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   // Self-loop next, so as to make Ref not active.
 307   java_lang_ref_Reference::set_next_raw(_current_discovered, _current_discovered);
 308 
 309   HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_current_discovered,
 310                                             java_lang_ref_Reference::discovered_offset,
 311                                             _next_discovered);
 312 }
 313 
 314 void DiscoveredListIterator::complete_enqeue() {
 315   if (_prev_discovered != NULL) {
 316     // This is the last object.
 317     // Swap refs_list into pending list and set obj's
 318     // discovered to what we read from the pending list.
 319     oop old = Universe::swap_reference_pending_list(_refs_list.head());
 320     HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_prev_discovered, java_lang_ref_Reference::discovered_offset, old);
 321   }
 322 }
 323 
 324 // NOTE: process_phase*() are largely similar, and at a high level
 325 // merely iterate over the extant list applying a predicate to
 326 // each of its elements and possibly removing that element from the
 327 // list and applying some further closures to that element.
 328 // We should consider the possibility of replacing these
 329 // process_phase*() methods by abstracting them into
 330 // a single general iterator invocation that receives appropriate
 331 // closures that accomplish this work.
 332 
 333 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 334 // referents are not alive, but that should be kept alive for policy reasons.
 335 // Keep alive the transitive closure of all such referents.
 336 void
 337 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 338                                    ReferencePolicy*   policy,
 339                                    BoolObjectClosure* is_alive,
 340                                    OopClosure*        keep_alive,
 341                                    VoidClosure*       complete_gc) {
 342   assert(policy != NULL, "Must have a non-NULL policy");
 343   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 344   // Decide which softly reachable refs should be kept alive.
 345   while (iter.has_next()) {
 346     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 347     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 348     if (referent_is_dead &&
 349         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 350       log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 351                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
 352       // Remove Reference object from list
 353       iter.remove();
 354       // keep the referent around
 355       iter.make_referent_alive();
 356       iter.move_to_next();
 357     } else {
 358       iter.next();
 359     }
 360   }
 361   // Close the reachable set
 362   complete_gc->do_void();
 363   log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
 364                              iter.removed(), iter.processed(), p2i(&refs_list));
 365 }
 366 
 367 void ReferenceProcessor::process_phase2(DiscoveredList&    refs_list,
 368                                         BoolObjectClosure* is_alive,
 369                                         OopClosure*        keep_alive,
 370                                         VoidClosure*       complete_gc) {
 371   if (discovery_is_atomic()) {
 372     // complete_gc is ignored in this case for this phase
 373     pp2_work(refs_list, is_alive, keep_alive);
 374   } else {
 375     assert(complete_gc != NULL, "Error");
 376     pp2_work_concurrent_discovery(refs_list, is_alive,
 377                                   keep_alive, complete_gc);
 378   }
 379 }
 380 // Traverse the list and remove any Refs that are not active, or
 381 // whose referents are either alive or NULL.
 382 void
 383 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 384                              BoolObjectClosure* is_alive,
 385                              OopClosure*        keep_alive) {
 386   assert(discovery_is_atomic(), "Error");
 387   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 388   while (iter.has_next()) {
 389     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 390     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 391     assert(next == NULL, "Should not discover inactive Reference");
 392     if (iter.is_referent_alive()) {
 393       log_develop_trace(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 394                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
 395       // The referent is reachable after all.
 396       // Remove Reference object from list.
 397       iter.remove();
 398       // Update the referent pointer as necessary: Note that this
 399       // should not entail any recursive marking because the
 400       // referent must already have been traversed.
 401       iter.make_referent_alive();
 402       iter.move_to_next();
 403     } else {
 404       iter.next();
 405     }
 406   }
 407   NOT_PRODUCT(
 408     if (iter.processed() > 0) {
 409       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 410         " Refs in discovered list " INTPTR_FORMAT,
 411         iter.removed(), iter.processed(), p2i(&refs_list));
 412     }
 413   )
 414 }
 415 
 416 void
 417 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 418                                                   BoolObjectClosure* is_alive,
 419                                                   OopClosure*        keep_alive,
 420                                                   VoidClosure*       complete_gc) {
 421   assert(!discovery_is_atomic(), "Error");
 422   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 423   while (iter.has_next()) {
 424     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 425     HeapWord* next_addr = java_lang_ref_Reference::next_addr_raw(iter.obj());
 426     oop next = java_lang_ref_Reference::next(iter.obj());
 427     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 428          next != NULL)) {
 429       assert(oopDesc::is_oop_or_null(next), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
 430       // Remove Reference object from list
 431       iter.remove();
 432       // Trace the cohorts
 433       iter.make_referent_alive();
 434       if (UseCompressedOops) {
 435         keep_alive->do_oop((narrowOop*)next_addr);
 436       } else {
 437         keep_alive->do_oop((oop*)next_addr);
 438       }
 439       iter.move_to_next();
 440     } else {
 441       iter.next();
 442     }
 443   }
 444   // Now close the newly reachable set
 445   complete_gc->do_void();
 446   NOT_PRODUCT(
 447     if (iter.processed() > 0) {
 448       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 449         " Refs in discovered list " INTPTR_FORMAT,
 450         iter.removed(), iter.processed(), p2i(&refs_list));
 451     }
 452   )
 453 }
 454 
 455 void ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 456                                         bool               clear_referent,
 457                                         BoolObjectClosure* is_alive,
 458                                         OopClosure*        keep_alive,
 459                                         VoidClosure*       complete_gc) {
 460   ResourceMark rm;
 461   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 462   while (iter.has_next()) {
 463     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 464     if (clear_referent) {
 465       // NULL out referent pointer
 466       iter.clear_referent();
 467     } else {
 468       // keep the referent around
 469       iter.make_referent_alive();
 470     }
 471     iter.enqueue();
 472     log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 473                                clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name());
 474     assert(oopDesc::is_oop(iter.obj(), UseConcMarkSweepGC), "Adding a bad reference");
 475     iter.next();
 476   }
 477   iter.complete_enqeue();
 478   // Close the reachable set
 479   complete_gc->do_void();
 480   // Clear the list.
 481   refs_list.set_head(NULL);
 482   refs_list.set_length(0);
 483 }
 484 
 485 void
 486 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 487   oop obj = NULL;
 488   oop next = refs_list.head();
 489   while (next != obj) {
 490     obj = next;
 491     next = java_lang_ref_Reference::discovered(obj);
 492     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 493   }
 494   refs_list.set_head(NULL);
 495   refs_list.set_length(0);
 496 }
 497 
 498 void ReferenceProcessor::abandon_partial_discovery() {
 499   // loop over the lists
 500   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
 501     if ((i % _max_num_queues) == 0) {
 502       log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
 503     }
 504     clear_discovered_references(_discovered_refs[i]);
 505   }
 506 }
 507 
 508 size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
 509   DiscoveredList* list = NULL;
 510 
 511   switch (type) {
 512     case REF_SOFT:
 513       list = _discoveredSoftRefs;
 514       break;
 515     case REF_WEAK:
 516       list = _discoveredWeakRefs;
 517       break;
 518     case REF_FINAL:
 519       list = _discoveredFinalRefs;
 520       break;
 521     case REF_PHANTOM:
 522       list = _discoveredPhantomRefs;
 523       break;
 524     case REF_OTHER:
 525     case REF_NONE:
 526     default:
 527       ShouldNotReachHere();
 528   }
 529   return total_count(list);
 530 }
 531 
 532 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 533 public:
 534   RefProcPhase1Task(ReferenceProcessor&           ref_processor,
 535                     DiscoveredList                refs_lists[],
 536                     ReferencePolicy*              policy,
 537                     bool                          marks_oops_alive,
 538                     ReferenceProcessorPhaseTimes* phase_times)
 539     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
 540       _policy(policy)
 541   { }
 542   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 543                     OopClosure& keep_alive,
 544                     VoidClosure& complete_gc)
 545   {
 546     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, _phase_times, i);
 547 
 548     _ref_processor.process_phase1(_refs_lists[i], _policy,
 549                                   &is_alive, &keep_alive, &complete_gc);
 550   }
 551 private:
 552   ReferencePolicy* _policy;
 553 };
 554 
 555 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 556 public:
 557   RefProcPhase2Task(ReferenceProcessor&           ref_processor,
 558                     DiscoveredList                refs_lists[],
 559                     bool                          marks_oops_alive,
 560                     ReferenceProcessorPhaseTimes* phase_times)
 561     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times)
 562   { }
 563   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 564                     OopClosure& keep_alive,
 565                     VoidClosure& complete_gc)
 566   {
 567     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, _phase_times, i);
 568 
 569     _ref_processor.process_phase2(_refs_lists[i],
 570                                   &is_alive, &keep_alive, &complete_gc);
 571   }
 572 };
 573 
 574 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 575 public:
 576   RefProcPhase3Task(ReferenceProcessor&           ref_processor,
 577                     DiscoveredList                refs_lists[],
 578                     bool                         clear_referent,
 579                     bool                          marks_oops_alive,
 580                     ReferenceProcessorPhaseTimes* phase_times)
 581     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
 582       _clear_referent(clear_referent)
 583   { }
 584   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 585                     OopClosure& keep_alive,
 586                     VoidClosure& complete_gc)
 587   {
 588     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, _phase_times, i);
 589 
 590     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 591                                   &is_alive, &keep_alive, &complete_gc);
 592   }
 593 private:
 594   bool _clear_referent;
 595 };
 596 
 597 #ifndef PRODUCT
 598 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint active_length, size_t total_refs) {
 599   if (!log_is_enabled(Trace, gc, ref)) {
 600     return;
 601   }
 602 
 603   stringStream st;
 604   for (uint i = 0; i < active_length; ++i) {
 605     st.print(SIZE_FORMAT " ", ref_lists[i].length());
 606   }
 607   log_develop_trace(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs);
 608 #ifdef ASSERT
 609   for (uint i = active_length; i < _max_num_queues; i++) {
 610     assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
 611            ref_lists[i].length(), i);
 612   }
 613 #endif
 614 }
 615 #endif
 616 
 617 void ReferenceProcessor::set_active_mt_degree(uint v) {
 618   _num_queues = v;
 619   _next_id = 0;
 620 }
 621 
 622 // Balances reference queues.
 623 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 624 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 625 // corresponding to the active workers will be processed.
 626 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 627 {
 628   // calculate total length
 629   size_t total_refs = 0;
 630   log_develop_trace(gc, ref)("Balance ref_lists ");
 631 
 632   for (uint i = 0; i < _max_num_queues; ++i) {
 633     total_refs += ref_lists[i].length();
 634   }
 635   log_reflist_counts(ref_lists, _max_num_queues, total_refs);
 636   size_t avg_refs = total_refs / _num_queues + 1;
 637   uint to_idx = 0;
 638   for (uint from_idx = 0; from_idx < _max_num_queues; from_idx++) {
 639     bool move_all = false;
 640     if (from_idx >= _num_queues) {
 641       move_all = ref_lists[from_idx].length() > 0;
 642     }
 643     while ((ref_lists[from_idx].length() > avg_refs) ||
 644            move_all) {
 645       assert(to_idx < _num_queues, "Sanity Check!");
 646       if (ref_lists[to_idx].length() < avg_refs) {
 647         // move superfluous refs
 648         size_t refs_to_move;
 649         // Move all the Ref's if the from queue will not be processed.
 650         if (move_all) {
 651           refs_to_move = MIN2(ref_lists[from_idx].length(),
 652                               avg_refs - ref_lists[to_idx].length());
 653         } else {
 654           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 655                               avg_refs - ref_lists[to_idx].length());
 656         }
 657 
 658         assert(refs_to_move > 0, "otherwise the code below will fail");
 659 
 660         oop move_head = ref_lists[from_idx].head();
 661         oop move_tail = move_head;
 662         oop new_head  = move_head;
 663         // find an element to split the list on
 664         for (size_t j = 0; j < refs_to_move; ++j) {
 665           move_tail = new_head;
 666           new_head = java_lang_ref_Reference::discovered(new_head);
 667         }
 668 
 669         // Add the chain to the to list.
 670         if (ref_lists[to_idx].head() == NULL) {
 671           // to list is empty. Make a loop at the end.
 672           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 673         } else {
 674           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 675         }
 676         ref_lists[to_idx].set_head(move_head);
 677         ref_lists[to_idx].inc_length(refs_to_move);
 678 
 679         // Remove the chain from the from list.
 680         if (move_tail == new_head) {
 681           // We found the end of the from list.
 682           ref_lists[from_idx].set_head(NULL);
 683         } else {
 684           ref_lists[from_idx].set_head(new_head);
 685         }
 686         ref_lists[from_idx].dec_length(refs_to_move);
 687         if (ref_lists[from_idx].length() == 0) {
 688           break;
 689         }
 690       } else {
 691         to_idx = (to_idx + 1) % _num_queues;
 692       }
 693     }
 694   }
 695 #ifdef ASSERT
 696   size_t balanced_total_refs = 0;
 697   for (uint i = 0; i < _num_queues; ++i) {
 698     balanced_total_refs += ref_lists[i].length();
 699   }
 700   log_reflist_counts(ref_lists, _num_queues, balanced_total_refs);
 701   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 702 #endif
 703 }
 704 
 705 void ReferenceProcessor::process_discovered_reflist(
 706   DiscoveredList                refs_lists[],
 707   ReferencePolicy*              policy,
 708   bool                          clear_referent,
 709   BoolObjectClosure*            is_alive,
 710   OopClosure*                   keep_alive,
 711   VoidClosure*                  complete_gc,
 712   AbstractRefProcTaskExecutor*  task_executor,
 713   ReferenceProcessorPhaseTimes* phase_times)
 714 {
 715   bool mt_processing = task_executor != NULL && _processing_is_mt;
 716 
 717   phase_times->set_processing_is_mt(mt_processing);
 718 
 719   if (mt_processing && ParallelRefProcBalancingEnabled) {
 720     RefProcBalanceQueuesTimeTracker tt(phase_times);
 721     balance_queues(refs_lists);
 722   }
 723 
 724   // Phase 1 (soft refs only):
 725   // . Traverse the list and remove any SoftReferences whose
 726   //   referents are not alive, but that should be kept alive for
 727   //   policy reasons. Keep alive the transitive closure of all
 728   //   such referents.
 729   if (policy != NULL) {
 730     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, phase_times);
 731 
 732     if (mt_processing) {
 733       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/, phase_times);
 734       task_executor->execute(phase1);
 735     } else {
 736       for (uint i = 0; i < _max_num_queues; i++) {
 737         process_phase1(refs_lists[i], policy,
 738                        is_alive, keep_alive, complete_gc);
 739       }
 740     }
 741   } else { // policy == NULL
 742     assert(refs_lists != _discoveredSoftRefs,
 743            "Policy must be specified for soft references.");
 744   }
 745 
 746   // Phase 2:
 747   // . Traverse the list and remove any refs whose referents are alive.
 748   {
 749     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, phase_times);
 750 
 751     if (mt_processing) {
 752       RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/, phase_times);
 753       task_executor->execute(phase2);
 754     } else {
 755       for (uint i = 0; i < _max_num_queues; i++) {
 756         process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 757       }
 758     }
 759   }
 760 
 761   // Phase 3:
 762   // . Traverse the list and process referents as appropriate.
 763   {
 764     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, phase_times);
 765 
 766     if (mt_processing) {
 767       RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/, phase_times);
 768       task_executor->execute(phase3);
 769     } else {
 770       for (uint i = 0; i < _max_num_queues; i++) {
 771         process_phase3(refs_lists[i], clear_referent,
 772                        is_alive, keep_alive, complete_gc);
 773       }
 774     }
 775   }
 776 }
 777 
 778 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
 779   uint id = 0;
 780   // Determine the queue index to use for this object.
 781   if (_discovery_is_mt) {
 782     // During a multi-threaded discovery phase,
 783     // each thread saves to its "own" list.
 784     Thread* thr = Thread::current();
 785     id = thr->as_Worker_thread()->id();
 786   } else {
 787     // single-threaded discovery, we save in round-robin
 788     // fashion to each of the lists.
 789     if (_processing_is_mt) {
 790       id = next_id();
 791     }
 792   }
 793   assert(id < _max_num_queues, "Id is out-of-bounds id %u and max id %u)", id, _max_num_queues);
 794 
 795   // Get the discovered queue to which we will add
 796   DiscoveredList* list = NULL;
 797   switch (rt) {
 798     case REF_OTHER:
 799       // Unknown reference type, no special treatment
 800       break;
 801     case REF_SOFT:
 802       list = &_discoveredSoftRefs[id];
 803       break;
 804     case REF_WEAK:
 805       list = &_discoveredWeakRefs[id];
 806       break;
 807     case REF_FINAL:
 808       list = &_discoveredFinalRefs[id];
 809       break;
 810     case REF_PHANTOM:
 811       list = &_discoveredPhantomRefs[id];
 812       break;
 813     case REF_NONE:
 814       // we should not reach here if we are an InstanceRefKlass
 815     default:
 816       ShouldNotReachHere();
 817   }
 818   log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
 819   return list;
 820 }
 821 
 822 inline void
 823 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
 824                                               oop             obj,
 825                                               HeapWord*       discovered_addr) {
 826   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
 827   // First we must make sure this object is only enqueued once. CAS in a non null
 828   // discovered_addr.
 829   oop current_head = refs_list.head();
 830   // The last ref must have its discovered field pointing to itself.
 831   oop next_discovered = (current_head != NULL) ? current_head : obj;
 832 
 833   oop retest = RawAccess<>::oop_atomic_cmpxchg(next_discovered, discovered_addr, oop(NULL));
 834 
 835   if (retest == NULL) {
 836     // This thread just won the right to enqueue the object.
 837     // We have separate lists for enqueueing, so no synchronization
 838     // is necessary.
 839     refs_list.set_head(obj);
 840     refs_list.inc_length(1);
 841 
 842     log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
 843                                p2i(obj), obj->klass()->internal_name());
 844   } else {
 845     // If retest was non NULL, another thread beat us to it:
 846     // The reference has already been discovered...
 847     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
 848                                p2i(obj), obj->klass()->internal_name());
 849   }
 850 }
 851 
 852 #ifndef PRODUCT
 853 // Non-atomic (i.e. concurrent) discovery might allow us
 854 // to observe j.l.References with NULL referents, being those
 855 // cleared concurrently by mutators during (or after) discovery.
 856 void ReferenceProcessor::verify_referent(oop obj) {
 857   bool da = discovery_is_atomic();
 858   oop referent = java_lang_ref_Reference::referent(obj);
 859   assert(da ? oopDesc::is_oop(referent) : oopDesc::is_oop_or_null(referent),
 860          "Bad referent " INTPTR_FORMAT " found in Reference "
 861          INTPTR_FORMAT " during %satomic discovery ",
 862          p2i(referent), p2i(obj), da ? "" : "non-");
 863 }
 864 #endif
 865 
 866 bool ReferenceProcessor::is_subject_to_discovery(oop const obj) const {
 867   return _is_subject_to_discovery->do_object_b(obj);
 868 }
 869 
 870 // We mention two of several possible choices here:
 871 // #0: if the reference object is not in the "originating generation"
 872 //     (or part of the heap being collected, indicated by our "span"
 873 //     we don't treat it specially (i.e. we scan it as we would
 874 //     a normal oop, treating its references as strong references).
 875 //     This means that references can't be discovered unless their
 876 //     referent is also in the same span. This is the simplest,
 877 //     most "local" and most conservative approach, albeit one
 878 //     that may cause weak references to be enqueued least promptly.
 879 //     We call this choice the "ReferenceBasedDiscovery" policy.
 880 // #1: the reference object may be in any generation (span), but if
 881 //     the referent is in the generation (span) being currently collected
 882 //     then we can discover the reference object, provided
 883 //     the object has not already been discovered by
 884 //     a different concurrently running collector (as may be the
 885 //     case, for instance, if the reference object is in CMS and
 886 //     the referent in DefNewGeneration), and provided the processing
 887 //     of this reference object by the current collector will
 888 //     appear atomic to every other collector in the system.
 889 //     (Thus, for instance, a concurrent collector may not
 890 //     discover references in other generations even if the
 891 //     referent is in its own generation). This policy may,
 892 //     in certain cases, enqueue references somewhat sooner than
 893 //     might Policy #0 above, but at marginally increased cost
 894 //     and complexity in processing these references.
 895 //     We call this choice the "RefeferentBasedDiscovery" policy.
 896 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
 897   // Make sure we are discovering refs (rather than processing discovered refs).
 898   if (!_discovering_refs || !RegisterReferences) {
 899     return false;
 900   }
 901   // We only discover active references.
 902   oop next = java_lang_ref_Reference::next(obj);
 903   if (next != NULL) {   // Ref is no longer active
 904     return false;
 905   }
 906 
 907   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
 908       !is_subject_to_discovery(obj)) {
 909     // Reference is not in the originating generation;
 910     // don't treat it specially (i.e. we want to scan it as a normal
 911     // object with strong references).
 912     return false;
 913   }
 914 
 915   // We only discover references whose referents are not (yet)
 916   // known to be strongly reachable.
 917   if (is_alive_non_header() != NULL) {
 918     verify_referent(obj);
 919     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
 920       return false;  // referent is reachable
 921     }
 922   }
 923   if (rt == REF_SOFT) {
 924     // For soft refs we can decide now if these are not
 925     // current candidates for clearing, in which case we
 926     // can mark through them now, rather than delaying that
 927     // to the reference-processing phase. Since all current
 928     // time-stamp policies advance the soft-ref clock only
 929     // at a full collection cycle, this is always currently
 930     // accurate.
 931     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
 932       return false;
 933     }
 934   }
 935 
 936   ResourceMark rm;      // Needed for tracing.
 937 
 938   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr_raw(obj);
 939   const oop  discovered = java_lang_ref_Reference::discovered(obj);
 940   assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
 941   if (discovered != NULL) {
 942     // The reference has already been discovered...
 943     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
 944                                p2i(obj), obj->klass()->internal_name());
 945     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
 946       // assumes that an object is not processed twice;
 947       // if it's been already discovered it must be on another
 948       // generation's discovered list; so we won't discover it.
 949       return false;
 950     } else {
 951       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
 952              "Unrecognized policy");
 953       // Check assumption that an object is not potentially
 954       // discovered twice except by concurrent collectors that potentially
 955       // trace the same Reference object twice.
 956       assert(UseConcMarkSweepGC || UseG1GC,
 957              "Only possible with a concurrent marking collector");
 958       return true;
 959     }
 960   }
 961 
 962   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
 963     verify_referent(obj);
 964     // Discover if and only if EITHER:
 965     // .. reference is in our span, OR
 966     // .. we are an atomic collector and referent is in our span
 967     if (is_subject_to_discovery(obj) ||
 968         (discovery_is_atomic() &&
 969          is_subject_to_discovery(java_lang_ref_Reference::referent(obj)))) {
 970     } else {
 971       return false;
 972     }
 973   } else {
 974     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
 975            is_subject_to_discovery(obj), "code inconsistency");
 976   }
 977 
 978   // Get the right type of discovered queue head.
 979   DiscoveredList* list = get_discovered_list(rt);
 980   if (list == NULL) {
 981     return false;   // nothing special needs to be done
 982   }
 983 
 984   if (_discovery_is_mt) {
 985     add_to_discovered_list_mt(*list, obj, discovered_addr);
 986   } else {
 987     // We do a raw store here: the field will be visited later when processing
 988     // the discovered references.
 989     oop current_head = list->head();
 990     // The last ref must have its discovered field pointing to itself.
 991     oop next_discovered = (current_head != NULL) ? current_head : obj;
 992 
 993     assert(discovered == NULL, "control point invariant");
 994     RawAccess<>::oop_store(discovered_addr, next_discovered);
 995     list->set_head(obj);
 996     list->inc_length(1);
 997 
 998     log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
 999   }
1000   assert(oopDesc::is_oop(obj), "Discovered a bad reference");
1001   verify_referent(obj);
1002   return true;
1003 }
1004 
1005 bool ReferenceProcessor::has_discovered_references() {
1006   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
1007     if (!_discovered_refs[i].is_empty()) {
1008       return true;
1009     }
1010   }
1011   return false;
1012 }
1013 
1014 // Preclean the discovered references by removing those
1015 // whose referents are alive, and by marking from those that
1016 // are not active. These lists can be handled here
1017 // in any order and, indeed, concurrently.
1018 void ReferenceProcessor::preclean_discovered_references(
1019   BoolObjectClosure* is_alive,
1020   OopClosure* keep_alive,
1021   VoidClosure* complete_gc,
1022   YieldClosure* yield,
1023   GCTimer* gc_timer) {
1024 
1025   // Soft references
1026   {
1027     GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
1028     for (uint i = 0; i < _max_num_queues; i++) {
1029       if (yield->should_return()) {
1030         return;
1031       }
1032       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1033                                   keep_alive, complete_gc, yield);
1034     }
1035   }
1036 
1037   // Weak references
1038   {
1039     GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
1040     for (uint i = 0; i < _max_num_queues; i++) {
1041       if (yield->should_return()) {
1042         return;
1043       }
1044       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1045                                   keep_alive, complete_gc, yield);
1046     }
1047   }
1048 
1049   // Final references
1050   {
1051     GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
1052     for (uint i = 0; i < _max_num_queues; i++) {
1053       if (yield->should_return()) {
1054         return;
1055       }
1056       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1057                                   keep_alive, complete_gc, yield);
1058     }
1059   }
1060 
1061   // Phantom references
1062   {
1063     GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
1064     for (uint i = 0; i < _max_num_queues; i++) {
1065       if (yield->should_return()) {
1066         return;
1067       }
1068       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1069                                   keep_alive, complete_gc, yield);
1070     }
1071   }
1072 }
1073 
1074 // Walk the given discovered ref list, and remove all reference objects
1075 // whose referents are still alive, whose referents are NULL or which
1076 // are not active (have a non-NULL next field). NOTE: When we are
1077 // thus precleaning the ref lists (which happens single-threaded today),
1078 // we do not disable refs discovery to honor the correct semantics of
1079 // java.lang.Reference. As a result, we need to be careful below
1080 // that ref removal steps interleave safely with ref discovery steps
1081 // (in this thread).
1082 void
1083 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1084                                                 BoolObjectClosure* is_alive,
1085                                                 OopClosure*        keep_alive,
1086                                                 VoidClosure*       complete_gc,
1087                                                 YieldClosure*      yield) {
1088   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1089   while (iter.has_next()) {
1090     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1091     oop obj = iter.obj();
1092     oop next = java_lang_ref_Reference::next(obj);
1093     if (iter.referent() == NULL || iter.is_referent_alive() ||
1094         next != NULL) {
1095       // The referent has been cleared, or is alive, or the Reference is not
1096       // active; we need to trace and mark its cohort.
1097       log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1098                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
1099       // Remove Reference object from list
1100       iter.remove();
1101       // Keep alive its cohort.
1102       iter.make_referent_alive();
1103       if (UseCompressedOops) {
1104         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr_raw(obj);
1105         keep_alive->do_oop(next_addr);
1106       } else {
1107         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr_raw(obj);
1108         keep_alive->do_oop(next_addr);
1109       }
1110       iter.move_to_next();
1111     } else {
1112       iter.next();
1113     }
1114   }
1115   // Close the reachable set
1116   complete_gc->do_void();
1117 
1118   NOT_PRODUCT(
1119     if (iter.processed() > 0) {
1120       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
1121         iter.removed(), iter.processed(), p2i(&refs_list));
1122     }
1123   )
1124 }
1125 
1126 const char* ReferenceProcessor::list_name(uint i) {
1127    assert(i <= _max_num_queues * number_of_subclasses_of_ref(),
1128           "Out of bounds index");
1129 
1130    int j = i / _max_num_queues;
1131    switch (j) {
1132      case 0: return "SoftRef";
1133      case 1: return "WeakRef";
1134      case 2: return "FinalRef";
1135      case 3: return "PhantomRef";
1136    }
1137    ShouldNotReachHere();
1138    return NULL;
1139 }