mcs/mcs/async.cs

   1 //
   2 // async.cs: Asynchronous functions
   3 //
   4 // Author:
   5 //   Marek Safar (marek.safar@gmail.com)
   6 //
   7 // Dual licensed under the terms of the MIT X11 or GNU GPL
   8 //
   9 // Copyright 2011 Novell, Inc.
  10 // Copyright 2011-2012 Xamarin Inc.
  11 //
  12
  13 using System;
  14 using System.Collections.Generic;
  15 using System.Linq;
  16 using System.Collections;
  17
  18 #if STATIC
  19 using IKVM.Reflection;
  20 using IKVM.Reflection.Emit;
  21 #else
  22 using System.Reflection;
  23 using System.Reflection.Emit;
  24 #endif
  25
  26 namespace Mono.CSharp
  27 {
  28         public class Await : ExpressionStatement
  29         {
  30                 Expression expr;
  31                 AwaitStatement stmt;
  32
  33                 public Await (Expression expr, Location loc)
  34                 {
  35                         this.expr = expr;
  36                         this.loc = loc;
  37                 }
  38
  39                 public Expression Expr {
  40                         get {
  41                                 return expr;
  42                         }
  43                 }
  44
  45                 public AwaitStatement Statement {
  46                         get {
  47                                 return stmt;
  48                         }
  49                 }
  50
  51                 protected override void CloneTo (CloneContext clonectx, Expression target)
  52                 {
  53                         var t = (Await) target;
  54
  55                         t.expr = expr.Clone (clonectx);
  56                 }
  57
  58                 public override Expression CreateExpressionTree (ResolveContext ec)
  59                 {
  60                         throw new NotImplementedException ("ET");
  61                 }
  62
  63                 public override bool ContainsEmitWithAwait ()
  64                 {
  65                         return true;
  66                 }
  67
  68                 protected override Expression DoResolve (ResolveContext rc)
  69                 {
  70                         if (rc.HasSet (ResolveContext.Options.LockScope)) {
  71                                 rc.Report.Error (1996, loc,
  72                                         "The `await' operator cannot be used in the body of a lock statement");
  73                         }
  74
  75                         if (rc.IsUnsafe) {
  76                                 rc.Report.Error (4004, loc,
  77                                         "The `await' operator cannot be used in an unsafe context");
  78                         }
  79
  80                         var bc = (BlockContext) rc;
  81
  82                         stmt = new AwaitStatement (expr, loc);
  83                         if (!stmt.Resolve (bc))
  84                                 return null;
  85
  86                         type = stmt.ResultType;
  87                         eclass = ExprClass.Variable;
  88                         return this;
  89                 }
  90
  91                 public override void Emit (EmitContext ec)
  92                 {
  93                         stmt.EmitPrologue (ec);
  94
  95                         using (ec.With (BuilderContext.Options.OmitDebugInfo, true)) {
  96                                 stmt.Emit (ec);
  97                         }
  98                 }
  99
 100                 public override Expression EmitToField (EmitContext ec)
 101                 {
 102                         stmt.EmitPrologue (ec);
 103                         return stmt.GetResultExpression (ec);
 104                 }
 105
 106                 public void EmitAssign (EmitContext ec, FieldExpr field)
 107                 {
 108                         stmt.EmitPrologue (ec);
 109                         field.InstanceExpression.Emit (ec);
 110                         stmt.Emit (ec);
 111                 }
 112
 113                 public override void EmitStatement (EmitContext ec)
 114                 {
 115                         stmt.EmitStatement (ec);
 116                 }
 117
 118                 public override object Accept (StructuralVisitor visitor)
 119                 {
 120                         return visitor.Visit (this);
 121                 }
 122         }
 123
 124         public class AwaitStatement : YieldStatement<AsyncInitializer>
 125         {
 126                 public sealed class AwaitableMemberAccess : MemberAccess
 127                 {
 128                         public AwaitableMemberAccess (Expression expr)
 129                                 : base (expr, "GetAwaiter")
 130                         {
 131                         }
 132
 133                         public bool ProbingMode { get; set; }
 134
 135                         protected override void Error_TypeDoesNotContainDefinition (ResolveContext rc, TypeSpec type, string name)
 136                         {
 137                                 Error_OperatorCannotBeApplied (rc, type);
 138                         }
 139
 140                         protected override void Error_OperatorCannotBeApplied (ResolveContext rc, TypeSpec type)
 141                         {
 142                                 if (ProbingMode)
 143                                         return;
 144
 145                                 var invocation = LeftExpression as Invocation;
 146                                 if (invocation != null && invocation.MethodGroup != null && (invocation.MethodGroup.BestCandidate.Modifiers & Modifiers.ASYNC) != 0) {
 147                                         rc.Report.Error (4008, loc, "Cannot await void method `{0}'. Consider changing method return type to `Task'",
 148                                                 invocation.GetSignatureForError ());
 149                                 } else {
 150                                         rc.Report.Error (4001, loc, "Cannot await `{0}' expression", type.GetSignatureForError ());
 151                                 }
 152                         }
 153                 }
 154
 155                 sealed class GetResultInvocation : Invocation
 156                 {
 157                         public GetResultInvocation (MethodGroupExpr mge, Arguments arguments)
 158                                 : base (null, arguments)
 159                         {
 160                                 mg = mge;
 161                                 type = mg.BestCandidateReturnType;
 162                         }
 163
 164                         public override Expression EmitToField (EmitContext ec)
 165                         {
 166                                 return this;
 167                         }
 168                 }
 169
 170                 Field awaiter;
 171                 AwaiterDefinition awaiter_definition;
 172                 TypeSpec type;
 173                 TypeSpec result_type;
 174
 175                 public AwaitStatement (Expression expr, Location loc)
 176                         : base (expr, loc)
 177                 {
 178                 }
 179
 180                 #region Properties
 181
 182                 bool IsDynamic {
 183                         get {
 184                                 return awaiter_definition == null;
 185                         }
 186                 }
 187
 188                 public TypeSpec ResultType {
 189                         get {
 190                                 return result_type;
 191                         }
 192                 }
 193
 194                 #endregion
 195
 196                 protected override void DoEmit (EmitContext ec)
 197                 {
 198                         GetResultExpression (ec).Emit (ec);
 199                 }
 200
 201                 public Expression GetResultExpression (EmitContext ec)
 202                 {
 203                         var fe_awaiter = new FieldExpr (awaiter, loc);
 204                         fe_awaiter.InstanceExpression = new CompilerGeneratedThis (ec.CurrentType, loc);
 205
 206                         //
 207                         // result = awaiter.GetResult ();
 208                         //
 209                         if (IsDynamic) {
 210                                 var rc = new ResolveContext (ec.MemberContext);
 211                                 return new Invocation (new MemberAccess (fe_awaiter, "GetResult"), new Arguments (0)).Resolve (rc);
 212                         }
 213
 214                         var mg_result = MethodGroupExpr.CreatePredefined (awaiter_definition.GetResult, fe_awaiter.Type, loc);
 215                         mg_result.InstanceExpression = fe_awaiter;
 216
 217                         return new GetResultInvocation (mg_result, new Arguments (0));
 218                 }
 219
 220                 public void EmitPrologue (EmitContext ec)
 221                 {
 222                         awaiter = ((AsyncTaskStorey) machine_initializer.Storey).AddAwaiter (expr.Type, loc);
 223
 224                         var fe_awaiter = new FieldExpr (awaiter, loc);
 225                         fe_awaiter.InstanceExpression = new CompilerGeneratedThis (ec.CurrentType, loc);
 226
 227                         //
 228                         // awaiter = expr.GetAwaiter ();
 229                         //
 230                         using (ec.With (BuilderContext.Options.OmitDebugInfo, true)) {
 231                                 fe_awaiter.EmitAssign (ec, expr, false, false);
 232                         }
 233
 234                         Label skip_continuation = ec.DefineLabel ();
 235
 236                         Expression completed_expr;
 237                         if (IsDynamic) {
 238                                 var rc = new ResolveContext (ec.MemberContext);
 239
 240                                 Arguments dargs = new Arguments (1);
 241                                 dargs.Add (new Argument (fe_awaiter));
 242                                 completed_expr = new DynamicMemberBinder ("IsCompleted", dargs, loc).Resolve (rc);
 243
 244                                 dargs = new Arguments (1);
 245                                 dargs.Add (new Argument (completed_expr));
 246                                 completed_expr = new DynamicConversion (ec.Module.Compiler.BuiltinTypes.Bool, 0, dargs, loc).Resolve (rc);
 247                         } else {
 248                                 var pe = PropertyExpr.CreatePredefined (awaiter_definition.IsCompleted, loc);
 249                                 pe.InstanceExpression = fe_awaiter;
 250                                 completed_expr = pe;
 251                         }
 252
 253                         completed_expr.EmitBranchable (ec, skip_continuation, true);
 254
 255                         base.DoEmit (ec);
 256
 257                         //
 258                         // The stack has to be empty before calling await continuation. We handle this
 259                         // by lifting values which would be left on stack into class fields. The process
 260                         // is quite complicated and quite hard to test because any expression can possibly
 261                         // leave a value on the stack.
 262                         //
 263                         // Following assert fails when some of expression called before is missing EmitToField
 264                         // or parent expression fails to find await in children expressions
 265                         //
 266                         ec.AssertEmptyStack ();
 267
 268                         var storey = (AsyncTaskStorey) machine_initializer.Storey;
 269                         if (IsDynamic) {
 270                                 storey.EmitAwaitOnCompletedDynamic (ec, fe_awaiter);
 271                         } else {
 272                                 storey.EmitAwaitOnCompleted (ec, fe_awaiter);
 273                         }
 274
 275                         // Return ok
 276                         machine_initializer.EmitLeave (ec, unwind_protect);
 277
 278                         ec.MarkLabel (resume_point);
 279                         ec.MarkLabel (skip_continuation);
 280                 }
 281
 282                 public void EmitStatement (EmitContext ec)
 283                 {
 284                         EmitPrologue (ec);
 285                         DoEmit (ec);
 286
 287                         awaiter.IsAvailableForReuse = true;
 288
 289                         if (ResultType.Kind != MemberKind.Void)
 290                                 ec.Emit (OpCodes.Pop);
 291                 }
 292
 293                 void Error_WrongAwaiterPattern (ResolveContext rc, TypeSpec awaiter)
 294                 {
 295                         rc.Report.Error (4011, loc, "The awaiter type `{0}' must have suitable IsCompleted and GetResult members",
 296                                 awaiter.GetSignatureForError ());
 297                 }
 298
 299                 public override bool Resolve (BlockContext bc)
 300                 {
 301                         if (bc.CurrentBlock is Linq.QueryBlock) {
 302                                 bc.Report.Error (1995, loc,
 303                                         "The `await' operator may only be used in a query expression within the first collection expression of the initial `from' clause or within the collection expression of a `join' clause");
 304                                 return false;
 305                         }
 306
 307                         if (!base.Resolve (bc))
 308                                 return false;
 309
 310                         type = expr.Type;
 311                         Arguments args = new Arguments (0);
 312
 313                         //
 314                         // The await expression is of dynamic type
 315                         //
 316                         if (type.BuiltinType == BuiltinTypeSpec.Type.Dynamic) {
 317                                 result_type = type;
 318                                 expr = new Invocation (new MemberAccess (expr, "GetAwaiter"), args).Resolve (bc);
 319                                 return true;
 320                         }
 321
 322                         //
 323                         // Check whether the expression is awaitable
 324                         //
 325                         Expression ama = new AwaitableMemberAccess (expr).Resolve (bc);
 326                         if (ama == null)
 327                                 return false;
 328
 329                         var errors_printer = new SessionReportPrinter ();
 330                         var old = bc.Report.SetPrinter (errors_printer);
 331                         ama = new Invocation (ama, args).Resolve (bc);
 332                         bc.Report.SetPrinter (old);
 333
 334                         if (errors_printer.ErrorsCount > 0 || !MemberAccess.IsValidDotExpression (ama.Type)) {
 335                                 bc.Report.Error (1986, expr.Location,
 336                                         "The `await' operand type `{0}' must have suitable GetAwaiter method",
 337                                         expr.Type.GetSignatureForError ());
 338
 339                                 return false;
 340                         }
 341
 342                         var awaiter_type = ama.Type;
 343
 344                         awaiter_definition = bc.Module.GetAwaiter (awaiter_type);
 345
 346                         if (!awaiter_definition.IsValidPattern) {
 347                                 Error_WrongAwaiterPattern (bc, awaiter_type);
 348                                 return false;
 349                         }
 350
 351                         if (!awaiter_definition.INotifyCompletion) {
 352                                 bc.Report.Error (4027, loc, "The awaiter type `{0}' must implement interface `{1}'",
 353                                         awaiter_type.GetSignatureForError (), bc.Module.PredefinedTypes.INotifyCompletion.GetSignatureForError ());
 354                                 return false;
 355                         }
 356
 357                         expr = ama;
 358                         result_type = awaiter_definition.GetResult.ReturnType;
 359
 360                         return true;
 361                 }
 362         }
 363
 364         public class AsyncInitializer : StateMachineInitializer
 365         {
 366                 TypeInferenceContext return_inference;
 367
 368                 public AsyncInitializer (ParametersBlock block, TypeDefinition host, TypeSpec returnType)
 369                         : base (block, host, returnType)
 370                 {
 371                 }
 372
 373                 #region Properties
 374
 375                 public override string ContainerType {
 376                         get {
 377                                 return "async state machine block";
 378                         }
 379                 }
 380
 381                 public TypeSpec DelegateType {
 382                         get; set;
 383                 }
 384
 385                 public override bool IsIterator {
 386                         get {
 387                                 return false;
 388                         }
 389                 }
 390
 391                 public TypeInferenceContext ReturnTypeInference {
 392                         get {
 393                                 return return_inference;
 394                         }
 395                 }
 396
 397                 #endregion
 398
 399                 protected override BlockContext CreateBlockContext (ResolveContext rc)
 400                 {
 401                         var ctx = base.CreateBlockContext (rc);
 402                         var lambda = rc.CurrentAnonymousMethod as LambdaMethod;
 403                         if (lambda != null)
 404                                 return_inference = lambda.ReturnTypeInference;
 405
 406                         ctx.StartFlowBranching (this, rc.CurrentBranching);
 407                         return ctx;
 408                 }
 409
 410                 public override Expression CreateExpressionTree (ResolveContext ec)
 411                 {
 412                         return base.CreateExpressionTree (ec);
 413                 }
 414
 415                 public override void Emit (EmitContext ec)
 416                 {
 417                         throw new NotImplementedException ();
 418                 }
 419
 420                 protected override void EmitMoveNextEpilogue (EmitContext ec)
 421                 {
 422                         var storey = (AsyncTaskStorey) Storey;
 423                         storey.EmitSetResult (ec);
 424                 }
 425
 426                 public override void EmitStatement (EmitContext ec)
 427                 {
 428                         var storey = (AsyncTaskStorey) Storey;
 429                         storey.EmitInitializer (ec);
 430                         ec.Emit (OpCodes.Ret);
 431                 }
 432         }
 433
 434         class AsyncTaskStorey : StateMachine
 435         {
 436                 int awaiters;
 437                 Field builder;
 438                 readonly TypeSpec return_type;
 439                 MethodSpec set_result;
 440                 MethodSpec set_exception;
 441                 MethodSpec builder_factory;
 442                 MethodSpec builder_start;
 443                 PropertySpec task;
 444                 LocalVariable hoisted_return;
 445                 int locals_captured;
 446                 Dictionary<TypeSpec, List<Field>> stack_fields;
 447                 Dictionary<TypeSpec, List<Field>> awaiter_fields;
 448
 449                 public AsyncTaskStorey (ParametersBlock block, IMemberContext context, AsyncInitializer initializer, TypeSpec type)
 450                         : base (block, initializer.Host, context.CurrentMemberDefinition as MemberBase, context.CurrentTypeParameters, "async", MemberKind.Struct)
 451                 {
 452                         return_type = type;
 453                         awaiter_fields = new Dictionary<TypeSpec, List<Field>> ();
 454                 }
 455
 456                 #region Properties
 457
 458                 public LocalVariable HoistedReturn {
 459                         get {
 460                                 return hoisted_return;
 461                         }
 462                 }
 463
 464                 public TypeSpec ReturnType {
 465                         get {
 466                                 return return_type;
 467                         }
 468                 }
 469
 470                 public PropertySpec Task {
 471                         get {
 472                                 return task;
 473                         }
 474                 }
 475
 476                 protected override TypeAttributes TypeAttr {
 477                         get {
 478                                 return base.TypeAttr & ~TypeAttributes.SequentialLayout;
 479                         }
 480                 }
 481
 482                 #endregion
 483
 484                 public Field AddAwaiter (TypeSpec type, Location loc)
 485                 {
 486                         if (mutator != null)
 487                                 type = mutator.Mutate (type);
 488
 489                         List<Field> existing_fields = null;
 490                         if (awaiter_fields.TryGetValue (type, out existing_fields)) {
 491                                 foreach (var f in existing_fields) {
 492                                         if (f.IsAvailableForReuse) {
 493                                                 f.IsAvailableForReuse = false;
 494                                                 return f;
 495                                         }
 496                                 }
 497                         }
 498
 499                         var field = AddCompilerGeneratedField ("$awaiter" + awaiters++.ToString ("X"), new TypeExpression (type, Location), true);
 500                         field.Define ();
 501
 502                         if (existing_fields == null) {
 503                                 existing_fields = new List<Field> ();
 504                                 awaiter_fields.Add (type, existing_fields);
 505                         }
 506
 507                         existing_fields.Add (field);
 508                         return field;
 509                 }
 510
 511                 public Field AddCapturedLocalVariable (TypeSpec type)
 512                 {
 513                         if (mutator != null)
 514                                 type = mutator.Mutate (type);
 515
 516                         List<Field> existing_fields = null;
 517                         if (stack_fields == null) {
 518                                 stack_fields = new Dictionary<TypeSpec, List<Field>> ();
 519                         } else if (stack_fields.TryGetValue (type, out existing_fields)) {
 520                                 foreach (var f in existing_fields) {
 521                                         if (f.IsAvailableForReuse) {
 522                                                 f.IsAvailableForReuse = false;
 523                                                 return f;
 524                                         }
 525                                 }
 526                         }
 527
 528                         var field = AddCompilerGeneratedField ("$stack" + locals_captured++.ToString ("X"), new TypeExpression (type, Location), true);
 529                         field.Define ();
 530
 531                         if (existing_fields == null) {
 532                                 existing_fields = new List<Field> ();
 533                                 stack_fields.Add (type, existing_fields);
 534                         }
 535
 536                         existing_fields.Add (field);
 537
 538                         return field;
 539                 }
 540
 541                 protected override bool DoDefineMembers ()
 542                 {
 543                         PredefinedType builder_type;
 544                         PredefinedMember<MethodSpec> bf;
 545                         PredefinedMember<MethodSpec> bs;
 546                         PredefinedMember<MethodSpec> sr;
 547                         PredefinedMember<MethodSpec> se;
 548                         PredefinedMember<MethodSpec> sm;
 549                         bool has_task_return_type = false;
 550                         var pred_members = Module.PredefinedMembers;
 551
 552                         if (return_type.Kind == MemberKind.Void) {
 553                                 builder_type = Module.PredefinedTypes.AsyncVoidMethodBuilder;
 554                                 bf = pred_members.AsyncVoidMethodBuilderCreate;
 555                                 bs = pred_members.AsyncVoidMethodBuilderStart;
 556                                 sr = pred_members.AsyncVoidMethodBuilderSetResult;
 557                                 se = pred_members.AsyncVoidMethodBuilderSetException;
 558                                 sm = pred_members.AsyncVoidMethodBuilderSetStateMachine;
 559                         } else if (return_type == Module.PredefinedTypes.Task.TypeSpec) {
 560                                 builder_type = Module.PredefinedTypes.AsyncTaskMethodBuilder;
 561                                 bf = pred_members.AsyncTaskMethodBuilderCreate;
 562                                 bs = pred_members.AsyncTaskMethodBuilderStart;
 563                                 sr = pred_members.AsyncTaskMethodBuilderSetResult;
 564                                 se = pred_members.AsyncTaskMethodBuilderSetException;
 565                                 sm = pred_members.AsyncTaskMethodBuilderSetStateMachine;
 566                                 task = pred_members.AsyncTaskMethodBuilderTask.Get ();
 567                         } else {
 568                                 builder_type = Module.PredefinedTypes.AsyncTaskMethodBuilderGeneric;
 569                                 bf = pred_members.AsyncTaskMethodBuilderGenericCreate;
 570                                 bs = pred_members.AsyncTaskMethodBuilderGenericStart;
 571                                 sr = pred_members.AsyncTaskMethodBuilderGenericSetResult;
 572                                 se = pred_members.AsyncTaskMethodBuilderGenericSetException;
 573                                 sm = pred_members.AsyncTaskMethodBuilderGenericSetStateMachine;
 574                                 task = pred_members.AsyncTaskMethodBuilderGenericTask.Get ();
 575                                 has_task_return_type = true;
 576                         }
 577
 578                         set_result = sr.Get ();
 579                         set_exception = se.Get ();
 580                         builder_factory = bf.Get ();
 581                         builder_start = bs.Get ();
 582
 583                         var istate_machine = Module.PredefinedTypes.IAsyncStateMachine;
 584                         var set_statemachine = sm.Get ();
 585
 586                         if (!builder_type.Define () || !istate_machine.Define () || set_result == null || builder_factory == null ||
 587                                 set_exception == null || set_statemachine == null || builder_start == null ||
 588                                 !Module.PredefinedTypes.INotifyCompletion.Define ()) {
 589                                 Report.Error (1993, Location,
 590                                         "Cannot find compiler required types for asynchronous functions support. Are you targeting the wrong framework version?");
 591                                 return base.DoDefineMembers ();
 592                         }
 593
 594                         var bt = builder_type.TypeSpec;
 595
 596                         //
 597                         // Inflate generic Task types
 598                         //
 599                         if (has_task_return_type) {
 600                                 var task_return_type = return_type.TypeArguments;
 601                                 if (mutator != null)
 602                                         task_return_type = mutator.Mutate (task_return_type);
 603
 604                                 bt = bt.MakeGenericType (Module, task_return_type);
 605                                 set_result = MemberCache.GetMember (bt, set_result);
 606                                 set_exception = MemberCache.GetMember (bt, set_exception);
 607                                 set_statemachine = MemberCache.GetMember (bt, set_statemachine);
 608
 609                                 if (task != null)
 610                                         task = MemberCache.GetMember (bt, task);
 611                         }
 612
 613                         builder = AddCompilerGeneratedField ("$builder", new TypeExpression (bt, Location));
 614
 615                         var set_state_machine = new Method (this, new TypeExpression (Compiler.BuiltinTypes.Void, Location),
 616                                 Modifiers.COMPILER_GENERATED | Modifiers.DEBUGGER_HIDDEN | Modifiers.PUBLIC,
 617                                 new MemberName ("SetStateMachine"),
 618                                 ParametersCompiled.CreateFullyResolved (
 619                                         new Parameter (new TypeExpression (istate_machine.TypeSpec, Location), "stateMachine", Parameter.Modifier.NONE, null, Location),
 620                                         istate_machine.TypeSpec),
 621                                 null);
 622
 623                         ToplevelBlock block = new ToplevelBlock (Compiler, set_state_machine.ParameterInfo, Location);
 624                         block.IsCompilerGenerated = true;
 625                         set_state_machine.Block = block;
 626
 627                         Members.Add (set_state_machine);
 628
 629                         if (!base.DoDefineMembers ())
 630                                 return false;
 631
 632                         //
 633                         // Fabricates SetStateMachine method
 634                         //
 635                         // public void SetStateMachine (IAsyncStateMachine stateMachine)
 636                         // {
 637                         //    $builder.SetStateMachine (stateMachine);
 638                         // }
 639                         //
 640                         var mg = MethodGroupExpr.CreatePredefined (set_statemachine, bt, Location);
 641                         mg.InstanceExpression = new FieldExpr (builder, Location);
 642
 643                         var param_reference = block.GetParameterReference (0, Location);
 644                         param_reference.Type = istate_machine.TypeSpec;
 645                         param_reference.eclass = ExprClass.Variable;
 646
 647                         var args = new Arguments (1);
 648                         args.Add (new Argument (param_reference));
 649                         set_state_machine.Block.AddStatement (new StatementExpression (new Invocation (mg, args)));
 650
 651                         if (has_task_return_type) {
 652                                 hoisted_return = LocalVariable.CreateCompilerGenerated (bt.TypeArguments[0], StateMachineMethod.Block, Location);
 653                         }
 654
 655                         return true;
 656                 }
 657
 658                 public void EmitAwaitOnCompletedDynamic (EmitContext ec, FieldExpr awaiter)
 659                 {
 660                         var critical = Module.PredefinedTypes.ICriticalNotifyCompletion;
 661                         if (!critical.Define ()) {
 662                                 throw new NotImplementedException ();
 663                         }
 664
 665                         var temp_critical = new LocalTemporary (critical.TypeSpec);
 666                         var label_critical = ec.DefineLabel ();
 667                         var label_end = ec.DefineLabel ();
 668
 669                         //
 670                         // Special path for dynamic awaiters
 671                         //
 672                         // var awaiter = this.$awaiter as ICriticalNotifyCompletion;
 673                         // if (awaiter == null) {
 674                         //    var completion = (INotifyCompletion) this.$awaiter;
 675                         //    this.$builder.AwaitOnCompleted (ref completion, ref this);
 676                         // } else {
 677                         //    this.$builder.AwaitUnsafeOnCompleted (ref awaiter, ref this);
 678                         // }
 679                         //
 680                         awaiter.Emit (ec);
 681                         ec.Emit (OpCodes.Isinst, critical.TypeSpec);
 682                         temp_critical.Store (ec);
 683                         temp_critical.Emit (ec);
 684                         ec.Emit (OpCodes.Brtrue_S, label_critical);
 685
 686                         var temp = new LocalTemporary (Module.PredefinedTypes.INotifyCompletion.TypeSpec);
 687                         awaiter.Emit (ec);
 688                         ec.Emit (OpCodes.Castclass, temp.Type);
 689                         temp.Store (ec);
 690                         EmitOnCompleted (ec, temp, false);
 691                         temp.Release (ec);
 692                         ec.Emit (OpCodes.Br_S, label_end);
 693
 694                         ec.MarkLabel (label_critical);
 695
 696                         EmitOnCompleted (ec, temp_critical, true);
 697
 698                         ec.MarkLabel (label_end);
 699
 700                         temp_critical.Release (ec);
 701                 }
 702
 703                 public void EmitAwaitOnCompleted (EmitContext ec, FieldExpr awaiter)
 704                 {
 705                         bool unsafe_version = false;
 706                         if (Module.PredefinedTypes.ICriticalNotifyCompletion.Define ()) {
 707                                 unsafe_version = awaiter.Type.ImplementsInterface (Module.PredefinedTypes.ICriticalNotifyCompletion.TypeSpec, false);
 708                         }
 709
 710                         EmitOnCompleted (ec, awaiter, unsafe_version);
 711                 }
 712
 713                 void EmitOnCompleted (EmitContext ec, Expression awaiter, bool unsafeVersion)
 714                 {
 715                         var pm = Module.PredefinedMembers;
 716                         PredefinedMember<MethodSpec> predefined;
 717                         bool has_task_return_type = false;
 718                         if (return_type.Kind == MemberKind.Void) {
 719                                 predefined = unsafeVersion ? pm.AsyncVoidMethodBuilderOnCompletedUnsafe : pm.AsyncVoidMethodBuilderOnCompleted;
 720                         } else if (return_type == Module.PredefinedTypes.Task.TypeSpec) {
 721                                 predefined = unsafeVersion ? pm.AsyncTaskMethodBuilderOnCompletedUnsafe : pm.AsyncTaskMethodBuilderOnCompleted;
 722                         } else {
 723                                 predefined = unsafeVersion ? pm.AsyncTaskMethodBuilderGenericOnCompletedUnsafe : pm.AsyncTaskMethodBuilderGenericOnCompleted;
 724                                 has_task_return_type = true;
 725                         }
 726
 727                         var on_completed = predefined.Resolve (Location);
 728                         if (on_completed == null)
 729                                 return;
 730
 731                         if (has_task_return_type)
 732                                 on_completed = MemberCache.GetMember<MethodSpec> (set_result.DeclaringType, on_completed);
 733
 734                         on_completed = on_completed.MakeGenericMethod (this, awaiter.Type, ec.CurrentType);
 735
 736                         var mg = MethodGroupExpr.CreatePredefined (on_completed, on_completed.DeclaringType, Location);
 737                         mg.InstanceExpression = new FieldExpr (builder, Location) {
 738                                 InstanceExpression = new CompilerGeneratedThis (ec.CurrentType, Location)
 739                         };
 740
 741                         var args = new Arguments (2);
 742                         args.Add (new Argument (awaiter, Argument.AType.Ref));
 743                         args.Add (new Argument (new CompilerGeneratedThis (CurrentType, Location), Argument.AType.Ref));
 744                         mg.EmitCall (ec, args);
 745                 }
 746
 747                 public void EmitInitializer (EmitContext ec)
 748                 {
 749                         //
 750                         // Some predefined types are missing
 751                         //
 752                         if (builder == null)
 753                                 return;
 754
 755                         var instance = (TemporaryVariableReference) Instance;
 756                         var builder_field = builder.Spec;
 757                         if (MemberName.Arity > 0) {
 758                                 builder_field = MemberCache.GetMember (instance.Type, builder_field);
 759                         }
 760
 761                         //
 762                         // Inflated factory method when task is of generic type
 763                         //
 764                         if (builder_factory.DeclaringType.IsGeneric) {
 765                                 var task_return_type = return_type.TypeArguments;
 766                                 var bt = builder_factory.DeclaringType.MakeGenericType (Module, task_return_type);
 767                                 builder_factory = MemberCache.GetMember (bt, builder_factory);
 768                                 builder_start = MemberCache.GetMember (bt, builder_start);
 769                         }
 770
 771                         //
 772                         // stateMachine.$builder = AsyncTaskMethodBuilder<{task-type}>.Create();
 773                         //
 774                         instance.AddressOf (ec, AddressOp.Store);
 775                         ec.Emit (OpCodes.Call, builder_factory);
 776                         ec.Emit (OpCodes.Stfld, builder_field);
 777
 778                         //
 779                         // stateMachine.$builder.Start<{storey-type}>(ref stateMachine);
 780                         //
 781                         instance.AddressOf (ec, AddressOp.Store);
 782                         ec.Emit (OpCodes.Ldflda, builder_field);
 783                         if (Task != null)
 784                                 ec.Emit (OpCodes.Dup);
 785                         instance.AddressOf (ec, AddressOp.Store);
 786                         ec.Emit (OpCodes.Call, builder_start.MakeGenericMethod (Module, instance.Type));
 787
 788                         //
 789                         // Emits return stateMachine.$builder.Task;
 790                         //
 791                         if (Task != null) {
 792                                 var task_get = Task.Get;
 793
 794                                 if (MemberName.Arity > 0) {
 795                                         task_get = MemberCache.GetMember (builder_field.MemberType, task_get);
 796                                 }
 797
 798                                 var pe_task = new PropertyExpr (Task, Location) {
 799                                         InstanceExpression = EmptyExpression.Null,      // Comes from the dup above
 800                                         Getter = task_get
 801                                 };
 802
 803                                 pe_task.Emit (ec);
 804                         }
 805                 }
 806
 807                 public void EmitSetException (EmitContext ec, LocalVariableReference exceptionVariable)
 808                 {
 809                         //
 810                         // $builder.SetException (Exception)
 811                         //
 812                         var mg = MethodGroupExpr.CreatePredefined (set_exception, set_exception.DeclaringType, Location);
 813                         mg.InstanceExpression = new FieldExpr (builder, Location) {
 814                                 InstanceExpression = new CompilerGeneratedThis (ec.CurrentType, Location)
 815                         };
 816
 817                         Arguments args = new Arguments (1);
 818                         args.Add (new Argument (exceptionVariable));
 819
 820                         mg.EmitCall (ec, args);
 821                 }
 822
 823                 public void EmitSetResult (EmitContext ec)
 824                 {
 825                         //
 826                         // $builder.SetResult ();
 827                         // $builder.SetResult<return-type> (value);
 828                         //
 829                         var mg = MethodGroupExpr.CreatePredefined (set_result, set_result.DeclaringType, Location);
 830                         mg.InstanceExpression = new FieldExpr (builder, Location) {
 831                                 InstanceExpression = new CompilerGeneratedThis (ec.CurrentType, Location)
 832                         };
 833
 834                         Arguments args;
 835                         if (hoisted_return == null) {
 836                                 args = new Arguments (0);
 837                         } else {
 838                                 args = new Arguments (1);
 839                                 args.Add (new Argument (new LocalVariableReference (hoisted_return, Location)));
 840                         }
 841
 842                         mg.EmitCall (ec, args);
 843                 }
 844
 845                 protected override TypeSpec[] ResolveBaseTypes (out FullNamedExpression base_class)
 846                 {
 847                         base_type = Compiler.BuiltinTypes.ValueType;
 848                         base_class = null;
 849
 850                         var istate_machine = Module.PredefinedTypes.IAsyncStateMachine;
 851                         if (istate_machine.Define ()) {
 852                                 return new[] { istate_machine.TypeSpec };
 853                         }
 854
 855                         return null;
 856                 }
 857         }
 858
 859         class StackFieldExpr : FieldExpr, IExpressionCleanup
 860         {
 861                 public StackFieldExpr (Field field)
 862                         : base (field, Location.Null)
 863                 {
 864                 }
 865
 866                 public override void AddressOf (EmitContext ec, AddressOp mode)
 867                 {
 868                         base.AddressOf (ec, mode);
 869
 870                         if (mode == AddressOp.Load) {
 871                                 var field = (Field) spec.MemberDefinition;
 872                                 field.IsAvailableForReuse = true;
 873                         }
 874                 }
 875
 876                 public override void Emit (EmitContext ec)
 877                 {
 878                         base.Emit (ec);
 879
 880                         var field = (Field) spec.MemberDefinition;
 881                         field.IsAvailableForReuse = true;
 882
 883                         //
 884                         // Release any captured reference type stack variables
 885                         // to imitate real stack behavour and help GC stuff early
 886                         //
 887                         if (TypeSpec.IsReferenceType (type)) {
 888                                 ec.AddStatementEpilog (this);
 889                         }
 890                 }
 891
 892                 void IExpressionCleanup.EmitCleanup (EmitContext ec)
 893                 {
 894                         EmitAssign (ec, new NullConstant (type, loc), false, false);
 895                 }
 896         }
 897 }