1 /*
   2  * Copyright (c) 2001, 2016, 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.hpp"
  36 #include "memory/resourceArea.hpp"
  37 #include "oops/oop.inline.hpp"
  38 #include "runtime/java.hpp"
  39 #include "runtime/jniHandles.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 defined(COMPILER2) || INCLUDE_JVMCI
  61   _default_soft_ref_policy      = new LRUMaxHeapPolicy();
  62 #else
  63   _default_soft_ref_policy      = new LRUCurrentHeapPolicy();
  64 #endif
  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[]) {
 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   GCTimer*                     gc_timer) {
 196 
 197   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 198   // Stop treating discovered references specially.
 199   disable_discovery();
 200 
 201   // If discovery was concurrent, someone could have modified
 202   // the value of the static field in the j.l.r.SoftReference
 203   // class that holds the soft reference timestamp clock using
 204   // reflection or Unsafe between when discovery was enabled and
 205   // now. Unconditionally update the static field in ReferenceProcessor
 206   // here so that we use the new value during processing of the
 207   // discovered soft refs.
 208 
 209   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 210 
 211   ReferenceProcessorStats stats(
 212       total_count(_discoveredSoftRefs),
 213       total_count(_discoveredWeakRefs),
 214       total_count(_discoveredFinalRefs),
 215       total_count(_discoveredPhantomRefs));
 216 
 217   // Soft references
 218   {
 219     GCTraceTime(Debug, gc, ref) tt("SoftReference", gc_timer);
 220     process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 221                                is_alive, keep_alive, complete_gc, task_executor);
 222   }
 223 
 224   update_soft_ref_master_clock();
 225 
 226   // Weak references
 227   {
 228     GCTraceTime(Debug, gc, ref) tt("WeakReference", gc_timer);
 229     process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 230                                is_alive, keep_alive, complete_gc, task_executor);
 231   }
 232 
 233   // Final references
 234   {
 235     GCTraceTime(Debug, gc, ref) tt("FinalReference", gc_timer);
 236     process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 237                                is_alive, keep_alive, complete_gc, task_executor);
 238   }
 239 
 240   // Phantom references
 241   {
 242     GCTraceTime(Debug, gc, ref) tt("PhantomReference", gc_timer);
 243     process_discovered_reflist(_discoveredPhantomRefs, NULL, true,
 244                                is_alive, keep_alive, complete_gc, task_executor);
 245   }
 246 
 247   // Weak global JNI references. It would make more sense (semantically) to
 248   // traverse these simultaneously with the regular weak references above, but
 249   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 250   // thus use JNI weak references to circumvent the phantom references and
 251   // resurrect a "post-mortem" object.
 252   {
 253     GCTraceTime(Debug, gc, ref) tt("JNI Weak Reference", gc_timer);
 254     if (task_executor != NULL) {
 255       task_executor->set_single_threaded_mode();
 256     }
 257     process_phaseJNI(is_alive, keep_alive, complete_gc);
 258   }
 259 
 260   log_debug(gc, ref)("Ref Counts: Soft: " SIZE_FORMAT " Weak: " SIZE_FORMAT " Final: " SIZE_FORMAT " Phantom: " SIZE_FORMAT,
 261                      stats.soft_count(), stats.weak_count(), stats.final_count(), stats.phantom_count());
 262   log_develop_trace(gc, ref)("JNI Weak Reference count: " SIZE_FORMAT, count_jni_refs());
 263 
 264   return stats;
 265 }
 266 
 267 #ifndef PRODUCT
 268 // Calculate the number of jni handles.
 269 size_t ReferenceProcessor::count_jni_refs() {
 270   class CountHandleClosure: public OopClosure {
 271   private:
 272     size_t _count;
 273   public:
 274     CountHandleClosure(): _count(0) {}
 275     void do_oop(oop* unused)       { _count++; }
 276     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 277     size_t count() { return _count; }
 278   };
 279   CountHandleClosure global_handle_count;
 280   JNIHandles::weak_oops_do(&global_handle_count);
 281   return global_handle_count.count();
 282 }
 283 #endif
 284 
 285 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 286                                           OopClosure*        keep_alive,
 287                                           VoidClosure*       complete_gc) {
 288   JNIHandles::weak_oops_do(is_alive, keep_alive);
 289   complete_gc->do_void();
 290 }
 291 
 292 
 293 template <class T>
 294 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 295                                    AbstractRefProcTaskExecutor* task_executor) {
 296 
 297   // Remember old value of pending references list
 298   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 299   T old_pending_list_value = *pending_list_addr;
 300 
 301   // Enqueue references that are not made active again, and
 302   // clear the decks for the next collection (cycle).
 303   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 304   // Do the post-barrier on pending_list_addr missed in
 305   // enqueue_discovered_reflist.
 306   oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 307 
 308   // Stop treating discovered references specially.
 309   ref->disable_discovery();
 310 
 311   // Return true if new pending references were added
 312   return old_pending_list_value != *pending_list_addr;
 313 }
 314 
 315 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 316   if (UseCompressedOops) {
 317     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 318   } else {
 319     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 320   }
 321 }
 322 
 323 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 324                                                     HeapWord* pending_list_addr) {
 325   // Given a list of refs linked through the "discovered" field
 326   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 327   // thus distinguishing them from active References, then
 328   // prepend them to the pending list.
 329   //
 330   // The Java threads will see the Reference objects linked together through
 331   // the discovered field. Instead of trying to do the write barrier updates
 332   // in all places in the reference processor where we manipulate the discovered
 333   // field we make sure to do the barrier here where we anyway iterate through
 334   // all linked Reference objects. Note that it is important to not dirty any
 335   // cards during reference processing since this will cause card table
 336   // verification to fail for G1.
 337   log_develop_trace(gc, ref)("ReferenceProcessor::enqueue_discovered_reflist list " INTPTR_FORMAT, p2i(&refs_list));
 338 
 339   oop obj = NULL;
 340   oop next_d = refs_list.head();
 341   // Walk down the list, self-looping the next field
 342   // so that the References are not considered active.
 343   while (obj != next_d) {
 344     obj = next_d;
 345     assert(obj->is_instance(), "should be an instance object");
 346     assert(InstanceKlass::cast(obj->klass())->is_reference_instance_klass(), "should be reference object");
 347     next_d = java_lang_ref_Reference::discovered(obj);
 348     log_develop_trace(gc, ref)("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, p2i(obj), p2i(next_d));
 349     assert(java_lang_ref_Reference::next(obj) == NULL,
 350            "Reference not active; should not be discovered");
 351     // Self-loop next, so as to make Ref not active.
 352     java_lang_ref_Reference::set_next_raw(obj, obj);
 353     if (next_d != obj) {
 354       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
 355     } else {
 356       // This is the last object.
 357       // Swap refs_list into pending_list_addr and
 358       // set obj's discovered to what we read from pending_list_addr.
 359       oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 360       // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.
 361       java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 362       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 363     }
 364   }
 365 }
 366 
 367 // Parallel enqueue task
 368 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 369 public:
 370   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 371                      DiscoveredList      discovered_refs[],
 372                      HeapWord*           pending_list_addr,
 373                      int                 n_queues)
 374     : EnqueueTask(ref_processor, discovered_refs,
 375                   pending_list_addr, n_queues)
 376   { }
 377 
 378   virtual void work(unsigned int work_id) {
 379     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 380     // Simplest first cut: static partitioning.
 381     int index = work_id;
 382     // The increment on "index" must correspond to the maximum number of queues
 383     // (n_queues) with which that ReferenceProcessor was created.  That
 384     // is because of the "clever" way the discovered references lists were
 385     // allocated and are indexed into.
 386     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 387     for (int j = 0;
 388          j < ReferenceProcessor::number_of_subclasses_of_ref();
 389          j++, index += _n_queues) {
 390       _ref_processor.enqueue_discovered_reflist(
 391         _refs_lists[index], _pending_list_addr);
 392       _refs_lists[index].set_head(NULL);
 393       _refs_lists[index].set_length(0);
 394     }
 395   }
 396 };
 397 
 398 // Enqueue references that are not made active again
 399 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 400   AbstractRefProcTaskExecutor* task_executor) {
 401   if (_processing_is_mt && task_executor != NULL) {
 402     // Parallel code
 403     RefProcEnqueueTask tsk(*this, _discovered_refs,
 404                            pending_list_addr, _max_num_q);
 405     task_executor->execute(tsk);
 406   } else {
 407     // Serial code: call the parent class's implementation
 408     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 409       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
 410       _discovered_refs[i].set_head(NULL);
 411       _discovered_refs[i].set_length(0);
 412     }
 413   }
 414 }
 415 
 416 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 417   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 418   oop discovered = java_lang_ref_Reference::discovered(_ref);
 419   assert(_discovered_addr && discovered->is_oop_or_null(),
 420          "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
 421   _next = discovered;
 422   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 423   _referent = java_lang_ref_Reference::referent(_ref);
 424   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 425          "Wrong oop found in java.lang.Reference object");
 426   assert(allow_null_referent ?
 427              _referent->is_oop_or_null()
 428            : _referent->is_oop(),
 429          "Expected an oop%s for referent field at " PTR_FORMAT,
 430          (allow_null_referent ? " or NULL" : ""),
 431          p2i(_referent));
 432 }
 433 
 434 void DiscoveredListIterator::remove() {
 435   assert(_ref->is_oop(), "Dropping a bad reference");
 436   oop_store_raw(_discovered_addr, NULL);
 437 
 438   // First _prev_next ref actually points into DiscoveredList (gross).
 439   oop new_next;
 440   if (_next == _ref) {
 441     // At the end of the list, we should make _prev point to itself.
 442     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 443     // and _prev will be NULL.
 444     new_next = _prev;
 445   } else {
 446     new_next = _next;
 447   }
 448   // Remove Reference object from discovered list. Note that G1 does not need a
 449   // pre-barrier here because we know the Reference has already been found/marked,
 450   // that's how it ended up in the discovered list in the first place.
 451   oop_store_raw(_prev_next, new_next);
 452   NOT_PRODUCT(_removed++);
 453   _refs_list.dec_length(1);
 454 }
 455 
 456 void DiscoveredListIterator::clear_referent() {
 457   oop_store_raw(_referent_addr, NULL);
 458 }
 459 
 460 // NOTE: process_phase*() are largely similar, and at a high level
 461 // merely iterate over the extant list applying a predicate to
 462 // each of its elements and possibly removing that element from the
 463 // list and applying some further closures to that element.
 464 // We should consider the possibility of replacing these
 465 // process_phase*() methods by abstracting them into
 466 // a single general iterator invocation that receives appropriate
 467 // closures that accomplish this work.
 468 
 469 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 470 // referents are not alive, but that should be kept alive for policy reasons.
 471 // Keep alive the transitive closure of all such referents.
 472 void
 473 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 474                                    ReferencePolicy*   policy,
 475                                    BoolObjectClosure* is_alive,
 476                                    OopClosure*        keep_alive,
 477                                    VoidClosure*       complete_gc) {
 478   assert(policy != NULL, "Must have a non-NULL policy");
 479   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 480   // Decide which softly reachable refs should be kept alive.
 481   while (iter.has_next()) {
 482     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 483     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 484     if (referent_is_dead &&
 485         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 486       log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 487                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
 488       // Remove Reference object from list
 489       iter.remove();
 490       // keep the referent around
 491       iter.make_referent_alive();
 492       iter.move_to_next();
 493     } else {
 494       iter.next();
 495     }
 496   }
 497   // Close the reachable set
 498   complete_gc->do_void();
 499   log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
 500                              iter.removed(), iter.processed(), p2i(&refs_list));
 501     }
 502 
 503 // Traverse the list and remove any Refs that are not active, or
 504 // whose referents are either alive or NULL.
 505 void
 506 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 507                              BoolObjectClosure* is_alive,
 508                              OopClosure*        keep_alive) {
 509   assert(discovery_is_atomic(), "Error");
 510   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 511   while (iter.has_next()) {
 512     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 513     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 514     assert(next == NULL, "Should not discover inactive Reference");
 515     if (iter.is_referent_alive()) {
 516       log_develop_trace(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 517                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
 518       // The referent is reachable after all.
 519       // Remove Reference object from list.
 520       iter.remove();
 521       // Update the referent pointer as necessary: Note that this
 522       // should not entail any recursive marking because the
 523       // referent must already have been traversed.
 524       iter.make_referent_alive();
 525       iter.move_to_next();
 526     } else {
 527       iter.next();
 528     }
 529   }
 530   NOT_PRODUCT(
 531     if (iter.processed() > 0) {
 532       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 533         " Refs in discovered list " INTPTR_FORMAT,
 534         iter.removed(), iter.processed(), p2i(&refs_list));
 535     }
 536   )
 537 }
 538 
 539 void
 540 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 541                                                   BoolObjectClosure* is_alive,
 542                                                   OopClosure*        keep_alive,
 543                                                   VoidClosure*       complete_gc) {
 544   assert(!discovery_is_atomic(), "Error");
 545   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 546   while (iter.has_next()) {
 547     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 548     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 549     oop next = java_lang_ref_Reference::next(iter.obj());
 550     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 551          next != NULL)) {
 552       assert(next->is_oop_or_null(), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
 553       // Remove Reference object from list
 554       iter.remove();
 555       // Trace the cohorts
 556       iter.make_referent_alive();
 557       if (UseCompressedOops) {
 558         keep_alive->do_oop((narrowOop*)next_addr);
 559       } else {
 560         keep_alive->do_oop((oop*)next_addr);
 561       }
 562       iter.move_to_next();
 563     } else {
 564       iter.next();
 565     }
 566   }
 567   // Now close the newly reachable set
 568   complete_gc->do_void();
 569   NOT_PRODUCT(
 570     if (iter.processed() > 0) {
 571       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 572         " Refs in discovered list " INTPTR_FORMAT,
 573         iter.removed(), iter.processed(), p2i(&refs_list));
 574     }
 575   )
 576 }
 577 
 578 // Traverse the list and process the referents, by either
 579 // clearing them or keeping them (and their reachable
 580 // closure) alive.
 581 void
 582 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 583                                    bool               clear_referent,
 584                                    BoolObjectClosure* is_alive,
 585                                    OopClosure*        keep_alive,
 586                                    VoidClosure*       complete_gc) {
 587   ResourceMark rm;
 588   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 589   while (iter.has_next()) {
 590     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 591     if (clear_referent) {
 592       // NULL out referent pointer
 593       iter.clear_referent();
 594     } else {
 595       // keep the referent around
 596       iter.make_referent_alive();
 597     }
 598     log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 599                                clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name());
 600     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 601     iter.next();
 602   }
 603   // Close the reachable set
 604   complete_gc->do_void();
 605 }
 606 
 607 void
 608 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 609   oop obj = NULL;
 610   oop next = refs_list.head();
 611   while (next != obj) {
 612     obj = next;
 613     next = java_lang_ref_Reference::discovered(obj);
 614     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 615   }
 616   refs_list.set_head(NULL);
 617   refs_list.set_length(0);
 618 }
 619 
 620 void ReferenceProcessor::abandon_partial_discovery() {
 621   // loop over the lists
 622   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 623     if ((i % _max_num_q) == 0) {
 624       log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
 625     }
 626     clear_discovered_references(_discovered_refs[i]);
 627   }
 628 }
 629 
 630 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 631 public:
 632   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 633                     DiscoveredList      refs_lists[],
 634                     ReferencePolicy*    policy,
 635                     bool                marks_oops_alive)
 636     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 637       _policy(policy)
 638   { }
 639   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 640                     OopClosure& keep_alive,
 641                     VoidClosure& complete_gc)
 642   {
 643     _ref_processor.process_phase1(_refs_lists[i], _policy,
 644                                   &is_alive, &keep_alive, &complete_gc);
 645   }
 646 private:
 647   ReferencePolicy* _policy;
 648 };
 649 
 650 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 651 public:
 652   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 653                     DiscoveredList      refs_lists[],
 654                     bool                marks_oops_alive)
 655     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 656   { }
 657   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 658                     OopClosure& keep_alive,
 659                     VoidClosure& complete_gc)
 660   {
 661     _ref_processor.process_phase2(_refs_lists[i],
 662                                   &is_alive, &keep_alive, &complete_gc);
 663   }
 664 };
 665 
 666 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 667 public:
 668   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 669                     DiscoveredList      refs_lists[],
 670                     bool                clear_referent,
 671                     bool                marks_oops_alive)
 672     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 673       _clear_referent(clear_referent)
 674   { }
 675   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 676                     OopClosure& keep_alive,
 677                     VoidClosure& complete_gc)
 678   {
 679     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 680                                   &is_alive, &keep_alive, &complete_gc);
 681   }
 682 private:
 683   bool _clear_referent;
 684 };
 685 
 686 #ifndef PRODUCT
 687 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint active_length, size_t total_refs) {
 688   if (!log_is_enabled(Trace, gc, ref)) {
 689     return;
 690   }
 691 
 692   stringStream st;
 693   for (uint i = 0; i < active_length; ++i) {
 694     st.print(SIZE_FORMAT " ", ref_lists[i].length());
 695   }
 696   log_develop_trace(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs);
 697 #ifdef ASSERT
 698   for (uint i = active_length; i < _max_num_q; i++) {
 699     assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
 700            ref_lists[i].length(), i);
 701   }
 702 #endif
 703 }
 704 #endif
 705 
 706 void ReferenceProcessor::set_active_mt_degree(uint v) {
 707   _num_q = v;
 708   _next_id = 0;
 709 }
 710 
 711 // Balances reference queues.
 712 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 713 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 714 // corresponding to the active workers will be processed.
 715 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 716 {
 717   // calculate total length
 718   size_t total_refs = 0;
 719   log_develop_trace(gc, ref)("Balance ref_lists ");
 720 
 721   for (uint i = 0; i < _max_num_q; ++i) {
 722     total_refs += ref_lists[i].length();
 723   }
 724   log_reflist_counts(ref_lists, _max_num_q, total_refs);
 725   size_t avg_refs = total_refs / _num_q + 1;
 726   uint to_idx = 0;
 727   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 728     bool move_all = false;
 729     if (from_idx >= _num_q) {
 730       move_all = ref_lists[from_idx].length() > 0;
 731     }
 732     while ((ref_lists[from_idx].length() > avg_refs) ||
 733            move_all) {
 734       assert(to_idx < _num_q, "Sanity Check!");
 735       if (ref_lists[to_idx].length() < avg_refs) {
 736         // move superfluous refs
 737         size_t refs_to_move;
 738         // Move all the Ref's if the from queue will not be processed.
 739         if (move_all) {
 740           refs_to_move = MIN2(ref_lists[from_idx].length(),
 741                               avg_refs - ref_lists[to_idx].length());
 742         } else {
 743           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 744                               avg_refs - ref_lists[to_idx].length());
 745         }
 746 
 747         assert(refs_to_move > 0, "otherwise the code below will fail");
 748 
 749         oop move_head = ref_lists[from_idx].head();
 750         oop move_tail = move_head;
 751         oop new_head  = move_head;
 752         // find an element to split the list on
 753         for (size_t j = 0; j < refs_to_move; ++j) {
 754           move_tail = new_head;
 755           new_head = java_lang_ref_Reference::discovered(new_head);
 756         }
 757 
 758         // Add the chain to the to list.
 759         if (ref_lists[to_idx].head() == NULL) {
 760           // to list is empty. Make a loop at the end.
 761           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 762         } else {
 763           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 764         }
 765         ref_lists[to_idx].set_head(move_head);
 766         ref_lists[to_idx].inc_length(refs_to_move);
 767 
 768         // Remove the chain from the from list.
 769         if (move_tail == new_head) {
 770           // We found the end of the from list.
 771           ref_lists[from_idx].set_head(NULL);
 772         } else {
 773           ref_lists[from_idx].set_head(new_head);
 774         }
 775         ref_lists[from_idx].dec_length(refs_to_move);
 776         if (ref_lists[from_idx].length() == 0) {
 777           break;
 778         }
 779       } else {
 780         to_idx = (to_idx + 1) % _num_q;
 781       }
 782     }
 783   }
 784 #ifdef ASSERT
 785   size_t balanced_total_refs = 0;
 786   for (uint i = 0; i < _num_q; ++i) {
 787     balanced_total_refs += ref_lists[i].length();
 788   }
 789   log_reflist_counts(ref_lists, _num_q, balanced_total_refs);
 790   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 791 #endif
 792 }
 793 
 794 void ReferenceProcessor::balance_all_queues() {
 795   balance_queues(_discoveredSoftRefs);
 796   balance_queues(_discoveredWeakRefs);
 797   balance_queues(_discoveredFinalRefs);
 798   balance_queues(_discoveredPhantomRefs);
 799 }
 800 
 801 void ReferenceProcessor::process_discovered_reflist(
 802   DiscoveredList               refs_lists[],
 803   ReferencePolicy*             policy,
 804   bool                         clear_referent,
 805   BoolObjectClosure*           is_alive,
 806   OopClosure*                  keep_alive,
 807   VoidClosure*                 complete_gc,
 808   AbstractRefProcTaskExecutor* task_executor)
 809 {
 810   bool mt_processing = task_executor != NULL && _processing_is_mt;
 811   // If discovery used MT and a dynamic number of GC threads, then
 812   // the queues must be balanced for correctness if fewer than the
 813   // maximum number of queues were used.  The number of queue used
 814   // during discovery may be different than the number to be used
 815   // for processing so don't depend of _num_q < _max_num_q as part
 816   // of the test.
 817   bool must_balance = _discovery_is_mt;
 818 
 819   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 820       must_balance) {
 821     balance_queues(refs_lists);
 822   }
 823 
 824   // Phase 1 (soft refs only):
 825   // . Traverse the list and remove any SoftReferences whose
 826   //   referents are not alive, but that should be kept alive for
 827   //   policy reasons. Keep alive the transitive closure of all
 828   //   such referents.
 829   if (policy != NULL) {
 830     if (mt_processing) {
 831       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 832       task_executor->execute(phase1);
 833     } else {
 834       for (uint i = 0; i < _max_num_q; i++) {
 835         process_phase1(refs_lists[i], policy,
 836                        is_alive, keep_alive, complete_gc);
 837       }
 838     }
 839   } else { // policy == NULL
 840     assert(refs_lists != _discoveredSoftRefs,
 841            "Policy must be specified for soft references.");
 842   }
 843 
 844   // Phase 2:
 845   // . Traverse the list and remove any refs whose referents are alive.
 846   if (mt_processing) {
 847     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 848     task_executor->execute(phase2);
 849   } else {
 850     for (uint i = 0; i < _max_num_q; i++) {
 851       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 852     }
 853   }
 854 
 855   // Phase 3:
 856   // . Traverse the list and process referents as appropriate.
 857   if (mt_processing) {
 858     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 859     task_executor->execute(phase3);
 860   } else {
 861     for (uint i = 0; i < _max_num_q; i++) {
 862       process_phase3(refs_lists[i], clear_referent,
 863                      is_alive, keep_alive, complete_gc);
 864     }
 865   }
 866 }
 867 
 868 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
 869   uint id = 0;
 870   // Determine the queue index to use for this object.
 871   if (_discovery_is_mt) {
 872     // During a multi-threaded discovery phase,
 873     // each thread saves to its "own" list.
 874     Thread* thr = Thread::current();
 875     id = thr->as_Worker_thread()->id();
 876   } else {
 877     // single-threaded discovery, we save in round-robin
 878     // fashion to each of the lists.
 879     if (_processing_is_mt) {
 880       id = next_id();
 881     }
 882   }
 883   assert(id < _max_num_q, "Id is out-of-bounds id %u and max id %u)", id, _max_num_q);
 884 
 885   // Get the discovered queue to which we will add
 886   DiscoveredList* list = NULL;
 887   switch (rt) {
 888     case REF_OTHER:
 889       // Unknown reference type, no special treatment
 890       break;
 891     case REF_SOFT:
 892       list = &_discoveredSoftRefs[id];
 893       break;
 894     case REF_WEAK:
 895       list = &_discoveredWeakRefs[id];
 896       break;
 897     case REF_FINAL:
 898       list = &_discoveredFinalRefs[id];
 899       break;
 900     case REF_PHANTOM:
 901       list = &_discoveredPhantomRefs[id];
 902       break;
 903     case REF_NONE:
 904       // we should not reach here if we are an InstanceRefKlass
 905     default:
 906       ShouldNotReachHere();
 907   }
 908   log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
 909   return list;
 910 }
 911 
 912 inline void
 913 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
 914                                               oop             obj,
 915                                               HeapWord*       discovered_addr) {
 916   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
 917   // First we must make sure this object is only enqueued once. CAS in a non null
 918   // discovered_addr.
 919   oop current_head = refs_list.head();
 920   // The last ref must have its discovered field pointing to itself.
 921   oop next_discovered = (current_head != NULL) ? current_head : obj;
 922 
 923   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
 924                                                     NULL);
 925   if (retest == NULL) {
 926     // This thread just won the right to enqueue the object.
 927     // We have separate lists for enqueueing, so no synchronization
 928     // is necessary.
 929     refs_list.set_head(obj);
 930     refs_list.inc_length(1);
 931 
 932     log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
 933                                p2i(obj), obj->klass()->internal_name());
 934   } else {
 935     // If retest was non NULL, another thread beat us to it:
 936     // The reference has already been discovered...
 937     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
 938                                p2i(obj), obj->klass()->internal_name());
 939     }
 940   }
 941 
 942 #ifndef PRODUCT
 943 // Non-atomic (i.e. concurrent) discovery might allow us
 944 // to observe j.l.References with NULL referents, being those
 945 // cleared concurrently by mutators during (or after) discovery.
 946 void ReferenceProcessor::verify_referent(oop obj) {
 947   bool da = discovery_is_atomic();
 948   oop referent = java_lang_ref_Reference::referent(obj);
 949   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
 950          "Bad referent " INTPTR_FORMAT " found in Reference "
 951          INTPTR_FORMAT " during %satomic discovery ",
 952          p2i(referent), p2i(obj), da ? "" : "non-");
 953 }
 954 #endif
 955 
 956 // We mention two of several possible choices here:
 957 // #0: if the reference object is not in the "originating generation"
 958 //     (or part of the heap being collected, indicated by our "span"
 959 //     we don't treat it specially (i.e. we scan it as we would
 960 //     a normal oop, treating its references as strong references).
 961 //     This means that references can't be discovered unless their
 962 //     referent is also in the same span. This is the simplest,
 963 //     most "local" and most conservative approach, albeit one
 964 //     that may cause weak references to be enqueued least promptly.
 965 //     We call this choice the "ReferenceBasedDiscovery" policy.
 966 // #1: the reference object may be in any generation (span), but if
 967 //     the referent is in the generation (span) being currently collected
 968 //     then we can discover the reference object, provided
 969 //     the object has not already been discovered by
 970 //     a different concurrently running collector (as may be the
 971 //     case, for instance, if the reference object is in CMS and
 972 //     the referent in DefNewGeneration), and provided the processing
 973 //     of this reference object by the current collector will
 974 //     appear atomic to every other collector in the system.
 975 //     (Thus, for instance, a concurrent collector may not
 976 //     discover references in other generations even if the
 977 //     referent is in its own generation). This policy may,
 978 //     in certain cases, enqueue references somewhat sooner than
 979 //     might Policy #0 above, but at marginally increased cost
 980 //     and complexity in processing these references.
 981 //     We call this choice the "RefeferentBasedDiscovery" policy.
 982 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
 983   // Make sure we are discovering refs (rather than processing discovered refs).
 984   if (!_discovering_refs || !RegisterReferences) {
 985     return false;
 986   }
 987   // We only discover active references.
 988   oop next = java_lang_ref_Reference::next(obj);
 989   if (next != NULL) {   // Ref is no longer active
 990     return false;
 991   }
 992 
 993   HeapWord* obj_addr = (HeapWord*)obj;
 994   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
 995       !_span.contains(obj_addr)) {
 996     // Reference is not in the originating generation;
 997     // don't treat it specially (i.e. we want to scan it as a normal
 998     // object with strong references).
 999     return false;
1000   }
1001 
1002   // We only discover references whose referents are not (yet)
1003   // known to be strongly reachable.
1004   if (is_alive_non_header() != NULL) {
1005     verify_referent(obj);
1006     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1007       return false;  // referent is reachable
1008     }
1009   }
1010   if (rt == REF_SOFT) {
1011     // For soft refs we can decide now if these are not
1012     // current candidates for clearing, in which case we
1013     // can mark through them now, rather than delaying that
1014     // to the reference-processing phase. Since all current
1015     // time-stamp policies advance the soft-ref clock only
1016     // at a full collection cycle, this is always currently
1017     // accurate.
1018     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1019       return false;
1020     }
1021   }
1022 
1023   ResourceMark rm;      // Needed for tracing.
1024 
1025   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1026   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1027   assert(discovered->is_oop_or_null(), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
1028   if (discovered != NULL) {
1029     // The reference has already been discovered...
1030     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
1031                                p2i(obj), obj->klass()->internal_name());
1032     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1033       // assumes that an object is not processed twice;
1034       // if it's been already discovered it must be on another
1035       // generation's discovered list; so we won't discover it.
1036       return false;
1037     } else {
1038       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1039              "Unrecognized policy");
1040       // Check assumption that an object is not potentially
1041       // discovered twice except by concurrent collectors that potentially
1042       // trace the same Reference object twice.
1043       assert(UseConcMarkSweepGC || UseG1GC,
1044              "Only possible with a concurrent marking collector");
1045       return true;
1046     }
1047   }
1048 
1049   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1050     verify_referent(obj);
1051     // Discover if and only if EITHER:
1052     // .. reference is in our span, OR
1053     // .. we are an atomic collector and referent is in our span
1054     if (_span.contains(obj_addr) ||
1055         (discovery_is_atomic() &&
1056          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1057       // should_enqueue = true;
1058     } else {
1059       return false;
1060     }
1061   } else {
1062     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1063            _span.contains(obj_addr), "code inconsistency");
1064   }
1065 
1066   // Get the right type of discovered queue head.
1067   DiscoveredList* list = get_discovered_list(rt);
1068   if (list == NULL) {
1069     return false;   // nothing special needs to be done
1070   }
1071 
1072   if (_discovery_is_mt) {
1073     add_to_discovered_list_mt(*list, obj, discovered_addr);
1074   } else {
1075     // We do a raw store here: the field will be visited later when processing
1076     // the discovered references.
1077     oop current_head = list->head();
1078     // The last ref must have its discovered field pointing to itself.
1079     oop next_discovered = (current_head != NULL) ? current_head : obj;
1080 
1081     assert(discovered == NULL, "control point invariant");
1082     oop_store_raw(discovered_addr, next_discovered);
1083     list->set_head(obj);
1084     list->inc_length(1);
1085 
1086     log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
1087   }
1088   assert(obj->is_oop(), "Discovered a bad reference");
1089   verify_referent(obj);
1090   return true;
1091 }
1092 
1093 // Preclean the discovered references by removing those
1094 // whose referents are alive, and by marking from those that
1095 // are not active. These lists can be handled here
1096 // in any order and, indeed, concurrently.
1097 void ReferenceProcessor::preclean_discovered_references(
1098   BoolObjectClosure* is_alive,
1099   OopClosure* keep_alive,
1100   VoidClosure* complete_gc,
1101   YieldClosure* yield,
1102   GCTimer* gc_timer) {
1103 
1104   // Soft references
1105   {
1106     GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
1107     for (uint i = 0; i < _max_num_q; i++) {
1108       if (yield->should_return()) {
1109         return;
1110       }
1111       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1112                                   keep_alive, complete_gc, yield);
1113     }
1114   }
1115 
1116   // Weak references
1117   {
1118     GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
1119     for (uint i = 0; i < _max_num_q; i++) {
1120       if (yield->should_return()) {
1121         return;
1122       }
1123       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1124                                   keep_alive, complete_gc, yield);
1125     }
1126   }
1127 
1128   // Final references
1129   {
1130     GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
1131     for (uint i = 0; i < _max_num_q; i++) {
1132       if (yield->should_return()) {
1133         return;
1134       }
1135       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1136                                   keep_alive, complete_gc, yield);
1137     }
1138   }
1139 
1140   // Phantom references
1141   {
1142     GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
1143     for (uint i = 0; i < _max_num_q; i++) {
1144       if (yield->should_return()) {
1145         return;
1146       }
1147       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1148                                   keep_alive, complete_gc, yield);
1149     }
1150   }
1151 }
1152 
1153 // Walk the given discovered ref list, and remove all reference objects
1154 // whose referents are still alive, whose referents are NULL or which
1155 // are not active (have a non-NULL next field). NOTE: When we are
1156 // thus precleaning the ref lists (which happens single-threaded today),
1157 // we do not disable refs discovery to honor the correct semantics of
1158 // java.lang.Reference. As a result, we need to be careful below
1159 // that ref removal steps interleave safely with ref discovery steps
1160 // (in this thread).
1161 void
1162 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1163                                                 BoolObjectClosure* is_alive,
1164                                                 OopClosure*        keep_alive,
1165                                                 VoidClosure*       complete_gc,
1166                                                 YieldClosure*      yield) {
1167   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1168   while (iter.has_next()) {
1169     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1170     oop obj = iter.obj();
1171     oop next = java_lang_ref_Reference::next(obj);
1172     if (iter.referent() == NULL || iter.is_referent_alive() ||
1173         next != NULL) {
1174       // The referent has been cleared, or is alive, or the Reference is not
1175       // active; we need to trace and mark its cohort.
1176       log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1177                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
1178       // Remove Reference object from list
1179       iter.remove();
1180       // Keep alive its cohort.
1181       iter.make_referent_alive();
1182       if (UseCompressedOops) {
1183         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1184         keep_alive->do_oop(next_addr);
1185       } else {
1186         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1187         keep_alive->do_oop(next_addr);
1188       }
1189       iter.move_to_next();
1190     } else {
1191       iter.next();
1192     }
1193   }
1194   // Close the reachable set
1195   complete_gc->do_void();
1196 
1197   NOT_PRODUCT(
1198     if (iter.processed() > 0) {
1199       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
1200         iter.removed(), iter.processed(), p2i(&refs_list));
1201     }
1202   )
1203 }
1204 
1205 const char* ReferenceProcessor::list_name(uint i) {
1206    assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1207           "Out of bounds index");
1208 
1209    int j = i / _max_num_q;
1210    switch (j) {
1211      case 0: return "SoftRef";
1212      case 1: return "WeakRef";
1213      case 2: return "FinalRef";
1214      case 3: return "PhantomRef";
1215    }
1216    ShouldNotReachHere();
1217    return NULL;
1218 }
1219