
            init.Add (new ElementInitializer (ht.Field.Name, from, loc));
          }

          if (host.hoisted_params != null) {
            if (init == null)
              init = new List<Expression> (host.HoistedParameters.Count);

            for (int i = 0; i < host.hoisted_params.Count; ++i) {
              HoistedParameter hp = host.hoisted_params [i];
              HoistedParameter hp_cp = host.hoisted_params_copy [i] ?? hp;

              FieldExpr from = new FieldExpr (hp_cp.Field, loc);
              from.InstanceExpression = new CompilerGeneratedThis (ec.CurrentType, loc);

              init.Add (new ElementInitializer (hp.Field.Name, from, loc));
            }
          }

          if (init != null) {
            new_storey = new NewInitialize (storey_type_expr, null,
              new CollectionOrObjectInitializers (init, loc), loc);
          } else {
            new_storey = new New (storey_type_expr, null, loc);
          }

          new_storey = new_storey.Resolve (ec);
          if (new_storey != null)
            new_storey = Convert.ImplicitConversionRequired (ec, new_storey, host_method.MemberType, loc);

          return true;
        }

        protected override void DoEmit (EmitContext ec)
        {
          Label label_init = ec.DefineLabel ();

          ec.EmitThis ();
          ec.Emit (OpCodes.Ldflda, host.PC.Spec);
          ec.EmitInt ((int) State.Start);
          ec.EmitInt ((int) State.Uninitialized);

          var m = ec.Module.PredefinedMembers.InterlockedCompareExchange.Resolve (loc);
          if (m != null)
            ec.Emit (OpCodes.Call, m);

          ec.EmitInt ((int) State.Uninitialized);
          ec.Emit (OpCodes.Bne_Un_S, label_init);

          ec.EmitThis ();
          ec.Emit (OpCodes.Ret);

          ec.MarkLabel (label_init);

          new_storey.Emit (ec);
          ec.Emit (OpCodes.Ret);
        }

        protected override bool DoFlowAnalysis (FlowAnalysisContext fc)
        {
          throw new NotImplementedException ();
        }

        public override Reachability MarkReachable (Reachability rc)
        {
          base.MarkReachable (rc);
          return Reachability.CreateUnreachable ();
        }
      }

      GetEnumeratorMethod (IteratorStorey host, FullNamedExpression returnType, MemberName name)
        : base (host, null, returnType, Modifiers.DEBUGGER_HIDDEN, name, ToplevelBlock.Flags.CompilerGenerated | ToplevelBlock.Flags.NoFlowAnalysis)
      {
      }

      public static GetEnumeratorMethod Create (IteratorStorey host, FullNamedExpression returnType, MemberName name)
      {
        return Create (host, returnType, name, null);
      }

      public static GetEnumeratorMethod Create (IteratorStorey host, FullNamedExpression returnType, MemberName name, Statement statement)
      {
        var m = new GetEnumeratorMethod (host, returnType, name);
        var stmt = statement ?? new GetEnumeratorStatement (host, m);
        m.block.AddStatement (stmt);
        return m;
      }
    }

    class DisposeMethod : StateMachineMethod
    {
      sealed class DisposeMethodStatement : Statement
      {
        Iterator iterator;

        public DisposeMethodStatement (Iterator iterator)
        {
          this.iterator = iterator;
          this.loc = iterator.Location;
        }

        protected override void CloneTo (CloneContext clonectx, Statement target)
        {
          throw new NotSupportedException ();
        }

        public override bool Resolve (BlockContext ec)
        {
          return true;
        }

        protected override void DoEmit (EmitContext ec)
        {
          ec.CurrentAnonymousMethod = iterator;
          iterator.EmitDispose (ec);
        }

        protected override bool DoFlowAnalysis (FlowAnalysisContext fc)
        {
          throw new NotImplementedException ();
        }
      }

      public DisposeMethod (IteratorStorey host)
        : base (host, null, new TypeExpression (host.Compiler.BuiltinTypes.Void, host.Location), Modifiers.PUBLIC | Modifiers.DEBUGGER_HIDDEN,
          new MemberName ("Dispose", host.Location), ToplevelBlock.Flags.CompilerGenerated | ToplevelBlock.Flags.NoFlowAnalysis)
      {
        host.Members.Add (this);

        Block.AddStatement (new DisposeMethodStatement (host.Iterator));
      }
    }

    //
    // Uses Method as method info
    //
    class DynamicMethodGroupExpr : MethodGroupExpr
    {
      readonly Method method;

      public DynamicMethodGroupExpr (Method method, Location loc)
        : base ((IList<MemberSpec>) null, null, loc)
      {
        this.method = method;
        eclass = ExprClass.Unresolved;
      }

      protected override Expression DoResolve (ResolveContext ec)
      {
        Methods = new List<MemberSpec> (1) { method.Spec };
        type = method.Parent.Definition;
        InstanceExpression = new CompilerGeneratedThis (type, Location);
        return base.DoResolve (ec);
      }
    }

    class DynamicFieldExpr : FieldExpr
    {
      readonly Field field;

      public DynamicFieldExpr (Field field, Location loc)
        : base (loc)
      {
        this.field = field;
      }

      protected override Expression DoResolve (ResolveContext ec)
      {
        spec = field.Spec;
        type = spec.MemberType;
        InstanceExpression = new CompilerGeneratedThis (type, Location);
        return base.DoResolve (ec);
      }
    }

    public readonly Iterator Iterator;

    List<HoistedParameter> hoisted_params_copy;

    TypeExpr iterator_type_expr;
    Field current_field;
    Field disposing_field;

    TypeSpec generic_enumerator_type;
    TypeSpec generic_enumerable_type;

    public IteratorStorey (Iterator iterator)
      : base (iterator.Container.ParametersBlock, iterator.Host,
        iterator.OriginalMethod as MemberBase, iterator.OriginalMethod.CurrentTypeParameters, "Iterator", MemberKind.Class)
    {
      this.Iterator = iterator;
    }

    public Field CurrentField {
      get {
        return current_field;
      }
    }

    public Field DisposingField {
      get {
        return disposing_field;
      }
    }

    public IList<HoistedParameter> HoistedParameters {
      get { return hoisted_params; }
    }

    protected override Constructor DefineDefaultConstructor (bool is_static)
    {
      var ctor = base.DefineDefaultConstructor (is_static);
      ctor.ModFlags |= Modifiers.DEBUGGER_HIDDEN;
      return ctor;
    }

    protected override TypeSpec[] ResolveBaseTypes (out FullNamedExpression base_class)
    {
      var mtype = Iterator.OriginalIteratorType;
      if (Mutator != null)
        mtype = Mutator.Mutate (mtype);

      iterator_type_expr = new TypeExpression (mtype, Location);

      var ifaces = new List<TypeSpec> (5);
      if (Iterator.IsEnumerable) {
        ifaces.Add (Compiler.BuiltinTypes.IEnumerable);

        if (Module.PredefinedTypes.IEnumerableGeneric.Define ()) {
          generic_enumerable_type = Module.PredefinedTypes.IEnumerableGeneric.TypeSpec.MakeGenericType (Module, new[] { mtype });
          ifaces.Add (generic_enumerable_type);
        }
      }

      ifaces.Add (Compiler.BuiltinTypes.IEnumerator);
      ifaces.Add (Compiler.BuiltinTypes.IDisposable);

      var ienumerator_generic = Module.PredefinedTypes.IEnumeratorGeneric;
      if (ienumerator_generic.Define ()) {
        generic_enumerator_type = ienumerator_generic.TypeSpec.MakeGenericType (Module, new [] { mtype });
        ifaces.Add (generic_enumerator_type);
      }

      base_class = null;

      base_type = Compiler.BuiltinTypes.Object;
      return ifaces.ToArray ();
    }

    protected override bool DoDefineMembers ()
    {
      current_field = AddCompilerGeneratedField ("$current", iterator_type_expr);
      disposing_field = AddCompilerGeneratedField ("$disposing", new TypeExpression (Compiler.BuiltinTypes.Bool, Location));

      if (Iterator.IsEnumerable && hoisted_params != null) {
        //
        // Iterators are independent, each GetEnumerator call has to
        // create same enumerator therefore we have to keep original values
        // around for re-initialization
        //
        hoisted_params_copy = new List<HoistedParameter> (hoisted_params.Count);
        foreach (HoistedParameter hp in hoisted_params) {

          //
          // Don't create field copy for unmodified captured parameters
          //
          HoistedParameter hp_copy;
          if (hp.IsAssigned) {
            hp_copy = new HoistedParameter (hp, "<$>" + hp.Field.Name);
          } else {
            hp_copy = null;
          }

          hoisted_params_copy.Add (hp_copy);
        }
      }

      if (generic_enumerator_type != null)
        Define_Current (true);

      Define_Current (false);
      new DisposeMethod (this);
      Define_Reset ();

      if (Iterator.IsEnumerable) {
        FullNamedExpression explicit_iface = new TypeExpression (Compiler.BuiltinTypes.IEnumerable, Location);
        var name = new MemberName ("GetEnumerator", null, explicit_iface, Location.Null);

        if (generic_enumerator_type != null) {
          explicit_iface = new TypeExpression (generic_enumerable_type, Location);
          var gname = new MemberName ("GetEnumerator", null, explicit_iface, Location.Null);
          Method gget_enumerator = GetEnumeratorMethod.Create (this, new TypeExpression (generic_enumerator_type, Location), gname);

          //
          // Just call generic GetEnumerator implementation
          //
          var stmt = new Return (new Invocation (new DynamicMethodGroupExpr (gget_enumerator, Location), null), Location);
          Method get_enumerator = GetEnumeratorMethod.Create (this, new TypeExpression (Compiler.BuiltinTypes.IEnumerator, Location), name, stmt);

          Members.Add (get_enumerator);
          Members.Add (gget_enumerator);
        } else {
          Members.Add (GetEnumeratorMethod.Create (this, new TypeExpression (Compiler.BuiltinTypes.IEnumerator, Location), name));
        }
      }

      return base.DoDefineMembers ();
    }

    void Define_Current (bool is_generic)
    {
      TypeExpr type;
      FullNamedExpression explicit_iface;

      if (is_generic) {
        explicit_iface = new TypeExpression (generic_enumerator_type, Location);
        type = iterator_type_expr;
      } else {
        explicit_iface = new TypeExpression (Module.Compiler.BuiltinTypes.IEnumerator, Location);
        type = new TypeExpression (Compiler.BuiltinTypes.Object, Location);
      }

      var name = new MemberName ("Current", null, explicit_iface, Location);

      ToplevelBlock get_block = new ToplevelBlock (Compiler, ParametersCompiled.EmptyReadOnlyParameters, Location,
        Block.Flags.CompilerGenerated | Block.Flags.NoFlowAnalysis);
      get_block.AddStatement (new Return (new DynamicFieldExpr (CurrentField, Location), Location));
        
      Property current = new Property (this, type, Modifiers.DEBUGGER_HIDDEN | Modifiers.COMPILER_GENERATED, name, null);
      current.Get = new Property.GetMethod (current, Modifiers.COMPILER_GENERATED, null, Location);
      current.Get.Block = get_block;

      Members.Add (current);
    }

    void Define_Reset ()
    {
      Method reset = new Method (
        this, new TypeExpression (Compiler.BuiltinTypes.Void, Location),
        Modifiers.PUBLIC | Modifiers.DEBUGGER_HIDDEN | Modifiers.COMPILER_GENERATED,
        new MemberName ("Reset", Location),
        ParametersCompiled.EmptyReadOnlyParameters, null);
      Members.Add (reset);

      reset.Block = new ToplevelBlock (Compiler, reset.ParameterInfo, Location,
        Block.Flags.CompilerGenerated | Block.Flags.NoFlowAnalysis);

      TypeSpec ex_type = Module.PredefinedTypes.NotSupportedException.Resolve ();
      if (ex_type == null)
        return;

      reset.Block.AddStatement (new Throw (new New (new TypeExpression (ex_type, Location), null, Location), Location));
    }

    protected override void EmitHoistedParameters (EmitContext ec, List<HoistedParameter> hoisted)
    {
      base.EmitHoistedParameters (ec, hoisted);
      if (hoisted_params_copy != null)
        base.EmitHoistedParameters (ec, hoisted_params_copy);
    }
  }

  public class StateMachineMethod : Method
  {
    readonly StateMachineInitializer expr;

    public StateMachineMethod (StateMachine host, StateMachineInitializer expr, FullNamedExpression returnType,
      Modifiers mod, MemberName name, ToplevelBlock.Flags blockFlags)
      : base (host, returnType, mod | Modifiers.COMPILER_GENERATED,
        name, ParametersCompiled.EmptyReadOnlyParameters, null)
    {
      this.expr = expr;
      Block = new ToplevelBlock (host.Compiler, ParametersCompiled.EmptyReadOnlyParameters, Location.Null, blockFlags);
    }

    public override EmitContext CreateEmitContext (ILGenerator ig, SourceMethodBuilder sourceMethod)
    {
      EmitContext ec = new EmitContext (this, ig, MemberType, sourceMethod);
      ec.CurrentAnonymousMethod = expr;

      if (expr is AsyncInitializer)
        ec.With (BuilderContext.Options.AsyncBody, true);

      return ec;
    }
  }

  public abstract class StateMachineInitializer : AnonymousExpression
  {
    sealed class MoveNextBodyStatement : Statement
    {
      readonly StateMachineInitializer state_machine;

      public MoveNextBodyStatement (StateMachineInitializer stateMachine)
      {
        this.state_machine = stateMachine;
        this.loc = stateMachine.Location;
      }

      protected override void CloneTo (CloneContext clonectx, Statement target)
      {
        throw new NotSupportedException ();
      }

      public override bool Resolve (BlockContext ec)
      {
        return true;
      }

      protected override void DoEmit (EmitContext ec)
      {
        state_machine.EmitMoveNext (ec);
      }

      public override void Emit (EmitContext ec)
      {
        // Don't create sequence point
        DoEmit (ec);
      }

      protected override bool DoFlowAnalysis (FlowAnalysisContext fc)
      {
        return state_machine.ReturnType.Kind != MemberKind.Void;
      }

      public override Reachability MarkReachable (Reachability rc)
      {
        base.MarkReachable (rc);

        if (state_machine.ReturnType.Kind != MemberKind.Void)
          rc = Reachability.CreateUnreachable ();

        return rc;
      }
    }

    public readonly TypeDefinition Host;
    protected StateMachine storey;

    //
    // The state as we generate the machine
    //
    protected Label move_next_ok;
    protected Label move_next_error;
    LocalBuilder skip_finally;
    protected LocalBuilder current_pc;
    protected List<ResumableStatement> resume_points;

    protected StateMachineInitializer (ParametersBlock block, TypeDefinition host, TypeSpec returnType)
      : base (block, returnType, block.StartLocation)
    {
      this.Host = host;
    }

    #region Properties

    public Label BodyEnd { get; set; }

    public LocalBuilder CurrentPC
    {
      get {
        return current_pc;
      }
    }

    public LocalBuilder SkipFinally {
      get {
        return skip_finally;
      }
    }

    public override AnonymousMethodStorey Storey {
      get {
        return storey;
      }
    }

    #endregion

    public int AddResumePoint (ResumableStatement stmt)
    {
      if (resume_points == null)
        resume_points = new List<ResumableStatement> ();

      resume_points.Add (stmt);
      return resume_points.Count;
    }

    public override Expression CreateExpressionTree (ResolveContext ec)
    {
      throw new NotSupportedException ("ET");
    }

    protected virtual BlockContext CreateBlockContext (BlockContext bc)
    {
      var ctx = new BlockContext (bc, block, bc.ReturnType);
      ctx.CurrentAnonymousMethod = this;

      ctx.AssignmentInfoOffset = bc.AssignmentInfoOffset;
      ctx.EnclosingLoop = bc.EnclosingLoop;
      ctx.EnclosingLoopOrSwitch = bc.EnclosingLoopOrSwitch;
      ctx.Switch = bc.Switch;

      return ctx;
    }

    protected override Expression DoResolve (ResolveContext rc)
    {
      var bc = (BlockContext) rc;
      var ctx = CreateBlockContext (bc);

      Block.Resolve (ctx);

      if (!rc.IsInProbingMode) {
        var move_next = new StateMachineMethod (storey, this, new TypeExpression (ReturnType, loc), Modifiers.PUBLIC, new MemberName ("MoveNext", loc), 0);
        move_next.Block.AddStatement (new MoveNextBodyStatement (this));
        storey.AddEntryMethod (move_next);
      }

      bc.AssignmentInfoOffset = ctx.AssignmentInfoOffset;
      eclass = ExprClass.Value;
      return this;
    }

    public override void Emit (EmitContext ec)
    {
      //
      // Load state machine instance
      //
      storey.Instance.Emit (ec);
    }

    void EmitMoveNext_NoResumePoints (EmitContext ec)
    {
      ec.EmitThis ();
      ec.Emit (OpCodes.Ldfld, storey.PC.Spec);

      ec.EmitThis ();
      ec.EmitInt ((int) IteratorStorey.State.After);
      ec.Emit (OpCodes.Stfld, storey.PC.Spec);

      // We only care if the PC is zero (start executing) or non-zero (don't do anything)
      ec.Emit (OpCodes.Brtrue, move_next_error);

      BodyEnd = ec.DefineLabel ();

      var async_init = this as AsyncInitializer;
      if (async_init != null)
        ec.BeginExceptionBlock ();

      block.EmitEmbedded (ec);

      if (async_init != null)
        async_init.EmitCatchBlock (ec);

      ec.MarkLabel (BodyEnd);

      EmitMoveNextEpilogue (ec);

      ec.MarkLabel (move_next_error);

      if (ReturnType.Kind != MemberKind.Void) {
        ec.EmitInt (0);
        ec.Emit (OpCodes.Ret);
      }

      ec.MarkLabel (move_next_ok);
    }

    void EmitMoveNext (EmitContext ec)
    {
      move_next_ok = ec.DefineLabel ();
      move_next_error = ec.DefineLabel ();

      if (resume_points == null) {
        EmitMoveNext_NoResumePoints (ec);
        return;
      }
      
      current_pc = ec.GetTemporaryLocal (ec.BuiltinTypes.UInt);
      ec.EmitThis ();
      ec.Emit (OpCodes.Ldfld, storey.PC.Spec);
      ec.Emit (OpCodes.Stloc, current_pc);

      // We're actually in state 'running', but this is as good a PC value as any if there's an abnormal exit
      ec.EmitThis ();
      ec.EmitInt ((int) IteratorStorey.State.After);
      ec.Emit (OpCodes.Stfld, storey.PC.Spec);

      Label[] labels = new Label[1 + resume_points.Count];
      labels[0] = ec.DefineLabel ();

      bool need_skip_finally = false;
      for (int i = 0; i < resume_points.Count; ++i) {
        ResumableStatement s = resume_points[i];
        need_skip_finally |= s is ExceptionStatement;
        labels[i + 1] = s.PrepareForEmit (ec);
      }

      if (need_skip_finally) {
        skip_finally = ec.GetTemporaryLocal (ec.BuiltinTypes.Bool);
        ec.EmitInt (0);
        ec.Emit (OpCodes.Stloc, skip_finally);
      }

      var async_init = this as AsyncInitializer;
      if (async_init != null)
        ec.BeginExceptionBlock ();

      ec.Emit (OpCodes.Ldloc, current_pc);
      ec.Emit (OpCodes.Switch, labels);

      ec.Emit (async_init != null ? OpCodes.Leave : OpCodes.Br, move_next_error);

      ec.MarkLabel (labels[0]);

      BodyEnd = ec.DefineLabel ();

      block.EmitEmbedded (ec);

      ec.MarkLabel (BodyEnd);

      if (async_init != null) {
        async_init.EmitCatchBlock (ec);
      }

      ec.Mark (Block.Original.EndLocation);
      ec.EmitThis ();
      ec.EmitInt ((int) IteratorStorey.State.After);
      ec.Emit (OpCodes.Stfld, storey.PC.Spec);

      EmitMoveNextEpilogue (ec);

      ec.MarkLabel (move_next_error);
      
      if (ReturnType.Kind != MemberKind.Void) {
        ec.EmitInt (0);
        ec.Emit (OpCodes.Ret);
      }

      ec.MarkLabel (move_next_ok);

      if (ReturnType.Kind != MemberKind.Void) {
        ec.EmitInt (1);
        ec.Emit (OpCodes.Ret);
      }
    }

    protected virtual void EmitMoveNextEpilogue (EmitContext ec)
    {
    }

    public void EmitLeave (EmitContext ec, bool unwind_protect)
    {
      // Return ok
      ec.Emit (unwind_protect ? OpCodes.Leave : OpCodes.Br, move_next_ok);
    }

    //
    // Called back from YieldStatement
    //
    public virtual void InjectYield (EmitContext ec, Expression expr, int resume_pc, bool unwind_protect, Label resume_point)
    {
      //
      // Guard against being disposed meantime
      //
      Label disposed = ec.DefineLabel ();
      var iterator = storey as IteratorStorey;
      if (iterator != null) {
        ec.EmitThis ();
        ec.Emit (OpCodes.Ldfld, iterator.DisposingField.Spec);
        ec.Emit (OpCodes.Brtrue_S, disposed);
      }

      //
      // store resume program-counter
      //
      ec.EmitThis ();
      ec.EmitInt (resume_pc);
      ec.Emit (OpCodes.Stfld, storey.PC.Spec);

      if (iterator != null) {
        ec.MarkLabel (disposed);
      }

      // mark finally blocks as disabled
      if (unwind_protect && skip_finally != null) {
        ec.EmitInt (1);
        ec.Emit (OpCodes.Stloc, skip_finally);
      }
    }

    public void SetStateMachine (StateMachine stateMachine)
    {
      this.storey = stateMachine;
    }
  }

  //
  // Iterators are implemented as state machine blocks
  //
  public class Iterator : StateMachineInitializer
  {
    sealed class TryFinallyBlockProxyStatement : Statement
    {
      TryFinallyBlock block;
      Iterator iterator;

      public TryFinallyBlockProxyStatement (Iterator iterator, TryFinallyBlock block)
      {
        this.iterator = iterator;
        this.block = block;
      }

      protected override void CloneTo (CloneContext clonectx, Statement target)
      {
        throw new NotSupportedException ();
      }

      protected override bool DoFlowAnalysis (FlowAnalysisContext fc)
      {
        throw new NotSupportedException ();
      }

      protected override void DoEmit (EmitContext ec)
      {
        //
        // Restore redirection for any captured variables
        //
        ec.CurrentAnonymousMethod = iterator;

        using (ec.With (BuilderContext.Options.OmitDebugInfo, !ec.HasMethodSymbolBuilder)) {
          block.EmitFinallyBody (ec);
        }
      }
    }

    public readonly IMethodData OriginalMethod;
    public readonly bool IsEnumerable;
    public readonly TypeSpec OriginalIteratorType;
    int finally_hosts_counter;

    public Iterator (ParametersBlock block, IMethodData method, TypeDefinition host, TypeSpec iterator_type, bool is_enumerable)
      : base (block, host, host.Compiler.BuiltinTypes.Bool)
    {
      this.OriginalMethod = method;
      this.OriginalIteratorType = iterator_type;
      this.IsEnumerable = is_enumerable;
      this.type = method.ReturnType;
    }

    #region Properties

    public ToplevelBlock Container {
      get { return OriginalMethod.Block; }
    }

    public override string ContainerType {
      get { return "iterator"; }
    }

    public override bool IsIterator {
      get { return true; }
    }

    #endregion

    public Method CreateFinallyHost (TryFinallyBlock block)
    {
      var method = new Method (storey, new TypeExpression (storey.Compiler.BuiltinTypes.Void, loc),
        Modifiers.COMPILER_GENERATED, new MemberName (CompilerGeneratedContainer.MakeName (null, null, "Finally", finally_hosts_counter++), loc),
        ParametersCompiled.EmptyReadOnlyParameters, null);

      method.Block = new ToplevelBlock (method.Compiler, method.ParameterInfo, loc,
        ToplevelBlock.Flags.CompilerGenerated | ToplevelBlock.Flags.NoFlowAnalysis);
      method.Block.AddStatement (new TryFinallyBlockProxyStatement (this, block));

      // Cannot it add to storey because it'd be emitted before nested
      // anonoymous methods which could capture shared variable

      return method;
    }

    public void EmitYieldBreak (EmitContext ec, bool unwind_protect)
    {
      ec.Emit (unwind_protect ? OpCodes.Leave : OpCodes.Br, move_next_error);
    }

    public override string GetSignatureForError ()
    {
      return OriginalMethod.GetSignatureForError ();
    }

    public override void Emit (EmitContext ec)
    {
      //
      // Load Iterator storey instance
      //
      storey.Instance.Emit (ec);

      //
      // Initialize iterator PC when it's unitialized
      //
      if (IsEnumerable) {
        ec.Emit (OpCodes.Dup);
        ec.EmitInt ((int)IteratorStorey.State.Uninitialized);

        var field = storey.PC.Spec;
        if (storey.MemberName.IsGeneric) {
          field = MemberCache.GetMember (Storey.Instance.Type, field);
        }

        ec.Emit (OpCodes.Stfld, field);
      }
    }

    public void EmitDispose (EmitContext ec)
    {
      if (resume_points == null)
        return;

      Label end = ec.DefineLabel ();

      Label[] labels = null;
      for (int i = 0; i < resume_points.Count; ++i) {
        ResumableStatement s = resume_points[i];
        Label ret = s.PrepareForDispose (ec, end);
        if (ret.Equals (end) && labels == null)
          continue;
        if (labels == null) {
          labels = new Label[resume_points.Count + 1];
          for (int j = 0; j <= i; ++j)
            labels[j] = end;
        }

        labels[i + 1] = ret;
      }

      if (labels != null) {
        current_pc = ec.GetTemporaryLocal (ec.BuiltinTypes.UInt);
        ec.EmitThis ();
        ec.Emit (OpCodes.Ldfld, storey.PC.Spec);
        ec.Emit (OpCodes.Stloc, current_pc);
      }

      ec.EmitThis ();
      ec.EmitInt (1);
      ec.Emit (OpCodes.Stfld, ((IteratorStorey) storey).DisposingField.Spec);

      ec.EmitThis ();
      ec.EmitInt ((int) IteratorStorey.State.After);
      ec.Emit (OpCodes.Stfld, storey.PC.Spec);

      if (labels != null) {
        //SymbolWriter.StartIteratorDispatcher (ec.ig);
        ec.Emit (OpCodes.Ldloc, current_pc);
        ec.Emit (OpCodes.Switch, labels);
        //SymbolWriter.EndIteratorDispatcher (ec.ig);

        foreach (ResumableStatement s in resume_points)
          s.EmitForDispose (ec, current_pc, end, true);
      }

      ec.MarkLabel (end);
    }

    public override void EmitStatement (EmitContext ec)
    {
      throw new NotImplementedException ();
    }

    public override void InjectYield (EmitContext ec, Expression expr, int resume_pc, bool unwind_protect, Label resume_point)
    {
      // Store the new value into current
      var fe = new FieldExpr (((IteratorStorey) storey).CurrentField, loc);
      fe.InstanceExpression = new CompilerGeneratedThis (storey.CurrentType, loc);
      fe.EmitAssign (ec, expr, false, false);

      base.InjectYield (ec, expr, resume_pc, unwind_protect, resume_point);

      EmitLeave (ec, unwind_protect);

      ec.MarkLabel (resume_point);
    }

    public static void CreateIterator (IMethodData method, TypeDefinition parent, Modifiers modifiers)
    {
      bool is_enumerable;
      TypeSpec iterator_type;

      TypeSpec ret = method.ReturnType;
      if (ret == null)
        return;

      if (!CheckType (ret, parent, out iterator_type, out is_enumerable)) {
        parent.Compiler.Report.Error (1624, method.Location,
                "The body of `{0}' cannot be an iterator block " +
                "because `{1}' is not an iterator interface type",
                method.GetSignatureForError (),
                ret.GetSignatureForError ());
        return;
      }

      ParametersCompiled parameters = method.ParameterInfo;
      for (int i = 0; i < parameters.Count; i++) {
        Parameter p = parameters [i];
        Parameter.Modifier mod = p.ModFlags;
        if ((mod & Parameter.Modifier.RefOutMask) != 0) {
          parent.Compiler.Report.Error (1623, p.Location,
            "Iterators cannot have ref or out parameters");
          return;
        }

        if (p is ArglistParameter) {
          parent.Compiler.Report.Error (1636, method.Location,
            "__arglist is not allowed in parameter list of iterators");
          return;
        }

        if (parameters.Types [i].IsPointer) {
          parent.Compiler.Report.Error (1637, p.Location,
            "Iterators cannot have unsafe parameters or yield types");
          return;
        }
      }

      if ((modifiers & Modifiers.UNSAFE) != 0) {
        parent.Compiler.Report.Error (1629, method.Location, "Unsafe code may not appear in iterators");
      }

      method.Block = method.Block.ConvertToIterator (method, parent, iterator_type, is_enumerable);
    }

    static bool CheckType (TypeSpec ret, TypeContainer parent, out TypeSpec original_iterator_type, out bool is_enumerable)
    {
      original_iterator_type = null;
      is_enumerable = false;

      if (ret.BuiltinType == BuiltinTypeSpec.Type.IEnumerable) {
        original_iterator_type = parent.Compiler.BuiltinTypes.Object;
        is_enumerable = true;
        return true;
      }
      if (ret.BuiltinType == BuiltinTypeSpec.Type.IEnumerator) {
        original_iterator_type = parent.Compiler.BuiltinTypes.Object;
        is_enumerable = false;
        return true;
      }

      InflatedTypeSpec inflated = ret as InflatedTypeSpec;
      if (inflated == null)
        return false;

      var member_definition = inflated.MemberDefinition;
      PredefinedType ptype = parent.Module.PredefinedTypes.IEnumerableGeneric;

      if (ptype.Define () && ptype.TypeSpec.MemberDefinition == member_definition) {
        original_iterator_type = inflated.TypeArguments[0];
        is_enumerable = true;
        return true;
      }

      ptype = parent.Module.PredefinedTypes.IEnumeratorGeneric;
      if (ptype.Define () && ptype.TypeSpec.MemberDefinition == member_definition) {
        original_iterator_type = inflated.TypeArguments[0];
        is_enumerable = false;
        return true;
      }

      return false;
    }
  }
}