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