1 /*
   2  * Copyright (c) 2005, 2012, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #ifndef SHARE_VM_CODE_DEPENDENCIES_HPP
  26 #define SHARE_VM_CODE_DEPENDENCIES_HPP
  27 
  28 #include "ci/ciCallSite.hpp"
  29 #include "ci/ciKlass.hpp"

  30 #include "ci/ciMethodHandle.hpp"
  31 #include "classfile/systemDictionary.hpp"
  32 #include "code/compressedStream.hpp"
  33 #include "code/nmethod.hpp"
  34 #include "utilities/growableArray.hpp"
  35 
  36 //** Dependencies represent assertions (approximate invariants) within
  37 // the runtime system, e.g. class hierarchy changes.  An example is an
  38 // assertion that a given method is not overridden; another example is
  39 // that a type has only one concrete subtype.  Compiled code which
  40 // relies on such assertions must be discarded if they are overturned
  41 // by changes in the runtime system.  We can think of these assertions
  42 // as approximate invariants, because we expect them to be overturned
  43 // very infrequently.  We are willing to perform expensive recovery
  44 // operations when they are overturned.  The benefit, of course, is
  45 // performing optimistic optimizations (!) on the object code.
  46 //
  47 // Changes in the class hierarchy due to dynamic linking or
  48 // class evolution can violate dependencies.  There is enough
  49 // indexing between classes and nmethods to make dependency
  50 // checking reasonably efficient.
  51 
  52 class ciEnv;
  53 class nmethod;
  54 class OopRecorder;
  55 class xmlStream;
  56 class CompileLog;
  57 class DepChange;
  58 class   KlassDepChange;
  59 class   CallSiteDepChange;
  60 class No_Safepoint_Verifier;
  61 
  62 class Dependencies: public ResourceObj {
  63  public:
  64   // Note: In the comments on dependency types, most uses of the terms
  65   // subtype and supertype are used in a "non-strict" or "inclusive"
  66   // sense, and are starred to remind the reader of this fact.
  67   // Strict uses of the terms use the word "proper".
  68   //
  69   // Specifically, every class is its own subtype* and supertype*.
  70   // (This trick is easier than continually saying things like "Y is a
  71   // subtype of X or X itself".)
  72   //
  73   // Sometimes we write X > Y to mean X is a proper supertype of Y.
  74   // The notation X > {Y, Z} means X has proper subtypes Y, Z.
  75   // The notation X.m > Y means that Y inherits m from X, while
  76   // X.m > Y.m means Y overrides X.m.  A star denotes abstractness,
  77   // as *I > A, meaning (abstract) interface I is a super type of A,
  78   // or A.*m > B.m, meaning B.m implements abstract method A.m.
  79   //
  80   // In this module, the terms "subtype" and "supertype" refer to
  81   // Java-level reference type conversions, as detected by
  82   // "instanceof" and performed by "checkcast" operations.  The method
  83   // Klass::is_subtype_of tests these relations.  Note that "subtype"
  84   // is richer than "subclass" (as tested by Klass::is_subclass_of),
  85   // since it takes account of relations involving interface and array
  86   // types.
  87   //
  88   // To avoid needless complexity, dependencies involving array types
  89   // are not accepted.  If you need to make an assertion about an
  90   // array type, make the assertion about its corresponding element
  91   // types.  Any assertion that might change about an array type can
  92   // be converted to an assertion about its element type.
  93   //
  94   // Most dependencies are evaluated over a "context type" CX, which
  95   // stands for the set Subtypes(CX) of every Java type that is a subtype*
  96   // of CX.  When the system loads a new class or interface N, it is
  97   // responsible for re-evaluating changed dependencies whose context
  98   // type now includes N, that is, all super types of N.
  99   //
 100   enum DepType {
 101     end_marker = 0,
 102 
 103     // An 'evol' dependency simply notes that the contents of the
 104     // method were used.  If it evolves (is replaced), the nmethod
 105     // must be recompiled.  No other dependencies are implied.
 106     evol_method,
 107     FIRST_TYPE = evol_method,
 108 
 109     // A context type CX is a leaf it if has no proper subtype.
 110     leaf_type,
 111 
 112     // An abstract class CX has exactly one concrete subtype CC.
 113     abstract_with_unique_concrete_subtype,
 114 
 115     // The type CX is purely abstract, with no concrete subtype* at all.
 116     abstract_with_no_concrete_subtype,
 117 
 118     // The concrete CX is free of concrete proper subtypes.
 119     concrete_with_no_concrete_subtype,
 120 
 121     // Given a method M1 and a context class CX, the set MM(CX, M1) of
 122     // "concrete matching methods" in CX of M1 is the set of every
 123     // concrete M2 for which it is possible to create an invokevirtual
 124     // or invokeinterface call site that can reach either M1 or M2.
 125     // That is, M1 and M2 share a name, signature, and vtable index.
 126     // We wish to notice when the set MM(CX, M1) is just {M1}, or
 127     // perhaps a set of two {M1,M2}, and issue dependencies on this.
 128 
 129     // The set MM(CX, M1) can be computed by starting with any matching
 130     // concrete M2 that is inherited into CX, and then walking the
 131     // subtypes* of CX looking for concrete definitions.
 132 
 133     // The parameters to this dependency are the method M1 and the
 134     // context class CX.  M1 must be either inherited in CX or defined
 135     // in a subtype* of CX.  It asserts that MM(CX, M1) is no greater
 136     // than {M1}.
 137     unique_concrete_method,       // one unique concrete method under CX
 138 
 139     // An "exclusive" assertion concerns two methods or subtypes, and
 140     // declares that there are at most two (or perhaps later N>2)
 141     // specific items that jointly satisfy the restriction.
 142     // We list all items explicitly rather than just giving their
 143     // count, for robustness in the face of complex schema changes.
 144 
 145     // A context class CX (which may be either abstract or concrete)
 146     // has two exclusive concrete subtypes* C1, C2 if every concrete
 147     // subtype* of CX is either C1 or C2.  Note that if neither C1 or C2
 148     // are equal to CX, then CX itself must be abstract.  But it is
 149     // also possible (for example) that C1 is CX (a concrete class)
 150     // and C2 is a proper subtype of C1.
 151     abstract_with_exclusive_concrete_subtypes_2,
 152 
 153     // This dependency asserts that MM(CX, M1) is no greater than {M1,M2}.
 154     exclusive_concrete_methods_2,
 155 
 156     // This dependency asserts that no instances of class or it's
 157     // subclasses require finalization registration.
 158     no_finalizable_subclasses,
 159 
 160     // This dependency asserts when the CallSite.target value changed.
 161     call_site_target_value,
 162 
 163     TYPE_LIMIT
 164   };
 165   enum {
 166     LG2_TYPE_LIMIT = 4,  // assert(TYPE_LIMIT <= (1<<LG2_TYPE_LIMIT))
 167 
 168     // handy categorizations of dependency types:
 169     all_types           = ((1 << TYPE_LIMIT) - 1) & ((-1) << FIRST_TYPE),
 170 
 171     non_klass_types     = (1 << call_site_target_value),
 172     klass_types         = all_types & ~non_klass_types,
 173 
 174     non_ctxk_types      = (1 << evol_method),
 175     implicit_ctxk_types = (1 << call_site_target_value),
 176     explicit_ctxk_types = all_types & ~(non_ctxk_types | implicit_ctxk_types),
 177 
 178     max_arg_count = 3,   // current maximum number of arguments (incl. ctxk)
 179 
 180     // A "context type" is a class or interface that
 181     // provides context for evaluating a dependency.
 182     // When present, it is one of the arguments (dep_context_arg).
 183     //
 184     // If a dependency does not have a context type, there is a
 185     // default context, depending on the type of the dependency.
 186     // This bit signals that a default context has been compressed away.
 187     default_context_type_bit = (1<<LG2_TYPE_LIMIT)
 188   };
 189 
 190   static const char* dep_name(DepType dept);
 191   static int         dep_args(DepType dept);
 192 
 193   static bool is_klass_type(           DepType dept) { return dept_in_mask(dept, klass_types        ); }
 194 
 195   static bool has_explicit_context_arg(DepType dept) { return dept_in_mask(dept, explicit_ctxk_types); }
 196   static bool has_implicit_context_arg(DepType dept) { return dept_in_mask(dept, implicit_ctxk_types); }
 197 
 198   static int           dep_context_arg(DepType dept) { return has_explicit_context_arg(dept) ? 0 : -1; }
 199   static int  dep_implicit_context_arg(DepType dept) { return has_implicit_context_arg(dept) ? 0 : -1; }
 200 
 201   static void check_valid_dependency_type(DepType dept);
 202 











































 203  private:
 204   // State for writing a new set of dependencies:
 205   GrowableArray<int>*       _dep_seen;  // (seen[h->ident] & (1<<dept))
 206   GrowableArray<ciBaseObject*>*  _deps[TYPE_LIMIT];
 207 
 208   static const char* _dep_name[TYPE_LIMIT];
 209   static int         _dep_args[TYPE_LIMIT];
 210 
 211   static bool dept_in_mask(DepType dept, int mask) {
 212     return (int)dept >= 0 && dept < TYPE_LIMIT && ((1<<dept) & mask) != 0;
 213   }
 214 
 215   bool note_dep_seen(int dept, ciBaseObject* x) {
 216     assert(dept < BitsPerInt, "oob");
 217     int x_id = x->ident();

 218     assert(_dep_seen != NULL, "deps must be writable");
 219     int seen = _dep_seen->at_grow(x_id, 0);
 220     _dep_seen->at_put(x_id, seen | (1<<dept));
 221     // return true if we've already seen dept/x
 222     return (seen & (1<<dept)) != 0;
 223   }
 224 
 225   bool maybe_merge_ctxk(GrowableArray<ciBaseObject*>* deps,
 226                         int ctxk_i, ciKlass* ctxk);
 227 
 228   void sort_all_deps();
 229   size_t estimate_size_in_bytes();
 230 
 231   // Initialize _deps, etc.
 232   void initialize(ciEnv* env);
 233 
 234   // State for making a new set of dependencies:
 235   OopRecorder* _oop_recorder;
 236 
 237   // Logging support
 238   CompileLog* _log;
 239 
 240   address  _content_bytes;  // everything but the oop references, encoded
 241   size_t   _size_in_bytes;
 242 
 243  public:
 244   // Make a new empty dependencies set.
 245   Dependencies(ciEnv* env) {
 246     initialize(env);
 247   }
 248 
 249  private:
 250   // Check for a valid context type.
 251   // Enforce the restriction against array types.
 252   static void check_ctxk(ciKlass* ctxk) {
 253     assert(ctxk->is_instance_klass(), "java types only");
 254   }
 255   static void check_ctxk_concrete(ciKlass* ctxk) {
 256     assert(is_concrete_klass(ctxk->as_instance_klass()), "must be concrete");

 257   }
 258   static void check_ctxk_abstract(ciKlass* ctxk) {
 259     check_ctxk(ctxk);
 260     assert(!is_concrete_klass(ctxk->as_instance_klass()), "must be abstract");
 261   }
 262 
 263   void assert_common_1(DepType dept, ciBaseObject* x);
 264   void assert_common_2(DepType dept, ciBaseObject* x0, ciBaseObject* x1);
 265   void assert_common_3(DepType dept, ciKlass* ctxk, ciBaseObject* x1, ciBaseObject* x2);
 266 
 267  public:
 268   // Adding assertions to a new dependency set at compile time:
 269   void assert_evol_method(ciMethod* m);
 270   void assert_leaf_type(ciKlass* ctxk);
 271   void assert_abstract_with_unique_concrete_subtype(ciKlass* ctxk, ciKlass* conck);
 272   void assert_abstract_with_no_concrete_subtype(ciKlass* ctxk);
 273   void assert_concrete_with_no_concrete_subtype(ciKlass* ctxk);
 274   void assert_unique_concrete_method(ciKlass* ctxk, ciMethod* uniqm);
 275   void assert_abstract_with_exclusive_concrete_subtypes(ciKlass* ctxk, ciKlass* k1, ciKlass* k2);
 276   void assert_exclusive_concrete_methods(ciKlass* ctxk, ciMethod* m1, ciMethod* m2);
 277   void assert_has_no_finalizable_subclasses(ciKlass* ctxk);
 278   void assert_call_site_target_value(ciCallSite* call_site, ciMethodHandle* method_handle);
































 279 
 280   // Define whether a given method or type is concrete.
 281   // These methods define the term "concrete" as used in this module.
 282   // For this module, an "abstract" class is one which is non-concrete.
 283   //
 284   // Future optimizations may allow some classes to remain
 285   // non-concrete until their first instantiation, and allow some
 286   // methods to remain non-concrete until their first invocation.
 287   // In that case, there would be a middle ground between concrete
 288   // and abstract (as defined by the Java language and VM).
 289   static bool is_concrete_klass(Klass* k);    // k is instantiable
 290   static bool is_concrete_method(Method* m);  // m is invocable
 291   static Klass* find_finalizable_subclass(Klass* k);
 292 
 293   // These versions of the concreteness queries work through the CI.
 294   // The CI versions are allowed to skew sometimes from the VM
 295   // (oop-based) versions.  The cost of such a difference is a
 296   // (safely) aborted compilation, or a deoptimization, or a missed
 297   // optimization opportunity.
 298   //
 299   // In order to prevent spurious assertions, query results must
 300   // remain stable within any single ciEnv instance.  (I.e., they must
 301   // not go back into the VM to get their value; they must cache the
 302   // bit in the CI, either eagerly or lazily.)
 303   static bool is_concrete_klass(ciInstanceKlass* k); // k appears instantiable
 304   static bool is_concrete_method(ciMethod* m);       // m appears invocable
 305   static bool has_finalizable_subclass(ciInstanceKlass* k);
 306 
 307   // As a general rule, it is OK to compile under the assumption that
 308   // a given type or method is concrete, even if it at some future
 309   // point becomes abstract.  So dependency checking is one-sided, in
 310   // that it permits supposedly concrete classes or methods to turn up
 311   // as really abstract.  (This shouldn't happen, except during class
 312   // evolution, but that's the logic of the checking.)  However, if a
 313   // supposedly abstract class or method suddenly becomes concrete, a
 314   // dependency on it must fail.
 315 
 316   // Checking old assertions at run-time (in the VM only):
 317   static Klass* check_evol_method(Method* m);
 318   static Klass* check_leaf_type(Klass* ctxk);
 319   static Klass* check_abstract_with_unique_concrete_subtype(Klass* ctxk, Klass* conck,
 320                                                               KlassDepChange* changes = NULL);
 321   static Klass* check_abstract_with_no_concrete_subtype(Klass* ctxk,
 322                                                           KlassDepChange* changes = NULL);
 323   static Klass* check_concrete_with_no_concrete_subtype(Klass* ctxk,
 324                                                           KlassDepChange* changes = NULL);
 325   static Klass* check_unique_concrete_method(Klass* ctxk, Method* uniqm,
 326                                                KlassDepChange* changes = NULL);
 327   static Klass* check_abstract_with_exclusive_concrete_subtypes(Klass* ctxk, Klass* k1, Klass* k2,
 328                                                                   KlassDepChange* changes = NULL);
 329   static Klass* check_exclusive_concrete_methods(Klass* ctxk, Method* m1, Method* m2,
 330                                                    KlassDepChange* changes = NULL);
 331   static Klass* check_has_no_finalizable_subclasses(Klass* ctxk, KlassDepChange* changes = NULL);
 332   static Klass* check_call_site_target_value(oop call_site, oop method_handle, CallSiteDepChange* changes = NULL);
 333   // A returned Klass* is NULL if the dependency assertion is still
 334   // valid.  A non-NULL Klass* is a 'witness' to the assertion
 335   // failure, a point in the class hierarchy where the assertion has
 336   // been proven false.  For example, if check_leaf_type returns
 337   // non-NULL, the value is a subtype of the supposed leaf type.  This
 338   // witness value may be useful for logging the dependency failure.
 339   // Note that, when a dependency fails, there may be several possible
 340   // witnesses to the failure.  The value returned from the check_foo
 341   // method is chosen arbitrarily.
 342 
 343   // The 'changes' value, if non-null, requests a limited spot-check
 344   // near the indicated recent changes in the class hierarchy.
 345   // It is used by DepStream::spot_check_dependency_at.
 346 
 347   // Detecting possible new assertions:
 348   static Klass*    find_unique_concrete_subtype(Klass* ctxk);
 349   static Method*   find_unique_concrete_method(Klass* ctxk, Method* m);
 350   static int       find_exclusive_concrete_subtypes(Klass* ctxk, int klen, Klass* k[]);
 351   static int       find_exclusive_concrete_methods(Klass* ctxk, int mlen, Method* m[]);
 352 
 353   // Create the encoding which will be stored in an nmethod.
 354   void encode_content_bytes();
 355 
 356   address content_bytes() {
 357     assert(_content_bytes != NULL, "encode it first");
 358     return _content_bytes;
 359   }
 360   size_t size_in_bytes() {
 361     assert(_content_bytes != NULL, "encode it first");
 362     return _size_in_bytes;
 363   }
 364 
 365   OopRecorder* oop_recorder() { return _oop_recorder; }
 366   CompileLog*  log()          { return _log; }
 367 
 368   void copy_to(nmethod* nm);
 369 
 370   void log_all_dependencies();
 371   void log_dependency(DepType dept, int nargs, ciBaseObject* args[]) {
 372     write_dependency_to(log(), dept, nargs, args);
 373   }
 374   void log_dependency(DepType dept,
 375                       ciBaseObject* x0,
 376                       ciBaseObject* x1 = NULL,
 377                       ciBaseObject* x2 = NULL) {
 378     if (log() == NULL)  return;
 379     ciBaseObject* args[max_arg_count];
 380     args[0] = x0;
 381     args[1] = x1;
 382     args[2] = x2;
 383     assert(2 < max_arg_count, "");
 384     log_dependency(dept, dep_args(dept), args);
 385   }
 386 
 387   class DepArgument : public ResourceObj {
 388    private:
 389     bool  _is_oop;
 390     bool  _valid;
 391     void* _value;
 392    public:
 393     DepArgument() : _is_oop(false), _value(NULL), _valid(false) {}
 394     DepArgument(oop v): _is_oop(true), _value(v), _valid(true) {}
 395     DepArgument(Metadata* v): _is_oop(false), _value(v), _valid(true) {}
 396 
 397     bool is_null() const               { return _value == NULL; }
 398     bool is_oop() const                { return _is_oop; }
 399     bool is_metadata() const           { return !_is_oop; }
 400     bool is_klass() const              { return is_metadata() && metadata_value()->is_klass(); }
 401     bool is_method() const              { return is_metadata() && metadata_value()->is_method(); }
 402 
 403     oop oop_value() const              { assert(_is_oop && _valid, "must be"); return (oop) _value; }
 404     Metadata* metadata_value() const { assert(!_is_oop && _valid, "must be"); return (Metadata*) _value; }
 405   };
 406 
 407   static void write_dependency_to(CompileLog* log,
 408                                   DepType dept,
 409                                   int nargs, ciBaseObject* args[],
 410                                   Klass* witness = NULL);
 411   static void write_dependency_to(CompileLog* log,
 412                                   DepType dept,
 413                                   int nargs, DepArgument args[],
 414                                   Klass* witness = NULL);




 415   static void write_dependency_to(xmlStream* xtty,
 416                                   DepType dept,
 417                                   int nargs, DepArgument args[],
 418                                   Klass* witness = NULL);
 419   static void print_dependency(DepType dept,
 420                                int nargs, DepArgument args[],
 421                                Klass* witness = NULL);
 422 
 423  private:
 424   // helper for encoding common context types as zero:
 425   static ciKlass* ctxk_encoded_as_null(DepType dept, ciBaseObject* x);
 426 
 427   static Klass* ctxk_encoded_as_null(DepType dept, Metadata* x);
 428 
 429  public:
 430   // Use this to iterate over an nmethod's dependency set.
 431   // Works on new and old dependency sets.
 432   // Usage:
 433   //
 434   // ;
 435   // Dependencies::DepType dept;
 436   // for (Dependencies::DepStream deps(nm); deps.next(); ) {
 437   //   ...
 438   // }
 439   //
 440   // The caller must be in the VM, since oops are not wrapped in handles.
 441   class DepStream {
 442   private:
 443     nmethod*              _code;   // null if in a compiler thread
 444     Dependencies*         _deps;   // null if not in a compiler thread
 445     CompressedReadStream  _bytes;
 446 #ifdef ASSERT
 447     size_t                _byte_limit;
 448 #endif
 449 
 450     // iteration variables:
 451     DepType               _type;
 452     int                   _xi[max_arg_count+1];
 453 
 454     void initial_asserts(size_t byte_limit) NOT_DEBUG({});
 455 
 456     inline Metadata* recorded_metadata_at(int i);
 457     inline oop recorded_oop_at(int i);
 458 
 459     Klass* check_klass_dependency(KlassDepChange* changes);
 460     Klass* check_call_site_dependency(CallSiteDepChange* changes);
 461 
 462     void trace_and_log_witness(Klass* witness);
 463 
 464   public:
 465     DepStream(Dependencies* deps)
 466       : _deps(deps),
 467         _code(NULL),
 468         _bytes(deps->content_bytes())
 469     {
 470       initial_asserts(deps->size_in_bytes());
 471     }
 472     DepStream(nmethod* code)
 473       : _deps(NULL),
 474         _code(code),
 475         _bytes(code->dependencies_begin())
 476     {
 477       initial_asserts(code->dependencies_size());
 478     }
 479 
 480     bool next();
 481 
 482     DepType type()               { return _type; }
 483     int argument_count()         { return dep_args(type()); }
 484     int argument_index(int i)    { assert(0 <= i && i < argument_count(), "oob");
 485                                    return _xi[i]; }
 486     Metadata* argument(int i);     // => recorded_oop_at(argument_index(i))
 487     oop argument_oop(int i);         // => recorded_oop_at(argument_index(i))
 488     Klass* context_type();
 489 
 490     bool is_klass_type()         { return Dependencies::is_klass_type(type()); }
 491 
 492     Method* method_argument(int i) {
 493       Metadata* x = argument(i);
 494       assert(x->is_method(), "type");
 495       return (Method*) x;
 496     }
 497     Klass* type_argument(int i) {
 498       Metadata* x = argument(i);
 499       assert(x->is_klass(), "type");
 500       return (Klass*) x;
 501     }
 502 
 503     // The point of the whole exercise:  Is this dep still OK?
 504     Klass* check_dependency() {
 505       Klass* result = check_klass_dependency(NULL);
 506       if (result != NULL)  return result;
 507       return check_call_site_dependency(NULL);
 508     }
 509 
 510     // A lighter version:  Checks only around recent changes in a class
 511     // hierarchy.  (See Universe::flush_dependents_on.)
 512     Klass* spot_check_dependency_at(DepChange& changes);
 513 
 514     // Log the current dependency to xtty or compilation log.
 515     void log_dependency(Klass* witness = NULL);
 516 
 517     // Print the current dependency to tty.
 518     void print_dependency(Klass* witness = NULL, bool verbose = false);
 519   };
 520   friend class Dependencies::DepStream;
 521 
 522   static void print_statistics() PRODUCT_RETURN;
 523 };
 524 
 525 
 526 // Every particular DepChange is a sub-class of this class.
 527 class DepChange : public StackObj {
 528  public:
 529   // What kind of DepChange is this?
 530   virtual bool is_klass_change()     const { return false; }
 531   virtual bool is_call_site_change() const { return false; }
 532 
 533   // Subclass casting with assertions.
 534   KlassDepChange*    as_klass_change() {
 535     assert(is_klass_change(), "bad cast");
 536     return (KlassDepChange*) this;
 537   }
 538   CallSiteDepChange* as_call_site_change() {
 539     assert(is_call_site_change(), "bad cast");
 540     return (CallSiteDepChange*) this;
 541   }
 542 
 543   void print();
 544 
 545  public:
 546   enum ChangeType {
 547     NO_CHANGE = 0,              // an uninvolved klass
 548     Change_new_type,            // a newly loaded type
 549     Change_new_sub,             // a super with a new subtype
 550     Change_new_impl,            // an interface with a new implementation
 551     CHANGE_LIMIT,
 552     Start_Klass = CHANGE_LIMIT  // internal indicator for ContextStream
 553   };
 554 
 555   // Usage:
 556   // for (DepChange::ContextStream str(changes); str.next(); ) {
 557   //   Klass* k = str.klass();
 558   //   switch (str.change_type()) {
 559   //     ...
 560   //   }
 561   // }
 562   class ContextStream : public StackObj {
 563    private:
 564     DepChange&  _changes;
 565     friend class DepChange;
 566 
 567     // iteration variables:
 568     ChangeType  _change_type;
 569     Klass*      _klass;
 570     Array<Klass*>* _ti_base;    // i.e., transitive_interfaces
 571     int         _ti_index;
 572     int         _ti_limit;
 573 
 574     // start at the beginning:
 575     void start();
 576 
 577    public:
 578     ContextStream(DepChange& changes)
 579       : _changes(changes)
 580     { start(); }
 581 
 582     ContextStream(DepChange& changes, No_Safepoint_Verifier& nsv)
 583       : _changes(changes)
 584       // the nsv argument makes it safe to hold oops like _klass
 585     { start(); }
 586 
 587     bool next();
 588 
 589     ChangeType change_type()     { return _change_type; }
 590     Klass*     klass()           { return _klass; }
 591   };
 592   friend class DepChange::ContextStream;
 593 };
 594 
 595 
 596 // A class hierarchy change coming through the VM (under the Compile_lock).
 597 // The change is structured as a single new type with any number of supers
 598 // and implemented interface types.  Other than the new type, any of the
 599 // super types can be context types for a relevant dependency, which the
 600 // new type could invalidate.
 601 class KlassDepChange : public DepChange {
 602  private:
 603   // each change set is rooted in exactly one new type (at present):
 604   KlassHandle _new_type;
 605 
 606   void initialize();
 607 
 608  public:
 609   // notes the new type, marks it and all its super-types
 610   KlassDepChange(KlassHandle new_type)
 611     : _new_type(new_type)
 612   {
 613     initialize();
 614   }
 615 
 616   // cleans up the marks
 617   ~KlassDepChange();
 618 
 619   // What kind of DepChange is this?
 620   virtual bool is_klass_change() const { return true; }
 621 
 622   Klass* new_type() { return _new_type(); }
 623 
 624   // involves_context(k) is true if k is new_type or any of the super types
 625   bool involves_context(Klass* k);
 626 };
 627 
 628 
 629 // A CallSite has changed its target.
 630 class CallSiteDepChange : public DepChange {
 631  private:
 632   Handle _call_site;
 633   Handle _method_handle;
 634 
 635  public:
 636   CallSiteDepChange(Handle call_site, Handle method_handle)
 637     : _call_site(call_site),
 638       _method_handle(method_handle)
 639   {
 640     assert(_call_site()    ->is_a(SystemDictionary::CallSite_klass()),     "must be");
 641     assert(_method_handle()->is_a(SystemDictionary::MethodHandle_klass()), "must be");
 642   }
 643 
 644   // What kind of DepChange is this?
 645   virtual bool is_call_site_change() const { return true; }
 646 
 647   oop call_site()     const { return _call_site();     }
 648   oop method_handle() const { return _method_handle(); }
 649 };
 650 
 651 #endif // SHARE_VM_CODE_DEPENDENCIES_HPP
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