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 "gc/shared/referenceProcessorPhaseTimes.hpp"
  35 #include "logging/log.hpp"
  36 #include "memory/allocation.inline.hpp"
  37 #include "memory/resourceArea.hpp"
  38 #include "oops/access.inline.hpp"
  39 #include "oops/oop.inline.hpp"
  40 #include "runtime/java.hpp"
  41 
  42 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  43 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  44 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  45 
  46 void referenceProcessor_init() {
  47   ReferenceProcessor::init_statics();
  48 }
  49 
  50 void ReferenceProcessor::init_statics() {
  51   // We need a monotonically non-decreasing time in ms but
  52   // os::javaTimeMillis() does not guarantee monotonicity.
  53   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  54 
  55   // Initialize the soft ref timestamp clock.
  56   _soft_ref_timestamp_clock = now;
  57   // Also update the soft ref clock in j.l.r.SoftReference
  58   java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
  59 
  60   _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  61   if (is_server_compilation_mode_vm()) {
  62     _default_soft_ref_policy = new LRUMaxHeapPolicy();
  63   } else {
  64     _default_soft_ref_policy = new LRUCurrentHeapPolicy();
  65   }
  66   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  67     vm_exit_during_initialization("Could not allocate reference policy object");
  68   }
  69   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  70             RefDiscoveryPolicy == ReferentBasedDiscovery,
  71             "Unrecognized RefDiscoveryPolicy");
  72 }
  73 
  74 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
  75 #ifdef ASSERT
  76   // Verify that we're not currently discovering refs
  77   assert(!_discovering_refs, "nested call?");
  78 
  79   if (check_no_refs) {
  80     // Verify that the discovered lists are empty
  81     verify_no_references_recorded();
  82   }
  83 #endif // ASSERT
  84 
  85   // Someone could have modified the value of the static
  86   // field in the j.l.r.SoftReference class that holds the
  87   // soft reference timestamp clock using reflection or
  88   // Unsafe between GCs. Unconditionally update the static
  89   // field in ReferenceProcessor here so that we use the new
  90   // value during reference discovery.
  91 
  92   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
  93   _discovering_refs = true;
  94 }
  95 
  96 ReferenceProcessor::ReferenceProcessor(BoolObjectClosure* is_subject_to_discovery,
  97                                        bool      mt_processing,
  98                                        uint      mt_processing_degree,
  99                                        bool      mt_discovery,
 100                                        uint      mt_discovery_degree,
 101                                        bool      atomic_discovery,
 102                                        BoolObjectClosure* is_alive_non_header,
 103                                        bool      adjust_no_of_processing_threads)  :
 104   _is_subject_to_discovery(is_subject_to_discovery),
 105   _discovering_refs(false),
 106   _enqueuing_is_done(false),
 107   _is_alive_non_header(is_alive_non_header),
 108   _processing_is_mt(mt_processing),
 109   _next_id(0),
 110   _adjust_no_of_processing_threads(adjust_no_of_processing_threads)
 111 {
 112   assert(is_subject_to_discovery != NULL, "must be set");
 113 
 114   _discovery_is_atomic = atomic_discovery;
 115   _discovery_is_mt     = mt_discovery;
 116   _num_queues          = MAX2(1U, mt_processing_degree);
 117   _max_num_queues      = MAX2(_num_queues, mt_discovery_degree);
 118   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 119             _max_num_queues * number_of_subclasses_of_ref(), mtGC);
 120 
 121   if (_discovered_refs == NULL) {
 122     vm_exit_during_initialization("Could not allocated RefProc Array");
 123   }
 124   _discoveredSoftRefs    = &_discovered_refs[0];
 125   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_queues];
 126   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_queues];
 127   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_queues];
 128 
 129   // Initialize all entries to NULL
 130   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
 131     _discovered_refs[i].clear();
 132   }
 133 
 134   setup_policy(false /* default soft ref policy */);
 135 }
 136 
 137 #ifndef PRODUCT
 138 void ReferenceProcessor::verify_no_references_recorded() {
 139   guarantee(!_discovering_refs, "Discovering refs?");
 140   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
 141     guarantee(_discovered_refs[i].is_empty(),
 142               "Found non-empty discovered list at %u", i);
 143   }
 144 }
 145 #endif
 146 
 147 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 148   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
 149     if (UseCompressedOops) {
 150       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 151     } else {
 152       f->do_oop((oop*)_discovered_refs[i].adr_head());
 153     }
 154   }
 155 }
 156 
 157 void ReferenceProcessor::update_soft_ref_master_clock() {
 158   // Update (advance) the soft ref master clock field. This must be done
 159   // after processing the soft ref list.
 160 
 161   // We need a monotonically non-decreasing time in ms but
 162   // os::javaTimeMillis() does not guarantee monotonicity.
 163   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 164   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 165   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 166 
 167   NOT_PRODUCT(
 168   if (now < _soft_ref_timestamp_clock) {
 169     log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
 170                     _soft_ref_timestamp_clock, now);
 171   }
 172   )
 173   // The values of now and _soft_ref_timestamp_clock are set using
 174   // javaTimeNanos(), which is guaranteed to be monotonically
 175   // non-decreasing provided the underlying platform provides such
 176   // a time source (and it is bug free).
 177   // In product mode, however, protect ourselves from non-monotonicity.
 178   if (now > _soft_ref_timestamp_clock) {
 179     _soft_ref_timestamp_clock = now;
 180     java_lang_ref_SoftReference::set_clock(now);
 181   }
 182   // Else leave clock stalled at its old value until time progresses
 183   // past clock value.
 184 }
 185 
 186 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) const {
 187   size_t total = 0;
 188   for (uint i = 0; i < _max_num_queues; ++i) {
 189     total += lists[i].length();
 190   }
 191   return total;
 192 }
 193 
 194 #ifdef ASSERT
 195 void ReferenceProcessor::verify_total_count_zero(DiscoveredList lists[], const char* type) {
 196   size_t count = total_count(lists);
 197   assert(count == 0, "%ss must be empty but has " SIZE_FORMAT " elements", type, count);
 198 }
 199 #endif
 200 
 201 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 202   BoolObjectClosure*            is_alive,
 203   OopClosure*                   keep_alive,
 204   VoidClosure*                  complete_gc,
 205   AbstractRefProcTaskExecutor*  task_executor,
 206   ReferenceProcessorPhaseTimes* phase_times) {
 207 
 208   double start_time = os::elapsedTime();
 209 
 210   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 211   // Stop treating discovered references specially.
 212   disable_discovery();
 213 
 214   // If discovery was concurrent, someone could have modified
 215   // the value of the static field in the j.l.r.SoftReference
 216   // class that holds the soft reference timestamp clock using
 217   // reflection or Unsafe between when discovery was enabled and
 218   // now. Unconditionally update the static field in ReferenceProcessor
 219   // here so that we use the new value during processing of the
 220   // discovered soft refs.
 221 
 222   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 223 
 224   ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
 225                                 total_count(_discoveredWeakRefs),
 226                                 total_count(_discoveredFinalRefs),
 227                                 total_count(_discoveredPhantomRefs));
 228 
 229   {
 230     RefProcTotalPhaseTimesTracker tt(RefPhase1, phase_times, this);
 231     process_soft_ref_reconsider(is_alive, keep_alive, complete_gc,
 232                                 task_executor, phase_times);
 233   }
 234 
 235   update_soft_ref_master_clock();
 236 
 237   {
 238     RefProcTotalPhaseTimesTracker tt(RefPhase2, phase_times, this);
 239     process_soft_weak_final_refs(is_alive, keep_alive, complete_gc, task_executor, phase_times);
 240   }
 241 
 242   {
 243     RefProcTotalPhaseTimesTracker tt(RefPhase3, phase_times, this);
 244     process_final_keep_alive(keep_alive, complete_gc, task_executor, phase_times);
 245   }
 246 
 247   {
 248     RefProcTotalPhaseTimesTracker tt(RefPhase4, phase_times, this);
 249     process_phantom_refs(is_alive, keep_alive, complete_gc, task_executor, phase_times);
 250   }
 251 
 252   if (task_executor != NULL) {
 253     // Record the work done by the parallel workers.
 254     task_executor->set_single_threaded_mode();
 255   }
 256 
 257   phase_times->set_total_time_ms((os::elapsedTime() - start_time) * 1000);
 258 
 259   return stats;
 260 }
 261 
 262 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 263   _current_discovered_addr = java_lang_ref_Reference::discovered_addr_raw(_current_discovered);
 264   oop discovered = java_lang_ref_Reference::discovered(_current_discovered);
 265   assert(_current_discovered_addr && oopDesc::is_oop_or_null(discovered),
 266          "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
 267   _next_discovered = discovered;
 268 
 269   _referent_addr = java_lang_ref_Reference::referent_addr_raw(_current_discovered);
 270   _referent = java_lang_ref_Reference::referent(_current_discovered);
 271   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 272          "Wrong oop found in java.lang.Reference object");
 273   assert(allow_null_referent ?
 274              oopDesc::is_oop_or_null(_referent)
 275            : oopDesc::is_oop(_referent),
 276          "Expected an oop%s for referent field at " PTR_FORMAT,
 277          (allow_null_referent ? " or NULL" : ""),
 278          p2i(_referent));
 279 }
 280 
 281 void DiscoveredListIterator::remove() {
 282   assert(oopDesc::is_oop(_current_discovered), "Dropping a bad reference");
 283   RawAccess<>::oop_store(_current_discovered_addr, oop(NULL));
 284 
 285   // First _prev_next ref actually points into DiscoveredList (gross).
 286   oop new_next;
 287   if (oopDesc::unsafe_equals(_next_discovered, _current_discovered)) {
 288     // At the end of the list, we should make _prev point to itself.
 289     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 290     // and _prev will be NULL.
 291     new_next = _prev_discovered;
 292   } else {
 293     new_next = _next_discovered;
 294   }
 295   // Remove Reference object from discovered list. Note that G1 does not need a
 296   // pre-barrier here because we know the Reference has already been found/marked,
 297   // that's how it ended up in the discovered list in the first place.
 298   RawAccess<>::oop_store(_prev_discovered_addr, new_next);
 299   _removed++;
 300   _refs_list.dec_length(1);
 301 }
 302 
 303 void DiscoveredListIterator::clear_referent() {
 304   RawAccess<>::oop_store(_referent_addr, oop(NULL));
 305 }
 306 
 307 void DiscoveredListIterator::enqueue() {
 308   HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_current_discovered,
 309                                             java_lang_ref_Reference::discovered_offset,
 310                                             _next_discovered);
 311 }
 312 
 313 void DiscoveredListIterator::complete_enqueue() {
 314   if (_prev_discovered != NULL) {
 315     // This is the last object.
 316     // Swap refs_list into pending list and set obj's
 317     // discovered to what we read from the pending list.
 318     oop old = Universe::swap_reference_pending_list(_refs_list.head());
 319     HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(_prev_discovered, java_lang_ref_Reference::discovered_offset, old);
 320   }
 321 }
 322 
 323 inline void log_dropped_ref(const DiscoveredListIterator& iter, const char* reason) {
 324   if (log_develop_is_enabled(Trace, gc, ref)) {
 325     ResourceMark rm;
 326     log_develop_trace(gc, ref)("Dropping %s reference " PTR_FORMAT ": %s",
 327                                reason, p2i(iter.obj()),
 328                                iter.obj()->klass()->internal_name());
 329   }
 330 }
 331 
 332 inline void log_enqueued_ref(const DiscoveredListIterator& iter, const char* reason) {
 333   if (log_develop_is_enabled(Trace, gc, ref)) {
 334     ResourceMark rm;
 335     log_develop_trace(gc, ref)("Enqueue %s reference (" INTPTR_FORMAT ": %s)",
 336                                reason, p2i(iter.obj()), iter.obj()->klass()->internal_name());
 337   }
 338   assert(oopDesc::is_oop(iter.obj(), UseConcMarkSweepGC), "Adding a bad reference");
 339 }
 340 
 341 size_t ReferenceProcessor::process_soft_ref_reconsider_work(DiscoveredList&    refs_list,
 342                                                             ReferencePolicy*   policy,
 343                                                             BoolObjectClosure* is_alive,
 344                                                             OopClosure*        keep_alive,
 345                                                             VoidClosure*       complete_gc) {
 346   assert(policy != NULL, "Must have a non-NULL policy");
 347   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 348   // Decide which softly reachable refs should be kept alive.
 349   while (iter.has_next()) {
 350     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 351     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 352     if (referent_is_dead &&
 353         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 354       log_dropped_ref(iter, "by policy");
 355       // Remove Reference object from list
 356       iter.remove();
 357       // keep the referent around
 358       iter.make_referent_alive();
 359       iter.move_to_next();
 360     } else {
 361       iter.next();
 362     }
 363   }
 364   // Close the reachable set
 365   complete_gc->do_void();
 366 
 367   log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
 368                              iter.removed(), iter.processed(), p2i(&refs_list));
 369   return iter.removed();
 370 }
 371 
 372 size_t ReferenceProcessor::process_soft_weak_final_refs_work(DiscoveredList&    refs_list,
 373                                                              BoolObjectClosure* is_alive,
 374                                                              OopClosure*        keep_alive,
 375                                                              bool               do_enqueue_and_clear) {
 376   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 377   while (iter.has_next()) {
 378     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 379     if (iter.referent() == NULL) {
 380       // Reference has been cleared since discovery; only possible if
 381       // discovery is not atomic (checked by load_ptrs).  Remove
 382       // reference from list.
 383       log_dropped_ref(iter, "cleared");
 384       iter.remove();
 385       iter.move_to_next();
 386     } else if (iter.is_referent_alive()) {
 387       // The referent is reachable after all.
 388       // Remove reference from list.
 389       log_dropped_ref(iter, "reachable");
 390       iter.remove();
 391       // Update the referent pointer as necessary.  Note that this
 392       // should not entail any recursive marking because the
 393       // referent must already have been traversed.
 394       iter.make_referent_alive();
 395       iter.move_to_next();
 396     } else {
 397       if (do_enqueue_and_clear) {
 398         iter.clear_referent();
 399         iter.enqueue();
 400         log_enqueued_ref(iter, "cleared");
 401       }
 402       // Keep in discovered list
 403       iter.next();
 404     }
 405   }
 406   if (do_enqueue_and_clear) {
 407     iter.complete_enqueue();
 408     refs_list.clear();
 409   }
 410 
 411   log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 412                              " Refs in discovered list " INTPTR_FORMAT,
 413                              iter.removed(), iter.processed(), p2i(&refs_list));
 414   return iter.removed();
 415 }
 416 
 417 size_t ReferenceProcessor::process_final_keep_alive_work(DiscoveredList& refs_list,
 418                                                          OopClosure*     keep_alive,
 419                                                          VoidClosure*    complete_gc) {
 420   DiscoveredListIterator iter(refs_list, keep_alive, NULL);
 421   while (iter.has_next()) {
 422     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 423     // keep the referent and followers around
 424     iter.make_referent_alive();
 425 
 426     // Self-loop next, to mark the FinalReference 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_enqueued_ref(iter, "Final");
 432     iter.next();
 433   }
 434   iter.complete_enqueue();
 435   // Close the reachable set
 436   complete_gc->do_void();
 437   refs_list.clear();
 438 
 439   assert(iter.removed() == 0, "This phase does not remove anything.");
 440   return iter.removed();
 441 }
 442 
 443 size_t ReferenceProcessor::process_phantom_refs_work(DiscoveredList&    refs_list,
 444                                           BoolObjectClosure* is_alive,
 445                                           OopClosure*        keep_alive,
 446                                           VoidClosure*       complete_gc) {
 447   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 448   while (iter.has_next()) {
 449     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 450 
 451     oop const referent = iter.referent();
 452 
 453     if (referent == NULL || iter.is_referent_alive()) {
 454       iter.make_referent_alive();
 455       iter.remove();
 456       iter.move_to_next();
 457     } else {
 458       iter.clear_referent();
 459       iter.enqueue();
 460       log_enqueued_ref(iter, "cleared Phantom");
 461       iter.next();
 462     }
 463   }
 464   iter.complete_enqueue();
 465   // Close the reachable set; needed for collectors which keep_alive_closure do
 466   // not immediately complete their work.
 467   complete_gc->do_void();
 468   refs_list.clear();
 469 
 470   return iter.removed();
 471 }
 472 
 473 void
 474 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 475   oop obj = NULL;
 476   oop next = refs_list.head();
 477   while (! oopDesc::unsafe_equals(next, obj)) {
 478     obj = next;
 479     next = java_lang_ref_Reference::discovered(obj);
 480     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 481   }
 482   refs_list.clear();
 483 }
 484 
 485 void ReferenceProcessor::abandon_partial_discovery() {
 486   // loop over the lists
 487   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
 488     if ((i % _max_num_queues) == 0) {
 489       log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
 490     }
 491     clear_discovered_references(_discovered_refs[i]);
 492   }
 493 }
 494 
 495 size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
 496   DiscoveredList* list = NULL;
 497 
 498   switch (type) {
 499     case REF_SOFT:
 500       list = _discoveredSoftRefs;
 501       break;
 502     case REF_WEAK:
 503       list = _discoveredWeakRefs;
 504       break;
 505     case REF_FINAL:
 506       list = _discoveredFinalRefs;
 507       break;
 508     case REF_PHANTOM:
 509       list = _discoveredPhantomRefs;
 510       break;
 511     case REF_OTHER:
 512     case REF_NONE:
 513     default:
 514       ShouldNotReachHere();
 515   }
 516   return total_count(list);
 517 }
 518 
 519 class RefProcPhase1Task : public AbstractRefProcTaskExecutor::ProcessTask {
 520 public:
 521   RefProcPhase1Task(ReferenceProcessor&           ref_processor,
 522                     ReferenceProcessorPhaseTimes* phase_times,
 523                     ReferencePolicy*              policy)
 524     : ProcessTask(ref_processor, true /* marks_oops_alive */, phase_times),
 525       _policy(policy) { }
 526 
 527   virtual void work(uint worker_id,
 528                     BoolObjectClosure& is_alive,
 529                     OopClosure& keep_alive,
 530                     VoidClosure& complete_gc)
 531   {
 532     RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::SoftRefSubPhase1, _phase_times, worker_id);
 533     size_t const removed = _ref_processor.process_soft_ref_reconsider_work(_ref_processor._discoveredSoftRefs[worker_id],
 534                                                                            _policy,
 535                                                                            &is_alive,
 536                                                                            &keep_alive,
 537                                                                            &complete_gc);
 538     _phase_times->add_ref_cleared(REF_SOFT, removed);
 539   }
 540 private:
 541   ReferencePolicy* _policy;
 542 };
 543 
 544 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 545   void run_phase2(uint worker_id,
 546                   DiscoveredList list[],
 547                   BoolObjectClosure& is_alive,
 548                   OopClosure& keep_alive,
 549                   bool do_enqueue_and_clear,
 550                   ReferenceType ref_type) {
 551     size_t const removed = _ref_processor.process_soft_weak_final_refs_work(list[worker_id],
 552                                                                             &is_alive,
 553                                                                             &keep_alive,
 554                                                                             do_enqueue_and_clear);
 555     _phase_times->add_ref_cleared(ref_type, removed);
 556   }
 557 
 558 public:
 559   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 560                     ReferenceProcessorPhaseTimes* phase_times)
 561     : ProcessTask(ref_processor, false /* marks_oops_alive */, phase_times) { }
 562 
 563   virtual void work(uint worker_id,
 564                     BoolObjectClosure& is_alive,
 565                     OopClosure& keep_alive,
 566                     VoidClosure& complete_gc) {
 567     RefProcWorkerTimeTracker t(_phase_times->phase2_worker_time_sec(), worker_id);
 568     {
 569       RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::SoftRefSubPhase2, _phase_times, worker_id);
 570       run_phase2(worker_id, _ref_processor._discoveredSoftRefs, is_alive, keep_alive, true /* do_enqueue_and_clear */, REF_SOFT);
 571     }
 572     {
 573       RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::WeakRefSubPhase2, _phase_times, worker_id);
 574       run_phase2(worker_id, _ref_processor._discoveredWeakRefs, is_alive, keep_alive, true /* do_enqueue_and_clear */, REF_WEAK);
 575     }
 576     {
 577       RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::FinalRefSubPhase2, _phase_times, worker_id);
 578       run_phase2(worker_id, _ref_processor._discoveredFinalRefs, is_alive, keep_alive, false /* do_enqueue_and_clear */, REF_FINAL);
 579     }
 580     // Close the reachable set; needed for collectors which keep_alive_closure do
 581     // not immediately complete their work.
 582     complete_gc.do_void();
 583   }
 584 };
 585 
 586 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 587 public:
 588   RefProcPhase3Task(ReferenceProcessor&           ref_processor,
 589                     ReferenceProcessorPhaseTimes* phase_times)
 590     : ProcessTask(ref_processor, true /* marks_oops_alive */, phase_times) { }
 591 
 592   virtual void work(uint worker_id,
 593                     BoolObjectClosure& is_alive,
 594                     OopClosure& keep_alive,
 595                     VoidClosure& complete_gc)
 596   {
 597     RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::FinalRefSubPhase3, _phase_times, worker_id);
 598     _ref_processor.process_final_keep_alive_work(_ref_processor._discoveredFinalRefs[worker_id], &keep_alive, &complete_gc);
 599   }
 600 };
 601 
 602 class RefProcPhase4Task: public AbstractRefProcTaskExecutor::ProcessTask {
 603 public:
 604   RefProcPhase4Task(ReferenceProcessor&           ref_processor,
 605                     ReferenceProcessorPhaseTimes* phase_times)
 606     : ProcessTask(ref_processor, false /* marks_oops_alive */, phase_times) { }
 607 
 608   virtual void work(uint worker_id,
 609                     BoolObjectClosure& is_alive,
 610                     OopClosure& keep_alive,
 611                     VoidClosure& complete_gc)
 612   {
 613     RefProcSubPhasesWorkerTimeTracker tt(ReferenceProcessor::PhantomRefSubPhase4, _phase_times, worker_id);
 614     size_t const removed = _ref_processor.process_phantom_refs_work(_ref_processor._discoveredPhantomRefs[worker_id],
 615                                                                     &is_alive,
 616                                                                     &keep_alive,
 617                                                                     &complete_gc);
 618     _phase_times->add_ref_cleared(REF_PHANTOM, removed);
 619   }
 620 };
 621 
 622 void ReferenceProcessor::log_reflist(const char* prefix, DiscoveredList list[], uint num_active_queues) {
 623   LogTarget(Trace, gc, ref) lt;
 624 
 625   if (!lt.is_enabled()) {
 626     return;
 627   }
 628 
 629   size_t total = 0;
 630 
 631   LogStream ls(lt);
 632   ls.print("%s", prefix);
 633   for (uint i = 0; i < num_active_queues; i++) {
 634     ls.print(SIZE_FORMAT " ", list[i].length());
 635     total += list[i].length();
 636   }
 637   ls.print_cr("(" SIZE_FORMAT ")", total);
 638 }
 639 
 640 #ifndef PRODUCT
 641 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint num_active_queues) {
 642   if (!log_is_enabled(Trace, gc, ref)) {
 643     return;
 644   }
 645 
 646   log_reflist("", ref_lists, num_active_queues);
 647 #ifdef ASSERT
 648   for (uint i = num_active_queues; i < _max_num_queues; i++) {
 649     assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
 650            ref_lists[i].length(), i);
 651   }
 652 #endif
 653 }
 654 #endif
 655 
 656 void ReferenceProcessor::set_active_mt_degree(uint v) {
 657   _num_queues = v;
 658   _next_id = 0;
 659 }
 660 
 661 bool ReferenceProcessor::need_balance_queues(DiscoveredList refs_lists[]) {
 662   assert(_processing_is_mt, "why balance non-mt processing?");
 663   // _num_queues is the processing degree.  Only list entries up to
 664   // _num_queues will be processed, so any non-empty lists beyond
 665   // that must be redistributed to lists in that range.  Even if not
 666   // needed for that, balancing may be desirable to eliminate poor
 667   // distribution of references among the lists.
 668   if (ParallelRefProcBalancingEnabled) {
 669     return true;                // Configuration says do it.
 670   } else {
 671     // Configuration says don't balance, but if there are non-empty
 672     // lists beyond the processing degree, then must ignore the
 673     // configuration and balance anyway.
 674     for (uint i = _num_queues; i < _max_num_queues; ++i) {
 675       if (!refs_lists[i].is_empty()) {
 676         return true;            // Must balance despite configuration.
 677       }
 678     }
 679     return false;               // Safe to obey configuration and not balance.
 680   }
 681 }
 682 
 683 void ReferenceProcessor::maybe_balance_queues(DiscoveredList refs_lists[]) {
 684   assert(_processing_is_mt, "Should not call this otherwise");
 685   if (need_balance_queues(refs_lists)) {
 686     balance_queues(refs_lists);
 687   }
 688 }
 689 
 690 // Balances reference queues.
 691 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 692 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 693 // corresponding to the active workers will be processed.
 694 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 695 {
 696   // calculate total length
 697   size_t total_refs = 0;
 698   log_develop_trace(gc, ref)("Balance ref_lists ");
 699 
 700   log_reflist_counts(ref_lists, _max_num_queues);
 701 
 702   for (uint i = 0; i < _max_num_queues; ++i) {
 703     total_refs += ref_lists[i].length();
 704   }
 705   size_t avg_refs = total_refs / _num_queues + 1;
 706   uint to_idx = 0;
 707   for (uint from_idx = 0; from_idx < _max_num_queues; from_idx++) {
 708     bool move_all = false;
 709     if (from_idx >= _num_queues) {
 710       move_all = ref_lists[from_idx].length() > 0;
 711     }
 712     while ((ref_lists[from_idx].length() > avg_refs) ||
 713            move_all) {
 714       assert(to_idx < _num_queues, "Sanity Check!");
 715       if (ref_lists[to_idx].length() < avg_refs) {
 716         // move superfluous refs
 717         size_t refs_to_move;
 718         // Move all the Ref's if the from queue will not be processed.
 719         if (move_all) {
 720           refs_to_move = MIN2(ref_lists[from_idx].length(),
 721                               avg_refs - ref_lists[to_idx].length());
 722         } else {
 723           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 724                               avg_refs - ref_lists[to_idx].length());
 725         }
 726 
 727         assert(refs_to_move > 0, "otherwise the code below will fail");
 728 
 729         oop move_head = ref_lists[from_idx].head();
 730         oop move_tail = move_head;
 731         oop new_head  = move_head;
 732         // find an element to split the list on
 733         for (size_t j = 0; j < refs_to_move; ++j) {
 734           move_tail = new_head;
 735           new_head = java_lang_ref_Reference::discovered(new_head);
 736         }
 737 
 738         // Add the chain to the to list.
 739         if (ref_lists[to_idx].head() == NULL) {
 740           // to list is empty. Make a loop at the end.
 741           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 742         } else {
 743           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 744         }
 745         ref_lists[to_idx].set_head(move_head);
 746         ref_lists[to_idx].inc_length(refs_to_move);
 747 
 748         // Remove the chain from the from list.
 749         if (oopDesc::unsafe_equals(move_tail, new_head)) {
 750           // We found the end of the from list.
 751           ref_lists[from_idx].set_head(NULL);
 752         } else {
 753           ref_lists[from_idx].set_head(new_head);
 754         }
 755         ref_lists[from_idx].dec_length(refs_to_move);
 756         if (ref_lists[from_idx].length() == 0) {
 757           break;
 758         }
 759       } else {
 760         to_idx = (to_idx + 1) % _num_queues;
 761       }
 762     }
 763   }
 764 #ifdef ASSERT
 765   log_reflist_counts(ref_lists, _num_queues);
 766   size_t balanced_total_refs = 0;
 767   for (uint i = 0; i < _num_queues; ++i) {
 768     balanced_total_refs += ref_lists[i].length();
 769   }
 770   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 771 #endif
 772 }
 773 
 774 bool ReferenceProcessor::is_mt_processing_set_up(AbstractRefProcTaskExecutor* task_executor) const {
 775   return task_executor != NULL && _processing_is_mt;
 776 }
 777 
 778 void ReferenceProcessor::process_soft_ref_reconsider(BoolObjectClosure* is_alive,
 779                                                      OopClosure* keep_alive,
 780                                                      VoidClosure* complete_gc,
 781                                                      AbstractRefProcTaskExecutor* task_executor,
 782                                                      ReferenceProcessorPhaseTimes* phase_times) {
 783   assert(!_processing_is_mt || task_executor != NULL, "Task executor must not be NULL when mt processing is set.");
 784 
 785   size_t const num_soft_refs = total_count(_discoveredSoftRefs);
 786   phase_times->set_ref_discovered(REF_SOFT, num_soft_refs);
 787 
 788   phase_times->set_processing_is_mt(_processing_is_mt);
 789 
 790   if (num_soft_refs == 0 || _current_soft_ref_policy == NULL) {
 791     log_debug(gc, ref)("Skipped phase1 of Reference Processing due to unavailable references");
 792     return;
 793   }
 794 
 795   RefProcMTDegreeAdjuster a(this, RefPhase1, num_soft_refs);
 796 
 797   if (_processing_is_mt) {
 798     RefProcBalanceQueuesTimeTracker tt(RefPhase1, phase_times);
 799     maybe_balance_queues(_discoveredSoftRefs);
 800   }
 801 
 802   RefProcPhaseTimeTracker tt(RefPhase1, phase_times);
 803 
 804   log_reflist("Phase1 Soft before", _discoveredSoftRefs, _max_num_queues);
 805   if (_processing_is_mt) {
 806     RefProcPhase1Task phase1(*this, phase_times, _current_soft_ref_policy);
 807     task_executor->execute(phase1, num_queues());
 808   } else {
 809     size_t removed = 0;
 810 
 811     RefProcSubPhasesWorkerTimeTracker tt2(SoftRefSubPhase1, phase_times, 0);
 812     for (uint i = 0; i < _max_num_queues; i++) {
 813       removed += process_soft_ref_reconsider_work(_discoveredSoftRefs[i], _current_soft_ref_policy,
 814                                                   is_alive, keep_alive, complete_gc);
 815     }
 816 
 817     phase_times->add_ref_cleared(REF_SOFT, removed);
 818   }
 819   log_reflist("Phase1 Soft after", _discoveredSoftRefs, _max_num_queues);
 820 }
 821 
 822 void ReferenceProcessor::process_soft_weak_final_refs(BoolObjectClosure* is_alive,
 823                                                       OopClosure* keep_alive,
 824                                                       VoidClosure* complete_gc,
 825                                                       AbstractRefProcTaskExecutor*  task_executor,
 826                                                       ReferenceProcessorPhaseTimes* phase_times) {
 827   assert(!_processing_is_mt || task_executor != NULL, "Task executor must not be NULL when mt processing is set.");
 828 
 829   size_t const num_soft_refs = total_count(_discoveredSoftRefs);
 830   size_t const num_weak_refs = total_count(_discoveredWeakRefs);
 831   size_t const num_final_refs = total_count(_discoveredFinalRefs);
 832   size_t const num_total_refs = num_soft_refs + num_weak_refs + num_final_refs;
 833   phase_times->set_ref_discovered(REF_WEAK, num_weak_refs);
 834   phase_times->set_ref_discovered(REF_FINAL, num_final_refs);
 835 
 836   phase_times->set_processing_is_mt(_processing_is_mt);
 837 
 838   if (num_total_refs == 0) {
 839     log_debug(gc, ref)("Skipped phase2 of Reference Processing due to unavailable references");
 840     return;
 841   }
 842 
 843   RefProcMTDegreeAdjuster a(this, RefPhase2, num_total_refs);
 844 
 845   if (_processing_is_mt) {
 846     RefProcBalanceQueuesTimeTracker tt(RefPhase2, phase_times);
 847     maybe_balance_queues(_discoveredSoftRefs);
 848     maybe_balance_queues(_discoveredWeakRefs);
 849     maybe_balance_queues(_discoveredFinalRefs);
 850   }
 851 
 852   RefProcPhaseTimeTracker tt(RefPhase2, phase_times);
 853 
 854   log_reflist("Phase2 Soft before", _discoveredSoftRefs, _max_num_queues);
 855   log_reflist("Phase2 Weak before", _discoveredWeakRefs, _max_num_queues);
 856   log_reflist("Phase2 Final before", _discoveredFinalRefs, _max_num_queues);
 857   if (_processing_is_mt) {
 858     RefProcPhase2Task phase2(*this, phase_times);
 859     task_executor->execute(phase2, num_queues());
 860   } else {
 861     RefProcWorkerTimeTracker t(phase_times->phase2_worker_time_sec(), 0);
 862     {
 863       size_t removed = 0;
 864 
 865       RefProcSubPhasesWorkerTimeTracker tt2(SoftRefSubPhase2, phase_times, 0);
 866       for (uint i = 0; i < _max_num_queues; i++) {
 867         removed += process_soft_weak_final_refs_work(_discoveredSoftRefs[i], is_alive, keep_alive, true /* do_enqueue */);
 868       }
 869 
 870       phase_times->add_ref_cleared(REF_SOFT, removed);
 871     }
 872     {
 873       size_t removed = 0;
 874 
 875       RefProcSubPhasesWorkerTimeTracker tt2(WeakRefSubPhase2, phase_times, 0);
 876       for (uint i = 0; i < _max_num_queues; i++) {
 877         removed += process_soft_weak_final_refs_work(_discoveredWeakRefs[i], is_alive, keep_alive, true /* do_enqueue */);
 878       }
 879 
 880       phase_times->add_ref_cleared(REF_WEAK, removed);
 881     }
 882     {
 883       size_t removed = 0;
 884 
 885       RefProcSubPhasesWorkerTimeTracker tt2(FinalRefSubPhase2, phase_times, 0);
 886       for (uint i = 0; i < _max_num_queues; i++) {
 887         removed += process_soft_weak_final_refs_work(_discoveredFinalRefs[i], is_alive, keep_alive, false /* do_enqueue */);
 888       }
 889 
 890       phase_times->add_ref_cleared(REF_FINAL, removed);
 891     }
 892     complete_gc->do_void();
 893   }
 894   verify_total_count_zero(_discoveredSoftRefs, "SoftReference");
 895   verify_total_count_zero(_discoveredWeakRefs, "WeakReference");
 896   log_reflist("Phase2 Final after", _discoveredFinalRefs, _max_num_queues);
 897 }
 898 
 899 void ReferenceProcessor::process_final_keep_alive(OopClosure* keep_alive,
 900                                                   VoidClosure* complete_gc,
 901                                                   AbstractRefProcTaskExecutor*  task_executor,
 902                                                   ReferenceProcessorPhaseTimes* phase_times) {
 903   assert(!_processing_is_mt || task_executor != NULL, "Task executor must not be NULL when mt processing is set.");
 904 
 905   size_t const num_final_refs = total_count(_discoveredFinalRefs);
 906 
 907   phase_times->set_processing_is_mt(_processing_is_mt);
 908 
 909   if (num_final_refs == 0) {
 910     log_debug(gc, ref)("Skipped phase3 of Reference Processing due to unavailable references");
 911     return;
 912   }
 913 
 914   RefProcMTDegreeAdjuster a(this, RefPhase3, num_final_refs);
 915 
 916   if (_processing_is_mt) {
 917     RefProcBalanceQueuesTimeTracker tt(RefPhase3, phase_times);
 918     maybe_balance_queues(_discoveredFinalRefs);
 919   }
 920 
 921   // Phase 3:
 922   // . Traverse referents of final references and keep them and followers alive.
 923   RefProcPhaseTimeTracker tt(RefPhase3, phase_times);
 924 
 925   if (_processing_is_mt) {
 926     RefProcPhase3Task phase3(*this, phase_times);
 927     task_executor->execute(phase3, num_queues());
 928   } else {
 929     RefProcSubPhasesWorkerTimeTracker tt2(FinalRefSubPhase3, phase_times, 0);
 930     for (uint i = 0; i < _max_num_queues; i++) {
 931       process_final_keep_alive_work(_discoveredFinalRefs[i], keep_alive, complete_gc);
 932     }
 933   }
 934   verify_total_count_zero(_discoveredFinalRefs, "FinalReference");
 935 }
 936 
 937 void ReferenceProcessor::process_phantom_refs(BoolObjectClosure* is_alive,
 938                                               OopClosure* keep_alive,
 939                                               VoidClosure* complete_gc,
 940                                               AbstractRefProcTaskExecutor* task_executor,
 941                                               ReferenceProcessorPhaseTimes* phase_times) {
 942   assert(!_processing_is_mt || task_executor != NULL, "Task executor must not be NULL when mt processing is set.");
 943 
 944   size_t const num_phantom_refs = total_count(_discoveredPhantomRefs);
 945   phase_times->set_ref_discovered(REF_PHANTOM, num_phantom_refs);
 946 
 947   phase_times->set_processing_is_mt(_processing_is_mt);
 948 
 949   if (num_phantom_refs == 0) {
 950     log_debug(gc, ref)("Skipped phase4 of Reference Processing due to unavailable references");
 951     return;
 952   }
 953 
 954   RefProcMTDegreeAdjuster a(this, RefPhase4, num_phantom_refs);
 955 
 956   if (_processing_is_mt) {
 957     RefProcBalanceQueuesTimeTracker tt(RefPhase4, phase_times);
 958     maybe_balance_queues(_discoveredPhantomRefs);
 959   }
 960 
 961   // Phase 4: Walk phantom references appropriately.
 962   RefProcPhaseTimeTracker tt(RefPhase4, phase_times);
 963 
 964   log_reflist("Phase4 Phantom before", _discoveredPhantomRefs, _max_num_queues);
 965   if (_processing_is_mt) {
 966     RefProcPhase4Task phase4(*this, phase_times);
 967     task_executor->execute(phase4, num_queues());
 968   } else {
 969     size_t removed = 0;
 970 
 971     RefProcSubPhasesWorkerTimeTracker tt(PhantomRefSubPhase4, phase_times, 0);
 972     for (uint i = 0; i < _max_num_queues; i++) {
 973       removed += process_phantom_refs_work(_discoveredPhantomRefs[i], is_alive, keep_alive, complete_gc);
 974     }
 975 
 976     phase_times->add_ref_cleared(REF_PHANTOM, removed);
 977   }
 978   verify_total_count_zero(_discoveredPhantomRefs, "PhantomReference");
 979 }
 980 
 981 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
 982   uint id = 0;
 983   // Determine the queue index to use for this object.
 984   if (_discovery_is_mt) {
 985     // During a multi-threaded discovery phase,
 986     // each thread saves to its "own" list.
 987     Thread* thr = Thread::current();
 988     id = thr->as_Worker_thread()->id();
 989   } else {
 990     // single-threaded discovery, we save in round-robin
 991     // fashion to each of the lists.
 992     if (_processing_is_mt) {
 993       id = next_id();
 994     }
 995   }
 996   assert(id < _max_num_queues, "Id is out of bounds id %u and max id %u)", id, _max_num_queues);
 997 
 998   // Get the discovered queue to which we will add
 999   DiscoveredList* list = NULL;
1000   switch (rt) {
1001     case REF_OTHER:
1002       // Unknown reference type, no special treatment
1003       break;
1004     case REF_SOFT:
1005       list = &_discoveredSoftRefs[id];
1006       break;
1007     case REF_WEAK:
1008       list = &_discoveredWeakRefs[id];
1009       break;
1010     case REF_FINAL:
1011       list = &_discoveredFinalRefs[id];
1012       break;
1013     case REF_PHANTOM:
1014       list = &_discoveredPhantomRefs[id];
1015       break;
1016     case REF_NONE:
1017       // we should not reach here if we are an InstanceRefKlass
1018     default:
1019       ShouldNotReachHere();
1020   }
1021   log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
1022   return list;
1023 }
1024 
1025 inline void
1026 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1027                                               oop             obj,
1028                                               HeapWord*       discovered_addr) {
1029   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1030   // First we must make sure this object is only enqueued once. CAS in a non null
1031   // discovered_addr.
1032   oop current_head = refs_list.head();
1033   // The last ref must have its discovered field pointing to itself.
1034   oop next_discovered = (current_head != NULL) ? current_head : obj;
1035 
1036   oop retest = HeapAccess<AS_NO_KEEPALIVE>::oop_atomic_cmpxchg(next_discovered, discovered_addr, oop(NULL));
1037 
1038   if (retest == NULL) {
1039     // This thread just won the right to enqueue the object.
1040     // We have separate lists for enqueueing, so no synchronization
1041     // is necessary.
1042     refs_list.set_head(obj);
1043     refs_list.inc_length(1);
1044 
1045     log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1046                                p2i(obj), obj->klass()->internal_name());
1047   } else {
1048     // If retest was non NULL, another thread beat us to it:
1049     // The reference has already been discovered...
1050     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
1051                                p2i(obj), obj->klass()->internal_name());
1052   }
1053 }
1054 
1055 #ifndef PRODUCT
1056 // Non-atomic (i.e. concurrent) discovery might allow us
1057 // to observe j.l.References with NULL referents, being those
1058 // cleared concurrently by mutators during (or after) discovery.
1059 void ReferenceProcessor::verify_referent(oop obj) {
1060   bool da = discovery_is_atomic();
1061   oop referent = java_lang_ref_Reference::referent(obj);
1062   assert(da ? oopDesc::is_oop(referent) : oopDesc::is_oop_or_null(referent),
1063          "Bad referent " INTPTR_FORMAT " found in Reference "
1064          INTPTR_FORMAT " during %satomic discovery ",
1065          p2i(referent), p2i(obj), da ? "" : "non-");
1066 }
1067 #endif
1068 
1069 bool ReferenceProcessor::is_subject_to_discovery(oop const obj) const {
1070   return _is_subject_to_discovery->do_object_b(obj);
1071 }
1072 
1073 // We mention two of several possible choices here:
1074 // #0: if the reference object is not in the "originating generation"
1075 //     (or part of the heap being collected, indicated by our "span"
1076 //     we don't treat it specially (i.e. we scan it as we would
1077 //     a normal oop, treating its references as strong references).
1078 //     This means that references can't be discovered unless their
1079 //     referent is also in the same span. This is the simplest,
1080 //     most "local" and most conservative approach, albeit one
1081 //     that may cause weak references to be enqueued least promptly.
1082 //     We call this choice the "ReferenceBasedDiscovery" policy.
1083 // #1: the reference object may be in any generation (span), but if
1084 //     the referent is in the generation (span) being currently collected
1085 //     then we can discover the reference object, provided
1086 //     the object has not already been discovered by
1087 //     a different concurrently running collector (as may be the
1088 //     case, for instance, if the reference object is in CMS and
1089 //     the referent in DefNewGeneration), and provided the processing
1090 //     of this reference object by the current collector will
1091 //     appear atomic to every other collector in the system.
1092 //     (Thus, for instance, a concurrent collector may not
1093 //     discover references in other generations even if the
1094 //     referent is in its own generation). This policy may,
1095 //     in certain cases, enqueue references somewhat sooner than
1096 //     might Policy #0 above, but at marginally increased cost
1097 //     and complexity in processing these references.
1098 //     We call this choice the "RefeferentBasedDiscovery" policy.
1099 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1100   // Make sure we are discovering refs (rather than processing discovered refs).
1101   if (!_discovering_refs || !RegisterReferences) {
1102     return false;
1103   }
1104 
1105   if ((rt == REF_FINAL) && (java_lang_ref_Reference::next(obj) != NULL)) {
1106     // Don't rediscover non-active FinalReferences.
1107     return false;
1108   }
1109 
1110   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1111       !is_subject_to_discovery(obj)) {
1112     // Reference is not in the originating generation;
1113     // don't treat it specially (i.e. we want to scan it as a normal
1114     // object with strong references).
1115     return false;
1116   }
1117 
1118   // We only discover references whose referents are not (yet)
1119   // known to be strongly reachable.
1120   if (is_alive_non_header() != NULL) {
1121     verify_referent(obj);
1122     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1123       return false;  // referent is reachable
1124     }
1125   }
1126   if (rt == REF_SOFT) {
1127     // For soft refs we can decide now if these are not
1128     // current candidates for clearing, in which case we
1129     // can mark through them now, rather than delaying that
1130     // to the reference-processing phase. Since all current
1131     // time-stamp policies advance the soft-ref clock only
1132     // at a full collection cycle, this is always currently
1133     // accurate.
1134     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1135       return false;
1136     }
1137   }
1138 
1139   ResourceMark rm;      // Needed for tracing.
1140 
1141   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr_raw(obj);
1142   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1143   assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
1144   if (discovered != NULL) {
1145     // The reference has already been discovered...
1146     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
1147                                p2i(obj), obj->klass()->internal_name());
1148     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1149       // assumes that an object is not processed twice;
1150       // if it's been already discovered it must be on another
1151       // generation's discovered list; so we won't discover it.
1152       return false;
1153     } else {
1154       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1155              "Unrecognized policy");
1156       // Check assumption that an object is not potentially
1157       // discovered twice except by concurrent collectors that potentially
1158       // trace the same Reference object twice.
1159       assert(UseConcMarkSweepGC || UseG1GC || UseShenandoahGC,
1160              "Only possible with a concurrent marking collector");
1161       return true;
1162     }
1163   }
1164 
1165   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1166     verify_referent(obj);
1167     // Discover if and only if EITHER:
1168     // .. reference is in our span, OR
1169     // .. we are an atomic collector and referent is in our span
1170     if (is_subject_to_discovery(obj) ||
1171         (discovery_is_atomic() &&
1172          is_subject_to_discovery(java_lang_ref_Reference::referent(obj)))) {
1173     } else {
1174       return false;
1175     }
1176   } else {
1177     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1178            is_subject_to_discovery(obj), "code inconsistency");
1179   }
1180 
1181   // Get the right type of discovered queue head.
1182   DiscoveredList* list = get_discovered_list(rt);
1183   if (list == NULL) {
1184     return false;   // nothing special needs to be done
1185   }
1186 
1187   if (_discovery_is_mt) {
1188     add_to_discovered_list_mt(*list, obj, discovered_addr);
1189   } else {
1190     // We do a raw store here: the field will be visited later when processing
1191     // the discovered references.
1192     oop current_head = list->head();
1193     // The last ref must have its discovered field pointing to itself.
1194     oop next_discovered = (current_head != NULL) ? current_head : obj;
1195 
1196     assert(discovered == NULL, "control point invariant");
1197     RawAccess<>::oop_store(discovered_addr, next_discovered);
1198     list->set_head(obj);
1199     list->inc_length(1);
1200 
1201     log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
1202   }
1203   assert(oopDesc::is_oop(obj), "Discovered a bad reference");
1204   verify_referent(obj);
1205   return true;
1206 }
1207 
1208 bool ReferenceProcessor::has_discovered_references() {
1209   for (uint i = 0; i < _max_num_queues * number_of_subclasses_of_ref(); i++) {
1210     if (!_discovered_refs[i].is_empty()) {
1211       return true;
1212     }
1213   }
1214   return false;
1215 }
1216 
1217 void ReferenceProcessor::preclean_discovered_references(BoolObjectClosure* is_alive,
1218                                                         OopClosure* keep_alive,
1219                                                         VoidClosure* complete_gc,
1220                                                         YieldClosure* yield,
1221                                                         GCTimer* gc_timer) {
1222   // These lists can be handled here in any order and, indeed, concurrently.
1223 
1224   // Soft references
1225   {
1226     GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
1227     log_reflist("SoftRef before: ", _discoveredSoftRefs, _max_num_queues);
1228     for (uint i = 0; i < _max_num_queues; i++) {
1229       if (yield->should_return()) {
1230         return;
1231       }
1232       if (preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1233                                       keep_alive, complete_gc, yield)) {
1234         log_reflist("SoftRef abort: ", _discoveredSoftRefs, _max_num_queues);
1235         return;
1236       }
1237     }
1238     log_reflist("SoftRef after: ", _discoveredSoftRefs, _max_num_queues);
1239   }
1240 
1241   // Weak references
1242   {
1243     GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
1244     log_reflist("WeakRef before: ", _discoveredWeakRefs, _max_num_queues);
1245     for (uint i = 0; i < _max_num_queues; i++) {
1246       if (yield->should_return()) {
1247         return;
1248       }
1249       if (preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1250                                       keep_alive, complete_gc, yield)) {
1251         log_reflist("WeakRef abort: ", _discoveredWeakRefs, _max_num_queues);
1252         return;
1253       }
1254     }
1255     log_reflist("WeakRef after: ", _discoveredWeakRefs, _max_num_queues);
1256   }
1257 
1258   // Final references
1259   {
1260     GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
1261     log_reflist("FinalRef before: ", _discoveredFinalRefs, _max_num_queues);
1262     for (uint i = 0; i < _max_num_queues; i++) {
1263       if (yield->should_return()) {
1264         return;
1265       }
1266       if (preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1267                                       keep_alive, complete_gc, yield)) {
1268         log_reflist("FinalRef abort: ", _discoveredFinalRefs, _max_num_queues);
1269         return;
1270       }
1271     }
1272     log_reflist("FinalRef after: ", _discoveredFinalRefs, _max_num_queues);
1273   }
1274 
1275   // Phantom references
1276   {
1277     GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
1278     log_reflist("PhantomRef before: ", _discoveredPhantomRefs, _max_num_queues);
1279     for (uint i = 0; i < _max_num_queues; i++) {
1280       if (yield->should_return()) {
1281         return;
1282       }
1283       if (preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1284                                       keep_alive, complete_gc, yield)) {
1285         log_reflist("PhantomRef abort: ", _discoveredPhantomRefs, _max_num_queues);
1286         return;
1287       }
1288     }
1289     log_reflist("PhantomRef after: ", _discoveredPhantomRefs, _max_num_queues);
1290   }
1291 }
1292 
1293 // Walk the given discovered ref list, and remove all reference objects
1294 // whose referents are still alive, whose referents are NULL or which
1295 // are not active (have a non-NULL next field). NOTE: When we are
1296 // thus precleaning the ref lists (which happens single-threaded today),
1297 // we do not disable refs discovery to honor the correct semantics of
1298 // java.lang.Reference. As a result, we need to be careful below
1299 // that ref removal steps interleave safely with ref discovery steps
1300 // (in this thread).
1301 bool ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1302                                                      BoolObjectClosure* is_alive,
1303                                                      OopClosure*        keep_alive,
1304                                                      VoidClosure*       complete_gc,
1305                                                      YieldClosure*      yield) {
1306   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1307   while (iter.has_next()) {
1308     if (yield->should_return_fine_grain()) {
1309       return true;
1310     }
1311     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1312     if (iter.referent() == NULL || iter.is_referent_alive()) {
1313       // The referent has been cleared, or is alive; we need to trace
1314       // and mark its cohort.
1315       log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1316                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
1317       // Remove Reference object from list
1318       iter.remove();
1319       // Keep alive its cohort.
1320       iter.make_referent_alive();
1321       iter.move_to_next();
1322     } else {
1323       iter.next();
1324     }
1325   }
1326   // Close the reachable set
1327   complete_gc->do_void();
1328 
1329   if (iter.processed() > 0) {
1330     log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
1331                                iter.removed(), iter.processed(), p2i(&refs_list));
1332   }
1333   return false;
1334 }
1335 
1336 const char* ReferenceProcessor::list_name(uint i) {
1337    assert(i <= _max_num_queues * number_of_subclasses_of_ref(),
1338           "Out of bounds index");
1339 
1340    int j = i / _max_num_queues;
1341    switch (j) {
1342      case 0: return "SoftRef";
1343      case 1: return "WeakRef";
1344      case 2: return "FinalRef";
1345      case 3: return "PhantomRef";
1346    }
1347    ShouldNotReachHere();
1348    return NULL;
1349 }
1350 
1351 uint RefProcMTDegreeAdjuster::ergo_proc_thread_count(size_t ref_count,
1352                                                      uint max_threads,
1353                                                      RefProcPhases phase) const {
1354   assert(0 < max_threads, "must allow at least one thread");
1355 
1356   if (use_max_threads(phase) || (ReferencesPerThread == 0)) {
1357     return max_threads;
1358   }
1359 
1360   size_t thread_count = 1 + (ref_count / ReferencesPerThread);
1361   return (uint)MIN3(thread_count,
1362                     static_cast<size_t>(max_threads),
1363                     (size_t)os::active_processor_count());
1364 }
1365 
1366 bool RefProcMTDegreeAdjuster::use_max_threads(RefProcPhases phase) const {
1367   // Even a small number of references in either of those cases could produce large amounts of work.
1368   return (phase == ReferenceProcessor::RefPhase1 || phase == ReferenceProcessor::RefPhase3);
1369 }
1370 
1371 RefProcMTDegreeAdjuster::RefProcMTDegreeAdjuster(ReferenceProcessor* rp,
1372                                                  RefProcPhases phase,
1373                                                  size_t ref_count):
1374     _rp(rp),
1375     _saved_mt_processing(_rp->processing_is_mt()),
1376     _saved_num_queues(_rp->num_queues()) {
1377   if (!_rp->processing_is_mt() || !_rp->adjust_no_of_processing_threads() || (ReferencesPerThread == 0)) {
1378     return;
1379   }
1380 
1381   uint workers = ergo_proc_thread_count(ref_count, _rp->num_queues(), phase);
1382 
1383   _rp->set_mt_processing(workers > 1);
1384   _rp->set_active_mt_degree(workers);
1385 }
1386 
1387 RefProcMTDegreeAdjuster::~RefProcMTDegreeAdjuster() {
1388   // Revert to previous status.
1389   _rp->set_mt_processing(_saved_mt_processing);
1390   _rp->set_active_mt_degree(_saved_num_queues);
1391 }