1 /*
   2  * Copyright (c) 2001, 2013, 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.hpp"
  27 #include "classfile/systemDictionary.hpp"
  28 #include "gc_implementation/shared/gcTimer.hpp"
  29 #include "gc_implementation/shared/gcTraceTime.hpp"
  30 #include "gc_interface/collectedHeap.hpp"
  31 #include "gc_interface/collectedHeap.inline.hpp"
  32 #include "memory/referencePolicy.hpp"
  33 #include "memory/referenceProcessor.hpp"
  34 #include "oops/oop.inline.hpp"
  35 #include "runtime/java.hpp"
  36 #include "runtime/jniHandles.hpp"
  37 
  38 ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
  39 ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
  40 bool             ReferenceProcessor::_pending_list_uses_discovered_field = false;
  41 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;
  42 
  43 void referenceProcessor_init() {
  44   ReferenceProcessor::init_statics();
  45 }
  46 
  47 void ReferenceProcessor::init_statics() {
  48   // We need a monotonically non-deccreasing time in ms but
  49   // os::javaTimeMillis() does not guarantee monotonicity.
  50   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  51 
  52   // Initialize the soft ref timestamp clock.
  53   _soft_ref_timestamp_clock = now;
  54   // Also update the soft ref clock in j.l.r.SoftReference
  55   java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);
  56 
  57   _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  58   _default_soft_ref_policy      = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
  59                                       NOT_COMPILER2(LRUCurrentHeapPolicy());
  60   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
  61     vm_exit_during_initialization("Could not allocate reference policy object");
  62   }
  63   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
  64             RefDiscoveryPolicy == ReferentBasedDiscovery,
  65             "Unrecongnized RefDiscoveryPolicy");
  66   _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
  67 }
  68 
  69 void ReferenceProcessor::enable_discovery(bool verify_disabled, bool check_no_refs) {
  70 #ifdef ASSERT
  71   // Verify that we're not currently discovering refs
  72   assert(!verify_disabled || !_discovering_refs, "nested call?");
  73 
  74   if (check_no_refs) {
  75     // Verify that the discovered lists are empty
  76     verify_no_references_recorded();
  77   }
  78 #endif // ASSERT
  79 
  80   // Someone could have modified the value of the static
  81   // field in the j.l.r.SoftReference class that holds the
  82   // soft reference timestamp clock using reflection or
  83   // Unsafe between GCs. Unconditionally update the static
  84   // field in ReferenceProcessor here so that we use the new
  85   // value during reference discovery.
  86 
  87   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
  88   _discovering_refs = true;
  89 }
  90 
  91 ReferenceProcessor::ReferenceProcessor(MemRegion span,
  92                                        bool      mt_processing,
  93                                        uint      mt_processing_degree,
  94                                        bool      mt_discovery,
  95                                        uint      mt_discovery_degree,
  96                                        bool      atomic_discovery,
  97                                        BoolObjectClosure* is_alive_non_header,
  98                                        bool      discovered_list_needs_barrier)  :
  99   _discovering_refs(false),
 100   _enqueuing_is_done(false),
 101   _is_alive_non_header(is_alive_non_header),
 102   _discovered_list_needs_barrier(discovered_list_needs_barrier),
 103   _bs(NULL),
 104   _processing_is_mt(mt_processing),
 105   _next_id(0)
 106 {
 107   _span = span;
 108   _discovery_is_atomic = atomic_discovery;
 109   _discovery_is_mt     = mt_discovery;
 110   _num_q               = MAX2(1U, mt_processing_degree);
 111   _max_num_q           = MAX2(_num_q, mt_discovery_degree);
 112   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,
 113             _max_num_q * number_of_subclasses_of_ref(), mtGC);
 114 
 115   if (_discovered_refs == NULL) {
 116     vm_exit_during_initialization("Could not allocated RefProc Array");
 117   }
 118   _discoveredSoftRefs    = &_discovered_refs[0];
 119   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
 120   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
 121   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
 122 
 123   // Initialize all entries to NULL
 124   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 125     _discovered_refs[i].set_head(NULL);
 126     _discovered_refs[i].set_length(0);
 127   }
 128 
 129   // If we do barriers, cache a copy of the barrier set.
 130   if (discovered_list_needs_barrier) {
 131     _bs = Universe::heap()->barrier_set();
 132   }
 133   setup_policy(false /* default soft ref policy */);
 134 }
 135 
 136 #ifndef PRODUCT
 137 void ReferenceProcessor::verify_no_references_recorded() {
 138   guarantee(!_discovering_refs, "Discovering refs?");
 139   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 140     guarantee(_discovered_refs[i].is_empty(),
 141               "Found non-empty discovered list");
 142   }
 143 }
 144 #endif
 145 
 146 void ReferenceProcessor::weak_oops_do(OopClosure* f) {
 147   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 148     if (UseCompressedOops) {
 149       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());
 150     } else {
 151       f->do_oop((oop*)_discovered_refs[i].adr_head());
 152     }
 153   }
 154 }
 155 
 156 void ReferenceProcessor::update_soft_ref_master_clock() {
 157   // Update (advance) the soft ref master clock field. This must be done
 158   // after processing the soft ref list.
 159 
 160   // We need a monotonically non-deccreasing time in ms but
 161   // os::javaTimeMillis() does not guarantee monotonicity.
 162   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
 163   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();
 164   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");
 165 
 166   NOT_PRODUCT(
 167   if (now < _soft_ref_timestamp_clock) {
 168     warning("time warp: "INT64_FORMAT" to "INT64_FORMAT,
 169             _soft_ref_timestamp_clock, now);
 170   }
 171   )
 172   // The values of now and _soft_ref_timestamp_clock are set using
 173   // javaTimeNanos(), which is guaranteed to be monotonically
 174   // non-decreasing provided the underlying platform provides such
 175   // a time source (and it is bug free).
 176   // In product mode, however, protect ourselves from non-monotonicty.
 177   if (now > _soft_ref_timestamp_clock) {
 178     _soft_ref_timestamp_clock = now;
 179     java_lang_ref_SoftReference::set_clock(now);
 180   }
 181   // Else leave clock stalled at its old value until time progresses
 182   // past clock value.
 183 }
 184 
 185 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {
 186   size_t total = 0;
 187   for (uint i = 0; i < _max_num_q; ++i) {
 188     total += lists[i].length();
 189   }
 190   return total;
 191 }
 192 
 193 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(
 194   BoolObjectClosure*           is_alive,
 195   OopClosure*                  keep_alive,
 196   VoidClosure*                 complete_gc,
 197   AbstractRefProcTaskExecutor* task_executor,
 198   GCTimer*                     gc_timer) {
 199   NOT_PRODUCT(verify_ok_to_handle_reflists());
 200 
 201   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");
 202   // Stop treating discovered references specially.
 203   disable_discovery();
 204 
 205   // If discovery was concurrent, someone could have modified
 206   // the value of the static field in the j.l.r.SoftReference
 207   // class that holds the soft reference timestamp clock using
 208   // reflection or Unsafe between when discovery was enabled and
 209   // now. Unconditionally update the static field in ReferenceProcessor
 210   // here so that we use the new value during processing of the
 211   // discovered soft refs.
 212 
 213   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();
 214 
 215   bool trace_time = PrintGCDetails && PrintReferenceGC;
 216 
 217   // Soft references
 218   size_t soft_count = 0;
 219   {
 220     GCTraceTime tt("SoftReference", trace_time, false, gc_timer);
 221     soft_count =
 222       process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,
 223                                  is_alive, keep_alive, complete_gc, task_executor);
 224   }
 225 
 226   update_soft_ref_master_clock();
 227 
 228   // Weak references
 229   size_t weak_count = 0;
 230   {
 231     GCTraceTime tt("WeakReference", trace_time, false, gc_timer);
 232     weak_count =
 233       process_discovered_reflist(_discoveredWeakRefs, NULL, true,
 234                                  is_alive, keep_alive, complete_gc, task_executor);
 235   }
 236 
 237   // Final references
 238   size_t final_count = 0;
 239   {
 240     GCTraceTime tt("FinalReference", trace_time, false, gc_timer);
 241     final_count =
 242       process_discovered_reflist(_discoveredFinalRefs, NULL, false,
 243                                  is_alive, keep_alive, complete_gc, task_executor);
 244   }
 245 
 246   // Phantom references
 247   size_t phantom_count = 0;
 248   {
 249     GCTraceTime tt("PhantomReference", trace_time, false, gc_timer);
 250     phantom_count =
 251       process_discovered_reflist(_discoveredPhantomRefs, NULL, false,
 252                                  is_alive, keep_alive, complete_gc, task_executor);
 253   }
 254 
 255   // Weak global JNI references. It would make more sense (semantically) to
 256   // traverse these simultaneously with the regular weak references above, but
 257   // that is not how the JDK1.2 specification is. See #4126360. Native code can
 258   // thus use JNI weak references to circumvent the phantom references and
 259   // resurrect a "post-mortem" object.
 260   {
 261     GCTraceTime tt("JNI Weak Reference", trace_time, false, gc_timer);
 262     if (task_executor != NULL) {
 263       task_executor->set_single_threaded_mode();
 264     }
 265     process_phaseJNI(is_alive, keep_alive, complete_gc);
 266   }
 267 
 268   return ReferenceProcessorStats(soft_count, weak_count, final_count, phantom_count);
 269 }
 270 
 271 #ifndef PRODUCT
 272 // Calculate the number of jni handles.
 273 uint ReferenceProcessor::count_jni_refs() {
 274   class AlwaysAliveClosure: public BoolObjectClosure {
 275   public:
 276     virtual bool do_object_b(oop obj) { return true; }
 277   };
 278 
 279   class CountHandleClosure: public OopClosure {
 280   private:
 281     int _count;
 282   public:
 283     CountHandleClosure(): _count(0) {}
 284     void do_oop(oop* unused)       { _count++; }
 285     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }
 286     int count() { return _count; }
 287   };
 288   CountHandleClosure global_handle_count;
 289   AlwaysAliveClosure always_alive;
 290   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);
 291   return global_handle_count.count();
 292 }
 293 #endif
 294 
 295 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,
 296                                           OopClosure*        keep_alive,
 297                                           VoidClosure*       complete_gc) {
 298 #ifndef PRODUCT
 299   if (PrintGCDetails && PrintReferenceGC) {
 300     unsigned int count = count_jni_refs();
 301     gclog_or_tty->print(", %u refs", count);
 302   }
 303 #endif
 304   JNIHandles::weak_oops_do(is_alive, keep_alive);
 305   complete_gc->do_void();
 306 }
 307 
 308 
 309 template <class T>
 310 bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,
 311                                    AbstractRefProcTaskExecutor* task_executor) {
 312 
 313   // Remember old value of pending references list
 314   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();
 315   T old_pending_list_value = *pending_list_addr;
 316 
 317   // Enqueue references that are not made active again, and
 318   // clear the decks for the next collection (cycle).
 319   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);
 320   // Do the oop-check on pending_list_addr missed in
 321   // enqueue_discovered_reflist. We should probably
 322   // do a raw oop_check so that future such idempotent
 323   // oop_stores relying on the oop-check side-effect
 324   // may be elided automatically and safely without
 325   // affecting correctness.
 326   oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));
 327 
 328   // Stop treating discovered references specially.
 329   ref->disable_discovery();
 330 
 331   // Return true if new pending references were added
 332   return old_pending_list_value != *pending_list_addr;
 333 }
 334 
 335 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {
 336   NOT_PRODUCT(verify_ok_to_handle_reflists());
 337   if (UseCompressedOops) {
 338     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);
 339   } else {
 340     return enqueue_discovered_ref_helper<oop>(this, task_executor);
 341   }
 342 }
 343 
 344 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,
 345                                                     HeapWord* pending_list_addr) {
 346   // Given a list of refs linked through the "discovered" field
 347   // (java.lang.ref.Reference.discovered), self-loop their "next" field
 348   // thus distinguishing them from active References, then
 349   // prepend them to the pending list.
 350   // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
 351   // the "next" field is used to chain the pending list, not the discovered
 352   // field.
 353 
 354   if (TraceReferenceGC && PrintGCDetails) {
 355     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
 356                            INTPTR_FORMAT, (address)refs_list.head());
 357   }
 358 
 359   oop obj = NULL;
 360   oop next_d = refs_list.head();
 361   if (pending_list_uses_discovered_field()) { // New behaviour
 362     // Walk down the list, self-looping the next field
 363     // so that the References are not considered active.
 364     while (obj != next_d) {
 365       obj = next_d;
 366       assert(obj->is_instanceRef(), "should be reference object");
 367       next_d = java_lang_ref_Reference::discovered(obj);
 368       if (TraceReferenceGC && PrintGCDetails) {
 369         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 370                                obj, next_d);
 371       }
 372       assert(java_lang_ref_Reference::next(obj) == NULL,
 373              "Reference not active; should not be discovered");
 374       // Self-loop next, so as to make Ref not active.
 375       java_lang_ref_Reference::set_next(obj, obj);
 376       if (next_d == obj) {  // obj is last
 377         // Swap refs_list into pendling_list_addr and
 378         // set obj's discovered to what we read from pending_list_addr.
 379         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 380         // Need oop_check on pending_list_addr above;
 381         // see special oop-check code at the end of
 382         // enqueue_discovered_reflists() further below.
 383         java_lang_ref_Reference::set_discovered(obj, old); // old may be NULL
 384       }
 385     }
 386   } else { // Old behaviour
 387     // Walk down the list, copying the discovered field into
 388     // the next field and clearing the discovered field.
 389     while (obj != next_d) {
 390       obj = next_d;
 391       assert(obj->is_instanceRef(), "should be reference object");
 392       next_d = java_lang_ref_Reference::discovered(obj);
 393       if (TraceReferenceGC && PrintGCDetails) {
 394         gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
 395                                obj, next_d);
 396       }
 397       assert(java_lang_ref_Reference::next(obj) == NULL,
 398              "The reference should not be enqueued");
 399       if (next_d == obj) {  // obj is last
 400         // Swap refs_list into pendling_list_addr and
 401         // set obj's next to what we read from pending_list_addr.
 402         oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);
 403         // Need oop_check on pending_list_addr above;
 404         // see special oop-check code at the end of
 405         // enqueue_discovered_reflists() further below.
 406         if (old == NULL) {
 407           // obj should be made to point to itself, since
 408           // pending list was empty.
 409           java_lang_ref_Reference::set_next(obj, obj);
 410         } else {
 411           java_lang_ref_Reference::set_next(obj, old);
 412         }
 413       } else {
 414         java_lang_ref_Reference::set_next(obj, next_d);
 415       }
 416       java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
 417     }
 418   }
 419 }
 420 
 421 // Parallel enqueue task
 422 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
 423 public:
 424   RefProcEnqueueTask(ReferenceProcessor& ref_processor,
 425                      DiscoveredList      discovered_refs[],
 426                      HeapWord*           pending_list_addr,
 427                      int                 n_queues)
 428     : EnqueueTask(ref_processor, discovered_refs,
 429                   pending_list_addr, n_queues)
 430   { }
 431 
 432   virtual void work(unsigned int work_id) {
 433     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
 434     // Simplest first cut: static partitioning.
 435     int index = work_id;
 436     // The increment on "index" must correspond to the maximum number of queues
 437     // (n_queues) with which that ReferenceProcessor was created.  That
 438     // is because of the "clever" way the discovered references lists were
 439     // allocated and are indexed into.
 440     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
 441     for (int j = 0;
 442          j < ReferenceProcessor::number_of_subclasses_of_ref();
 443          j++, index += _n_queues) {
 444       _ref_processor.enqueue_discovered_reflist(
 445         _refs_lists[index], _pending_list_addr);
 446       _refs_lists[index].set_head(NULL);
 447       _refs_lists[index].set_length(0);
 448     }
 449   }
 450 };
 451 
 452 // Enqueue references that are not made active again
 453 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,
 454   AbstractRefProcTaskExecutor* task_executor) {
 455   if (_processing_is_mt && task_executor != NULL) {
 456     // Parallel code
 457     RefProcEnqueueTask tsk(*this, _discovered_refs,
 458                            pending_list_addr, _max_num_q);
 459     task_executor->execute(tsk);
 460   } else {
 461     // Serial code: call the parent class's implementation
 462     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 463       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);
 464       _discovered_refs[i].set_head(NULL);
 465       _discovered_refs[i].set_length(0);
 466     }
 467   }
 468 }
 469 
 470 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {
 471   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);
 472   oop discovered = java_lang_ref_Reference::discovered(_ref);
 473   assert(_discovered_addr && discovered->is_oop_or_null(),
 474          "discovered field is bad");
 475   _next = discovered;
 476   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);
 477   _referent = java_lang_ref_Reference::referent(_ref);
 478   assert(Universe::heap()->is_in_reserved_or_null(_referent),
 479          "Wrong oop found in java.lang.Reference object");
 480   assert(allow_null_referent ?
 481              _referent->is_oop_or_null()
 482            : _referent->is_oop(),
 483          "bad referent");
 484 }
 485 
 486 void DiscoveredListIterator::remove() {
 487   assert(_ref->is_oop(), "Dropping a bad reference");
 488   oop_store_raw(_discovered_addr, NULL);
 489 
 490   // First _prev_next ref actually points into DiscoveredList (gross).
 491   oop new_next;
 492   if (_next == _ref) {
 493     // At the end of the list, we should make _prev point to itself.
 494     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
 495     // and _prev will be NULL.
 496     new_next = _prev;
 497   } else {
 498     new_next = _next;
 499   }
 500 
 501   if (UseCompressedOops) {
 502     // Remove Reference object from list.
 503     oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
 504   } else {
 505     // Remove Reference object from list.
 506     oopDesc::store_heap_oop((oop*)_prev_next, new_next);
 507   }
 508   NOT_PRODUCT(_removed++);
 509   _refs_list.dec_length(1);
 510 }
 511 
 512 // Make the Reference object active again.
 513 void DiscoveredListIterator::make_active() {
 514   // For G1 we don't want to use set_next - it
 515   // will dirty the card for the next field of
 516   // the reference object and will fail
 517   // CT verification.
 518   if (UseG1GC) {
 519     BarrierSet* bs = oopDesc::bs();
 520     HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);
 521 
 522     if (UseCompressedOops) {
 523       bs->write_ref_field_pre((narrowOop*)next_addr, NULL);
 524     } else {
 525       bs->write_ref_field_pre((oop*)next_addr, NULL);
 526     }
 527     java_lang_ref_Reference::set_next_raw(_ref, NULL);
 528   } else {
 529     java_lang_ref_Reference::set_next(_ref, NULL);
 530   }
 531 }
 532 
 533 void DiscoveredListIterator::clear_referent() {
 534   oop_store_raw(_referent_addr, NULL);
 535 }
 536 
 537 // NOTE: process_phase*() are largely similar, and at a high level
 538 // merely iterate over the extant list applying a predicate to
 539 // each of its elements and possibly removing that element from the
 540 // list and applying some further closures to that element.
 541 // We should consider the possibility of replacing these
 542 // process_phase*() methods by abstracting them into
 543 // a single general iterator invocation that receives appropriate
 544 // closures that accomplish this work.
 545 
 546 // (SoftReferences only) Traverse the list and remove any SoftReferences whose
 547 // referents are not alive, but that should be kept alive for policy reasons.
 548 // Keep alive the transitive closure of all such referents.
 549 void
 550 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,
 551                                    ReferencePolicy*   policy,
 552                                    BoolObjectClosure* is_alive,
 553                                    OopClosure*        keep_alive,
 554                                    VoidClosure*       complete_gc) {
 555   assert(policy != NULL, "Must have a non-NULL policy");
 556   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 557   // Decide which softly reachable refs should be kept alive.
 558   while (iter.has_next()) {
 559     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));
 560     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();
 561     if (referent_is_dead &&
 562         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {
 563       if (TraceReferenceGC) {
 564         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",
 565                                iter.obj(), iter.obj()->klass()->internal_name());
 566       }
 567       // Remove Reference object from list
 568       iter.remove();
 569       // Make the Reference object active again
 570       iter.make_active();
 571       // keep the referent around
 572       iter.make_referent_alive();
 573       iter.move_to_next();
 574     } else {
 575       iter.next();
 576     }
 577   }
 578   // Close the reachable set
 579   complete_gc->do_void();
 580   NOT_PRODUCT(
 581     if (PrintGCDetails && TraceReferenceGC) {
 582       gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
 583         "discovered Refs by policy, from list " INTPTR_FORMAT,
 584         iter.removed(), iter.processed(), (address)refs_list.head());
 585     }
 586   )
 587 }
 588 
 589 // Traverse the list and remove any Refs that are not active, or
 590 // whose referents are either alive or NULL.
 591 void
 592 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,
 593                              BoolObjectClosure* is_alive,
 594                              OopClosure*        keep_alive) {
 595   assert(discovery_is_atomic(), "Error");
 596   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 597   while (iter.has_next()) {
 598     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 599     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)
 600     assert(next == NULL, "Should not discover inactive Reference");
 601     if (iter.is_referent_alive()) {
 602       if (TraceReferenceGC) {
 603         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",
 604                                iter.obj(), iter.obj()->klass()->internal_name());
 605       }
 606       // The referent is reachable after all.
 607       // Remove Reference object from list.
 608       iter.remove();
 609       // Update the referent pointer as necessary: Note that this
 610       // should not entail any recursive marking because the
 611       // referent must already have been traversed.
 612       iter.make_referent_alive();
 613       iter.move_to_next();
 614     } else {
 615       iter.next();
 616     }
 617   }
 618   NOT_PRODUCT(
 619     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 620       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 621         "Refs in discovered list " INTPTR_FORMAT,
 622         iter.removed(), iter.processed(), (address)refs_list.head());
 623     }
 624   )
 625 }
 626 
 627 void
 628 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,
 629                                                   BoolObjectClosure* is_alive,
 630                                                   OopClosure*        keep_alive,
 631                                                   VoidClosure*       complete_gc) {
 632   assert(!discovery_is_atomic(), "Error");
 633   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 634   while (iter.has_next()) {
 635     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 636     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());
 637     oop next = java_lang_ref_Reference::next(iter.obj());
 638     if ((iter.referent() == NULL || iter.is_referent_alive() ||
 639          next != NULL)) {
 640       assert(next->is_oop_or_null(), "bad next field");
 641       // Remove Reference object from list
 642       iter.remove();
 643       // Trace the cohorts
 644       iter.make_referent_alive();
 645       if (UseCompressedOops) {
 646         keep_alive->do_oop((narrowOop*)next_addr);
 647       } else {
 648         keep_alive->do_oop((oop*)next_addr);
 649       }
 650       iter.move_to_next();
 651     } else {
 652       iter.next();
 653     }
 654   }
 655   // Now close the newly reachable set
 656   complete_gc->do_void();
 657   NOT_PRODUCT(
 658     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
 659       gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
 660         "Refs in discovered list " INTPTR_FORMAT,
 661         iter.removed(), iter.processed(), (address)refs_list.head());
 662     }
 663   )
 664 }
 665 
 666 // Traverse the list and process the referents, by either
 667 // clearing them or keeping them (and their reachable
 668 // closure) alive.
 669 void
 670 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,
 671                                    bool               clear_referent,
 672                                    BoolObjectClosure* is_alive,
 673                                    OopClosure*        keep_alive,
 674                                    VoidClosure*       complete_gc) {
 675   ResourceMark rm;
 676   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
 677   while (iter.has_next()) {
 678     iter.update_discovered();
 679     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));
 680     if (clear_referent) {
 681       // NULL out referent pointer
 682       iter.clear_referent();
 683     } else {
 684       // keep the referent around
 685       iter.make_referent_alive();
 686     }
 687     if (TraceReferenceGC) {
 688       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",
 689                              clear_referent ? "cleared " : "",
 690                              iter.obj(), iter.obj()->klass()->internal_name());
 691     }
 692     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");
 693     iter.next();
 694   }
 695   // Remember to update the next pointer of the last ref.
 696   iter.update_discovered();
 697   // Close the reachable set
 698   complete_gc->do_void();
 699 }
 700 
 701 void
 702 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
 703   oop obj = NULL;
 704   oop next = refs_list.head();
 705   while (next != obj) {
 706     obj = next;
 707     next = java_lang_ref_Reference::discovered(obj);
 708     java_lang_ref_Reference::set_discovered_raw(obj, NULL);
 709   }
 710   refs_list.set_head(NULL);
 711   refs_list.set_length(0);
 712 }
 713 
 714 void
 715 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
 716   clear_discovered_references(refs_list);
 717 }
 718 
 719 void ReferenceProcessor::abandon_partial_discovery() {
 720   // loop over the lists
 721   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 722     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 723       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));
 724     }
 725     abandon_partial_discovered_list(_discovered_refs[i]);
 726   }
 727 }
 728 
 729 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {
 730 public:
 731   RefProcPhase1Task(ReferenceProcessor& ref_processor,
 732                     DiscoveredList      refs_lists[],
 733                     ReferencePolicy*    policy,
 734                     bool                marks_oops_alive)
 735     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 736       _policy(policy)
 737   { }
 738   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 739                     OopClosure& keep_alive,
 740                     VoidClosure& complete_gc)
 741   {
 742     Thread* thr = Thread::current();
 743     int refs_list_index = ((WorkerThread*)thr)->id();
 744     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,
 745                                   &is_alive, &keep_alive, &complete_gc);
 746   }
 747 private:
 748   ReferencePolicy* _policy;
 749 };
 750 
 751 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {
 752 public:
 753   RefProcPhase2Task(ReferenceProcessor& ref_processor,
 754                     DiscoveredList      refs_lists[],
 755                     bool                marks_oops_alive)
 756     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)
 757   { }
 758   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 759                     OopClosure& keep_alive,
 760                     VoidClosure& complete_gc)
 761   {
 762     _ref_processor.process_phase2(_refs_lists[i],
 763                                   &is_alive, &keep_alive, &complete_gc);
 764   }
 765 };
 766 
 767 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {
 768 public:
 769   RefProcPhase3Task(ReferenceProcessor& ref_processor,
 770                     DiscoveredList      refs_lists[],
 771                     bool                clear_referent,
 772                     bool                marks_oops_alive)
 773     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),
 774       _clear_referent(clear_referent)
 775   { }
 776   virtual void work(unsigned int i, BoolObjectClosure& is_alive,
 777                     OopClosure& keep_alive,
 778                     VoidClosure& complete_gc)
 779   {
 780     // Don't use "refs_list_index" calculated in this way because
 781     // balance_queues() has moved the Ref's into the first n queues.
 782     // Thread* thr = Thread::current();
 783     // int refs_list_index = ((WorkerThread*)thr)->id();
 784     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
 785     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
 786                                   &is_alive, &keep_alive, &complete_gc);
 787   }
 788 private:
 789   bool _clear_referent;
 790 };
 791 
 792 void ReferenceProcessor::set_discovered(oop ref, oop value) {
 793   if (_discovered_list_needs_barrier) {
 794     java_lang_ref_Reference::set_discovered(ref, value);
 795   } else {
 796     java_lang_ref_Reference::set_discovered_raw(ref, value);
 797   }
 798 }
 799 
 800 // Balances reference queues.
 801 // Move entries from all queues[0, 1, ..., _max_num_q-1] to
 802 // queues[0, 1, ..., _num_q-1] because only the first _num_q
 803 // corresponding to the active workers will be processed.
 804 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
 805 {
 806   // calculate total length
 807   size_t total_refs = 0;
 808   if (TraceReferenceGC && PrintGCDetails) {
 809     gclog_or_tty->print_cr("\nBalance ref_lists ");
 810   }
 811 
 812   for (uint i = 0; i < _max_num_q; ++i) {
 813     total_refs += ref_lists[i].length();
 814     if (TraceReferenceGC && PrintGCDetails) {
 815       gclog_or_tty->print("%d ", ref_lists[i].length());
 816     }
 817   }
 818   if (TraceReferenceGC && PrintGCDetails) {
 819     gclog_or_tty->print_cr(" = %d", total_refs);
 820   }
 821   size_t avg_refs = total_refs / _num_q + 1;
 822   uint to_idx = 0;
 823   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {
 824     bool move_all = false;
 825     if (from_idx >= _num_q) {
 826       move_all = ref_lists[from_idx].length() > 0;
 827     }
 828     while ((ref_lists[from_idx].length() > avg_refs) ||
 829            move_all) {
 830       assert(to_idx < _num_q, "Sanity Check!");
 831       if (ref_lists[to_idx].length() < avg_refs) {
 832         // move superfluous refs
 833         size_t refs_to_move;
 834         // Move all the Ref's if the from queue will not be processed.
 835         if (move_all) {
 836           refs_to_move = MIN2(ref_lists[from_idx].length(),
 837                               avg_refs - ref_lists[to_idx].length());
 838         } else {
 839           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
 840                               avg_refs - ref_lists[to_idx].length());
 841         }
 842 
 843         assert(refs_to_move > 0, "otherwise the code below will fail");
 844 
 845         oop move_head = ref_lists[from_idx].head();
 846         oop move_tail = move_head;
 847         oop new_head  = move_head;
 848         // find an element to split the list on
 849         for (size_t j = 0; j < refs_to_move; ++j) {
 850           move_tail = new_head;
 851           new_head = java_lang_ref_Reference::discovered(new_head);
 852         }
 853 
 854         // Add the chain to the to list.
 855         if (ref_lists[to_idx].head() == NULL) {
 856           // to list is empty. Make a loop at the end.
 857           set_discovered(move_tail, move_tail);
 858         } else {
 859           set_discovered(move_tail, ref_lists[to_idx].head());
 860         }
 861         ref_lists[to_idx].set_head(move_head);
 862         ref_lists[to_idx].inc_length(refs_to_move);
 863 
 864         // Remove the chain from the from list.
 865         if (move_tail == new_head) {
 866           // We found the end of the from list.
 867           ref_lists[from_idx].set_head(NULL);
 868         } else {
 869           ref_lists[from_idx].set_head(new_head);
 870         }
 871         ref_lists[from_idx].dec_length(refs_to_move);
 872         if (ref_lists[from_idx].length() == 0) {
 873           break;
 874         }
 875       } else {
 876         to_idx = (to_idx + 1) % _num_q;
 877       }
 878     }
 879   }
 880 #ifdef ASSERT
 881   size_t balanced_total_refs = 0;
 882   for (uint i = 0; i < _max_num_q; ++i) {
 883     balanced_total_refs += ref_lists[i].length();
 884     if (TraceReferenceGC && PrintGCDetails) {
 885       gclog_or_tty->print("%d ", ref_lists[i].length());
 886     }
 887   }
 888   if (TraceReferenceGC && PrintGCDetails) {
 889     gclog_or_tty->print_cr(" = %d", balanced_total_refs);
 890     gclog_or_tty->flush();
 891   }
 892   assert(total_refs == balanced_total_refs, "Balancing was incomplete");
 893 #endif
 894 }
 895 
 896 void ReferenceProcessor::balance_all_queues() {
 897   balance_queues(_discoveredSoftRefs);
 898   balance_queues(_discoveredWeakRefs);
 899   balance_queues(_discoveredFinalRefs);
 900   balance_queues(_discoveredPhantomRefs);
 901 }
 902 
 903 size_t
 904 ReferenceProcessor::process_discovered_reflist(
 905   DiscoveredList               refs_lists[],
 906   ReferencePolicy*             policy,
 907   bool                         clear_referent,
 908   BoolObjectClosure*           is_alive,
 909   OopClosure*                  keep_alive,
 910   VoidClosure*                 complete_gc,
 911   AbstractRefProcTaskExecutor* task_executor)
 912 {
 913   bool mt_processing = task_executor != NULL && _processing_is_mt;
 914   // If discovery used MT and a dynamic number of GC threads, then
 915   // the queues must be balanced for correctness if fewer than the
 916   // maximum number of queues were used.  The number of queue used
 917   // during discovery may be different than the number to be used
 918   // for processing so don't depend of _num_q < _max_num_q as part
 919   // of the test.
 920   bool must_balance = _discovery_is_mt;
 921 
 922   if ((mt_processing && ParallelRefProcBalancingEnabled) ||
 923       must_balance) {
 924     balance_queues(refs_lists);
 925   }
 926 
 927   size_t total_list_count = total_count(refs_lists);
 928 
 929   if (PrintReferenceGC && PrintGCDetails) {
 930     gclog_or_tty->print(", %u refs", total_list_count);
 931   }
 932 
 933   // Phase 1 (soft refs only):
 934   // . Traverse the list and remove any SoftReferences whose
 935   //   referents are not alive, but that should be kept alive for
 936   //   policy reasons. Keep alive the transitive closure of all
 937   //   such referents.
 938   if (policy != NULL) {
 939     if (mt_processing) {
 940       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
 941       task_executor->execute(phase1);
 942     } else {
 943       for (uint i = 0; i < _max_num_q; i++) {
 944         process_phase1(refs_lists[i], policy,
 945                        is_alive, keep_alive, complete_gc);
 946       }
 947     }
 948   } else { // policy == NULL
 949     assert(refs_lists != _discoveredSoftRefs,
 950            "Policy must be specified for soft references.");
 951   }
 952 
 953   // Phase 2:
 954   // . Traverse the list and remove any refs whose referents are alive.
 955   if (mt_processing) {
 956     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
 957     task_executor->execute(phase2);
 958   } else {
 959     for (uint i = 0; i < _max_num_q; i++) {
 960       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);
 961     }
 962   }
 963 
 964   // Phase 3:
 965   // . Traverse the list and process referents as appropriate.
 966   if (mt_processing) {
 967     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
 968     task_executor->execute(phase3);
 969   } else {
 970     for (uint i = 0; i < _max_num_q; i++) {
 971       process_phase3(refs_lists[i], clear_referent,
 972                      is_alive, keep_alive, complete_gc);
 973     }
 974   }
 975 
 976   return total_list_count;
 977 }
 978 
 979 void ReferenceProcessor::clean_up_discovered_references() {
 980   // loop over the lists
 981   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
 982     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {
 983       gclog_or_tty->print_cr(
 984         "\nScrubbing %s discovered list of Null referents",
 985         list_name(i));
 986     }
 987     clean_up_discovered_reflist(_discovered_refs[i]);
 988   }
 989 }
 990 
 991 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {
 992   assert(!discovery_is_atomic(), "Else why call this method?");
 993   DiscoveredListIterator iter(refs_list, NULL, NULL);
 994   while (iter.has_next()) {
 995     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
 996     oop next = java_lang_ref_Reference::next(iter.obj());
 997     assert(next->is_oop_or_null(), "bad next field");
 998     // If referent has been cleared or Reference is not active,
 999     // drop it.
1000     if (iter.referent() == NULL || next != NULL) {
1001       debug_only(
1002         if (PrintGCDetails && TraceReferenceGC) {
1003           gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "
1004             INTPTR_FORMAT " with next field: " INTPTR_FORMAT
1005             " and referent: " INTPTR_FORMAT,
1006             iter.obj(), next, iter.referent());
1007         }
1008       )
1009       // Remove Reference object from list
1010       iter.remove();
1011       iter.move_to_next();
1012     } else {
1013       iter.next();
1014     }
1015   }
1016   NOT_PRODUCT(
1017     if (PrintGCDetails && TraceReferenceGC) {
1018       gclog_or_tty->print(
1019         " Removed %d Refs with NULL referents out of %d discovered Refs",
1020         iter.removed(), iter.processed());
1021     }
1022   )
1023 }
1024 
1025 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {
1026   uint id = 0;
1027   // Determine the queue index to use for this object.
1028   if (_discovery_is_mt) {
1029     // During a multi-threaded discovery phase,
1030     // each thread saves to its "own" list.
1031     Thread* thr = Thread::current();
1032     id = thr->as_Worker_thread()->id();
1033   } else {
1034     // single-threaded discovery, we save in round-robin
1035     // fashion to each of the lists.
1036     if (_processing_is_mt) {
1037       id = next_id();
1038     }
1039   }
1040   assert(0 <= id && id < _max_num_q, "Id is out-of-bounds (call Freud?)");
1041 
1042   // Get the discovered queue to which we will add
1043   DiscoveredList* list = NULL;
1044   switch (rt) {
1045     case REF_OTHER:
1046       // Unknown reference type, no special treatment
1047       break;
1048     case REF_SOFT:
1049       list = &_discoveredSoftRefs[id];
1050       break;
1051     case REF_WEAK:
1052       list = &_discoveredWeakRefs[id];
1053       break;
1054     case REF_FINAL:
1055       list = &_discoveredFinalRefs[id];
1056       break;
1057     case REF_PHANTOM:
1058       list = &_discoveredPhantomRefs[id];
1059       break;
1060     case REF_NONE:
1061       // we should not reach here if we are an InstanceRefKlass
1062     default:
1063       ShouldNotReachHere();
1064   }
1065   if (TraceReferenceGC && PrintGCDetails) {
1066     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
1067   }
1068   return list;
1069 }
1070 
1071 inline void
1072 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,
1073                                               oop             obj,
1074                                               HeapWord*       discovered_addr) {
1075   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");
1076   // First we must make sure this object is only enqueued once. CAS in a non null
1077   // discovered_addr.
1078   oop current_head = refs_list.head();
1079   // The last ref must have its discovered field pointing to itself.
1080   oop next_discovered = (current_head != NULL) ? current_head : obj;
1081 
1082   // Note: In the case of G1, this specific pre-barrier is strictly
1083   // not necessary because the only case we are interested in
1084   // here is when *discovered_addr is NULL (see the CAS further below),
1085   // so this will expand to nothing. As a result, we have manually
1086   // elided this out for G1, but left in the test for some future
1087   // collector that might have need for a pre-barrier here, e.g.:-
1088   // _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1089   assert(!_discovered_list_needs_barrier || UseG1GC,
1090          "Need to check non-G1 collector: "
1091          "may need a pre-write-barrier for CAS from NULL below");
1092   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,
1093                                                     NULL);
1094   if (retest == NULL) {
1095     // This thread just won the right to enqueue the object.
1096     // We have separate lists for enqueueing, so no synchronization
1097     // is necessary.
1098     refs_list.set_head(obj);
1099     refs_list.inc_length(1);
1100     if (_discovered_list_needs_barrier) {
1101       _bs->write_ref_field((void*)discovered_addr, next_discovered);
1102     }
1103 
1104     if (TraceReferenceGC) {
1105       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
1106                              obj, obj->klass()->internal_name());
1107     }
1108   } else {
1109     // If retest was non NULL, another thread beat us to it:
1110     // The reference has already been discovered...
1111     if (TraceReferenceGC) {
1112       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1113                              obj, obj->klass()->internal_name());
1114     }
1115   }
1116 }
1117 
1118 #ifndef PRODUCT
1119 // Non-atomic (i.e. concurrent) discovery might allow us
1120 // to observe j.l.References with NULL referents, being those
1121 // cleared concurrently by mutators during (or after) discovery.
1122 void ReferenceProcessor::verify_referent(oop obj) {
1123   bool da = discovery_is_atomic();
1124   oop referent = java_lang_ref_Reference::referent(obj);
1125   assert(da ? referent->is_oop() : referent->is_oop_or_null(),
1126          err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
1127                  INTPTR_FORMAT " during %satomic discovery ",
1128                  (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
1129 }
1130 #endif
1131 
1132 // We mention two of several possible choices here:
1133 // #0: if the reference object is not in the "originating generation"
1134 //     (or part of the heap being collected, indicated by our "span"
1135 //     we don't treat it specially (i.e. we scan it as we would
1136 //     a normal oop, treating its references as strong references).
1137 //     This means that references can't be discovered unless their
1138 //     referent is also in the same span. This is the simplest,
1139 //     most "local" and most conservative approach, albeit one
1140 //     that may cause weak references to be enqueued least promptly.
1141 //     We call this choice the "ReferenceBasedDiscovery" policy.
1142 // #1: the reference object may be in any generation (span), but if
1143 //     the referent is in the generation (span) being currently collected
1144 //     then we can discover the reference object, provided
1145 //     the object has not already been discovered by
1146 //     a different concurrently running collector (as may be the
1147 //     case, for instance, if the reference object is in CMS and
1148 //     the referent in DefNewGeneration), and provided the processing
1149 //     of this reference object by the current collector will
1150 //     appear atomic to every other collector in the system.
1151 //     (Thus, for instance, a concurrent collector may not
1152 //     discover references in other generations even if the
1153 //     referent is in its own generation). This policy may,
1154 //     in certain cases, enqueue references somewhat sooner than
1155 //     might Policy #0 above, but at marginally increased cost
1156 //     and complexity in processing these references.
1157 //     We call this choice the "RefeferentBasedDiscovery" policy.
1158 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
1159   // Make sure we are discovering refs (rather than processing discovered refs).
1160   if (!_discovering_refs || !RegisterReferences) {
1161     return false;
1162   }
1163   // We only discover active references.
1164   oop next = java_lang_ref_Reference::next(obj);
1165   if (next != NULL) {   // Ref is no longer active
1166     return false;
1167   }
1168 
1169   HeapWord* obj_addr = (HeapWord*)obj;
1170   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1171       !_span.contains(obj_addr)) {
1172     // Reference is not in the originating generation;
1173     // don't treat it specially (i.e. we want to scan it as a normal
1174     // object with strong references).
1175     return false;
1176   }
1177 
1178   // We only discover references whose referents are not (yet)
1179   // known to be strongly reachable.
1180   if (is_alive_non_header() != NULL) {
1181     verify_referent(obj);
1182     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
1183       return false;  // referent is reachable
1184     }
1185   }
1186   if (rt == REF_SOFT) {
1187     // For soft refs we can decide now if these are not
1188     // current candidates for clearing, in which case we
1189     // can mark through them now, rather than delaying that
1190     // to the reference-processing phase. Since all current
1191     // time-stamp policies advance the soft-ref clock only
1192     // at a major collection cycle, this is always currently
1193     // accurate.
1194     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {
1195       return false;
1196     }
1197   }
1198 
1199   ResourceMark rm;      // Needed for tracing.
1200 
1201   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);
1202   const oop  discovered = java_lang_ref_Reference::discovered(obj);
1203   assert(discovered->is_oop_or_null(), "bad discovered field");
1204   if (discovered != NULL) {
1205     // The reference has already been discovered...
1206     if (TraceReferenceGC) {
1207       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",
1208                              obj, obj->klass()->internal_name());
1209     }
1210     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1211       // assumes that an object is not processed twice;
1212       // if it's been already discovered it must be on another
1213       // generation's discovered list; so we won't discover it.
1214       return false;
1215     } else {
1216       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,
1217              "Unrecognized policy");
1218       // Check assumption that an object is not potentially
1219       // discovered twice except by concurrent collectors that potentially
1220       // trace the same Reference object twice.
1221       assert(UseConcMarkSweepGC || UseG1GC,
1222              "Only possible with a concurrent marking collector");
1223       return true;
1224     }
1225   }
1226 
1227   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
1228     verify_referent(obj);
1229     // Discover if and only if EITHER:
1230     // .. reference is in our span, OR
1231     // .. we are an atomic collector and referent is in our span
1232     if (_span.contains(obj_addr) ||
1233         (discovery_is_atomic() &&
1234          _span.contains(java_lang_ref_Reference::referent(obj)))) {
1235       // should_enqueue = true;
1236     } else {
1237       return false;
1238     }
1239   } else {
1240     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&
1241            _span.contains(obj_addr), "code inconsistency");
1242   }
1243 
1244   // Get the right type of discovered queue head.
1245   DiscoveredList* list = get_discovered_list(rt);
1246   if (list == NULL) {
1247     return false;   // nothing special needs to be done
1248   }
1249 
1250   if (_discovery_is_mt) {
1251     add_to_discovered_list_mt(*list, obj, discovered_addr);
1252   } else {
1253     // If "_discovered_list_needs_barrier", we do write barriers when
1254     // updating the discovered reference list.  Otherwise, we do a raw store
1255     // here: the field will be visited later when processing the discovered
1256     // references.
1257     oop current_head = list->head();
1258     // The last ref must have its discovered field pointing to itself.
1259     oop next_discovered = (current_head != NULL) ? current_head : obj;
1260 
1261     // As in the case further above, since we are over-writing a NULL
1262     // pre-value, we can safely elide the pre-barrier here for the case of G1.
1263     // e.g.:- _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
1264     assert(discovered == NULL, "control point invariant");
1265     assert(!_discovered_list_needs_barrier || UseG1GC,
1266            "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
1267     oop_store_raw(discovered_addr, next_discovered);
1268     if (_discovered_list_needs_barrier) {
1269       _bs->write_ref_field((void*)discovered_addr, next_discovered);
1270     }
1271     list->set_head(obj);
1272     list->inc_length(1);
1273 
1274     if (TraceReferenceGC) {
1275       gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
1276                                 obj, obj->klass()->internal_name());
1277     }
1278   }
1279   assert(obj->is_oop(), "Discovered a bad reference");
1280   verify_referent(obj);
1281   return true;
1282 }
1283 
1284 // Preclean the discovered references by removing those
1285 // whose referents are alive, and by marking from those that
1286 // are not active. These lists can be handled here
1287 // in any order and, indeed, concurrently.
1288 void ReferenceProcessor::preclean_discovered_references(
1289   BoolObjectClosure* is_alive,
1290   OopClosure* keep_alive,
1291   VoidClosure* complete_gc,
1292   YieldClosure* yield,
1293   GCTimer* gc_timer) {
1294 
1295   NOT_PRODUCT(verify_ok_to_handle_reflists());
1296 
1297   // Soft references
1298   {
1299     GCTraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
1300               false, gc_timer);
1301     for (uint i = 0; i < _max_num_q; i++) {
1302       if (yield->should_return()) {
1303         return;
1304       }
1305       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
1306                                   keep_alive, complete_gc, yield);
1307     }
1308   }
1309 
1310   // Weak references
1311   {
1312     GCTraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
1313               false, gc_timer);
1314     for (uint i = 0; i < _max_num_q; i++) {
1315       if (yield->should_return()) {
1316         return;
1317       }
1318       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
1319                                   keep_alive, complete_gc, yield);
1320     }
1321   }
1322 
1323   // Final references
1324   {
1325     GCTraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
1326               false, gc_timer);
1327     for (uint i = 0; i < _max_num_q; i++) {
1328       if (yield->should_return()) {
1329         return;
1330       }
1331       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
1332                                   keep_alive, complete_gc, yield);
1333     }
1334   }
1335 
1336   // Phantom references
1337   {
1338     GCTraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
1339               false, gc_timer);
1340     for (uint i = 0; i < _max_num_q; i++) {
1341       if (yield->should_return()) {
1342         return;
1343       }
1344       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,
1345                                   keep_alive, complete_gc, yield);
1346     }
1347   }
1348 }
1349 
1350 // Walk the given discovered ref list, and remove all reference objects
1351 // whose referents are still alive, whose referents are NULL or which
1352 // are not active (have a non-NULL next field). NOTE: When we are
1353 // thus precleaning the ref lists (which happens single-threaded today),
1354 // we do not disable refs discovery to honour the correct semantics of
1355 // java.lang.Reference. As a result, we need to be careful below
1356 // that ref removal steps interleave safely with ref discovery steps
1357 // (in this thread).
1358 void
1359 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,
1360                                                 BoolObjectClosure* is_alive,
1361                                                 OopClosure*        keep_alive,
1362                                                 VoidClosure*       complete_gc,
1363                                                 YieldClosure*      yield) {
1364   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);
1365   while (iter.has_next()) {
1366     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));
1367     oop obj = iter.obj();
1368     oop next = java_lang_ref_Reference::next(obj);
1369     if (iter.referent() == NULL || iter.is_referent_alive() ||
1370         next != NULL) {
1371       // The referent has been cleared, or is alive, or the Reference is not
1372       // active; we need to trace and mark its cohort.
1373       if (TraceReferenceGC) {
1374         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",
1375                                iter.obj(), iter.obj()->klass()->internal_name());
1376       }
1377       // Remove Reference object from list
1378       iter.remove();
1379       // Keep alive its cohort.
1380       iter.make_referent_alive();
1381       if (UseCompressedOops) {
1382         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);
1383         keep_alive->do_oop(next_addr);
1384       } else {
1385         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);
1386         keep_alive->do_oop(next_addr);
1387       }
1388       iter.move_to_next();
1389     } else {
1390       iter.next();
1391     }
1392   }
1393   // Close the reachable set
1394   complete_gc->do_void();
1395 
1396   NOT_PRODUCT(
1397     if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
1398       gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
1399         "Refs in discovered list " INTPTR_FORMAT,
1400         iter.removed(), iter.processed(), (address)refs_list.head());
1401     }
1402   )
1403 }
1404 
1405 const char* ReferenceProcessor::list_name(uint i) {
1406    assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
1407           "Out of bounds index");
1408 
1409    int j = i / _max_num_q;
1410    switch (j) {
1411      case 0: return "SoftRef";
1412      case 1: return "WeakRef";
1413      case 2: return "FinalRef";
1414      case 3: return "PhantomRef";
1415    }
1416    ShouldNotReachHere();
1417    return NULL;
1418 }
1419 
1420 #ifndef PRODUCT
1421 void ReferenceProcessor::verify_ok_to_handle_reflists() {
1422   // empty for now
1423 }
1424 #endif
1425 
1426 #ifndef PRODUCT
1427 void ReferenceProcessor::clear_discovered_references() {
1428   guarantee(!_discovering_refs, "Discovering refs?");
1429   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
1430     clear_discovered_references(_discovered_refs[i]);
1431   }
1432 }
1433 
1434 #endif // PRODUCT