mcs/mcs/codegen.cs

   1 //
   2 // codegen.cs: The code generator
   3 //
   4 // Author:
   5 //   Miguel de Icaza (miguel@ximian.com)
   6 //
   7 // (C) 2001 Ximian, Inc.
   8 //
   9
  10 using System;
  11 using System.IO;
  12 using System.Collections;
  13 using System.Reflection;
  14 using System.Reflection.Emit;
  15
  16 namespace Mono.CSharp {
  17
  18         /// <summary>
  19         ///    Code generator class.
  20         /// </summary>
  21         public class CodeGen {
  22                 static AppDomain current_domain;
  23                 public static AssemblyBuilder AssemblyBuilder;
  24                 public static ModuleBuilder   ModuleBuilder;
  25
  26                 static public SymbolWriter SymbolWriter;
  27
  28                 public static string Basename (string name)
  29                 {
  30                         int pos = name.LastIndexOf ('/');
  31
  32                         if (pos != -1)
  33                                 return name.Substring (pos + 1);
  34
  35                         pos = name.LastIndexOf ('\\');
  36                         if (pos != -1)
  37                                 return name.Substring (pos + 1);
  38
  39                         return name;
  40                 }
  41
  42                 public static string Dirname (string name)
  43                 {
  44                         int pos = name.LastIndexOf ('/');
  45
  46                         if (pos != -1)
  47                                 return name.Substring (0, pos);
  48
  49                         pos = name.LastIndexOf ('\\');
  50                         if (pos != -1)
  51                                 return name.Substring (0, pos);
  52
  53                         return ".";
  54                 }
  55
  56                 static string TrimExt (string name)
  57                 {
  58                         int pos = name.LastIndexOf ('.');
  59
  60                         return name.Substring (0, pos);
  61                 }
  62
  63                 static public string FileName;
  64
  65                 //
  66                 // Initializes the symbol writer
  67                 //
  68                 static void InitializeSymbolWriter ()
  69                 {
  70                         SymbolWriter = SymbolWriter.GetSymbolWriter (ModuleBuilder);
  71
  72                         //
  73                         // If we got an ISymbolWriter instance, initialize it.
  74                         //
  75                         if (SymbolWriter == null) {
  76                                 Report.Warning (
  77                                         -18, "Could not find the symbol writer assembly (Mono.CSharp.Debugger.dll). This is normally an installation problem. Please make sure to compile and install the mcs/class/Mono.CSharp.Debugger directory.");
  78                                 return;
  79                         }
  80                 }
  81
  82                 //
  83                 // Initializes the code generator variables
  84                 //
  85                 static public void Init (string name, string output, bool want_debugging_support)
  86                 {
  87                         AssemblyName an;
  88
  89                         FileName = output;
  90                         an = new AssemblyName ();
  91                         an.Name = Path.GetFileNameWithoutExtension (name);
  92
  93                         current_domain = AppDomain.CurrentDomain;
  94                         AssemblyBuilder = current_domain.DefineDynamicAssembly (
  95                                 an, AssemblyBuilderAccess.Save, Dirname (name));
  96
  97                         //
  98                         // Pass a path-less name to DefineDynamicModule.  Wonder how
  99                         // this copes with output in different directories then.
 100                         // FIXME: figure out how this copes with --output /tmp/blah
 101                         //
 102                         // If the third argument is true, the ModuleBuilder will dynamically
 103                         // load the default symbol writer.
 104                         //
 105                         ModuleBuilder = AssemblyBuilder.DefineDynamicModule (
 106                                 Basename (name), Basename (output), want_debugging_support);
 107
 108                         if (want_debugging_support)
 109                                 InitializeSymbolWriter ();
 110                 }
 111
 112                 static public void Save (string name)
 113                 {
 114                         try {
 115                                 AssemblyBuilder.Save (Basename (name));
 116                         } catch (System.IO.IOException io){
 117                                 Report.Error (16, "Could not write to file `"+name+"', cause: " + io.Message);
 118                         }
 119                 }
 120         }
 121
 122         //
 123         // Provides "local" store across code that can yield: locals
 124         // or fields, notice that this should not be used by anonymous
 125         // methods to create local storage, those only require
 126         // variable mapping.
 127         //
 128         public class VariableStorage {
 129                 ILGenerator ig;
 130                 FieldBuilder fb;
 131                 LocalBuilder local;
 132
 133                 static int count;
 134
 135                 public VariableStorage (EmitContext ec, Type t)
 136                 {
 137                         count++;
 138                         if (ec.InIterator)
 139                                 fb = IteratorHandler.Current.MapVariable ("s_", count.ToString (), t);
 140                         else
 141                                 local = ec.ig.DeclareLocal (t);
 142                         ig = ec.ig;
 143                 }
 144
 145                 public void EmitThis ()
 146                 {
 147                         if (fb != null)
 148                                 ig.Emit (OpCodes.Ldarg_0);
 149                 }
 150
 151                 public void EmitStore ()
 152                 {
 153                         if (fb == null)
 154                                 ig.Emit (OpCodes.Stloc, local);
 155                         else
 156                                 ig.Emit (OpCodes.Stfld, fb);
 157                 }
 158
 159                 public void EmitLoad ()
 160                 {
 161                         if (fb == null)
 162                                 ig.Emit (OpCodes.Ldloc, local);
 163                         else
 164                                 ig.Emit (OpCodes.Ldfld, fb);
 165                 }
 166         }
 167
 168         /// <summary>
 169         ///   An Emit Context is created for each body of code (from methods,
 170         ///   properties bodies, indexer bodies or constructor bodies)
 171         /// </summary>
 172         public class EmitContext {
 173                 public DeclSpace DeclSpace;
 174                 public DeclSpace TypeContainer;
 175                 public ILGenerator   ig;
 176
 177                 /// <summary>
 178                 ///   This variable tracks the `checked' state of the compilation,
 179                 ///   it controls whether we should generate code that does overflow
 180                 ///   checking, or if we generate code that ignores overflows.
 181                 ///
 182                 ///   The default setting comes from the command line option to generate
 183                 ///   checked or unchecked code plus any source code changes using the
 184                 ///   checked/unchecked statements or expressions.   Contrast this with
 185                 ///   the ConstantCheckState flag.
 186                 /// </summary>
 187
 188                 public bool CheckState;
 189
 190                 /// <summary>
 191                 ///   The constant check state is always set to `true' and cant be changed
 192                 ///   from the command line.  The source code can change this setting with
 193                 ///   the `checked' and `unchecked' statements and expressions.
 194                 /// </summary>
 195                 public bool ConstantCheckState;
 196
 197                 /// <summary>
 198                 ///   Whether we are emitting code inside a static or instance method
 199                 /// </summary>
 200                 public bool IsStatic;
 201
 202                 /// <summary>
 203                 ///   Whether we are emitting a field initializer
 204                 /// </summary>
 205                 public bool IsFieldInitializer;
 206
 207                 /// <summary>
 208                 ///   The value that is allowed to be returned or NULL if there is no
 209                 ///   return type.
 210                 /// </summary>
 211                 public Type ReturnType;
 212
 213                 /// <summary>
 214                 ///   Points to the Type (extracted from the TypeContainer) that
 215                 ///   declares this body of code
 216                 /// </summary>
 217                 public Type ContainerType;
 218
 219                 /// <summary>
 220                 ///   Whether this is generating code for a constructor
 221                 /// </summary>
 222                 public bool IsConstructor;
 223
 224                 /// <summary>
 225                 ///   Whether we're control flow analysis enabled
 226                 /// </summary>
 227                 public bool DoFlowAnalysis;
 228
 229                 /// <summary>
 230                 ///   Keeps track of the Type to LocalBuilder temporary storage created
 231                 ///   to store structures (used to compute the address of the structure
 232                 ///   value on structure method invocations)
 233                 /// </summary>
 234                 public Hashtable temporary_storage;
 235
 236                 public Block CurrentBlock;
 237
 238                 public int CurrentFile;
 239
 240                 /// <summary>
 241                 ///   The location where we store the return value.
 242                 /// </summary>
 243                 LocalBuilder return_value;
 244
 245                 /// <summary>
 246                 ///   The location where return has to jump to return the
 247                 ///   value
 248                 /// </summary>
 249                 public Label ReturnLabel;
 250
 251                 /// <summary>
 252                 ///   If we already defined the ReturnLabel
 253                 /// </summary>
 254                 public bool HasReturnLabel;
 255
 256                 /// <summary>
 257                 ///   Whether we are in a Finally block
 258                 /// </summary>
 259                 public bool InFinally;
 260
 261                 /// <summary>
 262                 ///   Whether we are in a Try block
 263                 /// </summary>
 264                 public bool InTry;
 265
 266                 /// <summary>
 267                 ///   Whether we are inside an iterator block.
 268                 /// </summary>
 269                 public bool InIterator;
 270
 271                 /// <summary>
 272                 ///   Whether we need an explicit return statement at the end of the method.
 273                 /// </summary>
 274                 public bool NeedExplicitReturn;
 275
 276                 /// <summary>
 277                 ///   Whether remapping of locals, parameters and fields is turned on.
 278                 ///   Used by iterators and anonymous methods.
 279                 /// </summary>
 280                 public bool RemapToProxy;
 281
 282                 /// <summary>
 283                 ///   Whether we are in a Catch block
 284                 /// </summary>
 285                 public bool InCatch;
 286
 287                 /// <summary>
 288                 ///  Whether we are inside an unsafe block
 289                 /// </summary>
 290                 public bool InUnsafe;
 291
 292                 /// <summary>
 293                 ///  Whether we are in a `fixed' initialization
 294                 /// </summary>
 295                 public bool InFixedInitializer;
 296
 297                 /// <summary>
 298                 ///  Whether we are inside an anonymous method.
 299                 /// </summary>
 300                 public bool InAnonymousMethod;
 301
 302                 /// <summary>
 303                 ///   Location for this EmitContext
 304                 /// </summary>
 305                 public Location loc;
 306
 307                 /// <summary>
 308                 ///   Used to flag that it is ok to define types recursively, as the
 309                 ///   expressions are being evaluated as part of the type lookup
 310                 ///   during the type resolution process
 311                 /// </summary>
 312                 public bool ResolvingTypeTree;
 313
 314                 /// <summary>
 315                 ///   Inside an enum definition, we do not resolve enumeration values
 316                 ///   to their enumerations, but rather to the underlying type/value
 317                 ///   This is so EnumVal + EnumValB can be evaluated.
 318                 ///
 319                 ///   There is no "E operator + (E x, E y)", so during an enum evaluation
 320                 ///   we relax the rules
 321                 /// </summary>
 322                 public bool InEnumContext;
 323
 324                 protected Stack FlowStack;
 325
 326                 public EmitContext (DeclSpace parent, DeclSpace ds, Location l, ILGenerator ig,
 327                                     Type return_type, int code_flags, bool is_constructor)
 328                 {
 329                         this.ig = ig;
 330
 331                         TypeContainer = parent;
 332                         DeclSpace = ds;
 333                         CheckState = RootContext.Checked;
 334                         ConstantCheckState = true;
 335
 336                         IsStatic = (code_flags & Modifiers.STATIC) != 0;
 337                         InIterator = (code_flags & Modifiers.METHOD_YIELDS) != 0;
 338                         RemapToProxy = InIterator;
 339                         ReturnType = return_type;
 340                         IsConstructor = is_constructor;
 341                         CurrentBlock = null;
 342                         CurrentFile = 0;
 343
 344                         if (parent != null){
 345                                 // Can only be null for the ResolveType contexts.
 346                                 ContainerType = parent.TypeBuilder;
 347                                 if (parent.UnsafeContext)
 348                                         InUnsafe = true;
 349                                 else
 350                                         InUnsafe = (code_flags & Modifiers.UNSAFE) != 0;
 351                         }
 352                         loc = l;
 353
 354                         FlowStack = new Stack ();
 355
 356                         if (ReturnType == TypeManager.void_type)
 357                                 ReturnType = null;
 358                 }
 359
 360                 public EmitContext (TypeContainer tc, Location l, ILGenerator ig,
 361                                     Type return_type, int code_flags, bool is_constructor)
 362                         : this (tc, tc, l, ig, return_type, code_flags, is_constructor)
 363                 {
 364                 }
 365
 366                 public EmitContext (TypeContainer tc, Location l, ILGenerator ig,
 367                                     Type return_type, int code_flags)
 368                         : this (tc, tc, l, ig, return_type, code_flags, false)
 369                 {
 370                 }
 371
 372                 public FlowBranching CurrentBranching {
 373                         get {
 374                                 return (FlowBranching) FlowStack.Peek ();
 375                         }
 376                 }
 377
 378                 // <summary>
 379                 //   Starts a new code branching.  This inherits the state of all local
 380                 //   variables and parameters from the current branching.
 381                 // </summary>
 382                 public FlowBranching StartFlowBranching (FlowBranching.BranchingType type, Location loc)
 383                 {
 384                         FlowBranching cfb = FlowBranching.CreateBranching (CurrentBranching, type, null, loc);
 385
 386                         FlowStack.Push (cfb);
 387
 388                         return cfb;
 389                 }
 390
 391                 // <summary>
 392                 //   Starts a new code branching for block `block'.
 393                 // </summary>
 394                 public FlowBranching StartFlowBranching (Block block)
 395                 {
 396                         FlowBranching cfb;
 397                         FlowBranching.BranchingType type;
 398
 399                         if (CurrentBranching.Type == FlowBranching.BranchingType.Switch)
 400                                 type = FlowBranching.BranchingType.SwitchSection;
 401                         else
 402                                 type = FlowBranching.BranchingType.Block;
 403
 404                         cfb = FlowBranching.CreateBranching (CurrentBranching, type, block, block.StartLocation);
 405
 406                         FlowStack.Push (cfb);
 407
 408                         return cfb;
 409                 }
 410
 411                 // <summary>
 412                 //   Ends a code branching.  Merges the state of locals and parameters
 413                 //   from all the children of the ending branching.
 414                 // </summary>
 415                 public FlowBranching.UsageVector DoEndFlowBranching ()
 416                 {
 417                         FlowBranching cfb = (FlowBranching) FlowStack.Pop ();
 418
 419                         return CurrentBranching.MergeChild (cfb);
 420                 }
 421
 422                 // <summary>
 423                 //   Ends a code branching.  Merges the state of locals and parameters
 424                 //   from all the children of the ending branching.
 425                 // </summary>
 426                 public FlowBranching.Reachability EndFlowBranching ()
 427                 {
 428                         FlowBranching.UsageVector vector = DoEndFlowBranching ();
 429
 430                         return vector.Reachability;
 431                 }
 432
 433                 // <summary>
 434                 //   Kills the current code branching.  This throws away any changed state
 435                 //   information and should only be used in case of an error.
 436                 // </summary>
 437                 public void KillFlowBranching ()
 438                 {
 439                         FlowStack.Pop ();
 440                 }
 441
 442                 public void EmitTopBlock (Block block, InternalParameters ip, Location loc)
 443                 {
 444                         bool has_ret = false;
 445
 446                         if (!Location.IsNull (loc))
 447                                 CurrentFile = loc.File;
 448
 449                         if (block != null){
 450                             try {
 451                                 int errors = Report.Errors;
 452
 453                                 block.EmitMeta (this, ip);
 454
 455                                 if (Report.Errors == errors){
 456                                         bool old_do_flow_analysis = DoFlowAnalysis;
 457                                         DoFlowAnalysis = true;
 458
 459                                         FlowBranching cfb = FlowBranching.CreateBranching (
 460                                                 null, FlowBranching.BranchingType.Block, block, loc);
 461                                         FlowStack.Push (cfb);
 462
 463                                         if (!block.Resolve (this)) {
 464                                                 FlowStack.Pop ();
 465                                                 DoFlowAnalysis = old_do_flow_analysis;
 466                                                 return;
 467                                         }
 468
 469                                         cfb = (FlowBranching) FlowStack.Pop ();
 470                                         FlowBranching.Reachability reachability = cfb.MergeTopBlock ();
 471
 472                                         DoFlowAnalysis = old_do_flow_analysis;
 473
 474                                         block.Emit (this);
 475
 476                                         if ((reachability.Returns == FlowBranching.FlowReturns.Always) ||
 477                                             (reachability.Throws == FlowBranching.FlowReturns.Always) ||
 478                                             (reachability.Reachable == FlowBranching.FlowReturns.Never))
 479                                                 has_ret = true;
 480                                 }
 481                             } catch {
 482                                         Console.WriteLine ("Exception caught by the compiler while compiling:");
 483                                         Console.WriteLine ("   Block that caused the problem begin at: " + loc);
 484
 485                                         if (CurrentBlock != null){
 486                                                 Console.WriteLine ("                     Block being compiled: [{0},{1}]",
 487                                                                    CurrentBlock.StartLocation, CurrentBlock.EndLocation);
 488                                         }
 489                                         throw;
 490                             }
 491                         }
 492
 493                         if (ReturnType != null && !has_ret){
 494                                 //
 495                                 // FIXME: we need full flow analysis to implement this
 496                                 // correctly and emit an error instead of a warning.
 497                                 //
 498                                 //
 499                                 if (!InIterator){
 500                                         Report.Error (161, loc, "Not all code paths return a value");
 501                                         return;
 502                                 }
 503                         }
 504
 505                         if (HasReturnLabel)
 506                                 ig.MarkLabel (ReturnLabel);
 507                         if (return_value != null){
 508                                 ig.Emit (OpCodes.Ldloc, return_value);
 509                                 ig.Emit (OpCodes.Ret);
 510                         } else {
 511                                 if (!InTry){
 512                                         if (InIterator)
 513                                                 has_ret = true;
 514
 515                                         if (!has_ret || HasReturnLabel) {
 516                                                 ig.Emit (OpCodes.Ret);
 517                                                 NeedExplicitReturn = false;
 518                                         }
 519                                 }
 520
 521                                 // Unfortunately, System.Reflection.Emit automatically emits a leave
 522                                 // to the end of a finally block.  This is a problem if no code is
 523                                 // following the try/finally block since we may jump to a point after
 524                                 // the end of the method. As a workaround, emit an explicit ret here.
 525
 526                                 if (NeedExplicitReturn) {
 527                                         if (ReturnType != null)
 528                                                 ig.Emit (OpCodes.Ldloc, TemporaryReturn ());
 529                                         ig.Emit (OpCodes.Ret);
 530                                 }
 531                         }
 532                 }
 533
 534                 /// <summary>
 535                 ///   This is called immediately before emitting an IL opcode to tell the symbol
 536                 ///   writer to which source line this opcode belongs.
 537                 /// </summary>
 538                 public void Mark (Location loc, bool check_file)
 539                 {
 540                         if ((CodeGen.SymbolWriter == null) || Location.IsNull (loc))
 541                                 return;
 542
 543                         if (check_file && (CurrentFile != loc.File))
 544                                 return;
 545
 546                         ig.MarkSequencePoint (null, loc.Row, 0, 0, 0);
 547                 }
 548
 549                 /// <summary>
 550                 ///   Returns a temporary storage for a variable of type t as
 551                 ///   a local variable in the current body.
 552                 /// </summary>
 553                 public LocalBuilder GetTemporaryLocal (Type t)
 554                 {
 555                         LocalBuilder location = null;
 556
 557                         if (temporary_storage != null){
 558                                 object o = temporary_storage [t];
 559                                 if (o != null){
 560                                         if (o is ArrayList){
 561                                                 ArrayList al = (ArrayList) o;
 562
 563                                                 for (int i = 0; i < al.Count; i++){
 564                                                         if (al [i] != null){
 565                                                                 location = (LocalBuilder) al [i];
 566                                                                 al [i] = null;
 567                                                                 break;
 568                                                         }
 569                                                 }
 570                                         } else
 571                                                 location = (LocalBuilder) o;
 572                                         if (location != null)
 573                                                 return location;
 574                                 }
 575                         }
 576
 577                         return ig.DeclareLocal (t);
 578                 }
 579
 580                 public void FreeTemporaryLocal (LocalBuilder b, Type t)
 581                 {
 582                         if (temporary_storage == null){
 583                                 temporary_storage = new Hashtable ();
 584                                 temporary_storage [t] = b;
 585                                 return;
 586                         }
 587                         object o = temporary_storage [t];
 588                         if (o == null){
 589                                 temporary_storage [t] = b;
 590                                 return;
 591                         }
 592                         if (o is ArrayList){
 593                                 ArrayList al = (ArrayList) o;
 594                                 for (int i = 0; i < al.Count; i++){
 595                                         if (al [i] == null){
 596                                                 al [i] = b;
 597                                                 return;
 598                                         }
 599                                 }
 600                                 al.Add (b);
 601                                 return;
 602                         }
 603                         ArrayList replacement = new ArrayList ();
 604                         replacement.Add (o);
 605                         temporary_storage.Remove (t);
 606                         temporary_storage [t] = replacement;
 607                 }
 608
 609                 /// <summary>
 610                 ///   Current loop begin and end labels.
 611                 /// </summary>
 612                 public Label LoopBegin, LoopEnd;
 613
 614                 /// <summary>
 615                 ///   Whether we are inside a loop and break/continue are possible.
 616                 /// </summary>
 617                 public bool  InLoop;
 618
 619                 /// <summary>
 620                 ///   This is incremented each time we enter a try/catch block and
 621                 ///   decremented if we leave it.
 622                 /// </summary>
 623                 public int   TryCatchLevel;
 624
 625                 /// <summary>
 626                 ///   The TryCatchLevel at the begin of the current loop.
 627                 /// </summary>
 628                 public int   LoopBeginTryCatchLevel;
 629
 630                 /// <summary>
 631                 ///   Default target in a switch statement.   Only valid if
 632                 ///   InSwitch is true
 633                 /// </summary>
 634                 public Label DefaultTarget;
 635
 636                 /// <summary>
 637                 ///   If this is non-null, points to the current switch statement
 638                 /// </summary>
 639                 public Switch Switch;
 640
 641                 /// <summary>
 642                 ///   ReturnValue creates on demand the LocalBuilder for the
 643                 ///   return value from the function.  By default this is not
 644                 ///   used.  This is only required when returns are found inside
 645                 ///   Try or Catch statements.
 646                 /// </summary>
 647                 public LocalBuilder TemporaryReturn ()
 648                 {
 649                         if (return_value == null){
 650                                 return_value = ig.DeclareLocal (ReturnType);
 651                                 ReturnLabel = ig.DefineLabel ();
 652                                 HasReturnLabel = true;
 653                         }
 654
 655                         return return_value;
 656                 }
 657
 658                 //
 659                 // Creates a field `name' with the type `t' on the proxy class
 660                 //
 661                 public FieldBuilder MapVariable (string name, Type t)
 662                 {
 663                         if (InIterator){
 664                                 return IteratorHandler.Current.MapVariable ("v_", name, t);
 665                         }
 666
 667                         throw new Exception ("MapVariable for an unknown state");
 668                 }
 669
 670                 //
 671                 // Invoke this routine to remap a VariableInfo into the
 672                 // proper MemberAccess expression
 673                 //
 674                 public Expression RemapLocal (LocalInfo local_info)
 675                 {
 676                         FieldExpr fe = new FieldExpr (local_info.FieldBuilder, loc);
 677                         fe.InstanceExpression = new ProxyInstance ();
 678                         return fe.DoResolve (this);
 679                 }
 680
 681                 public Expression RemapLocalLValue (LocalInfo local_info, Expression right_side)
 682                 {
 683                         FieldExpr fe = new FieldExpr (local_info.FieldBuilder, loc);
 684                         fe.InstanceExpression = new ProxyInstance ();
 685                         return fe.DoResolveLValue (this, right_side);
 686                 }
 687
 688                 public Expression RemapParameter (int idx)
 689                 {
 690                         FieldExpr fe = new FieldExprNoAddress (IteratorHandler.Current.parameter_fields [idx], loc);
 691                         fe.InstanceExpression = new ProxyInstance ();
 692                         return fe.DoResolve (this);
 693                 }
 694
 695                 public Expression RemapParameterLValue (int idx, Expression right_side)
 696                 {
 697                         FieldExpr fe = new FieldExprNoAddress (IteratorHandler.Current.parameter_fields [idx], loc);
 698                         fe.InstanceExpression = new ProxyInstance ();
 699                         return fe.DoResolveLValue (this, right_side);
 700                 }
 701
 702                 //
 703                 // Emits the proper object to address fields on a remapped
 704                 // variable/parameter to field in anonymous-method/iterator proxy classes.
 705                 //
 706                 public void EmitThis ()
 707                 {
 708                         ig.Emit (OpCodes.Ldarg_0);
 709
 710                         if (!IsStatic){
 711                                 if (InIterator)
 712                                         ig.Emit (OpCodes.Ldfld, IteratorHandler.Current.this_field);
 713                                 else
 714                                         throw new Exception ("EmitThis for an unknown state");
 715                         }
 716                 }
 717
 718                 public Expression GetThis (Location loc)
 719                 {
 720                         This my_this;
 721                         if (CurrentBlock != null)
 722                                 my_this = new This (CurrentBlock, loc);
 723                         else
 724                                 my_this = new This (loc);
 725
 726                         if (!my_this.ResolveBase (this))
 727                                 my_this = null;
 728
 729                         return my_this;
 730                 }
 731         }
 732 }