1 /*
   2  * Copyright (c) 2001, 2017, 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 (is_server_compilation_mode_vm()) {
  61     _default_soft_ref_policy = new LRUMaxHeapPolicy();
  62   } else {
  63     _default_soft_ref_policy = new LRUCurrentHeapPolicy();
  64   }
  65   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  66     vm_exit_during_initialization("Could not allocate reference policy object");
  67   }
  68   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  69             RefDiscoveryPolicy == ReferentBasedDiscovery,
  70             "Unrecognized RefDiscoveryPolicy");
  71 }
  72 
  73 void ReferenceProcessor::enable_discovery(bool check_no_refs) {
  74 #ifdef ASSERT
  75   // Verify that we're not currently discovering refs
  76   assert(!_discovering_refs, "nested call?");
  77 
  78   if (check_no_refs) {
  79     // Verify that the discovered lists are empty
  80     verify_no_references_recorded();
  81   }
  82 #endif // ASSERT
  83 
  84   // Someone could have modified the value of the static
  85   // field in the j.l.r.SoftReference class that holds the
  86   // soft reference timestamp clock using reflection or
  87   // Unsafe between GCs. Unconditionally update the static
  88   // field in ReferenceProcessor here so that we use the new
  89   // value during reference discovery.
  90 
  91   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
  92   _discovering_refs = true;
  93 }
  94 
  95 ReferenceProcessor::ReferenceProcessor(MemRegion span,
  96                                        bool      mt_processing,
  97                                        uint      mt_processing_degree,
  98                                        bool      mt_discovery,
  99                                        uint      mt_discovery_degree,
 100                                        bool      atomic_discovery,
 101                                        BoolObjectClosure* is_alive_non_header)  :
 102   _discovering_refs(false),
 103   _enqueuing_is_done(false),
 104   _is_alive_non_header(is_alive_non_header),
 105   _processing_is_mt(mt_processing),
 106   _next_id(0)
 107 {
 108   _span = span;
 109   _discovery_is_atomic = atomic_discovery;
 110   _discovery_is_mt     = mt_discovery;
 111   _num_q               = MAX2(1U, mt_processing_degree);
 112   _max_num_q           = MAX2(_num_q, mt_discovery_degree);
 113   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 114             _max_num_q * number_of_subclasses_of_ref(), mtGC);
 115 
 116   if (_discovered_refs == NULL) {
 117     vm_exit_during_initialization("Could not allocated RefProc Array");
 118   }
 119   _discoveredSoftRefs    = &_discovered_refs[0];
 120   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
 121   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
 122   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
 123 
 124   // Initialize all entries to NULL
 125   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 126     _discovered_refs[i].set_head(NULL);
 127     _discovered_refs[i].set_length(0);
 128   }
 129 
 130   setup_policy(false /* default soft ref policy */);
 131 }
 132 
 133 #ifndef PRODUCT
 134 void ReferenceProcessor::verify_no_references_recorded() {
 135   guarantee(!_discovering_refs, "Discovering refs?");
 136   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 137     guarantee(_discovered_refs[i].is_empty(),
 138               "Found non-empty discovered list at %u", i);
 139   }
 140 }
 141 #endif
 142 
 143 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 144   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 145     if (UseCompressedOops) {
 146       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 147     } else {
 148       f->do_oop((oop*)_discovered_refs[i].adr_head());
 149     }
 150   }
 151 }
 152 
 153 void ReferenceProcessor::update_soft_ref_master_clock() {
 154   // Update (advance) the soft ref master clock field. This must be done
 155   // after processing the soft ref list.
 156 
 157   // We need a monotonically non-decreasing time in ms but
 158   // os::javaTimeMillis() does not guarantee monotonicity.
 159   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 160   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 161   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 162 
 163   NOT_PRODUCT(
 164   if (now < _soft_ref_timestamp_clock) {
 165     log_warning(gc)("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,
 166                     _soft_ref_timestamp_clock, now);
 167   }
 168   )
 169   // The values of now and _soft_ref_timestamp_clock are set using
 170   // javaTimeNanos(), which is guaranteed to be monotonically
 171   // non-decreasing provided the underlying platform provides such
 172   // a time source (and it is bug free).
 173   // In product mode, however, protect ourselves from non-monotonicity.
 174   if (now > _soft_ref_timestamp_clock) {
 175     _soft_ref_timestamp_clock = now;
 176     java_lang_ref_SoftReference::set_clock(now);
 177   }
 178   // Else leave clock stalled at its old value until time progresses
 179   // past clock value.
 180 }
 181 
 182 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) const {
 183   size_t total = 0;
 184   for (uint i = 0; i < _max_num_q; ++i) {
 185     total += lists[i].length();
 186   }
 187   return total;
 188 }
 189 
 190 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 191   BoolObjectClosure*            is_alive,
 192   OopClosure*                   keep_alive,
 193   VoidClosure*                  complete_gc,
 194   AbstractRefProcTaskExecutor*  task_executor,
 195   ReferenceProcessorPhaseTimes* phase_times) {
 196 
 197   double start_time = os::elapsedTime();
 198 
 199   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 200   // Stop treating discovered references specially.
 201   disable_discovery();
 202 
 203   // If discovery was concurrent, someone could have modified
 204   // the value of the static field in the j.l.r.SoftReference
 205   // class that holds the soft reference timestamp clock using
 206   // reflection or Unsafe between when discovery was enabled and
 207   // now. Unconditionally update the static field in ReferenceProcessor
 208   // here so that we use the new value during processing of the
 209   // discovered soft refs.
 210 
 211   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 212 
 213   ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
 214                                 total_count(_discoveredWeakRefs),
 215                                 total_count(_discoveredFinalRefs),
 216                                 total_count(_discoveredPhantomRefs));
 217 
 218   // Soft references
 219   {
 220     RefProcPhaseTimesTracker tt(REF_SOFT, phase_times, this);
 221     process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 222                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 223   }
 224 
 225   update_soft_ref_master_clock();
 226 
 227   // Weak references
 228   {
 229     RefProcPhaseTimesTracker tt(REF_WEAK, phase_times, this);
 230     process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 231                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 232   }
 233 
 234   // Final references
 235   {
 236     RefProcPhaseTimesTracker tt(REF_FINAL, phase_times, this);
 237     process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 238                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 239   }
 240 
 241   // Phantom references
 242   {
 243     RefProcPhaseTimesTracker tt(REF_PHANTOM, phase_times, this);
 244     process_discovered_reflist(_discoveredPhantomRefs, NULL, true,
 245                                is_alive, keep_alive, complete_gc, task_executor, phase_times);
 246   }
 247 
 248   // Weak global JNI references. It would make more sense (semantically) to
 249   // traverse these simultaneously with the regular weak references above, but
 250   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 251   // thus use JNI weak references to circumvent the phantom references and
 252   // resurrect a "post-mortem" object.
 253   {
 254     GCTraceTime(Debug, gc, ref) tt("JNI Weak Reference", phase_times->gc_timer());
 255     if (task_executor != NULL) {
 256       task_executor->set_single_threaded_mode();
 257     }
 258     process_phaseJNI(is_alive, keep_alive, complete_gc);
 259   }
 260 
 261   phase_times->set_total_time_ms((os::elapsedTime() - start_time) * 1000);
 262 
 263   log_develop_trace(gc, ref)("JNI Weak Reference count: " SIZE_FORMAT, count_jni_refs());
 264 
 265   return stats;
 266 }
 267 
 268 #ifndef PRODUCT
 269 // Calculate the number of jni handles.
 270 size_t ReferenceProcessor::count_jni_refs() {
 271   class CountHandleClosure: public OopClosure {
 272   private:
 273     size_t _count;
 274   public:
 275     CountHandleClosure(): _count(0) {}
 276     void do_oop(oop* unused)       { _count++; }
 277     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 278     size_t count() { return _count; }
 279   };
 280   CountHandleClosure global_handle_count;
 281   JNIHandles::weak_oops_do(&global_handle_count);
 282   return global_handle_count.count();
 283 }
 284 #endif
 285 
 286 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 287                                           OopClosure*        keep_alive,
 288                                           VoidClosure*       complete_gc) {
 289   JNIHandles::weak_oops_do(is_alive, keep_alive);
 290   complete_gc->do_void();
 291 }
 292 
 293 void ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor*  task_executor,
 294                                                        ReferenceProcessorPhaseTimes* phase_times) {
 295   // Enqueue references that are not made active again, and
 296   // clear the decks for the next collection (cycle).
 297   enqueue_discovered_reflists(task_executor, phase_times);
 298 
 299   // Stop treating discovered references specially.
 300   disable_discovery();
 301 }
 302 
 303 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list) {
 304   // Given a list of refs linked through the "discovered" field
 305   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 306   // thus distinguishing them from active References, then
 307   // prepend them to the pending list.
 308   //
 309   // The Java threads will see the Reference objects linked together through
 310   // the discovered field. Instead of trying to do the write barrier updates
 311   // in all places in the reference processor where we manipulate the discovered
 312   // field we make sure to do the barrier here where we anyway iterate through
 313   // all linked Reference objects. Note that it is important to not dirty any
 314   // cards during reference processing since this will cause card table
 315   // verification to fail for G1.
 316   log_develop_trace(gc, ref)("ReferenceProcessor::enqueue_discovered_reflist list " INTPTR_FORMAT, p2i(&refs_list));
 317 
 318   oop obj = NULL;
 319   oop next_d = refs_list.head();
 320   // Walk down the list, self-looping the next field
 321   // so that the References are not considered active.
 322   while (obj != next_d) {
 323     obj = next_d;
 324     assert(obj->is_instance(), "should be an instance object");
 325     assert(InstanceKlass::cast(obj->klass())->is_reference_instance_klass(), "should be reference object");
 326     next_d = java_lang_ref_Reference::discovered(obj);
 327     log_develop_trace(gc, ref)("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT, p2i(obj), p2i(next_d));
 328     assert(java_lang_ref_Reference::next(obj) == NULL,
 329            "Reference not active; should not be discovered");
 330     // Self-loop next, so as to make Ref not active.
 331     java_lang_ref_Reference::set_next_raw(obj, obj);
 332     if (next_d != obj) {
 333       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);
 334     } else {
 335       // This is the last object.
 336       // Swap refs_list into pending list and set obj's
 337       // discovered to what we read from the pending list.
 338       oop old = Universe::swap_reference_pending_list(refs_list.head());
 339       java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL
 340       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);
 341     }
 342   }
 343 }
 344 
 345 // Parallel enqueue task
 346 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 347 public:
 348   RefProcEnqueueTask(ReferenceProcessor&           ref_processor,
 349                      DiscoveredList                discovered_refs[],
 350                      int                           n_queues,
 351                      ReferenceProcessorPhaseTimes* phase_times)
 352     : EnqueueTask(ref_processor, discovered_refs, n_queues, phase_times)
 353   { }
 354 
 355   virtual void work(unsigned int work_id) {
 356     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefEnqueue, _phase_times, work_id);
 357 
 358     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 359     // Simplest first cut: static partitioning.
 360     int index = work_id;
 361     // The increment on "index" must correspond to the maximum number of queues
 362     // (n_queues) with which that ReferenceProcessor was created.  That
 363     // is because of the "clever" way the discovered references lists were
 364     // allocated and are indexed into.
 365     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 366     for (int j = 0;
 367          j < ReferenceProcessor::number_of_subclasses_of_ref();
 368          j++, index += _n_queues) {
 369       _ref_processor.enqueue_discovered_reflist(_refs_lists[index]);
 370       _refs_lists[index].set_head(NULL);
 371       _refs_lists[index].set_length(0);
 372     }
 373   }
 374 };
 375 
 376 // Enqueue references that are not made active again
 377 void ReferenceProcessor::enqueue_discovered_reflists(AbstractRefProcTaskExecutor*  task_executor,
 378                                                      ReferenceProcessorPhaseTimes* phase_times) {
 379 
 380   ReferenceProcessorStats stats(total_count(_discoveredSoftRefs),
 381                                 total_count(_discoveredWeakRefs),
 382                                 total_count(_discoveredFinalRefs),
 383                                 total_count(_discoveredPhantomRefs));
 384 
 385   RefProcEnqueueTimeTracker tt(phase_times, stats);
 386 
 387   if (_processing_is_mt && task_executor != NULL) {
 388     // Parallel code
 389     RefProcEnqueueTask tsk(*this, _discovered_refs, _max_num_q, phase_times);
 390     task_executor->execute(tsk);
 391   } else {
 392     // Serial code: call the parent class's implementation
 393     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 394       enqueue_discovered_reflist(_discovered_refs[i]);
 395       _discovered_refs[i].set_head(NULL);
 396       _discovered_refs[i].set_length(0);
 397     }
 398   }
 399 }
 400 
 401 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 402   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 403   oop discovered = java_lang_ref_Reference::discovered(_ref);
 404   assert(_discovered_addr && oopDesc::is_oop_or_null(discovered),
 405          "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
 406   _next = discovered;
 407   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 408   _referent = java_lang_ref_Reference::referent(_ref);
 409   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 410          "Wrong oop found in java.lang.Reference object");
 411   assert(allow_null_referent ?
 412              oopDesc::is_oop_or_null(_referent)
 413            : oopDesc::is_oop(_referent),
 414          "Expected an oop%s for referent field at " PTR_FORMAT,
 415          (allow_null_referent ? " or NULL" : ""),
 416          p2i(_referent));
 417 }
 418 
 419 void DiscoveredListIterator::remove() {
 420   assert(oopDesc::is_oop(_ref), "Dropping a bad reference");
 421   oop_store_raw(_discovered_addr, NULL);
 422 
 423   // First _prev_next ref actually points into DiscoveredList (gross).
 424   oop new_next;
 425   if (_next == _ref) {
 426     // At the end of the list, we should make _prev point to itself.
 427     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 428     // and _prev will be NULL.
 429     new_next = _prev;
 430   } else {
 431     new_next = _next;
 432   }
 433   // Remove Reference object from discovered list. Note that G1 does not need a
 434   // pre-barrier here because we know the Reference has already been found/marked,
 435   // that's how it ended up in the discovered list in the first place.
 436   oop_store_raw(_prev_next, new_next);
 437   NOT_PRODUCT(_removed++);
 438   _refs_list.dec_length(1);
 439 }
 440 
 441 void DiscoveredListIterator::clear_referent() {
 442   oop_store_raw(_referent_addr, NULL);
 443 }
 444 
 445 // NOTE: process_phase*() are largely similar, and at a high level
 446 // merely iterate over the extant list applying a predicate to
 447 // each of its elements and possibly removing that element from the
 448 // list and applying some further closures to that element.
 449 // We should consider the possibility of replacing these
 450 // process_phase*() methods by abstracting them into
 451 // a single general iterator invocation that receives appropriate
 452 // closures that accomplish this work.
 453 
 454 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 455 // referents are not alive, but that should be kept alive for policy reasons.
 456 // Keep alive the transitive closure of all such referents.
 457 void
 458 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 459                                    ReferencePolicy*   policy,
 460                                    BoolObjectClosure* is_alive,
 461                                    OopClosure*        keep_alive,
 462                                    VoidClosure*       complete_gc) {
 463   assert(policy != NULL, "Must have a non-NULL policy");
 464   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 465   // Decide which softly reachable refs should be kept alive.
 466   while (iter.has_next()) {
 467     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 468     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 469     if (referent_is_dead &&
 470         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 471       log_develop_trace(gc, ref)("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 472                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
 473       // Remove Reference object from list
 474       iter.remove();
 475       // keep the referent around
 476       iter.make_referent_alive();
 477       iter.move_to_next();
 478     } else {
 479       iter.next();
 480     }
 481   }
 482   // Close the reachable set
 483   complete_gc->do_void();
 484   log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT " discovered Refs by policy, from list " INTPTR_FORMAT,
 485                              iter.removed(), iter.processed(), p2i(&refs_list));
 486 }
 487 
 488 // Traverse the list and remove any Refs that are not active, or
 489 // whose referents are either alive or NULL.
 490 void
 491 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 492                              BoolObjectClosure* is_alive,
 493                              OopClosure*        keep_alive) {
 494   assert(discovery_is_atomic(), "Error");
 495   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 496   while (iter.has_next()) {
 497     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 498     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 499     assert(next == NULL, "Should not discover inactive Reference");
 500     if (iter.is_referent_alive()) {
 501       log_develop_trace(gc, ref)("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 502                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
 503       // The referent is reachable after all.
 504       // Remove Reference object from list.
 505       iter.remove();
 506       // Update the referent pointer as necessary: Note that this
 507       // should not entail any recursive marking because the
 508       // referent must already have been traversed.
 509       iter.make_referent_alive();
 510       iter.move_to_next();
 511     } else {
 512       iter.next();
 513     }
 514   }
 515   NOT_PRODUCT(
 516     if (iter.processed() > 0) {
 517       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 518         " Refs in discovered list " INTPTR_FORMAT,
 519         iter.removed(), iter.processed(), p2i(&refs_list));
 520     }
 521   )
 522 }
 523 
 524 void
 525 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 526                                                   BoolObjectClosure* is_alive,
 527                                                   OopClosure*        keep_alive,
 528                                                   VoidClosure*       complete_gc) {
 529   assert(!discovery_is_atomic(), "Error");
 530   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 531   while (iter.has_next()) {
 532     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 533     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 534     oop next = java_lang_ref_Reference::next(iter.obj());
 535     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 536          next != NULL)) {
 537       assert(oopDesc::is_oop_or_null(next), "Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next));
 538       // Remove Reference object from list
 539       iter.remove();
 540       // Trace the cohorts
 541       iter.make_referent_alive();
 542       if (UseCompressedOops) {
 543         keep_alive->do_oop((narrowOop*)next_addr);
 544       } else {
 545         keep_alive->do_oop((oop*)next_addr);
 546       }
 547       iter.move_to_next();
 548     } else {
 549       iter.next();
 550     }
 551   }
 552   // Now close the newly reachable set
 553   complete_gc->do_void();
 554   NOT_PRODUCT(
 555     if (iter.processed() > 0) {
 556       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT
 557         " Refs in discovered list " INTPTR_FORMAT,
 558         iter.removed(), iter.processed(), p2i(&refs_list));
 559     }
 560   )
 561 }
 562 
 563 // Traverse the list and process the referents, by either
 564 // clearing them or keeping them (and their reachable
 565 // closure) alive.
 566 void
 567 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 568                                    bool               clear_referent,
 569                                    BoolObjectClosure* is_alive,
 570                                    OopClosure*        keep_alive,
 571                                    VoidClosure*       complete_gc) {
 572   ResourceMark rm;
 573   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 574   while (iter.has_next()) {
 575     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 576     if (clear_referent) {
 577       // NULL out referent pointer
 578       iter.clear_referent();
 579     } else {
 580       // keep the referent around
 581       iter.make_referent_alive();
 582     }
 583     log_develop_trace(gc, ref)("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 584                                clear_referent ? "cleared " : "", p2i(iter.obj()), iter.obj()->klass()->internal_name());
 585     assert(oopDesc::is_oop(iter.obj(), UseConcMarkSweepGC), "Adding a bad reference");
 586     iter.next();
 587   }
 588   // Close the reachable set
 589   complete_gc->do_void();
 590 }
 591 
 592 void
 593 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 594   oop obj = NULL;
 595   oop next = refs_list.head();
 596   while (next != obj) {
 597     obj = next;
 598     next = java_lang_ref_Reference::discovered(obj);
 599     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 600   }
 601   refs_list.set_head(NULL);
 602   refs_list.set_length(0);
 603 }
 604 
 605 void ReferenceProcessor::abandon_partial_discovery() {
 606   // loop over the lists
 607   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 608     if ((i % _max_num_q) == 0) {
 609       log_develop_trace(gc, ref)("Abandoning %s discovered list", list_name(i));
 610     }
 611     clear_discovered_references(_discovered_refs[i]);
 612   }
 613 }
 614 
 615 size_t ReferenceProcessor::total_reference_count(ReferenceType type) const {
 616   DiscoveredList* list = NULL;
 617 
 618   switch (type) {
 619     case REF_SOFT:
 620       list = _discoveredSoftRefs;
 621       break;
 622     case REF_WEAK:
 623       list = _discoveredWeakRefs;
 624       break;
 625     case REF_FINAL:
 626       list = _discoveredFinalRefs;
 627       break;
 628     case REF_PHANTOM:
 629       list = _discoveredPhantomRefs;
 630       break;
 631     case REF_OTHER:
 632     case REF_NONE:
 633     default:
 634       ShouldNotReachHere();
 635   }
 636   return total_count(list);
 637 }
 638 
 639 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 640 public:
 641   RefProcPhase1Task(ReferenceProcessor&           ref_processor,
 642                     DiscoveredList                refs_lists[],
 643                     ReferencePolicy*              policy,
 644                     bool                          marks_oops_alive,
 645                     ReferenceProcessorPhaseTimes* phase_times)
 646     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
 647       _policy(policy)
 648   { }
 649   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 650                     OopClosure& keep_alive,
 651                     VoidClosure& complete_gc)
 652   {
 653     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, _phase_times, i);
 654 
 655     _ref_processor.process_phase1(_refs_lists[i], _policy,
 656                                   &is_alive, &keep_alive, &complete_gc);
 657   }
 658 private:
 659   ReferencePolicy* _policy;
 660 };
 661 
 662 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 663 public:
 664   RefProcPhase2Task(ReferenceProcessor&           ref_processor,
 665                     DiscoveredList                refs_lists[],
 666                     bool                          marks_oops_alive,
 667                     ReferenceProcessorPhaseTimes* phase_times)
 668     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times)
 669   { }
 670   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 671                     OopClosure& keep_alive,
 672                     VoidClosure& complete_gc)
 673   {
 674     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, _phase_times, i);
 675 
 676     _ref_processor.process_phase2(_refs_lists[i],
 677                                   &is_alive, &keep_alive, &complete_gc);
 678   }
 679 };
 680 
 681 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 682 public:
 683   RefProcPhase3Task(ReferenceProcessor&           ref_processor,
 684                     DiscoveredList                refs_lists[],
 685                     bool                         clear_referent,
 686                     bool                          marks_oops_alive,
 687                     ReferenceProcessorPhaseTimes* phase_times)
 688     : ProcessTask(ref_processor, refs_lists, marks_oops_alive, phase_times),
 689       _clear_referent(clear_referent)
 690   { }
 691   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 692                     OopClosure& keep_alive,
 693                     VoidClosure& complete_gc)
 694   {
 695     RefProcWorkerTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, _phase_times, i);
 696 
 697     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 698                                   &is_alive, &keep_alive, &complete_gc);
 699   }
 700 private:
 701   bool _clear_referent;
 702 };
 703 
 704 #ifndef PRODUCT
 705 void ReferenceProcessor::log_reflist_counts(DiscoveredList ref_lists[], uint active_length, size_t total_refs) {
 706   if (!log_is_enabled(Trace, gc, ref)) {
 707     return;
 708   }
 709 
 710   stringStream st;
 711   for (uint i = 0; i < active_length; ++i) {
 712     st.print(SIZE_FORMAT " ", ref_lists[i].length());
 713   }
 714   log_develop_trace(gc, ref)("%s= " SIZE_FORMAT, st.as_string(), total_refs);
 715 #ifdef ASSERT
 716   for (uint i = active_length; i < _max_num_q; i++) {
 717     assert(ref_lists[i].length() == 0, SIZE_FORMAT " unexpected References in %u",
 718            ref_lists[i].length(), i);
 719   }
 720 #endif
 721 }
 722 #endif
 723 
 724 void ReferenceProcessor::set_active_mt_degree(uint v) {
 725   _num_q = v;
 726   _next_id = 0;
 727 }
 728 
 729 // Balances reference queues.
 730 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 731 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 732 // corresponding to the active workers will be processed.
 733 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 734 {
 735   // calculate total length
 736   size_t total_refs = 0;
 737   log_develop_trace(gc, ref)("Balance ref_lists ");
 738 
 739   for (uint i = 0; i < _max_num_q; ++i) {
 740     total_refs += ref_lists[i].length();
 741   }
 742   log_reflist_counts(ref_lists, _max_num_q, total_refs);
 743   size_t avg_refs = total_refs / _num_q + 1;
 744   uint to_idx = 0;
 745   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 746     bool move_all = false;
 747     if (from_idx >= _num_q) {
 748       move_all = ref_lists[from_idx].length() > 0;
 749     }
 750     while ((ref_lists[from_idx].length() > avg_refs) ||
 751            move_all) {
 752       assert(to_idx < _num_q, "Sanity Check!");
 753       if (ref_lists[to_idx].length() < avg_refs) {
 754         // move superfluous refs
 755         size_t refs_to_move;
 756         // Move all the Ref's if the from queue will not be processed.
 757         if (move_all) {
 758           refs_to_move = MIN2(ref_lists[from_idx].length(),
 759                               avg_refs - ref_lists[to_idx].length());
 760         } else {
 761           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 762                               avg_refs - ref_lists[to_idx].length());
 763         }
 764 
 765         assert(refs_to_move > 0, "otherwise the code below will fail");
 766 
 767         oop move_head = ref_lists[from_idx].head();
 768         oop move_tail = move_head;
 769         oop new_head  = move_head;
 770         // find an element to split the list on
 771         for (size_t j = 0; j < refs_to_move; ++j) {
 772           move_tail = new_head;
 773           new_head = java_lang_ref_Reference::discovered(new_head);
 774         }
 775 
 776         // Add the chain to the to list.
 777         if (ref_lists[to_idx].head() == NULL) {
 778           // to list is empty. Make a loop at the end.
 779           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);
 780         } else {
 781           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());
 782         }
 783         ref_lists[to_idx].set_head(move_head);
 784         ref_lists[to_idx].inc_length(refs_to_move);
 785 
 786         // Remove the chain from the from list.
 787         if (move_tail == new_head) {
 788           // We found the end of the from list.
 789           ref_lists[from_idx].set_head(NULL);
 790         } else {
 791           ref_lists[from_idx].set_head(new_head);
 792         }
 793         ref_lists[from_idx].dec_length(refs_to_move);
 794         if (ref_lists[from_idx].length() == 0) {
 795           break;
 796         }
 797       } else {
 798         to_idx = (to_idx + 1) % _num_q;
 799       }
 800     }
 801   }
 802 #ifdef ASSERT
 803   size_t balanced_total_refs = 0;
 804   for (uint i = 0; i < _num_q; ++i) {
 805     balanced_total_refs += ref_lists[i].length();
 806   }
 807   log_reflist_counts(ref_lists, _num_q, balanced_total_refs);
 808   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 809 #endif
 810 }
 811 
 812 void ReferenceProcessor::balance_all_queues() {
 813   balance_queues(_discoveredSoftRefs);
 814   balance_queues(_discoveredWeakRefs);
 815   balance_queues(_discoveredFinalRefs);
 816   balance_queues(_discoveredPhantomRefs);
 817 }
 818 
 819 void ReferenceProcessor::process_discovered_reflist(
 820   DiscoveredList                refs_lists[],
 821   ReferencePolicy*              policy,
 822   bool                          clear_referent,
 823   BoolObjectClosure*            is_alive,
 824   OopClosure*                   keep_alive,
 825   VoidClosure*                  complete_gc,
 826   AbstractRefProcTaskExecutor*  task_executor,
 827   ReferenceProcessorPhaseTimes* phase_times)
 828 {
 829   bool mt_processing = task_executor != NULL && _processing_is_mt;
 830 
 831   phase_times->set_processing_is_mt(mt_processing);
 832 
 833   // If discovery used MT and a dynamic number of GC threads, then
 834   // the queues must be balanced for correctness if fewer than the
 835   // maximum number of queues were used.  The number of queue used
 836   // during discovery may be different than the number to be used
 837   // for processing so don't depend of _num_q < _max_num_q as part
 838   // of the test.
 839   bool must_balance = _discovery_is_mt;
 840 
 841   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 842       must_balance) {
 843     RefProcBalanceQueuesTimeTracker tt(phase_times);
 844     balance_queues(refs_lists);
 845   }
 846 
 847   // Phase 1 (soft refs only):
 848   // . Traverse the list and remove any SoftReferences whose
 849   //   referents are not alive, but that should be kept alive for
 850   //   policy reasons. Keep alive the transitive closure of all
 851   //   such referents.
 852   if (policy != NULL) {
 853     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase1, phase_times);
 854 
 855     if (mt_processing) {
 856       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/, phase_times);
 857       task_executor->execute(phase1);
 858     } else {
 859       for (uint i = 0; i < _max_num_q; i++) {
 860         process_phase1(refs_lists[i], policy,
 861                        is_alive, keep_alive, complete_gc);
 862       }
 863     }
 864   } else { // policy == NULL
 865     assert(refs_lists != _discoveredSoftRefs,
 866            "Policy must be specified for soft references.");
 867   }
 868 
 869   // Phase 2:
 870   // . Traverse the list and remove any refs whose referents are alive.
 871   {
 872     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase2, phase_times);
 873 
 874     if (mt_processing) {
 875       RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/, phase_times);
 876       task_executor->execute(phase2);
 877     } else {
 878       for (uint i = 0; i < _max_num_q; i++) {
 879         process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 880       }
 881     }
 882   }
 883 
 884   // Phase 3:
 885   // . Traverse the list and process referents as appropriate.
 886   {
 887     RefProcParPhaseTimeTracker tt(ReferenceProcessorPhaseTimes::RefPhase3, phase_times);
 888 
 889     if (mt_processing) {
 890       RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/, phase_times);
 891       task_executor->execute(phase3);
 892     } else {
 893       for (uint i = 0; i < _max_num_q; i++) {
 894         process_phase3(refs_lists[i], clear_referent,
 895                        is_alive, keep_alive, complete_gc);
 896       }
 897     }
 898   }
 899 }
 900 
 901 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
 902   uint id = 0;
 903   // Determine the queue index to use for this object.
 904   if (_discovery_is_mt) {
 905     // During a multi-threaded discovery phase,
 906     // each thread saves to its "own" list.
 907     Thread* thr = Thread::current();
 908     id = thr->as_Worker_thread()->id();
 909   } else {
 910     // single-threaded discovery, we save in round-robin
 911     // fashion to each of the lists.
 912     if (_processing_is_mt) {
 913       id = next_id();
 914     }
 915   }
 916   assert(id < _max_num_q, "Id is out-of-bounds id %u and max id %u)", id, _max_num_q);
 917 
 918   // Get the discovered queue to which we will add
 919   DiscoveredList* list = NULL;
 920   switch (rt) {
 921     case REF_OTHER:
 922       // Unknown reference type, no special treatment
 923       break;
 924     case REF_SOFT:
 925       list = &_discoveredSoftRefs[id];
 926       break;
 927     case REF_WEAK:
 928       list = &_discoveredWeakRefs[id];
 929       break;
 930     case REF_FINAL:
 931       list = &_discoveredFinalRefs[id];
 932       break;
 933     case REF_PHANTOM:
 934       list = &_discoveredPhantomRefs[id];
 935       break;
 936     case REF_NONE:
 937       // we should not reach here if we are an InstanceRefKlass
 938     default:
 939       ShouldNotReachHere();
 940   }
 941   log_develop_trace(gc, ref)("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));
 942   return list;
 943 }
 944 
 945 inline void
 946 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
 947                                               oop             obj,
 948                                               HeapWord*       discovered_addr) {
 949   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
 950   // First we must make sure this object is only enqueued once. CAS in a non null
 951   // discovered_addr.
 952   oop current_head = refs_list.head();
 953   // The last ref must have its discovered field pointing to itself.
 954   oop next_discovered = (current_head != NULL) ? current_head : obj;
 955 
 956   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
 957                                                     NULL);
 958   if (retest == NULL) {
 959     // This thread just won the right to enqueue the object.
 960     // We have separate lists for enqueueing, so no synchronization
 961     // is necessary.
 962     refs_list.set_head(obj);
 963     refs_list.inc_length(1);
 964 
 965     log_develop_trace(gc, ref)("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
 966                                p2i(obj), obj->klass()->internal_name());
 967   } else {
 968     // If retest was non NULL, another thread beat us to it:
 969     // The reference has already been discovered...
 970     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
 971                                p2i(obj), obj->klass()->internal_name());
 972     }
 973   }
 974 
 975 #ifndef PRODUCT
 976 // Non-atomic (i.e. concurrent) discovery might allow us
 977 // to observe j.l.References with NULL referents, being those
 978 // cleared concurrently by mutators during (or after) discovery.
 979 void ReferenceProcessor::verify_referent(oop obj) {
 980   bool da = discovery_is_atomic();
 981   oop referent = java_lang_ref_Reference::referent(obj);
 982   assert(da ? oopDesc::is_oop(referent) : oopDesc::is_oop_or_null(referent),
 983          "Bad referent " INTPTR_FORMAT " found in Reference "
 984          INTPTR_FORMAT " during %satomic discovery ",
 985          p2i(referent), p2i(obj), da ? "" : "non-");
 986 }
 987 #endif
 988 
 989 // We mention two of several possible choices here:
 990 // #0: if the reference object is not in the "originating generation"
 991 //     (or part of the heap being collected, indicated by our "span"
 992 //     we don't treat it specially (i.e. we scan it as we would
 993 //     a normal oop, treating its references as strong references).
 994 //     This means that references can't be discovered unless their
 995 //     referent is also in the same span. This is the simplest,
 996 //     most "local" and most conservative approach, albeit one
 997 //     that may cause weak references to be enqueued least promptly.
 998 //     We call this choice the "ReferenceBasedDiscovery" policy.
 999 // #1: the reference object may be in any generation (span), but if
1000 //     the referent is in the generation (span) being currently collected
1001 //     then we can discover the reference object, provided
1002 //     the object has not already been discovered by
1003 //     a different concurrently running collector (as may be the
1004 //     case, for instance, if the reference object is in CMS and
1005 //     the referent in DefNewGeneration), and provided the processing
1006 //     of this reference object by the current collector will
1007 //     appear atomic to every other collector in the system.
1008 //     (Thus, for instance, a concurrent collector may not
1009 //     discover references in other generations even if the
1010 //     referent is in its own generation). This policy may,
1011 //     in certain cases, enqueue references somewhat sooner than
1012 //     might Policy #0 above, but at marginally increased cost
1013 //     and complexity in processing these references.
1014 //     We call this choice the "RefeferentBasedDiscovery" policy.
1015 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1016   // Make sure we are discovering refs (rather than processing discovered refs).
1017   if (!_discovering_refs || !RegisterReferences) {
1018     return false;
1019   }
1020   // We only discover active references.
1021   oop next = java_lang_ref_Reference::next(obj);
1022   if (next != NULL) {   // Ref is no longer active
1023     return false;
1024   }
1025 
1026   HeapWord* obj_addr = (HeapWord*)obj;
1027   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1028       !_span.contains(obj_addr)) {
1029     // Reference is not in the originating generation;
1030     // don't treat it specially (i.e. we want to scan it as a normal
1031     // object with strong references).
1032     return false;
1033   }
1034 
1035   // We only discover references whose referents are not (yet)
1036   // known to be strongly reachable.
1037   if (is_alive_non_header() != NULL) {
1038     verify_referent(obj);
1039     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1040       return false;  // referent is reachable
1041     }
1042   }
1043   if (rt == REF_SOFT) {
1044     // For soft refs we can decide now if these are not
1045     // current candidates for clearing, in which case we
1046     // can mark through them now, rather than delaying that
1047     // to the reference-processing phase. Since all current
1048     // time-stamp policies advance the soft-ref clock only
1049     // at a full collection cycle, this is always currently
1050     // accurate.
1051     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1052       return false;
1053     }
1054   }
1055 
1056   ResourceMark rm;      // Needed for tracing.
1057 
1058   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1059   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1060   assert(oopDesc::is_oop_or_null(discovered), "Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered));
1061   if (discovered != NULL) {
1062     // The reference has already been discovered...
1063     log_develop_trace(gc, ref)("Already discovered reference (" INTPTR_FORMAT ": %s)",
1064                                p2i(obj), obj->klass()->internal_name());
1065     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1066       // assumes that an object is not processed twice;
1067       // if it's been already discovered it must be on another
1068       // generation's discovered list; so we won't discover it.
1069       return false;
1070     } else {
1071       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1072              "Unrecognized policy");
1073       // Check assumption that an object is not potentially
1074       // discovered twice except by concurrent collectors that potentially
1075       // trace the same Reference object twice.
1076       assert(UseConcMarkSweepGC || UseG1GC,
1077              "Only possible with a concurrent marking collector");
1078       return true;
1079     }
1080   }
1081 
1082   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1083     verify_referent(obj);
1084     // Discover if and only if EITHER:
1085     // .. reference is in our span, OR
1086     // .. we are an atomic collector and referent is in our span
1087     if (_span.contains(obj_addr) ||
1088         (discovery_is_atomic() &&
1089          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1090       // should_enqueue = true;
1091     } else {
1092       return false;
1093     }
1094   } else {
1095     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1096            _span.contains(obj_addr), "code inconsistency");
1097   }
1098 
1099   // Get the right type of discovered queue head.
1100   DiscoveredList* list = get_discovered_list(rt);
1101   if (list == NULL) {
1102     return false;   // nothing special needs to be done
1103   }
1104 
1105   if (_discovery_is_mt) {
1106     add_to_discovered_list_mt(*list, obj, discovered_addr);
1107   } else {
1108     // We do a raw store here: the field will be visited later when processing
1109     // the discovered references.
1110     oop current_head = list->head();
1111     // The last ref must have its discovered field pointing to itself.
1112     oop next_discovered = (current_head != NULL) ? current_head : obj;
1113 
1114     assert(discovered == NULL, "control point invariant");
1115     oop_store_raw(discovered_addr, next_discovered);
1116     list->set_head(obj);
1117     list->inc_length(1);
1118 
1119     log_develop_trace(gc, ref)("Discovered reference (" INTPTR_FORMAT ": %s)", p2i(obj), obj->klass()->internal_name());
1120   }
1121   assert(oopDesc::is_oop(obj), "Discovered a bad reference");
1122   verify_referent(obj);
1123   return true;
1124 }
1125 
1126 bool ReferenceProcessor::has_discovered_references() {
1127   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1128     if (!_discovered_refs[i].is_empty()) {
1129       return true;
1130     }
1131   }
1132   return false;
1133 }
1134 
1135 // Preclean the discovered references by removing those
1136 // whose referents are alive, and by marking from those that
1137 // are not active. These lists can be handled here
1138 // in any order and, indeed, concurrently.
1139 void ReferenceProcessor::preclean_discovered_references(
1140   BoolObjectClosure* is_alive,
1141   OopClosure* keep_alive,
1142   VoidClosure* complete_gc,
1143   YieldClosure* yield,
1144   GCTimer* gc_timer) {
1145 
1146   // Soft references
1147   {
1148     GCTraceTime(Debug, gc, ref) tm("Preclean SoftReferences", gc_timer);
1149     for (uint i = 0; i < _max_num_q; i++) {
1150       if (yield->should_return()) {
1151         return;
1152       }
1153       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1154                                   keep_alive, complete_gc, yield);
1155     }
1156   }
1157 
1158   // Weak references
1159   {
1160     GCTraceTime(Debug, gc, ref) tm("Preclean WeakReferences", gc_timer);
1161     for (uint i = 0; i < _max_num_q; i++) {
1162       if (yield->should_return()) {
1163         return;
1164       }
1165       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1166                                   keep_alive, complete_gc, yield);
1167     }
1168   }
1169 
1170   // Final references
1171   {
1172     GCTraceTime(Debug, gc, ref) tm("Preclean FinalReferences", gc_timer);
1173     for (uint i = 0; i < _max_num_q; i++) {
1174       if (yield->should_return()) {
1175         return;
1176       }
1177       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1178                                   keep_alive, complete_gc, yield);
1179     }
1180   }
1181 
1182   // Phantom references
1183   {
1184     GCTraceTime(Debug, gc, ref) tm("Preclean PhantomReferences", gc_timer);
1185     for (uint i = 0; i < _max_num_q; i++) {
1186       if (yield->should_return()) {
1187         return;
1188       }
1189       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1190                                   keep_alive, complete_gc, yield);
1191     }
1192   }
1193 }
1194 
1195 // Walk the given discovered ref list, and remove all reference objects
1196 // whose referents are still alive, whose referents are NULL or which
1197 // are not active (have a non-NULL next field). NOTE: When we are
1198 // thus precleaning the ref lists (which happens single-threaded today),
1199 // we do not disable refs discovery to honor the correct semantics of
1200 // java.lang.Reference. As a result, we need to be careful below
1201 // that ref removal steps interleave safely with ref discovery steps
1202 // (in this thread).
1203 void
1204 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1205                                                 BoolObjectClosure* is_alive,
1206                                                 OopClosure*        keep_alive,
1207                                                 VoidClosure*       complete_gc,
1208                                                 YieldClosure*      yield) {
1209   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1210   while (iter.has_next()) {
1211     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1212     oop obj = iter.obj();
1213     oop next = java_lang_ref_Reference::next(obj);
1214     if (iter.referent() == NULL || iter.is_referent_alive() ||
1215         next != NULL) {
1216       // The referent has been cleared, or is alive, or the Reference is not
1217       // active; we need to trace and mark its cohort.
1218       log_develop_trace(gc, ref)("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1219                                  p2i(iter.obj()), iter.obj()->klass()->internal_name());
1220       // Remove Reference object from list
1221       iter.remove();
1222       // Keep alive its cohort.
1223       iter.make_referent_alive();
1224       if (UseCompressedOops) {
1225         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1226         keep_alive->do_oop(next_addr);
1227       } else {
1228         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1229         keep_alive->do_oop(next_addr);
1230       }
1231       iter.move_to_next();
1232     } else {
1233       iter.next();
1234     }
1235   }
1236   // Close the reachable set
1237   complete_gc->do_void();
1238 
1239   NOT_PRODUCT(
1240     if (iter.processed() > 0) {
1241       log_develop_trace(gc, ref)(" Dropped " SIZE_FORMAT " Refs out of " SIZE_FORMAT " Refs in discovered list " INTPTR_FORMAT,
1242         iter.removed(), iter.processed(), p2i(&refs_list));
1243     }
1244   )
1245 }
1246 
1247 const char* ReferenceProcessor::list_name(uint i) {
1248    assert(i <= _max_num_q * number_of_subclasses_of_ref(),
1249           "Out of bounds index");
1250 
1251    int j = i / _max_num_q;
1252    switch (j) {
1253      case 0: return "SoftRef";
1254      case 1: return "WeakRef";
1255      case 2: return "FinalRef";
1256      case 3: return "PhantomRef";
1257    }
1258    ShouldNotReachHere();
1259    return NULL;
1260 }