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                 public void EmitCall (MethodInfo mi)
 168                 {
 169                         // FIXME : we should handle a call like tostring
 170                         // here, where boxing is needed. However, we will
 171                         // never encounter that with the current usage.
 172
 173                         bool value_type_call;
 174                         EmitThis ();
 175                         if (fb == null) {
 176                                 value_type_call = local.LocalType.IsValueType;
 177
 178                                 if (value_type_call)
 179                                         ig.Emit (OpCodes.Ldloca, local);
 180                                 else
 181                                         ig.Emit (OpCodes.Ldloc, local);
 182                         } else {
 183                                 value_type_call = fb.FieldType.IsValueType;
 184
 185                                 if (value_type_call)
 186                                         ig.Emit (OpCodes.Ldflda, fb);
 187                                 else
 188                                         ig.Emit (OpCodes.Ldfld, fb);
 189                         }
 190
 191                         ig.Emit (value_type_call ? OpCodes.Call : OpCodes.Callvirt, mi);
 192                 }
 193         }
 194
 195         /// <summary>
 196         ///   An Emit Context is created for each body of code (from methods,
 197         ///   properties bodies, indexer bodies or constructor bodies)
 198         /// </summary>
 199         public class EmitContext {
 200                 public DeclSpace DeclSpace;
 201                 public DeclSpace TypeContainer;
 202                 public ILGenerator   ig;
 203
 204                 /// <summary>
 205                 ///   This variable tracks the `checked' state of the compilation,
 206                 ///   it controls whether we should generate code that does overflow
 207                 ///   checking, or if we generate code that ignores overflows.
 208                 ///
 209                 ///   The default setting comes from the command line option to generate
 210                 ///   checked or unchecked code plus any source code changes using the
 211                 ///   checked/unchecked statements or expressions.   Contrast this with
 212                 ///   the ConstantCheckState flag.
 213                 /// </summary>
 214
 215                 public bool CheckState;
 216
 217                 /// <summary>
 218                 ///   The constant check state is always set to `true' and cant be changed
 219                 ///   from the command line.  The source code can change this setting with
 220                 ///   the `checked' and `unchecked' statements and expressions.
 221                 /// </summary>
 222                 public bool ConstantCheckState;
 223
 224                 /// <summary>
 225                 ///   Whether we are emitting code inside a static or instance method
 226                 /// </summary>
 227                 public bool IsStatic;
 228
 229                 /// <summary>
 230                 ///   Whether we are emitting a field initializer
 231                 /// </summary>
 232                 public bool IsFieldInitializer;
 233
 234                 /// <summary>
 235                 ///   The value that is allowed to be returned or NULL if there is no
 236                 ///   return type.
 237                 /// </summary>
 238                 public Type ReturnType;
 239
 240                 /// <summary>
 241                 ///   Points to the Type (extracted from the TypeContainer) that
 242                 ///   declares this body of code
 243                 /// </summary>
 244                 public Type ContainerType;
 245
 246                 /// <summary>
 247                 ///   Whether this is generating code for a constructor
 248                 /// </summary>
 249                 public bool IsConstructor;
 250
 251                 /// <summary>
 252                 ///   Whether we're control flow analysis enabled
 253                 /// </summary>
 254                 public bool DoFlowAnalysis;
 255
 256                 /// <summary>
 257                 ///   Keeps track of the Type to LocalBuilder temporary storage created
 258                 ///   to store structures (used to compute the address of the structure
 259                 ///   value on structure method invocations)
 260                 /// </summary>
 261                 public Hashtable temporary_storage;
 262
 263                 public Block CurrentBlock;
 264
 265                 public int CurrentFile;
 266
 267                 /// <summary>
 268                 ///   The location where we store the return value.
 269                 /// </summary>
 270                 LocalBuilder return_value;
 271
 272                 /// <summary>
 273                 ///   The location where return has to jump to return the
 274                 ///   value
 275                 /// </summary>
 276                 public Label ReturnLabel;
 277
 278                 /// <summary>
 279                 ///   If we already defined the ReturnLabel
 280                 /// </summary>
 281                 public bool HasReturnLabel;
 282
 283                 /// <summary>
 284                 ///   Whether we are inside an iterator block.
 285                 /// </summary>
 286                 public bool InIterator;
 287
 288                 public bool IsLastStatement;
 289
 290                 /// <summary>
 291                 ///   Whether remapping of locals, parameters and fields is turned on.
 292                 ///   Used by iterators and anonymous methods.
 293                 /// </summary>
 294                 public bool RemapToProxy;
 295
 296                 /// <summary>
 297                 ///  Whether we are inside an unsafe block
 298                 /// </summary>
 299                 public bool InUnsafe;
 300
 301                 /// <summary>
 302                 ///  Whether we are in a `fixed' initialization
 303                 /// </summary>
 304                 public bool InFixedInitializer;
 305
 306                 /// <summary>
 307                 ///  Whether we are inside an anonymous method.
 308                 /// </summary>
 309                 public bool InAnonymousMethod;
 310
 311                 /// <summary>
 312                 ///   Location for this EmitContext
 313                 /// </summary>
 314                 public Location loc;
 315
 316                 /// <summary>
 317                 ///   Used to flag that it is ok to define types recursively, as the
 318                 ///   expressions are being evaluated as part of the type lookup
 319                 ///   during the type resolution process
 320                 /// </summary>
 321                 public bool ResolvingTypeTree;
 322
 323                 /// <summary>
 324                 ///   Inside an enum definition, we do not resolve enumeration values
 325                 ///   to their enumerations, but rather to the underlying type/value
 326                 ///   This is so EnumVal + EnumValB can be evaluated.
 327                 ///
 328                 ///   There is no "E operator + (E x, E y)", so during an enum evaluation
 329                 ///   we relax the rules
 330                 /// </summary>
 331                 public bool InEnumContext;
 332
 333                 FlowBranching current_flow_branching;
 334
 335                 public EmitContext (DeclSpace parent, DeclSpace ds, Location l, ILGenerator ig,
 336                                     Type return_type, int code_flags, bool is_constructor)
 337                 {
 338                         this.ig = ig;
 339
 340                         TypeContainer = parent;
 341                         DeclSpace = ds;
 342                         CheckState = RootContext.Checked;
 343                         ConstantCheckState = true;
 344
 345                         IsStatic = (code_flags & Modifiers.STATIC) != 0;
 346                         InIterator = (code_flags & Modifiers.METHOD_YIELDS) != 0;
 347                         RemapToProxy = InIterator;
 348                         ReturnType = return_type;
 349                         IsConstructor = is_constructor;
 350                         CurrentBlock = null;
 351                         CurrentFile = 0;
 352
 353                         if (parent != null){
 354                                 // Can only be null for the ResolveType contexts.
 355                                 ContainerType = parent.TypeBuilder;
 356                                 if (parent.UnsafeContext)
 357                                         InUnsafe = true;
 358                                 else
 359                                         InUnsafe = (code_flags & Modifiers.UNSAFE) != 0;
 360                         }
 361                         loc = l;
 362
 363                         if (ReturnType == TypeManager.void_type)
 364                                 ReturnType = null;
 365                 }
 366
 367                 public EmitContext (TypeContainer tc, Location l, ILGenerator ig,
 368                                     Type return_type, int code_flags, bool is_constructor)
 369                         : this (tc, tc, l, ig, return_type, code_flags, is_constructor)
 370                 {
 371                 }
 372
 373                 public EmitContext (TypeContainer tc, Location l, ILGenerator ig,
 374                                     Type return_type, int code_flags)
 375                         : this (tc, tc, l, ig, return_type, code_flags, false)
 376                 {
 377                 }
 378
 379                 public FlowBranching CurrentBranching {
 380                         get {
 381                                 return current_flow_branching;
 382                         }
 383                 }
 384
 385                 // <summary>
 386                 //   Starts a new code branching.  This inherits the state of all local
 387                 //   variables and parameters from the current branching.
 388                 // </summary>
 389                 public FlowBranching StartFlowBranching (FlowBranching.BranchingType type, Location loc)
 390                 {
 391                         current_flow_branching = FlowBranching.CreateBranching (CurrentBranching, type, null, loc);
 392                         return current_flow_branching;
 393                 }
 394
 395                 // <summary>
 396                 //   Starts a new code branching for block `block'.
 397                 // </summary>
 398                 public FlowBranching StartFlowBranching (Block block)
 399                 {
 400                         FlowBranching.BranchingType type;
 401
 402                         if (CurrentBranching.Type == FlowBranching.BranchingType.Switch)
 403                                 type = FlowBranching.BranchingType.SwitchSection;
 404                         else
 405                                 type = FlowBranching.BranchingType.Block;
 406
 407                         current_flow_branching = FlowBranching.CreateBranching (CurrentBranching, type, block, block.StartLocation);
 408                         return current_flow_branching;
 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 old = current_flow_branching;
 418                         current_flow_branching = current_flow_branching.Parent;
 419
 420                         return current_flow_branching.MergeChild (old);
 421                 }
 422
 423                 // <summary>
 424                 //   Ends a code branching.  Merges the state of locals and parameters
 425                 //   from all the children of the ending branching.
 426                 // </summary>
 427                 public FlowBranching.Reachability EndFlowBranching ()
 428                 {
 429                         FlowBranching.UsageVector vector = DoEndFlowBranching ();
 430
 431                         return vector.Reachability;
 432                 }
 433
 434                 // <summary>
 435                 //   Kills the current code branching.  This throws away any changed state
 436                 //   information and should only be used in case of an error.
 437                 // </summary>
 438                 public void KillFlowBranching ()
 439                 {
 440                         current_flow_branching = current_flow_branching.Parent;
 441                 }
 442
 443                 public void EmitTopBlock (Block block, InternalParameters ip, Location loc)
 444                 {
 445                         bool unreachable = false;
 446
 447                         if (!Location.IsNull (loc))
 448                                 CurrentFile = loc.File;
 449
 450                         if (block != null){
 451                             try {
 452                                 int errors = Report.Errors;
 453
 454                                 block.EmitMeta (this, ip);
 455
 456                                 if (Report.Errors == errors){
 457                                         bool old_do_flow_analysis = DoFlowAnalysis;
 458                                         DoFlowAnalysis = true;
 459
 460                                         current_flow_branching = FlowBranching.CreateBranching (
 461                                                 null, FlowBranching.BranchingType.Block, block, loc);
 462
 463                                         if (!block.Resolve (this)) {
 464                                                 current_flow_branching = null;
 465                                                 DoFlowAnalysis = old_do_flow_analysis;
 466                                                 return;
 467                                         }
 468
 469                                         FlowBranching.Reachability reachability = current_flow_branching.MergeTopBlock ();
 470                                         current_flow_branching = null;
 471
 472                                         DoFlowAnalysis = old_do_flow_analysis;
 473
 474                                         block.Emit (this);
 475
 476                                         if (reachability.AlwaysReturns ||
 477                                             reachability.AlwaysThrows ||
 478                                             reachability.IsUnreachable)
 479                                                 unreachable = true;
 480                                 }
 481                             } catch (Exception e) {
 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                                         Console.WriteLine (e.GetType ().FullName + ": " + e.Message);
 490                                         Console.WriteLine (Report.FriendlyStackTrace (e));
 491
 492                                         Environment.Exit (1);
 493                             }
 494                         }
 495
 496                         if (ReturnType != null && !unreachable){
 497                                 if (!InIterator){
 498                                         Report.Error (161, loc, "Not all code paths return a value");
 499                                         return;
 500                                 }
 501                         }
 502
 503                         if (HasReturnLabel)
 504                                 ig.MarkLabel (ReturnLabel);
 505                         if (return_value != null){
 506                                 ig.Emit (OpCodes.Ldloc, return_value);
 507                                 ig.Emit (OpCodes.Ret);
 508                         } else {
 509                                 //
 510                                 // If `HasReturnLabel' is set, then we already emitted a
 511                                 // jump to the end of the method, so we must emit a `ret'
 512                                 // there.
 513                                 //
 514                                 // Unfortunately, System.Reflection.Emit automatically emits
 515                                 // a leave to the end of a finally block.  This is a problem
 516                                 // if no code is following the try/finally block since we may
 517                                 // jump to a point after the end of the method.
 518                                 // As a workaround, we're always creating a return label in
 519                                 // this case.
 520                                 //
 521
 522                                 if ((block != null) && block.IsDestructor) {
 523                                         // Nothing to do; S.R.E automatically emits a leave.
 524                                 } else if (HasReturnLabel || (!unreachable && !InIterator)) {
 525                                         if (ReturnType != null)
 526                                                 ig.Emit (OpCodes.Ldloc, TemporaryReturn ());
 527                                         ig.Emit (OpCodes.Ret);
 528                                 }
 529                         }
 530                 }
 531
 532                 /// <summary>
 533                 ///   This is called immediately before emitting an IL opcode to tell the symbol
 534                 ///   writer to which source line this opcode belongs.
 535                 /// </summary>
 536                 public void Mark (Location loc, bool check_file)
 537                 {
 538                         if ((CodeGen.SymbolWriter == null) || Location.IsNull (loc))
 539                                 return;
 540
 541                         if (check_file && (CurrentFile != loc.File))
 542                                 return;
 543
 544                         ig.MarkSequencePoint (null, loc.Row, 0, 0, 0);
 545                 }
 546
 547                 /// <summary>
 548                 ///   Returns a temporary storage for a variable of type t as
 549                 ///   a local variable in the current body.
 550                 /// </summary>
 551                 public LocalBuilder GetTemporaryLocal (Type t)
 552                 {
 553                         LocalBuilder location = null;
 554
 555                         if (temporary_storage != null){
 556                                 object o = temporary_storage [t];
 557                                 if (o != null){
 558                                         if (o is ArrayList){
 559                                                 ArrayList al = (ArrayList) o;
 560
 561                                                 for (int i = 0; i < al.Count; i++){
 562                                                         if (al [i] != null){
 563                                                                 location = (LocalBuilder) al [i];
 564                                                                 al [i] = null;
 565                                                                 break;
 566                                                         }
 567                                                 }
 568                                         } else
 569                                                 location = (LocalBuilder) o;
 570                                         if (location != null)
 571                                                 return location;
 572                                 }
 573                         }
 574
 575                         return ig.DeclareLocal (t);
 576                 }
 577
 578                 public void FreeTemporaryLocal (LocalBuilder b, Type t)
 579                 {
 580                         if (temporary_storage == null){
 581                                 temporary_storage = new Hashtable ();
 582                                 temporary_storage [t] = b;
 583                                 return;
 584                         }
 585                         object o = temporary_storage [t];
 586                         if (o == null){
 587                                 temporary_storage [t] = b;
 588                                 return;
 589                         }
 590                         if (o is ArrayList){
 591                                 ArrayList al = (ArrayList) o;
 592                                 for (int i = 0; i < al.Count; i++){
 593                                         if (al [i] == null){
 594                                                 al [i] = b;
 595                                                 return;
 596                                         }
 597                                 }
 598                                 al.Add (b);
 599                                 return;
 600                         }
 601                         ArrayList replacement = new ArrayList ();
 602                         replacement.Add (o);
 603                         temporary_storage.Remove (t);
 604                         temporary_storage [t] = replacement;
 605                 }
 606
 607                 /// <summary>
 608                 ///   Current loop begin and end labels.
 609                 /// </summary>
 610                 public Label LoopBegin, LoopEnd;
 611
 612                 /// <summary>
 613                 ///   Default target in a switch statement.   Only valid if
 614                 ///   InSwitch is true
 615                 /// </summary>
 616                 public Label DefaultTarget;
 617
 618                 /// <summary>
 619                 ///   If this is non-null, points to the current switch statement
 620                 /// </summary>
 621                 public Switch Switch;
 622
 623                 /// <summary>
 624                 ///   ReturnValue creates on demand the LocalBuilder for the
 625                 ///   return value from the function.  By default this is not
 626                 ///   used.  This is only required when returns are found inside
 627                 ///   Try or Catch statements.
 628                 /// </summary>
 629                 public LocalBuilder TemporaryReturn ()
 630                 {
 631                         if (return_value == null){
 632                                 return_value = ig.DeclareLocal (ReturnType);
 633                                 ReturnLabel = ig.DefineLabel ();
 634                                 HasReturnLabel = true;
 635                         }
 636
 637                         return return_value;
 638                 }
 639
 640                 public void NeedReturnLabel ()
 641                 {
 642                         if (!HasReturnLabel) {
 643                                 ReturnLabel = ig.DefineLabel ();
 644                                 HasReturnLabel = true;
 645                         }
 646                 }
 647
 648                 //
 649                 // Creates a field `name' with the type `t' on the proxy class
 650                 //
 651                 public FieldBuilder MapVariable (string name, Type t)
 652                 {
 653                         if (InIterator){
 654                                 return IteratorHandler.Current.MapVariable ("v_", name, t);
 655                         }
 656
 657                         throw new Exception ("MapVariable for an unknown state");
 658                 }
 659
 660                 //
 661                 // Invoke this routine to remap a VariableInfo into the
 662                 // proper MemberAccess expression
 663                 //
 664                 public Expression RemapLocal (LocalInfo local_info)
 665                 {
 666                         FieldExpr fe = new FieldExpr (local_info.FieldBuilder, loc);
 667                         fe.InstanceExpression = new ProxyInstance ();
 668                         return fe.DoResolve (this);
 669                 }
 670
 671                 public Expression RemapLocalLValue (LocalInfo local_info, Expression right_side)
 672                 {
 673                         FieldExpr fe = new FieldExpr (local_info.FieldBuilder, loc);
 674                         fe.InstanceExpression = new ProxyInstance ();
 675                         return fe.DoResolveLValue (this, right_side);
 676                 }
 677
 678                 public Expression RemapParameter (int idx)
 679                 {
 680                         FieldExpr fe = new FieldExprNoAddress (IteratorHandler.Current.parameter_fields [idx], loc);
 681                         fe.InstanceExpression = new ProxyInstance ();
 682                         return fe.DoResolve (this);
 683                 }
 684
 685                 public Expression RemapParameterLValue (int idx, Expression right_side)
 686                 {
 687                         FieldExpr fe = new FieldExprNoAddress (IteratorHandler.Current.parameter_fields [idx], loc);
 688                         fe.InstanceExpression = new ProxyInstance ();
 689                         return fe.DoResolveLValue (this, right_side);
 690                 }
 691
 692                 //
 693                 // Emits the proper object to address fields on a remapped
 694                 // variable/parameter to field in anonymous-method/iterator proxy classes.
 695                 //
 696                 public void EmitThis ()
 697                 {
 698                         ig.Emit (OpCodes.Ldarg_0);
 699
 700                         if (!IsStatic){
 701                                 if (InIterator)
 702                                         ig.Emit (OpCodes.Ldfld, IteratorHandler.Current.this_field);
 703                                 else
 704                                         throw new Exception ("EmitThis for an unknown state");
 705                         }
 706                 }
 707
 708                 public Expression GetThis (Location loc)
 709                 {
 710                         This my_this;
 711                         if (CurrentBlock != null)
 712                                 my_this = new This (CurrentBlock, loc);
 713                         else
 714                                 my_this = new This (loc);
 715
 716                         if (!my_this.ResolveBase (this))
 717                                 my_this = null;
 718
 719                         return my_this;
 720                 }
 721         }
 722 }