rev 49684 : imported patch 8201487-do-not-rebalance-with-serial-processing

   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(MemRegion span,
  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   _discovering_refs(false),
 103   _enqueuing_is_done(false),
 104   _is_alive_non_header(is_alive_non_header),
 105   _processing_is_mt(mt_processing),
 106   _next_id(0)
 107 {
 108   _span = span;
 109   _discovery_is_atomic = atomic_discovery;
 110   _discovery_is_mt     = mt_discovery;
 111   _num_q               = MAX2(1U, mt_processing_degree);
 112   _max_num_q           = MAX2(_num_q, mt_discovery_degree);
 113   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 114             _max_num_q * number_of_subclasses_of_ref(), mtGC);
 115 
 116   if (_discovered_refs == NULL) {
 117     vm_exit_during_initialization("Could not allocated RefProc Array");
 118   }
 119   _discoveredSoftRefs    = &_discovered_refs[0];
 120   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
 121   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
 122   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
 123 
 124   // Initialize all entries to NULL
 125   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 126     _discovered_refs[i].set_head(NULL);
 127     _discovered_refs[i].set_length(0);
 128   }
 129 
 130   setup_policy(false /* default soft ref policy */);
 131 }
 132 
 133 #ifndef PRODUCT
 134 void ReferenceProcessor::verify_no_references_recorded() {
 135   guarantee(!_discovering_refs, "Discovering refs?");
 136   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 137     guarantee(_discovered_refs[i].is_empty(),
 138               "Found non-empty discovered list at %u", i);
 139   }
 140 }
 141 #endif
 142 
 143 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 144   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 145     if (UseCompressedOops) {
 146       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 147     } else {
 148       f->do_oop((oop*)_discovered_refs[i].adr_head());
 149     }
 150   }
 151 }
 152 
 153 void ReferenceProcessor::update_soft_ref_master_clock() {
 154   // Update (advance) the soft ref master clock field. This must be done
 155   // after processing the soft ref list.
 156 
 157   // We need a monotonically non-decreasing time in ms but
 158   // os::javaTimeMillis() does not guarantee monotonicity.
 159   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 160   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 161   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 162 
 163   NOT_PRODUCT(
 164   if (now < _soft_ref_timestamp_clock) {
 165     log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
 166                     _soft_ref_timestamp_clock, now);
 167   }
 168   )
 169   // The values of now and _soft_ref_timestamp_clock are set using
 170   // javaTimeNanos(), which is guaranteed to be monotonically
 171   // non-decreasing provided the underlying platform provides such
 172   // a time source (and it is bug free).
 173   // In product mode, however, protect ourselves from non-monotonicity.
 174   if (now > _soft_ref_timestamp_clock) {
 175     _soft_ref_timestamp_clock = now;
 176     java_lang_ref_SoftReference::set_clock(now);
 177   }
 178   // Else leave clock stalled at its old value until time progresses
 179   // past clock value.
 180 }
 181 
 182 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) const {
 183   size_t total = 0;
 184   for (uint i = 0; i < _max_num_q; ++i) {
 185     total += lists[i].length();
 186   }
 187   return total;
 188 }
 189 
 190 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 191   BoolObjectClosure*            is_alive,
 192   OopClosure*                   keep_alive,
 193   VoidClosure*                  complete_gc,
 194   AbstractRefProcTaskExecutor*  task_executor,
 195   ReferenceProcessorPhaseTimes* phase_times) {
 196 
 197   double start_time = os::elapsedTime();
 198 
 199   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 200   // Stop treating discovered references specially.
 201   disable_discovery();
 202 
 203   // If discovery was concurrent, someone could have modified
 204   // the value of the static field in the j.l.r.SoftReference
 205   // class that holds the soft reference timestamp clock using
 206   // reflection or Unsafe between when discovery was enabled and
 207   // now. Unconditionally update the static field in ReferenceProcessor
 208   // here so that we use the new value during processing of the
 209   // discovered soft refs.
 210 
 211   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 212 
 213   ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
 214                                 total_count(_discoveredWeakRefs),
 215                                 total_count(_discoveredFinalRefs),
 216                                 total_count(_discoveredPhantomRefs));
 217 
 218   // Soft references
 219   {
 220     RefProcPhaseTimesTracker tt(REF_SOFT, phase_times, this);
 221     process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 222                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 223   }
 224 
 225   update_soft_ref_master_clock();
 226 
 227   // Weak references
 228   {
 229     RefProcPhaseTimesTracker tt(REF_WEAK, phase_times, this);
 230     process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 231                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 232   }
 233 
 234   // Final references
 235   {
 236     RefProcPhaseTimesTracker tt(REF_FINAL, phase_times, this);
 237     process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 238                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 239   }
 240 
 241   // Phantom references
 242   {
 243     RefProcPhaseTimesTracker tt(REF_PHANTOM, phase_times, this);
 244     process_discovered_reflist(_discoveredPhantomRefs, NULL, true,
 245                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 246   }
 247 
 248   if (task_executor != NULL) {
 249     // Record the work done by the parallel workers.
 250     task_executor->set_single_threaded_mode();
 251   }
 252 
 253   phase_times->set_total_time_ms((os::elapsedTime() - start_time) * 1000);
 254 
 255   return stats;
 256 }
 257 
 258 void ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor*  task_executor,
 259                                                        ReferenceProcessorPhaseTimes* phase_times) {
 260   // Enqueue references that are not made active again, and
 261   // clear the decks for the next collection (cycle).
 262   enqueue_discovered_reflists(task_executor, phase_times);
 263 
 264   // Stop treating discovered references specially.
 265   disable_discovery();
 266 }
 267 
 268 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list) {
 269   // Given a list of refs linked through the "discovered" field
 270   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 271   // thus distinguishing them from active References, then
 272   // prepend them to the pending list.
 273   //
 274   // The Java threads will see the Reference objects linked together through
 275   // the discovered field. Instead of trying to do the write barrier updates
 276   // in all places in the reference processor where we manipulate the discovered
 277   // field we make sure to do the barrier here where we anyway iterate through
 278   // all linked Reference objects. Note that it is important to not dirty any
 279   // cards during reference processing since this will cause card table
 280   // verification to fail for G1.
 281   log_develop_trace(gc, ref)("ReferenceProcessor::enqueue_discovered_reflist list " INTPTR_FORMAT, p2i(&refs_list));
 282 
 283   oop obj = NULL;
 284   oop next_d = refs_list.head();
 285   // Walk down the list, self-looping the next field
 286   // so that the References are not considered active.
 287   while (obj != next_d) {
 288     obj = next_d;
 289     assert(obj->is_instance(), "should be an instance object");
 290     assert(InstanceKlass::cast(obj->klass())->is_reference_instance_klass(), "should be reference object");
 291     next_d = java_lang_ref_Reference::discovered(obj);
 292     log_develop_trace(gc, ref)("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, p2i(obj), p2i(next_d));
 293     assert(java_lang_ref_Reference::next(obj) == NULL,
 294            "Reference not active; should not be discovered");
 295     // Self-loop next, so as to make Ref not active.
 296     java_lang_ref_Reference::set_next_raw(obj, obj);
 297     if (next_d != obj) {
 298       HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(obj, java_lang_ref_Reference::discovered_offset, next_d);
 299     } else {
 300       // This is the last object.
 301       // Swap refs_list into pending list and set obj's
 302       // discovered to what we read from the pending list.
 303       oop old = Universe::swap_reference_pending_list(refs_list.head());
 304       HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(obj, java_lang_ref_Reference::discovered_offset, old);
 305     }
 306   }
 307 }
 308 
 309 // Parallel enqueue task
 310 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 311 public:
 312   RefProcEnqueueTask(ReferenceProcessor&           ref_processor,
 313                      DiscoveredList                discovered_refs[],
 314                      int                           n_queues,
 315                      ReferenceProcessorPhaseTimes* phase_times)
 316     : EnqueueTask(ref_processor, discovered_refs, n_queues, phase_times)
 317   { }
 318 
 319   virtual void work(unsigned int work_id) {
 320     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefEnqueue, _phase_times, work_id);
 321 
 322     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 323     // Simplest first cut: static partitioning.
 324     int index = work_id;
 325     // The increment on "index" must correspond to the maximum number of queues
 326     // (n_queues) with which that ReferenceProcessor was created.  That
 327     // is because of the "clever" way the discovered references lists were
 328     // allocated and are indexed into.
 329     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 330     for (int j = 0;
 331          j < ReferenceProcessor::number_of_subclasses_of_ref();
 332          j++, index += _n_queues) {
 333       _ref_processor.enqueue_discovered_reflist(_refs_lists[index]);
 334       _refs_lists[index].set_head(NULL);
 335       _refs_lists[index].set_length(0);
 336     }
 337   }
 338 };
 339 
 340 // Enqueue references that are not made active again
 341 void ReferenceProcessor::enqueue_discovered_reflists(AbstractRefProcTaskExecutor*  task_executor,
 342                                                      ReferenceProcessorPhaseTimes* phase_times) {
 343 
 344   ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
 345                                 total_count(_discoveredWeakRefs),
 346                                 total_count(_discoveredFinalRefs),
 347                                 total_count(_discoveredPhantomRefs));
 348 
 349   RefProcEnqueueTimeTracker tt(phase_times, stats);
 350 
 351   if (_processing_is_mt && task_executor != NULL) {
 352     // Parallel code
 353     RefProcEnqueueTask tsk(*this, _discovered_refs, _max_num_q, phase_times);
 354     task_executor->execute(tsk);
 355   } else {
 356     // Serial code: call the parent class's implementation
 357     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 358       enqueue_discovered_reflist(_discovered_refs[i]);
 359       _discovered_refs[i].set_head(NULL);
 360       _discovered_refs[i].set_length(0);
 361     }
 362   }
 363 }
 364 
 365 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 366   _discovered_addr = java_lang_ref_Reference::discovered_addr_raw(_ref);
 367   oop discovered = java_lang_ref_Reference::discovered(_ref);
 368   assert(_discovered_addr && oopDesc::is_oop_or_null(discovered),
 369          "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
 370   _next = discovered;
 371   _referent_addr = java_lang_ref_Reference::referent_addr_raw(_ref);
 372   _referent = java_lang_ref_Reference::referent(_ref);
 373   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 374          "Wrong oop found in java.lang.Reference object");
 375   assert(allow_null_referent ?
 376              oopDesc::is_oop_or_null(_referent)
 377            : oopDesc::is_oop(_referent),
 378          "Expected an oop%s for referent field at " PTR_FORMAT,
 379          (allow_null_referent ? " or NULL" : ""),
 380          p2i(_referent));
 381 }
 382 
 383 void DiscoveredListIterator::remove() {
 384   assert(oopDesc::is_oop(_ref), "Dropping a bad reference");
 385   RawAccess<>::oop_store(_discovered_addr, oop(NULL));
 386 
 387   // First _prev_next ref actually points into DiscoveredList (gross).
 388   oop new_next;
 389   if (_next == _ref) {
 390     // At the end of the list, we should make _prev point to itself.
 391     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 392     // and _prev will be NULL.
 393     new_next = _prev;
 394   } else {
 395     new_next = _next;
 396   }
 397   // Remove Reference object from discovered list. Note that G1 does not need a
 398   // pre-barrier here because we know the Reference has already been found/marked,
 399   // that's how it ended up in the discovered list in the first place.
 400   RawAccess<>::oop_store(_prev_next, new_next);
 401   NOT_PRODUCT(_removed++);
 402   _refs_list.dec_length(1);
 403 }
 404 
 405 void DiscoveredListIterator::clear_referent() {
 406   RawAccess<>::oop_store(_referent_addr, oop(NULL));
 407 }
 408 
 409 // NOTE: process_phase*() are largely similar, and at a high level
 410 // merely iterate over the extant list applying a predicate to
 411 // each of its elements and possibly removing that element from the
 412 // list and applying some further closures to that element.
 413 // We should consider the possibility of replacing these
 414 // process_phase*() methods by abstracting them into
 415 // a single general iterator invocation that receives appropriate
 416 // closures that accomplish this work.
 417 
 418 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 419 // referents are not alive, but that should be kept alive for policy reasons.
 420 // Keep alive the transitive closure of all such referents.
 421 void
 422 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 423                                    ReferencePolicy*   policy,
 424                                    BoolObjectClosure* is_alive,
 425                                    OopClosure*        keep_alive,
 426                                    VoidClosure*       complete_gc) {
 427   assert(policy != NULL, "Must have a non-NULL policy");
 428   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 429   // Decide which softly reachable refs should be kept alive.
 430   while (iter.has_next()) {
 431     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 432     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 433     if (referent_is_dead &&
 434         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 435       log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 436                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
 437       // Remove Reference object from list
 438       iter.remove();
 439       // keep the referent around
 440       iter.make_referent_alive();
 441       iter.move_to_next();
 442     } else {
 443       iter.next();
 444     }
 445   }
 446   // Close the reachable set
 447   complete_gc->do_void();
 448   log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
 449                              iter.removed(), iter.processed(), p2i(&refs_list));
 450 }
 451 
 452 // Traverse the list and remove any Refs that are not active, or
 453 // whose referents are either alive or NULL.
 454 void
 455 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 456                              BoolObjectClosure* is_alive,
 457                              OopClosure*        keep_alive) {
 458   assert(discovery_is_atomic(), "Error");
 459   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 460   while (iter.has_next()) {
 461     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 462     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 463     assert(next == NULL, "Should not discover inactive Reference");
 464     if (iter.is_referent_alive()) {
 465       log_develop_trace(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 466                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
 467       // The referent is reachable after all.
 468       // Remove Reference object from list.
 469       iter.remove();
 470       // Update the referent pointer as necessary: Note that this
 471       // should not entail any recursive marking because the
 472       // referent must already have been traversed.
 473       iter.make_referent_alive();
 474       iter.move_to_next();
 475     } else {
 476       iter.next();
 477     }
 478   }
 479   NOT_PRODUCT(
 480     if (iter.processed() > 0) {
 481       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 482         " Refs in discovered list " INTPTR_FORMAT,
 483         iter.removed(), iter.processed(), p2i(&refs_list));
 484     }
 485   )
 486 }
 487 
 488 void
 489 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 490                                                   BoolObjectClosure* is_alive,
 491                                                   OopClosure*        keep_alive,
 492                                                   VoidClosure*       complete_gc) {
 493   assert(!discovery_is_atomic(), "Error");
 494   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 495   while (iter.has_next()) {
 496     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 497     HeapWord* next_addr = java_lang_ref_Reference::next_addr_raw(iter.obj());
 498     oop next = java_lang_ref_Reference::next(iter.obj());
 499     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 500          next != NULL)) {
 501       assert(oopDesc::is_oop_or_null(next), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
 502       // Remove Reference object from list
 503       iter.remove();
 504       // Trace the cohorts
 505       iter.make_referent_alive();
 506       if (UseCompressedOops) {
 507         keep_alive->do_oop((narrowOop*)next_addr);
 508       } else {
 509         keep_alive->do_oop((oop*)next_addr);
 510       }
 511       iter.move_to_next();
 512     } else {
 513       iter.next();
 514     }
 515   }
 516   // Now close the newly reachable set
 517   complete_gc->do_void();
 518   NOT_PRODUCT(
 519     if (iter.processed() > 0) {
 520       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 521         " Refs in discovered list " INTPTR_FORMAT,
 522         iter.removed(), iter.processed(), p2i(&refs_list));
 523     }
 524   )
 525 }
 526 
 527 // Traverse the list and process the referents, by either
 528 // clearing them or keeping them (and their reachable
 529 // closure) alive.
 530 void
 531 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 532                                    bool               clear_referent,
 533                                    BoolObjectClosure* is_alive,
 534                                    OopClosure*        keep_alive,
 535                                    VoidClosure*       complete_gc) {
 536   ResourceMark rm;
 537   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 538   while (iter.has_next()) {
 539     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 540     if (clear_referent) {
 541       // NULL out referent pointer
 542       iter.clear_referent();
 543     } else {
 544       // keep the referent around
 545       iter.make_referent_alive();
 546     }
 547     log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 548                                clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name());
 549     assert(oopDesc::is_oop(iter.obj(), UseConcMarkSweepGC), "Adding a bad reference");
 550     iter.next();
 551   }
 552   // Close the reachable set
 553   complete_gc->do_void();
 554 }
 555 
 556 void
 557 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 558   oop obj = NULL;
 559   oop next = refs_list.head();
 560   while (next != obj) {
 561     obj = next;
 562     next = java_lang_ref_Reference::discovered(obj);
 563     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 564   }
 565   refs_list.set_head(NULL);
 566   refs_list.set_length(0);
 567 }
 568 
 569 void ReferenceProcessor::abandon_partial_discovery() {
 570   // loop over the lists
 571   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 572     if ((i % _max_num_q) == 0) {
 573       log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
 574     }
 575     clear_discovered_references(_discovered_refs[i]);
 576   }
 577 }
 578 
 579 size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
 580   DiscoveredList* list = NULL;
 581 
 582   switch (type) {
 583     case REF_SOFT:
 584       list = _discoveredSoftRefs;
 585       break;
 586     case REF_WEAK:
 587       list = _discoveredWeakRefs;
 588       break;
 589     case REF_FINAL:
 590       list = _discoveredFinalRefs;
 591       break;
 592     case REF_PHANTOM:
 593       list = _discoveredPhantomRefs;
 594       break;
 595     case REF_OTHER:
 596     case REF_NONE:
 597     default:
 598       ShouldNotReachHere();
 599   }
 600   return total_count(list);
 601 }
 602 
 603 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 604 public:
 605   RefProcPhase1Task(ReferenceProcessor&           ref_processor,
 606                     DiscoveredList                refs_lists[],
 607                     ReferencePolicy*              policy,
 608                     bool                          marks_oops_alive,
 609                     ReferenceProcessorPhaseTimes* phase_times)
 610     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
 611       _policy(policy)
 612   { }
 613   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 614                     OopClosure& keep_alive,
 615                     VoidClosure& complete_gc)
 616   {
 617     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, _phase_times, i);
 618 
 619     _ref_processor.process_phase1(_refs_lists[i], _policy,
 620                                   &is_alive, &keep_alive, &complete_gc);
 621   }
 622 private:
 623   ReferencePolicy* _policy;
 624 };
 625 
 626 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 627 public:
 628   RefProcPhase2Task(ReferenceProcessor&           ref_processor,
 629                     DiscoveredList                refs_lists[],
 630                     bool                          marks_oops_alive,
 631                     ReferenceProcessorPhaseTimes* phase_times)
 632     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times)
 633   { }
 634   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 635                     OopClosure& keep_alive,
 636                     VoidClosure& complete_gc)
 637   {
 638     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, _phase_times, i);
 639 
 640     _ref_processor.process_phase2(_refs_lists[i],
 641                                   &is_alive, &keep_alive, &complete_gc);
 642   }
 643 };
 644 
 645 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 646 public:
 647   RefProcPhase3Task(ReferenceProcessor&           ref_processor,
 648                     DiscoveredList                refs_lists[],
 649                     bool                         clear_referent,
 650                     bool                          marks_oops_alive,
 651                     ReferenceProcessorPhaseTimes* phase_times)
 652     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
 653       _clear_referent(clear_referent)
 654   { }
 655   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 656                     OopClosure& keep_alive,
 657                     VoidClosure& complete_gc)
 658   {
 659     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, _phase_times, i);
 660 
 661     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 662                                   &is_alive, &keep_alive, &complete_gc);
 663   }
 664 private:
 665   bool _clear_referent;
 666 };
 667 
 668 #ifndef PRODUCT
 669 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint active_length, size_t total_refs) {
 670   if (!log_is_enabled(Trace, gc, ref)) {
 671     return;
 672   }
 673 
 674   stringStream st;
 675   for (uint i = 0; i < active_length; ++i) {
 676     st.print(SIZE_FORMAT " ", ref_lists[i].length());
 677   }
 678   log_develop_trace(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs);
 679 #ifdef ASSERT
 680   for (uint i = active_length; i < _max_num_q; i++) {
 681     assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
 682            ref_lists[i].length(), i);
 683   }
 684 #endif
 685 }
 686 #endif
 687 
 688 void ReferenceProcessor::set_active_mt_degree(uint v) {
 689   _num_q = v;
 690   _next_id = 0;
 691 }
 692 
 693 // Balances reference queues.
 694 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 695 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 696 // corresponding to the active workers will be processed.
 697 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 698 {
 699   // calculate total length
 700   size_t total_refs = 0;
 701   log_develop_trace(gc, ref)("Balance ref_lists ");
 702 
 703   for (uint i = 0; i < _max_num_q; ++i) {
 704     total_refs += ref_lists[i].length();
 705   }
 706   log_reflist_counts(ref_lists, _max_num_q, total_refs);
 707   size_t avg_refs = total_refs / _num_q + 1;
 708   uint to_idx = 0;
 709   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 710     bool move_all = false;
 711     if (from_idx >= _num_q) {
 712       move_all = ref_lists[from_idx].length() > 0;
 713     }
 714     while ((ref_lists[from_idx].length() > avg_refs) ||
 715            move_all) {
 716       assert(to_idx < _num_q, "Sanity Check!");
 717       if (ref_lists[to_idx].length() < avg_refs) {
 718         // move superfluous refs
 719         size_t refs_to_move;
 720         // Move all the Ref's if the from queue will not be processed.
 721         if (move_all) {
 722           refs_to_move = MIN2(ref_lists[from_idx].length(),
 723                               avg_refs - ref_lists[to_idx].length());
 724         } else {
 725           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 726                               avg_refs - ref_lists[to_idx].length());
 727         }
 728 
 729         assert(refs_to_move > 0, "otherwise the code below will fail");
 730 
 731         oop move_head = ref_lists[from_idx].head();
 732         oop move_tail = move_head;
 733         oop new_head  = move_head;
 734         // find an element to split the list on
 735         for (size_t j = 0; j < refs_to_move; ++j) {
 736           move_tail = new_head;
 737           new_head = java_lang_ref_Reference::discovered(new_head);
 738         }
 739 
 740         // Add the chain to the to list.
 741         if (ref_lists[to_idx].head() == NULL) {
 742           // to list is empty. Make a loop at the end.
 743           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 744         } else {
 745           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 746         }
 747         ref_lists[to_idx].set_head(move_head);
 748         ref_lists[to_idx].inc_length(refs_to_move);
 749 
 750         // Remove the chain from the from list.
 751         if (move_tail == new_head) {
 752           // We found the end of the from list.
 753           ref_lists[from_idx].set_head(NULL);
 754         } else {
 755           ref_lists[from_idx].set_head(new_head);
 756         }
 757         ref_lists[from_idx].dec_length(refs_to_move);
 758         if (ref_lists[from_idx].length() == 0) {
 759           break;
 760         }
 761       } else {
 762         to_idx = (to_idx + 1) % _num_q;
 763       }
 764     }
 765   }
 766 #ifdef ASSERT
 767   size_t balanced_total_refs = 0;
 768   for (uint i = 0; i < _num_q; ++i) {
 769     balanced_total_refs += ref_lists[i].length();
 770   }
 771   log_reflist_counts(ref_lists, _num_q, balanced_total_refs);
 772   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 773 #endif
 774 }
 775 
 776 void ReferenceProcessor::balance_all_queues() {
 777   balance_queues(_discoveredSoftRefs);
 778   balance_queues(_discoveredWeakRefs);
 779   balance_queues(_discoveredFinalRefs);
 780   balance_queues(_discoveredPhantomRefs);
 781 }
 782 
 783 void ReferenceProcessor::process_discovered_reflist(
 784   DiscoveredList                refs_lists[],
 785   ReferencePolicy*              policy,
 786   bool                          clear_referent,
 787   BoolObjectClosure*            is_alive,
 788   OopClosure*                   keep_alive,
 789   VoidClosure*                  complete_gc,
 790   AbstractRefProcTaskExecutor*  task_executor,
 791   ReferenceProcessorPhaseTimes* phase_times)
 792 {
 793   bool mt_processing = task_executor != NULL && _processing_is_mt;
 794 
 795   phase_times->set_processing_is_mt(mt_processing);
 796 
 797   if (mt_processing && ParallelRefProcBalancingEnabled) {









 798     RefProcBalanceQueuesTimeTracker tt(phase_times);
 799     balance_queues(refs_lists);
 800   }
 801 
 802   // Phase 1 (soft refs only):
 803   // . Traverse the list and remove any SoftReferences whose
 804   //   referents are not alive, but that should be kept alive for
 805   //   policy reasons. Keep alive the transitive closure of all
 806   //   such referents.
 807   if (policy != NULL) {
 808     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, phase_times);
 809 
 810     if (mt_processing) {
 811       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/, phase_times);
 812       task_executor->execute(phase1);
 813     } else {
 814       for (uint i = 0; i < _max_num_q; i++) {
 815         process_phase1(refs_lists[i], policy,
 816                        is_alive, keep_alive, complete_gc);
 817       }
 818     }
 819   } else { // policy == NULL
 820     assert(refs_lists != _discoveredSoftRefs,
 821            "Policy must be specified for soft references.");
 822   }
 823 
 824   // Phase 2:
 825   // . Traverse the list and remove any refs whose referents are alive.
 826   {
 827     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, phase_times);
 828 
 829     if (mt_processing) {
 830       RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/, phase_times);
 831       task_executor->execute(phase2);
 832     } else {
 833       for (uint i = 0; i < _max_num_q; i++) {
 834         process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 835       }
 836     }
 837   }
 838 
 839   // Phase 3:
 840   // . Traverse the list and process referents as appropriate.
 841   {
 842     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, phase_times);
 843 
 844     if (mt_processing) {
 845       RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/, phase_times);
 846       task_executor->execute(phase3);
 847     } else {
 848       for (uint i = 0; i < _max_num_q; i++) {
 849         process_phase3(refs_lists[i], clear_referent,
 850                        is_alive, keep_alive, complete_gc);
 851       }
 852     }
 853   }
 854 }
 855 
 856 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
 857   uint id = 0;
 858   // Determine the queue index to use for this object.
 859   if (_discovery_is_mt) {
 860     // During a multi-threaded discovery phase,
 861     // each thread saves to its "own" list.
 862     Thread* thr = Thread::current();
 863     id = thr->as_Worker_thread()->id();
 864   } else {
 865     // single-threaded discovery, we save in round-robin
 866     // fashion to each of the lists.
 867     if (_processing_is_mt) {
 868       id = next_id();
 869     }
 870   }
 871   assert(id < _max_num_q, "Id is out-of-bounds id %u and max id %u)", id, _max_num_q);
 872 
 873   // Get the discovered queue to which we will add
 874   DiscoveredList* list = NULL;
 875   switch (rt) {
 876     case REF_OTHER:
 877       // Unknown reference type, no special treatment
 878       break;
 879     case REF_SOFT:
 880       list = &_discoveredSoftRefs[id];
 881       break;
 882     case REF_WEAK:
 883       list = &_discoveredWeakRefs[id];
 884       break;
 885     case REF_FINAL:
 886       list = &_discoveredFinalRefs[id];
 887       break;
 888     case REF_PHANTOM:
 889       list = &_discoveredPhantomRefs[id];
 890       break;
 891     case REF_NONE:
 892       // we should not reach here if we are an InstanceRefKlass
 893     default:
 894       ShouldNotReachHere();
 895   }
 896   log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
 897   return list;
 898 }
 899 
 900 inline void
 901 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
 902                                               oop             obj,
 903                                               HeapWord*       discovered_addr) {
 904   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
 905   // First we must make sure this object is only enqueued once. CAS in a non null
 906   // discovered_addr.
 907   oop current_head = refs_list.head();
 908   // The last ref must have its discovered field pointing to itself.
 909   oop next_discovered = (current_head != NULL) ? current_head : obj;
 910 
 911   oop retest = RawAccess<>::oop_atomic_cmpxchg(next_discovered, discovered_addr, oop(NULL));
 912 
 913   if (retest == NULL) {
 914     // This thread just won the right to enqueue the object.
 915     // We have separate lists for enqueueing, so no synchronization
 916     // is necessary.
 917     refs_list.set_head(obj);
 918     refs_list.inc_length(1);
 919 
 920     log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
 921                                p2i(obj), obj->klass()->internal_name());
 922   } else {
 923     // If retest was non NULL, another thread beat us to it:
 924     // The reference has already been discovered...
 925     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
 926                                p2i(obj), obj->klass()->internal_name());
 927   }
 928 }
 929 
 930 #ifndef PRODUCT
 931 // Non-atomic (i.e. concurrent) discovery might allow us
 932 // to observe j.l.References with NULL referents, being those
 933 // cleared concurrently by mutators during (or after) discovery.
 934 void ReferenceProcessor::verify_referent(oop obj) {
 935   bool da = discovery_is_atomic();
 936   oop referent = java_lang_ref_Reference::referent(obj);
 937   assert(da ? oopDesc::is_oop(referent) : oopDesc::is_oop_or_null(referent),
 938          "Bad referent " INTPTR_FORMAT " found in Reference "
 939          INTPTR_FORMAT " during %satomic discovery ",
 940          p2i(referent), p2i(obj), da ? "" : "non-");
 941 }
 942 #endif
 943 
 944 // We mention two of several possible choices here:
 945 // #0: if the reference object is not in the "originating generation"
 946 //     (or part of the heap being collected, indicated by our "span"
 947 //     we don't treat it specially (i.e. we scan it as we would
 948 //     a normal oop, treating its references as strong references).
 949 //     This means that references can't be discovered unless their
 950 //     referent is also in the same span. This is the simplest,
 951 //     most "local" and most conservative approach, albeit one
 952 //     that may cause weak references to be enqueued least promptly.
 953 //     We call this choice the "ReferenceBasedDiscovery" policy.
 954 // #1: the reference object may be in any generation (span), but if
 955 //     the referent is in the generation (span) being currently collected
 956 //     then we can discover the reference object, provided
 957 //     the object has not already been discovered by
 958 //     a different concurrently running collector (as may be the
 959 //     case, for instance, if the reference object is in CMS and
 960 //     the referent in DefNewGeneration), and provided the processing
 961 //     of this reference object by the current collector will
 962 //     appear atomic to every other collector in the system.
 963 //     (Thus, for instance, a concurrent collector may not
 964 //     discover references in other generations even if the
 965 //     referent is in its own generation). This policy may,
 966 //     in certain cases, enqueue references somewhat sooner than
 967 //     might Policy #0 above, but at marginally increased cost
 968 //     and complexity in processing these references.
 969 //     We call this choice the "RefeferentBasedDiscovery" policy.
 970 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
 971   // Make sure we are discovering refs (rather than processing discovered refs).
 972   if (!_discovering_refs || !RegisterReferences) {
 973     return false;
 974   }
 975   // We only discover active references.
 976   oop next = java_lang_ref_Reference::next(obj);
 977   if (next != NULL) {   // Ref is no longer active
 978     return false;
 979   }
 980 
 981   HeapWord* obj_addr = (HeapWord*)obj;
 982   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
 983       !_span.contains(obj_addr)) {
 984     // Reference is not in the originating generation;
 985     // don't treat it specially (i.e. we want to scan it as a normal
 986     // object with strong references).
 987     return false;
 988   }
 989 
 990   // We only discover references whose referents are not (yet)
 991   // known to be strongly reachable.
 992   if (is_alive_non_header() != NULL) {
 993     verify_referent(obj);
 994     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
 995       return false;  // referent is reachable
 996     }
 997   }
 998   if (rt == REF_SOFT) {
 999     // For soft refs we can decide now if these are not
1000     // current candidates for clearing, in which case we
1001     // can mark through them now, rather than delaying that
1002     // to the reference-processing phase. Since all current
1003     // time-stamp policies advance the soft-ref clock only
1004     // at a full collection cycle, this is always currently
1005     // accurate.
1006     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1007       return false;
1008     }
1009   }
1010 
1011   ResourceMark rm;      // Needed for tracing.
1012 
1013   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr_raw(obj);
1014   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1015   assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
1016   if (discovered != NULL) {
1017     // The reference has already been discovered...
1018     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
1019                                p2i(obj), obj->klass()->internal_name());
1020     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1021       // assumes that an object is not processed twice;
1022       // if it's been already discovered it must be on another
1023       // generation's discovered list; so we won't discover it.
1024       return false;
1025     } else {
1026       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1027              "Unrecognized policy");
1028       // Check assumption that an object is not potentially
1029       // discovered twice except by concurrent collectors that potentially
1030       // trace the same Reference object twice.
1031       assert(UseConcMarkSweepGC || UseG1GC,
1032              "Only possible with a concurrent marking collector");
1033       return true;
1034     }
1035   }
1036 
1037   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1038     verify_referent(obj);
1039     // Discover if and only if EITHER:
1040     // .. reference is in our span, OR
1041     // .. we are an atomic collector and referent is in our span
1042     if (_span.contains(obj_addr) ||
1043         (discovery_is_atomic() &&
1044          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1045       // should_enqueue = true;
1046     } else {
1047       return false;
1048     }
1049   } else {
1050     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1051            _span.contains(obj_addr), "code inconsistency");
1052   }
1053 
1054   // Get the right type of discovered queue head.
1055   DiscoveredList* list = get_discovered_list(rt);
1056   if (list == NULL) {
1057     return false;   // nothing special needs to be done
1058   }
1059 
1060   if (_discovery_is_mt) {
1061     add_to_discovered_list_mt(*list, obj, discovered_addr);
1062   } else {
1063     // We do a raw store here: the field will be visited later when processing
1064     // the discovered references.
1065     oop current_head = list->head();
1066     // The last ref must have its discovered field pointing to itself.
1067     oop next_discovered = (current_head != NULL) ? current_head : obj;
1068 
1069     assert(discovered == NULL, "control point invariant");
1070     RawAccess<>::oop_store(discovered_addr, next_discovered);
1071     list->set_head(obj);
1072     list->inc_length(1);
1073 
1074     log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
1075   }
1076   assert(oopDesc::is_oop(obj), "Discovered a bad reference");
1077   verify_referent(obj);
1078   return true;
1079 }
1080 
1081 bool ReferenceProcessor::has_discovered_references() {
1082   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1083     if (!_discovered_refs[i].is_empty()) {
1084       return true;
1085     }
1086   }
1087   return false;
1088 }
1089 
1090 // Preclean the discovered references by removing those
1091 // whose referents are alive, and by marking from those that
1092 // are not active. These lists can be handled here
1093 // in any order and, indeed, concurrently.
1094 void ReferenceProcessor::preclean_discovered_references(
1095   BoolObjectClosure* is_alive,
1096   OopClosure* keep_alive,
1097   VoidClosure* complete_gc,
1098   YieldClosure* yield,
1099   GCTimer* gc_timer) {
1100 
1101   // Soft references
1102   {
1103     GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
1104     for (uint i = 0; i < _max_num_q; i++) {
1105       if (yield->should_return()) {
1106         return;
1107       }
1108       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1109                                   keep_alive, complete_gc, yield);
1110     }
1111   }
1112 
1113   // Weak references
1114   {
1115     GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
1116     for (uint i = 0; i < _max_num_q; i++) {
1117       if (yield->should_return()) {
1118         return;
1119       }
1120       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1121                                   keep_alive, complete_gc, yield);
1122     }
1123   }
1124 
1125   // Final references
1126   {
1127     GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
1128     for (uint i = 0; i < _max_num_q; i++) {
1129       if (yield->should_return()) {
1130         return;
1131       }
1132       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1133                                   keep_alive, complete_gc, yield);
1134     }
1135   }
1136 
1137   // Phantom references
1138   {
1139     GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
1140     for (uint i = 0; i < _max_num_q; i++) {
1141       if (yield->should_return()) {
1142         return;
1143       }
1144       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1145                                   keep_alive, complete_gc, yield);
1146     }
1147   }
1148 }
1149 
1150 // Walk the given discovered ref list, and remove all reference objects
1151 // whose referents are still alive, whose referents are NULL or which
1152 // are not active (have a non-NULL next field). NOTE: When we are
1153 // thus precleaning the ref lists (which happens single-threaded today),
1154 // we do not disable refs discovery to honor the correct semantics of
1155 // java.lang.Reference. As a result, we need to be careful below
1156 // that ref removal steps interleave safely with ref discovery steps
1157 // (in this thread).
1158 void
1159 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1160                                                 BoolObjectClosure* is_alive,
1161                                                 OopClosure*        keep_alive,
1162                                                 VoidClosure*       complete_gc,
1163                                                 YieldClosure*      yield) {
1164   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1165   while (iter.has_next()) {
1166     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1167     oop obj = iter.obj();
1168     oop next = java_lang_ref_Reference::next(obj);
1169     if (iter.referent() == NULL || iter.is_referent_alive() ||
1170         next != NULL) {
1171       // The referent has been cleared, or is alive, or the Reference is not
1172       // active; we need to trace and mark its cohort.
1173       log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1174                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
1175       // Remove Reference object from list
1176       iter.remove();
1177       // Keep alive its cohort.
1178       iter.make_referent_alive();
1179       if (UseCompressedOops) {
1180         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr_raw(obj);
1181         keep_alive->do_oop(next_addr);
1182       } else {
1183         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr_raw(obj);
1184         keep_alive->do_oop(next_addr);
1185       }
1186       iter.move_to_next();
1187     } else {
1188       iter.next();
1189     }
1190   }
1191   // Close the reachable set
1192   complete_gc->do_void();
1193 
1194   NOT_PRODUCT(
1195     if (iter.processed() > 0) {
1196       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
1197         iter.removed(), iter.processed(), p2i(&refs_list));
1198     }
1199   )
1200 }
1201 
1202 const char* ReferenceProcessor::list_name(uint i) {
1203    assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1204           "Out of bounds index");
1205 
1206    int j = i / _max_num_q;
1207    switch (j) {
1208      case 0: return "SoftRef";
1209      case 1: return "WeakRef";
1210      case 2: return "FinalRef";
1211      case 3: return "PhantomRef";
1212    }
1213    ShouldNotReachHere();
1214    return NULL;
1215 }
--- EOF ---