rev 50108 : [mq]: tschatzl_1

   1 /*
   2  * Copyright (c) 2001, 2018, 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 #include "precompiled.hpp"
  26 #include "classfile/javaClasses.inline.hpp"
  27 #include "classfile/systemDictionary.hpp"
  28 #include "gc/shared/collectedHeap.hpp"
  29 #include "gc/shared/collectedHeap.inline.hpp"
  30 #include "gc/shared/gcTimer.hpp"
  31 #include "gc/shared/gcTraceTime.inline.hpp"
  32 #include "gc/shared/referencePolicy.hpp"
  33 #include "gc/shared/referenceProcessor.inline.hpp"
  34 #include "logging/log.hpp"
  35 #include "memory/allocation.inline.hpp"
  36 #include "memory/resourceArea.hpp"
  37 #include "oops/access.inline.hpp"
  38 #include "oops/oop.inline.hpp"
  39 #include "runtime/java.hpp"
  40 
  41 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  42 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  43 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  44 
  45 void referenceProcessor_init() {
  46   ReferenceProcessor::init_statics();
  47 }
  48 
  49 void ReferenceProcessor::init_statics() {
  50   // We need a monotonically non-decreasing time in ms but
  51   // os::javaTimeMillis() does not guarantee monotonicity.
  52   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  53 
  54   // Initialize the soft ref timestamp clock.
  55   _soft_ref_timestamp_clock = now;
  56   // Also update the soft ref clock in j.l.r.SoftReference
  57   java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
  58 
  59   _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  60   if (is_server_compilation_mode_vm()) {
  61     _default_soft_ref_policy = new LRUMaxHeapPolicy();
  62   } else {
  63     _default_soft_ref_policy = new LRUCurrentHeapPolicy();
  64   }
  65   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  66     vm_exit_during_initialization("Could not allocate reference policy object");
  67   }
  68   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  69             RefDiscoveryPolicy == ReferentBasedDiscovery,
  70             "Unrecognized RefDiscoveryPolicy");
  71 }
  72 
  73 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
  74 #ifdef ASSERT
  75   // Verify that we're not currently discovering refs
  76   assert(!_discovering_refs, "nested call?");
  77 
  78   if (check_no_refs) {
  79     // Verify that the discovered lists are empty
  80     verify_no_references_recorded();
  81   }
  82 #endif // ASSERT
  83 
  84   // Someone could have modified the value of the static
  85   // field in the j.l.r.SoftReference class that holds the
  86   // soft reference timestamp clock using reflection or
  87   // Unsafe between GCs. Unconditionally update the static
  88   // field in ReferenceProcessor here so that we use the new
  89   // value during reference discovery.
  90 
  91   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
  92   _discovering_refs = true;
  93 }
  94 
  95 ReferenceProcessor::ReferenceProcessor(BoolObjectClosure* is_subject_to_discovery,
  96                                        bool      mt_processing,
  97                                        uint      mt_processing_degree,
  98                                        bool      mt_discovery,
  99                                        uint      mt_discovery_degree,
 100                                        bool      atomic_discovery,
 101                                        BoolObjectClosure* is_alive_non_header)  :
 102   _is_subject_to_discovery(is_subject_to_discovery),
 103   _discovering_refs(false),
 104   _enqueuing_is_done(false),
 105   _is_alive_non_header(is_alive_non_header),
 106   _processing_is_mt(mt_processing),
 107   _next_id(0)
 108 {
 109   assert(is_subject_to_discovery != NULL, "must be set");
 110 
 111   _discovery_is_atomic = atomic_discovery;
 112   _discovery_is_mt     = mt_discovery;
 113   _num_queues          = MAX2(1U, mt_processing_degree);
 114   _max_num_queues      = MAX2(_num_queues, mt_discovery_degree);
 115   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 116             _max_num_queues * number_of_subclasses_of_ref(), mtGC);
 117 
 118   if (_discovered_refs == NULL) {
 119     vm_exit_during_initialization("Could not allocated RefProc Array");
 120   }
 121   _discoveredSoftRefs    = &_discovered_refs[0];
 122   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_queues];
 123   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_queues];
 124   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_queues];
 125 
 126   // Initialize all entries to NULL
 127   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
 128     _discovered_refs[i].set_head(NULL);
 129     _discovered_refs[i].set_length(0);
 130   }
 131 
 132   setup_policy(false /* default soft ref policy */);
 133 }
 134 
 135 #ifndef PRODUCT
 136 void ReferenceProcessor::verify_no_references_recorded() {
 137   guarantee(!_discovering_refs, "Discovering refs?");
 138   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
 139     guarantee(_discovered_refs[i].is_empty(),
 140               "Found non-empty discovered list at %u", i);
 141   }
 142 }
 143 #endif
 144 
 145 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 146   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
 147     if (UseCompressedOops) {
 148       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 149     } else {
 150       f->do_oop((oop*)_discovered_refs[i].adr_head());
 151     }
 152   }
 153 }
 154 
 155 void ReferenceProcessor::update_soft_ref_master_clock() {
 156   // Update (advance) the soft ref master clock field. This must be done
 157   // after processing the soft ref list.
 158 
 159   // We need a monotonically non-decreasing time in ms but
 160   // os::javaTimeMillis() does not guarantee monotonicity.
 161   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 162   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 163   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 164 
 165   NOT_PRODUCT(
 166   if (now < _soft_ref_timestamp_clock) {
 167     log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
 168                     _soft_ref_timestamp_clock, now);
 169   }
 170   )
 171   // The values of now and _soft_ref_timestamp_clock are set using
 172   // javaTimeNanos(), which is guaranteed to be monotonically
 173   // non-decreasing provided the underlying platform provides such
 174   // a time source (and it is bug free).
 175   // In product mode, however, protect ourselves from non-monotonicity.
 176   if (now > _soft_ref_timestamp_clock) {
 177     _soft_ref_timestamp_clock = now;
 178     java_lang_ref_SoftReference::set_clock(now);
 179   }
 180   // Else leave clock stalled at its old value until time progresses
 181   // past clock value.
 182 }
 183 
 184 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) const {
 185   size_t total = 0;
 186   for (uint i = 0; i < _max_num_queues; ++i) {
 187     total += lists[i].length();
 188   }
 189   return total;
 190 }
 191 
 192 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 193   BoolObjectClosure*            is_alive,
 194   OopClosure*                   keep_alive,
 195   VoidClosure*                  complete_gc,
 196   AbstractRefProcTaskExecutor*  task_executor,
 197   ReferenceProcessorPhaseTimes* phase_times) {
 198 
 199   double start_time = os::elapsedTime();
 200 
 201   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 202   // Stop treating discovered references specially.
 203   disable_discovery();
 204 
 205   // If discovery was concurrent, someone could have modified
 206   // the value of the static field in the j.l.r.SoftReference
 207   // class that holds the soft reference timestamp clock using
 208   // reflection or Unsafe between when discovery was enabled and
 209   // now. Unconditionally update the static field in ReferenceProcessor
 210   // here so that we use the new value during processing of the
 211   // discovered soft refs.
 212 
 213   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 214 
 215   ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
 216                                 total_count(_discoveredWeakRefs),
 217                                 total_count(_discoveredFinalRefs),
 218                                 total_count(_discoveredPhantomRefs));
 219 
 220   // Soft references
 221   {
 222     RefProcPhaseTimesTracker tt(REF_SOFT, phase_times, this);
 223     process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 224                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 225   }
 226 
 227   update_soft_ref_master_clock();
 228 
 229   // Weak references
 230   {
 231     RefProcPhaseTimesTracker tt(REF_WEAK, phase_times, this);
 232     process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 233                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 234   }
 235 
 236   // Final references
 237   {
 238     RefProcPhaseTimesTracker tt(REF_FINAL, phase_times, this);
 239     process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 240                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 241   }
 242 
 243   // Phantom references
 244   {
 245     RefProcPhaseTimesTracker tt(REF_PHANTOM, phase_times, this);
 246     process_discovered_reflist(_discoveredPhantomRefs, NULL, true,
 247                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 248   }
 249 
 250   if (task_executor != NULL) {
 251     // Record the work done by the parallel workers.
 252     task_executor->set_single_threaded_mode();
 253   }
 254 
 255   phase_times->set_total_time_ms((os::elapsedTime() - start_time) * 1000);
 256 
 257   return stats;
 258 }
 259 
 260 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 261   _current_discovered_addr = java_lang_ref_Reference::discovered_addr_raw(_current_discovered);
 262   oop discovered = java_lang_ref_Reference::discovered(_current_discovered);
 263   assert(_current_discovered_addr && oopDesc::is_oop_or_null(discovered),
 264          "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
 265   _next_discovered = discovered;
 266 
 267   _referent_addr = java_lang_ref_Reference::referent_addr_raw(_current_discovered);
 268   _referent = java_lang_ref_Reference::referent(_current_discovered);
 269   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 270          "Wrong oop found in java.lang.Reference object");
 271   assert(allow_null_referent ?
 272              oopDesc::is_oop_or_null(_referent)
 273            : oopDesc::is_oop(_referent),
 274          "Expected an oop%s for referent field at " PTR_FORMAT,
 275          (allow_null_referent ? " or NULL" : ""),
 276          p2i(_referent));
 277 }
 278 
 279 void DiscoveredListIterator::remove() {
 280   assert(oopDesc::is_oop(_current_discovered), "Dropping a bad reference");
 281   RawAccess<>::oop_store(_current_discovered_addr, oop(NULL));
 282 
 283   // First _prev_next ref actually points into DiscoveredList (gross).
 284   oop new_next;
 285   if (_next_discovered == _current_discovered) {
 286     // At the end of the list, we should make _prev point to itself.
 287     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 288     // and _prev will be NULL.
 289     new_next = _prev_discovered;
 290   } else {
 291     new_next = _next_discovered;
 292   }
 293   // Remove Reference object from discovered list. Note that G1 does not need a
 294   // pre-barrier here because we know the Reference has already been found/marked,
 295   // that's how it ended up in the discovered list in the first place.
 296   RawAccess<>::oop_store(_prev_discovered_addr, new_next);
 297   NOT_PRODUCT(_removed++);
 298   _refs_list.dec_length(1);
 299 }
 300 
 301 void DiscoveredListIterator::clear_referent() {
 302   RawAccess<>::oop_store(_referent_addr, oop(NULL));
 303 }
 304 
 305 void DiscoveredListIterator::enqueue() {
 306   HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_current_discovered,
 307                                             java_lang_ref_Reference::discovered_offset,
 308                                             _next_discovered);
 309 }
 310 
 311 void DiscoveredListIterator::complete_enqueue() {
 312   if (_prev_discovered != NULL) {
 313     // This is the last object.
 314     // Swap refs_list into pending list and set obj's
 315     // discovered to what we read from the pending list.
 316     oop old = Universe::swap_reference_pending_list(_refs_list.head());
 317     HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_prev_discovered, java_lang_ref_Reference::discovered_offset, old);
 318   }
 319 }
 320 
 321 // NOTE: process_phase*() are largely similar, and at a high level
 322 // merely iterate over the extant list applying a predicate to
 323 // each of its elements and possibly removing that element from the
 324 // list and applying some further closures to that element.
 325 // We should consider the possibility of replacing these
 326 // process_phase*() methods by abstracting them into
 327 // a single general iterator invocation that receives appropriate
 328 // closures that accomplish this work.
 329 
 330 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 331 // referents are not alive, but that should be kept alive for policy reasons.
 332 // Keep alive the transitive closure of all such referents.
 333 void
 334 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 335                                    ReferencePolicy*   policy,
 336                                    BoolObjectClosure* is_alive,
 337                                    OopClosure*        keep_alive,
 338                                    VoidClosure*       complete_gc) {
 339   assert(policy != NULL, "Must have a non-NULL policy");
 340   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 341   // Decide which softly reachable refs should be kept alive.
 342   while (iter.has_next()) {
 343     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 344     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 345     if (referent_is_dead &&
 346         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 347       log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 348                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
 349       // Remove Reference object from list
 350       iter.remove();
 351       // keep the referent around
 352       iter.make_referent_alive();
 353       iter.move_to_next();
 354     } else {
 355       iter.next();
 356     }
 357   }
 358   // Close the reachable set
 359   complete_gc->do_void();
 360   log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
 361                              iter.removed(), iter.processed(), p2i(&refs_list));
 362 }
 363 
 364 inline void log_dropped_ref(const DiscoveredListIterator& iter, const char* reason) {
 365   log_develop_trace(gc, ref)("Dropping %s reference " PTR_FORMAT ": %s",
 366                              reason, p2i(iter.obj()),
 367                              iter.obj()->klass()->internal_name());
 368 }
 369 
 370 // Traverse the list and remove any Refs whose referents are alive,
 371 // or NULL if discovery is not atomic.
 372 void ReferenceProcessor::process_phase2(DiscoveredList&    refs_list,
 373                                         BoolObjectClosure* is_alive,
 374                                         OopClosure*        keep_alive,
 375                                         VoidClosure*       complete_gc) {
 376   // complete_gc is unused.
 377   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 378   while (iter.has_next()) {
 379     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 380     if (iter.referent() == NULL) {
 381       // Reference has been cleared since discovery; only possible if
 382       // discovery is not atomic (checked by load_ptrs).  Remove
 383       // reference from list.
 384       log_dropped_ref(iter, "cleared");
 385       iter.remove();
 386       iter.move_to_next();
 387     } else if (iter.is_referent_alive()) {
 388       // The referent is reachable after all.
 389       // Remove reference from list.
 390       log_dropped_ref(iter, "reachable");
 391       iter.remove();
 392       // Update the referent pointer as necessary.  Note that this
 393       // should not entail any recursive marking because the
 394       // referent must already have been traversed.
 395       iter.make_referent_alive();
 396       iter.move_to_next();
 397     } else {
 398       iter.next();
 399     }
 400   }
 401   NOT_PRODUCT(
 402     if (iter.processed() > 0) {
 403       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 404         " Refs in discovered list " INTPTR_FORMAT,
 405         iter.removed(), iter.processed(), p2i(&refs_list));
 406     }
 407   )
 408 }
 409 
 410 void ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 411                                         bool               clear_referent,
 412                                         BoolObjectClosure* is_alive,
 413                                         OopClosure*        keep_alive,
 414                                         VoidClosure*       complete_gc) {
 415   ResourceMark rm;
 416   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 417   while (iter.has_next()) {
 418     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 419     if (clear_referent) {
 420       // NULL out referent pointer
 421       iter.clear_referent();
 422     } else {
 423       // Current reference is a FinalReference; that's the only kind we
 424       // don't clear the referent, instead keeping it for calling finalize.
 425       iter.make_referent_alive();
 426       // Self-loop next, to mark it not active.
 427       assert(java_lang_ref_Reference::next(iter.obj()) == NULL, "enqueued FinalReference");
 428       java_lang_ref_Reference::set_next_raw(iter.obj(), iter.obj());
 429     }
 430     iter.enqueue();
 431     log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 432                                clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name());
 433     assert(oopDesc::is_oop(iter.obj(), UseConcMarkSweepGC), "Adding a bad reference");
 434     iter.next();
 435   }
 436   iter.complete_enqueue();
 437   // Close the reachable set
 438   complete_gc->do_void();
 439   // Clear the list.
 440   refs_list.set_head(NULL);
 441   refs_list.set_length(0);
 442 }
 443 
 444 void
 445 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 446   oop obj = NULL;
 447   oop next = refs_list.head();
 448   while (next != obj) {
 449     obj = next;
 450     next = java_lang_ref_Reference::discovered(obj);
 451     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 452   }
 453   refs_list.set_head(NULL);
 454   refs_list.set_length(0);
 455 }
 456 
 457 void ReferenceProcessor::abandon_partial_discovery() {
 458   // loop over the lists
 459   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
 460     if ((i % _max_num_queues) == 0) {
 461       log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
 462     }
 463     clear_discovered_references(_discovered_refs[i]);
 464   }
 465 }
 466 
 467 size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
 468   DiscoveredList* list = NULL;
 469 
 470   switch (type) {
 471     case REF_SOFT:
 472       list = _discoveredSoftRefs;
 473       break;
 474     case REF_WEAK:
 475       list = _discoveredWeakRefs;
 476       break;
 477     case REF_FINAL:
 478       list = _discoveredFinalRefs;
 479       break;
 480     case REF_PHANTOM:
 481       list = _discoveredPhantomRefs;
 482       break;
 483     case REF_OTHER:
 484     case REF_NONE:
 485     default:
 486       ShouldNotReachHere();
 487   }
 488   return total_count(list);
 489 }
 490 
 491 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 492 public:
 493   RefProcPhase1Task(ReferenceProcessor&           ref_processor,
 494                     DiscoveredList                refs_lists[],
 495                     ReferencePolicy*              policy,
 496                     bool                          marks_oops_alive,
 497                     ReferenceProcessorPhaseTimes* phase_times)
 498     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
 499       _policy(policy)
 500   { }
 501   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 502                     OopClosure& keep_alive,
 503                     VoidClosure& complete_gc)
 504   {
 505     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, _phase_times, i);
 506 
 507     _ref_processor.process_phase1(_refs_lists[i], _policy,
 508                                   &is_alive, &keep_alive, &complete_gc);
 509   }
 510 private:
 511   ReferencePolicy* _policy;
 512 };
 513 
 514 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 515 public:
 516   RefProcPhase2Task(ReferenceProcessor&           ref_processor,
 517                     DiscoveredList                refs_lists[],
 518                     bool                          marks_oops_alive,
 519                     ReferenceProcessorPhaseTimes* phase_times)
 520     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times)
 521   { }
 522   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 523                     OopClosure& keep_alive,
 524                     VoidClosure& complete_gc)
 525   {
 526     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, _phase_times, i);
 527 
 528     _ref_processor.process_phase2(_refs_lists[i],
 529                                   &is_alive, &keep_alive, &complete_gc);
 530   }
 531 };
 532 
 533 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 534 public:
 535   RefProcPhase3Task(ReferenceProcessor&           ref_processor,
 536                     DiscoveredList                refs_lists[],
 537                     bool                         clear_referent,
 538                     bool                          marks_oops_alive,
 539                     ReferenceProcessorPhaseTimes* phase_times)
 540     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
 541       _clear_referent(clear_referent)
 542   { }
 543   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 544                     OopClosure& keep_alive,
 545                     VoidClosure& complete_gc)
 546   {
 547     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, _phase_times, i);
 548 
 549     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 550                                   &is_alive, &keep_alive, &complete_gc);
 551   }
 552 private:
 553   bool _clear_referent;
 554 };
 555 
 556 void ReferenceProcessor::log_reflist(const char* prefix, DiscoveredList list[], uint num_active_queues) {
 557   LogTarget(Trace, gc, ref) lt;
 558 
 559   if (!lt.is_enabled()) {
 560     return;
 561   }
 562 
 563   size_t total = 0;
 564 
 565   LogStream ls(lt);
 566   ls.print("%s", prefix);
 567   for (uint i = 0; i < num_active_queues; i++) {
 568     ls.print(SIZE_FORMAT " ", list[i].length());
 569     total += list[i].length();
 570   }
 571   ls.print_cr("(" SIZE_FORMAT ")", total);
 572 }
 573 
 574 #ifndef PRODUCT
 575 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint num_active_queues) {
 576   if (!log_is_enabled(Trace, gc, ref)) {
 577     return;
 578   }
 579 
 580   log_reflist("", ref_lists, num_active_queues);
 581 #ifdef ASSERT
 582   for (uint i = num_active_queues; i < _max_num_queues; i++) {
 583     assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
 584            ref_lists[i].length(), i);
 585   }
 586 #endif
 587 }
 588 #endif
 589 
 590 void ReferenceProcessor::set_active_mt_degree(uint v) {
 591   _num_queues = v;
 592   _next_id = 0;
 593 }
 594 
 595 // Balances reference queues.
 596 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 597 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 598 // corresponding to the active workers will be processed.
 599 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 600 {
 601   // calculate total length
 602   size_t total_refs = 0;
 603   log_develop_trace(gc, ref)("Balance ref_lists ");
 604 
 605   log_reflist_counts(ref_lists, _max_num_queues);
 606 
 607   for (uint i = 0; i < _max_num_queues; ++i) {
 608     total_refs += ref_lists[i].length();
 609   }
 610   size_t avg_refs = total_refs / _num_queues + 1;
 611   uint to_idx = 0;
 612   for (uint from_idx = 0; from_idx < _max_num_queues; from_idx++) {
 613     bool move_all = false;
 614     if (from_idx >= _num_queues) {
 615       move_all = ref_lists[from_idx].length() > 0;
 616     }
 617     while ((ref_lists[from_idx].length() > avg_refs) ||
 618            move_all) {
 619       assert(to_idx < _num_queues, "Sanity Check!");
 620       if (ref_lists[to_idx].length() < avg_refs) {
 621         // move superfluous refs
 622         size_t refs_to_move;
 623         // Move all the Ref's if the from queue will not be processed.
 624         if (move_all) {
 625           refs_to_move = MIN2(ref_lists[from_idx].length(),
 626                               avg_refs - ref_lists[to_idx].length());
 627         } else {
 628           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 629                               avg_refs - ref_lists[to_idx].length());
 630         }
 631 
 632         assert(refs_to_move > 0, "otherwise the code below will fail");
 633 
 634         oop move_head = ref_lists[from_idx].head();
 635         oop move_tail = move_head;
 636         oop new_head  = move_head;
 637         // find an element to split the list on
 638         for (size_t j = 0; j < refs_to_move; ++j) {
 639           move_tail = new_head;
 640           new_head = java_lang_ref_Reference::discovered(new_head);
 641         }
 642 
 643         // Add the chain to the to list.
 644         if (ref_lists[to_idx].head() == NULL) {
 645           // to list is empty. Make a loop at the end.
 646           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 647         } else {
 648           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 649         }
 650         ref_lists[to_idx].set_head(move_head);
 651         ref_lists[to_idx].inc_length(refs_to_move);
 652 
 653         // Remove the chain from the from list.
 654         if (move_tail == new_head) {
 655           // We found the end of the from list.
 656           ref_lists[from_idx].set_head(NULL);
 657         } else {
 658           ref_lists[from_idx].set_head(new_head);
 659         }
 660         ref_lists[from_idx].dec_length(refs_to_move);
 661         if (ref_lists[from_idx].length() == 0) {
 662           break;
 663         }
 664       } else {
 665         to_idx = (to_idx + 1) % _num_queues;
 666       }
 667     }
 668   }
 669 #ifdef ASSERT
 670   log_reflist_counts(ref_lists, _num_queues);
 671   size_t balanced_total_refs = 0;
 672   for (uint i = 0; i < _num_queues; ++i) {
 673     balanced_total_refs += ref_lists[i].length();
 674   }
 675   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 676 #endif
 677 }
 678 
 679 void ReferenceProcessor::process_discovered_reflist(
 680   DiscoveredList                refs_lists[],
 681   ReferencePolicy*              policy,
 682   bool                          clear_referent,
 683   BoolObjectClosure*            is_alive,
 684   OopClosure*                   keep_alive,
 685   VoidClosure*                  complete_gc,
 686   AbstractRefProcTaskExecutor*  task_executor,
 687   ReferenceProcessorPhaseTimes* phase_times)
 688 {
 689   bool mt_processing = task_executor != NULL && _processing_is_mt;
 690 
 691   phase_times->set_processing_is_mt(mt_processing);
 692 
 693   if (mt_processing && ParallelRefProcBalancingEnabled) {
 694     RefProcBalanceQueuesTimeTracker tt(phase_times);
 695     balance_queues(refs_lists);
 696   }
 697 
 698   // Phase 1 (soft refs only):
 699   // . Traverse the list and remove any SoftReferences whose
 700   //   referents are not alive, but that should be kept alive for
 701   //   policy reasons. Keep alive the transitive closure of all
 702   //   such referents.
 703   if (policy != NULL) {
 704     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, phase_times);
 705 
 706     if (mt_processing) {
 707       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/, phase_times);
 708       task_executor->execute(phase1);
 709     } else {
 710       for (uint i = 0; i < _max_num_queues; i++) {
 711         process_phase1(refs_lists[i], policy,
 712                        is_alive, keep_alive, complete_gc);
 713       }
 714     }
 715   } else { // policy == NULL
 716     assert(refs_lists != _discoveredSoftRefs,
 717            "Policy must be specified for soft references.");
 718   }
 719 
 720   // Phase 2:
 721   // . Traverse the list and remove any refs whose referents are alive.
 722   {
 723     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, phase_times);
 724 
 725     if (mt_processing) {
 726       RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/, phase_times);
 727       task_executor->execute(phase2);
 728     } else {
 729       for (uint i = 0; i < _max_num_queues; i++) {
 730         process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 731       }
 732     }
 733   }
 734 
 735   // Phase 3:
 736   // . Traverse the list and process referents as appropriate.
 737   {
 738     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, phase_times);
 739 
 740     if (mt_processing) {
 741       RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/, phase_times);
 742       task_executor->execute(phase3);
 743     } else {
 744       for (uint i = 0; i < _max_num_queues; i++) {
 745         process_phase3(refs_lists[i], clear_referent,
 746                        is_alive, keep_alive, complete_gc);
 747       }
 748     }
 749   }
 750 }
 751 
 752 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
 753   uint id = 0;
 754   // Determine the queue index to use for this object.
 755   if (_discovery_is_mt) {
 756     // During a multi-threaded discovery phase,
 757     // each thread saves to its "own" list.
 758     Thread* thr = Thread::current();
 759     id = thr->as_Worker_thread()->id();
 760   } else {
 761     // single-threaded discovery, we save in round-robin
 762     // fashion to each of the lists.
 763     if (_processing_is_mt) {
 764       id = next_id();
 765     }
 766   }
 767   assert(id < _max_num_queues, "Id is out of bounds id %u and max id %u)", id, _max_num_queues);
 768 
 769   // Get the discovered queue to which we will add
 770   DiscoveredList* list = NULL;
 771   switch (rt) {
 772     case REF_OTHER:
 773       // Unknown reference type, no special treatment
 774       break;
 775     case REF_SOFT:
 776       list = &_discoveredSoftRefs[id];
 777       break;
 778     case REF_WEAK:
 779       list = &_discoveredWeakRefs[id];
 780       break;
 781     case REF_FINAL:
 782       list = &_discoveredFinalRefs[id];
 783       break;
 784     case REF_PHANTOM:
 785       list = &_discoveredPhantomRefs[id];
 786       break;
 787     case REF_NONE:
 788       // we should not reach here if we are an InstanceRefKlass
 789     default:
 790       ShouldNotReachHere();
 791   }
 792   log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
 793   return list;
 794 }
 795 
 796 inline void
 797 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
 798                                               oop             obj,
 799                                               HeapWord*       discovered_addr) {
 800   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
 801   // First we must make sure this object is only enqueued once. CAS in a non null
 802   // discovered_addr.
 803   oop current_head = refs_list.head();
 804   // The last ref must have its discovered field pointing to itself.
 805   oop next_discovered = (current_head != NULL) ? current_head : obj;
 806 
 807   oop retest = RawAccess<>::oop_atomic_cmpxchg(next_discovered, discovered_addr, oop(NULL));
 808 
 809   if (retest == NULL) {
 810     // This thread just won the right to enqueue the object.
 811     // We have separate lists for enqueueing, so no synchronization
 812     // is necessary.
 813     refs_list.set_head(obj);
 814     refs_list.inc_length(1);
 815 
 816     log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
 817                                p2i(obj), obj->klass()->internal_name());
 818   } else {
 819     // If retest was non NULL, another thread beat us to it:
 820     // The reference has already been discovered...
 821     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
 822                                p2i(obj), obj->klass()->internal_name());
 823   }
 824 }
 825 
 826 #ifndef PRODUCT
 827 // Non-atomic (i.e. concurrent) discovery might allow us
 828 // to observe j.l.References with NULL referents, being those
 829 // cleared concurrently by mutators during (or after) discovery.
 830 void ReferenceProcessor::verify_referent(oop obj) {
 831   bool da = discovery_is_atomic();
 832   oop referent = java_lang_ref_Reference::referent(obj);
 833   assert(da ? oopDesc::is_oop(referent) : oopDesc::is_oop_or_null(referent),
 834          "Bad referent " INTPTR_FORMAT " found in Reference "
 835          INTPTR_FORMAT " during %satomic discovery ",
 836          p2i(referent), p2i(obj), da ? "" : "non-");
 837 }
 838 #endif
 839 
 840 bool ReferenceProcessor::is_subject_to_discovery(oop const obj) const {
 841   return _is_subject_to_discovery->do_object_b(obj);
 842 }
 843 
 844 // We mention two of several possible choices here:
 845 // #0: if the reference object is not in the "originating generation"
 846 //     (or part of the heap being collected, indicated by our "span"
 847 //     we don't treat it specially (i.e. we scan it as we would
 848 //     a normal oop, treating its references as strong references).
 849 //     This means that references can't be discovered unless their
 850 //     referent is also in the same span. This is the simplest,
 851 //     most "local" and most conservative approach, albeit one
 852 //     that may cause weak references to be enqueued least promptly.
 853 //     We call this choice the "ReferenceBasedDiscovery" policy.
 854 // #1: the reference object may be in any generation (span), but if
 855 //     the referent is in the generation (span) being currently collected
 856 //     then we can discover the reference object, provided
 857 //     the object has not already been discovered by
 858 //     a different concurrently running collector (as may be the
 859 //     case, for instance, if the reference object is in CMS and
 860 //     the referent in DefNewGeneration), and provided the processing
 861 //     of this reference object by the current collector will
 862 //     appear atomic to every other collector in the system.
 863 //     (Thus, for instance, a concurrent collector may not
 864 //     discover references in other generations even if the
 865 //     referent is in its own generation). This policy may,
 866 //     in certain cases, enqueue references somewhat sooner than
 867 //     might Policy #0 above, but at marginally increased cost
 868 //     and complexity in processing these references.
 869 //     We call this choice the "RefeferentBasedDiscovery" policy.
 870 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
 871   // Make sure we are discovering refs (rather than processing discovered refs).
 872   if (!_discovering_refs || !RegisterReferences) {
 873     return false;
 874   }
 875 
 876   if ((rt == REF_FINAL) && (java_lang_ref_Reference::next(obj) != NULL)) {
 877     // Don't rediscover non-active FinalReferences.
 878     return false;
 879   }
 880 
 881   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
 882       !is_subject_to_discovery(obj)) {
 883     // Reference is not in the originating generation;
 884     // don't treat it specially (i.e. we want to scan it as a normal
 885     // object with strong references).
 886     return false;
 887   }
 888 
 889   // We only discover references whose referents are not (yet)
 890   // known to be strongly reachable.
 891   if (is_alive_non_header() != NULL) {
 892     verify_referent(obj);
 893     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
 894       return false;  // referent is reachable
 895     }
 896   }
 897   if (rt == REF_SOFT) {
 898     // For soft refs we can decide now if these are not
 899     // current candidates for clearing, in which case we
 900     // can mark through them now, rather than delaying that
 901     // to the reference-processing phase. Since all current
 902     // time-stamp policies advance the soft-ref clock only
 903     // at a full collection cycle, this is always currently
 904     // accurate.
 905     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
 906       return false;
 907     }
 908   }
 909 
 910   ResourceMark rm;      // Needed for tracing.
 911 
 912   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr_raw(obj);
 913   const oop  discovered = java_lang_ref_Reference::discovered(obj);
 914   assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
 915   if (discovered != NULL) {
 916     // The reference has already been discovered...
 917     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
 918                                p2i(obj), obj->klass()->internal_name());
 919     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
 920       // assumes that an object is not processed twice;
 921       // if it's been already discovered it must be on another
 922       // generation's discovered list; so we won't discover it.
 923       return false;
 924     } else {
 925       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
 926              "Unrecognized policy");
 927       // Check assumption that an object is not potentially
 928       // discovered twice except by concurrent collectors that potentially
 929       // trace the same Reference object twice.
 930       assert(UseConcMarkSweepGC || UseG1GC,
 931              "Only possible with a concurrent marking collector");
 932       return true;
 933     }
 934   }
 935 
 936   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
 937     verify_referent(obj);
 938     // Discover if and only if EITHER:
 939     // .. reference is in our span, OR
 940     // .. we are an atomic collector and referent is in our span
 941     if (is_subject_to_discovery(obj) ||
 942         (discovery_is_atomic() &&
 943          is_subject_to_discovery(java_lang_ref_Reference::referent(obj)))) {
 944     } else {
 945       return false;
 946     }
 947   } else {
 948     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
 949            is_subject_to_discovery(obj), "code inconsistency");
 950   }
 951 
 952   // Get the right type of discovered queue head.
 953   DiscoveredList* list = get_discovered_list(rt);
 954   if (list == NULL) {
 955     return false;   // nothing special needs to be done
 956   }
 957 
 958   if (_discovery_is_mt) {
 959     add_to_discovered_list_mt(*list, obj, discovered_addr);
 960   } else {
 961     // We do a raw store here: the field will be visited later when processing
 962     // the discovered references.
 963     oop current_head = list->head();
 964     // The last ref must have its discovered field pointing to itself.
 965     oop next_discovered = (current_head != NULL) ? current_head : obj;
 966 
 967     assert(discovered == NULL, "control point invariant");
 968     RawAccess<>::oop_store(discovered_addr, next_discovered);
 969     list->set_head(obj);
 970     list->inc_length(1);
 971 
 972     log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
 973   }
 974   assert(oopDesc::is_oop(obj), "Discovered a bad reference");
 975   verify_referent(obj);
 976   return true;
 977 }
 978 
 979 bool ReferenceProcessor::has_discovered_references() {
 980   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
 981     if (!_discovered_refs[i].is_empty()) {
 982       return true;
 983     }
 984   }
 985   return false;
 986 }
 987 
 988 void ReferenceProcessor::preclean_discovered_references(BoolObjectClosure* is_alive,
 989                                                         OopClosure* keep_alive,
 990                                                         VoidClosure* complete_gc,
 991                                                         YieldClosure* yield,
 992                                                         GCTimer* gc_timer) {
 993   // These lists can be handled here in any order and, indeed, concurrently.
 994 
 995   // Soft references
 996   {
 997     GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
 998     log_reflist("SoftRef before: ", _discoveredSoftRefs, _max_num_queues);
 999     for (uint i = 0; i < _max_num_queues; i++) {
1000       if (yield->should_return()) {
1001         return;
1002       }
1003       if (preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1004                                       keep_alive, complete_gc, yield)) {
1005         log_reflist("SoftRef abort: ", _discoveredSoftRefs, _max_num_queues);
1006         return;
1007       }
1008     }
1009     log_reflist("SoftRef after: ", _discoveredSoftRefs, _max_num_queues);
1010   }
1011 
1012   // Weak references
1013   {
1014     GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
1015     log_reflist("WeakRef before: ", _discoveredWeakRefs, _max_num_queues);
1016     for (uint i = 0; i < _max_num_queues; i++) {
1017       if (yield->should_return()) {
1018         return;
1019       }
1020       if (preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1021                                       keep_alive, complete_gc, yield)) {
1022         log_reflist("WeakRef abort: ", _discoveredWeakRefs, _max_num_queues);
1023         return;
1024       }
1025     }
1026     log_reflist("WeakRef after: ", _discoveredWeakRefs, _max_num_queues);
1027   }
1028 
1029   // Final references
1030   {
1031     GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
1032     log_reflist("FinalRef before: ", _discoveredFinalRefs, _max_num_queues);
1033     for (uint i = 0; i < _max_num_queues; i++) {
1034       if (yield->should_return()) {
1035         return;
1036       }
1037       if (preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1038                                       keep_alive, complete_gc, yield)) {
1039         log_reflist("FinalRef abort: ", _discoveredFinalRefs, _max_num_queues);
1040         return;
1041       }
1042     }
1043     log_reflist("FinalRef after: ", _discoveredFinalRefs, _max_num_queues);
1044   }
1045 
1046   // Phantom references
1047   {
1048     GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
1049     log_reflist("PhantomRef before: ", _discoveredPhantomRefs, _max_num_queues);
1050     for (uint i = 0; i < _max_num_queues; i++) {
1051       if (yield->should_return()) {
1052         return;
1053       }
1054       if (preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1055                                       keep_alive, complete_gc, yield)) {
1056         log_reflist("PhantomRef abort: ", _discoveredPhantomRefs, _max_num_queues);
1057         return;
1058       }
1059     }
1060     log_reflist("PhantomRef after: ", _discoveredPhantomRefs, _max_num_queues);
1061   }
1062 }
1063 
1064 // Walk the given discovered ref list, and remove all reference objects
1065 // whose referents are still alive, whose referents are NULL or which
1066 // are not active (have a non-NULL next field). NOTE: When we are
1067 // thus precleaning the ref lists (which happens single-threaded today),
1068 // we do not disable refs discovery to honor the correct semantics of
1069 // java.lang.Reference. As a result, we need to be careful below
1070 // that ref removal steps interleave safely with ref discovery steps
1071 // (in this thread).
1072 bool ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1073                                                      BoolObjectClosure* is_alive,
1074                                                      OopClosure*        keep_alive,
1075                                                      VoidClosure*       complete_gc,
1076                                                      YieldClosure*      yield) {
1077   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1078   while (iter.has_next()) {
1079     if (yield->should_return_fine_grain()) {
1080       return true;
1081     }
1082     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1083     if (iter.referent() == NULL || iter.is_referent_alive()) {
1084       // The referent has been cleared, or is alive; we need to trace
1085       // and mark its cohort.
1086       log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1087                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
1088       // Remove Reference object from list
1089       iter.remove();
1090       // Keep alive its cohort.
1091       iter.make_referent_alive();
1092       iter.move_to_next();
1093     } else {
1094       iter.next();
1095     }
1096   }
1097   // Close the reachable set
1098   complete_gc->do_void();
1099 
1100   NOT_PRODUCT(
1101     if (iter.processed() > 0) {
1102       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
1103         iter.removed(), iter.processed(), p2i(&refs_list));
1104     }
1105   )
1106   return false;
1107 }
1108 
1109 const char* ReferenceProcessor::list_name(uint i) {
1110    assert(i <= _max_num_queues * number_of_subclasses_of_ref(),
1111           "Out of bounds index");
1112 
1113    int j = i / _max_num_queues;
1114    switch (j) {
1115      case 0: return "SoftRef";
1116      case 1: return "WeakRef";
1117      case 2: return "FinalRef";
1118      case 3: return "PhantomRef";
1119    }
1120    ShouldNotReachHere();
1121    return NULL;
1122 }
--- EOF ---