1 /*

     2  * Copyright (c) 2001, 2015, 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/shared/collectedHeap.hpp"

    29 #include "gc/shared/collectedHeap.inline.hpp"

    30 #include "gc/shared/gcTimer.hpp"

    31 #include "gc/shared/gcTraceTime.hpp"

    32 #include "gc/shared/referencePolicy.hpp"

    33 #include "gc/shared/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 jlong            ReferenceProcessor::_soft_ref_timestamp_clock = 0;

    41 

    42 void referenceProcessor_init() {

    43   ReferenceProcessor::init_statics();

    44 }

    45 

    46 void ReferenceProcessor::init_statics() {

    47   // We need a monotonically non-decreasing time in ms but

    48   // os::javaTimeMillis() does not guarantee monotonicity.

    49   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;

    50 

    51   // Initialize the soft ref timestamp clock.

    52   _soft_ref_timestamp_clock = now;

    53   // Also update the soft ref clock in j.l.r.SoftReference

    54   java_lang_ref_SoftReference::set_clock(_soft_ref_timestamp_clock);

    55 

    56   _always_clear_soft_ref_policy = new AlwaysClearPolicy();

    57   _default_soft_ref_policy      = new COMPILER2_PRESENT(LRUMaxHeapPolicy())

    58                                       NOT_COMPILER2(LRUCurrentHeapPolicy());

    59   if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {

    60     vm_exit_during_initialization("Could not allocate reference policy object");

    61   }

    62   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||

    63             RefDiscoveryPolicy == ReferentBasedDiscovery,

    64             "Unrecognized RefDiscoveryPolicy");

    65 }

    66 

    67 void ReferenceProcessor::enable_discovery(bool check_no_refs) {

    68 #ifdef ASSERT

    69   // Verify that we're not currently discovering refs

    70   assert(!_discovering_refs, "nested call?");

    71 

    72   if (check_no_refs) {

    73     // Verify that the discovered lists are empty

    74     verify_no_references_recorded();

    75   }

    76 #endif // ASSERT

    77 

    78   // Someone could have modified the value of the static

    79   // field in the j.l.r.SoftReference class that holds the

    80   // soft reference timestamp clock using reflection or

    81   // Unsafe between GCs. Unconditionally update the static

    82   // field in ReferenceProcessor here so that we use the new

    83   // value during reference discovery.

    84 

    85   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();

    86   _discovering_refs = true;

    87 }

    88 

    89 ReferenceProcessor::ReferenceProcessor(MemRegion span,

    90                                        bool      mt_processing,

    91                                        uint      mt_processing_degree,

    92                                        bool      mt_discovery,

    93                                        uint      mt_discovery_degree,

    94                                        bool      atomic_discovery,

    95                                        BoolObjectClosure* is_alive_non_header)  :

    96   _discovering_refs(false),

    97   _enqueuing_is_done(false),

    98   _is_alive_non_header(is_alive_non_header),

    99   _processing_is_mt(mt_processing),

   100   _next_id(0)

   101 {

   102   _span = span;

   103   _discovery_is_atomic = atomic_discovery;

   104   _discovery_is_mt     = mt_discovery;

   105   _num_q               = MAX2(1U, mt_processing_degree);

   106   _max_num_q           = MAX2(_num_q, mt_discovery_degree);

   107   _discovered_refs     = NEW_C_HEAP_ARRAY(DiscoveredList,

   108             _max_num_q * number_of_subclasses_of_ref(), mtGC);

   109 

   110   if (_discovered_refs == NULL) {

   111     vm_exit_during_initialization("Could not allocated RefProc Array");

   112   }

   113   _discoveredSoftRefs    = &_discovered_refs[0];

   114   _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];

   115   _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];

   116   _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];

   117   _discoveredCleanerRefs = &_discoveredPhantomRefs[_max_num_q];

   118 

   119   // Initialize all entries to NULL

   120   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {

   121     _discovered_refs[i].set_head(NULL);

   122     _discovered_refs[i].set_length(0);

   123   }

   124 

   125   setup_policy(false /* default soft ref policy */);

   126 }

   127 

   128 #ifndef PRODUCT

   129 void ReferenceProcessor::verify_no_references_recorded() {

   130   guarantee(!_discovering_refs, "Discovering refs?");

   131   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {

   132     guarantee(_discovered_refs[i].is_empty(),

   133               "Found non-empty discovered list");

   134   }

   135 }

   136 #endif

   137 

   138 void ReferenceProcessor::weak_oops_do(OopClosure* f) {

   139   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {

   140     if (UseCompressedOops) {

   141       f->do_oop((narrowOop*)_discovered_refs[i].adr_head());

   142     } else {

   143       f->do_oop((oop*)_discovered_refs[i].adr_head());

   144     }

   145   }

   146 }

   147 

   148 void ReferenceProcessor::update_soft_ref_master_clock() {

   149   // Update (advance) the soft ref master clock field. This must be done

   150   // after processing the soft ref list.

   151 

   152   // We need a monotonically non-decreasing time in ms but

   153   // os::javaTimeMillis() does not guarantee monotonicity.

   154   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;

   155   jlong soft_ref_clock = java_lang_ref_SoftReference::clock();

   156   assert(soft_ref_clock == _soft_ref_timestamp_clock, "soft ref clocks out of sync");

   157 

   158   NOT_PRODUCT(

   159   if (now < _soft_ref_timestamp_clock) {

   160     warning("time warp: " JLONG_FORMAT " to " JLONG_FORMAT,

   161             _soft_ref_timestamp_clock, now);

   162   }

   163   )

   164   // The values of now and _soft_ref_timestamp_clock are set using

   165   // javaTimeNanos(), which is guaranteed to be monotonically

   166   // non-decreasing provided the underlying platform provides such

   167   // a time source (and it is bug free).

   168   // In product mode, however, protect ourselves from non-monotonicity.

   169   if (now > _soft_ref_timestamp_clock) {

   170     _soft_ref_timestamp_clock = now;

   171     java_lang_ref_SoftReference::set_clock(now);

   172   }

   173   // Else leave clock stalled at its old value until time progresses

   174   // past clock value.

   175 }

   176 

   177 size_t ReferenceProcessor::total_count(DiscoveredList lists[]) {

   178   size_t total = 0;

   179   for (uint i = 0; i < _max_num_q; ++i) {

   180     total += lists[i].length();

   181   }

   182   return total;

   183 }

   184 

   185 ReferenceProcessorStats ReferenceProcessor::process_discovered_references(

   186   BoolObjectClosure*           is_alive,

   187   OopClosure*                  keep_alive,

   188   VoidClosure*                 complete_gc,

   189   AbstractRefProcTaskExecutor* task_executor,

   190   GCTimer*                     gc_timer,

   191   GCId                         gc_id) {

   192   NOT_PRODUCT(verify_ok_to_handle_reflists());

   193 

   194   assert(!enqueuing_is_done(), "If here enqueuing should not be complete");

   195   // Stop treating discovered references specially.

   196   disable_discovery();

   197 

   198   // If discovery was concurrent, someone could have modified

   199   // the value of the static field in the j.l.r.SoftReference

   200   // class that holds the soft reference timestamp clock using

   201   // reflection or Unsafe between when discovery was enabled and

   202   // now. Unconditionally update the static field in ReferenceProcessor

   203   // here so that we use the new value during processing of the

   204   // discovered soft refs.

   205 

   206   _soft_ref_timestamp_clock = java_lang_ref_SoftReference::clock();

   207 

   208   bool trace_time = PrintGCDetails && PrintReferenceGC;

   209 

   210   // Soft references

   211   size_t soft_count = 0;

   212   {

   213     GCTraceTime tt("SoftReference", trace_time, false, gc_timer, gc_id);

   214     soft_count =

   215       process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true,

   216                                  is_alive, keep_alive, complete_gc, task_executor);

   217   }

   218 

   219   update_soft_ref_master_clock();

   220 

   221   // Weak references

   222   size_t weak_count = 0;

   223   {

   224     GCTraceTime tt("WeakReference", trace_time, false, gc_timer, gc_id);

   225     weak_count =

   226       process_discovered_reflist(_discoveredWeakRefs, NULL, true,

   227                                  is_alive, keep_alive, complete_gc, task_executor);

   228   }

   229 

   230   // Final references

   231   size_t final_count = 0;

   232   {

   233     GCTraceTime tt("FinalReference", trace_time, false, gc_timer, gc_id);

   234     final_count =

   235       process_discovered_reflist(_discoveredFinalRefs, NULL, false,

   236                                  is_alive, keep_alive, complete_gc, task_executor);

   237   }

   238 

   239   // Phantom references

   240   size_t phantom_count = 0;

   241   {

   242     GCTraceTime tt("PhantomReference", trace_time, false, gc_timer, gc_id);

   243     phantom_count =

   244       process_discovered_reflist(_discoveredPhantomRefs, NULL, false,

   245                                  is_alive, keep_alive, complete_gc, task_executor);

   246 

   247     // Process cleaners, but include them in phantom statistics.  We expect

   248     // Cleaner references to be temporary, and don't want to deal with

   249     // possible incompatibilities arising from making it more visible.

   250     phantom_count +=

   251       process_discovered_reflist(_discoveredCleanerRefs, NULL, true,

   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, gc_id);

   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 post-barrier on pending_list_addr missed in

   321   // enqueue_discovered_reflist.

   322   oopDesc::bs()->write_ref_field(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));

   323 

   324   // Stop treating discovered references specially.

   325   ref->disable_discovery();

   326 

   327   // Return true if new pending references were added

   328   return old_pending_list_value != *pending_list_addr;

   329 }

   330 

   331 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {

   332   NOT_PRODUCT(verify_ok_to_handle_reflists());

   333   if (UseCompressedOops) {

   334     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);

   335   } else {

   336     return enqueue_discovered_ref_helper<oop>(this, task_executor);

   337   }

   338 }

   339 

   340 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,

   341                                                     HeapWord* pending_list_addr) {

   342   // Given a list of refs linked through the "discovered" field

   343   // (java.lang.ref.Reference.discovered), self-loop their "next" field

   344   // thus distinguishing them from active References, then

   345   // prepend them to the pending list.

   346   //

   347   // The Java threads will see the Reference objects linked together through

   348   // the discovered field. Instead of trying to do the write barrier updates

   349   // in all places in the reference processor where we manipulate the discovered

   350   // field we make sure to do the barrier here where we anyway iterate through

   351   // all linked Reference objects. Note that it is important to not dirty any

   352   // cards during reference processing since this will cause card table

   353   // verification to fail for G1.

   354   if (TraceReferenceGC && PrintGCDetails) {

   355     gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "

   356                            INTPTR_FORMAT, p2i(refs_list.head()));

   357   }

   358 

   359   oop obj = NULL;

   360   oop next_d = refs_list.head();

   361   // Walk down the list, self-looping the next field

   362   // so that the References are not considered active.

   363   while (obj != next_d) {

   364     obj = next_d;

   365     assert(obj->is_instanceRef(), "should be reference object");

   366     next_d = java_lang_ref_Reference::discovered(obj);

   367     if (TraceReferenceGC && PrintGCDetails) {

   368       gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,

   369                              p2i(obj), p2i(next_d));

   370     }

   371     assert(java_lang_ref_Reference::next(obj) == NULL,

   372            "Reference not active; should not be discovered");

   373     // Self-loop next, so as to make Ref not active.

   374     java_lang_ref_Reference::set_next_raw(obj, obj);

   375     if (next_d != obj) {

   376       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), next_d);

   377     } else {

   378       // This is the last object.

   379       // Swap refs_list into pending_list_addr and

   380       // set obj's discovered to what we read from pending_list_addr.

   381       oop old = oopDesc::atomic_exchange_oop(refs_list.head(), pending_list_addr);

   382       // Need post-barrier on pending_list_addr. See enqueue_discovered_ref_helper() above.

   383       java_lang_ref_Reference::set_discovered_raw(obj, old); // old may be NULL

   384       oopDesc::bs()->write_ref_field(java_lang_ref_Reference::discovered_addr(obj), old);

   385     }

   386   }

   387 }

   388 

   389 // Parallel enqueue task

   390 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {

   391 public:

   392   RefProcEnqueueTask(ReferenceProcessor& ref_processor,

   393                      DiscoveredList      discovered_refs[],

   394                      HeapWord*           pending_list_addr,

   395                      int                 n_queues)

   396     : EnqueueTask(ref_processor, discovered_refs,

   397                   pending_list_addr, n_queues)

   398   { }

   399 

   400   virtual void work(unsigned int work_id) {

   401     assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");

   402     // Simplest first cut: static partitioning.

   403     int index = work_id;

   404     // The increment on "index" must correspond to the maximum number of queues

   405     // (n_queues) with which that ReferenceProcessor was created.  That

   406     // is because of the "clever" way the discovered references lists were

   407     // allocated and are indexed into.

   408     assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");

   409     for (int j = 0;

   410          j < ReferenceProcessor::number_of_subclasses_of_ref();

   411          j++, index += _n_queues) {

   412       _ref_processor.enqueue_discovered_reflist(

   413         _refs_lists[index], _pending_list_addr);

   414       _refs_lists[index].set_head(NULL);

   415       _refs_lists[index].set_length(0);

   416     }

   417   }

   418 };

   419 

   420 // Enqueue references that are not made active again

   421 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,

   422   AbstractRefProcTaskExecutor* task_executor) {

   423   if (_processing_is_mt && task_executor != NULL) {

   424     // Parallel code

   425     RefProcEnqueueTask tsk(*this, _discovered_refs,

   426                            pending_list_addr, _max_num_q);

   427     task_executor->execute(tsk);

   428   } else {

   429     // Serial code: call the parent class's implementation

   430     for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {

   431       enqueue_discovered_reflist(_discovered_refs[i], pending_list_addr);

   432       _discovered_refs[i].set_head(NULL);

   433       _discovered_refs[i].set_length(0);

   434     }

   435   }

   436 }

   437 

   438 void DiscoveredListIterator::load_ptrs(DEBUG_ONLY(bool allow_null_referent)) {

   439   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);

   440   oop discovered = java_lang_ref_Reference::discovered(_ref);

   441   assert(_discovered_addr && discovered->is_oop_or_null(),

   442          err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));

   443   _next = discovered;

   444   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);

   445   _referent = java_lang_ref_Reference::referent(_ref);

   446   assert(Universe::heap()->is_in_reserved_or_null(_referent),

   447          "Wrong oop found in java.lang.Reference object");

   448   assert(allow_null_referent ?

   449              _referent->is_oop_or_null()

   450            : _referent->is_oop(),

   451          err_msg("Expected an oop%s for referent field at " PTR_FORMAT,

   452                  (allow_null_referent ? " or NULL" : ""),

   453                  p2i(_referent)));

   454 }

   455 

   456 void DiscoveredListIterator::remove() {

   457   assert(_ref->is_oop(), "Dropping a bad reference");

   458   oop_store_raw(_discovered_addr, NULL);

   459 

   460   // First _prev_next ref actually points into DiscoveredList (gross).

   461   oop new_next;

   462   if (_next == _ref) {

   463     // At the end of the list, we should make _prev point to itself.

   464     // If _ref is the first ref, then _prev_next will be in the DiscoveredList,

   465     // and _prev will be NULL.

   466     new_next = _prev;

   467   } else {

   468     new_next = _next;

   469   }

   470   // Remove Reference object from discovered list. Note that G1 does not need a

   471   // pre-barrier here because we know the Reference has already been found/marked,

   472   // that's how it ended up in the discovered list in the first place.

   473   oop_store_raw(_prev_next, new_next);

   474   NOT_PRODUCT(_removed++);

   475   _refs_list.dec_length(1);

   476 }

   477 

   478 void DiscoveredListIterator::clear_referent() {

   479   oop_store_raw(_referent_addr, NULL);

   480 }

   481 

   482 // NOTE: process_phase*() are largely similar, and at a high level

   483 // merely iterate over the extant list applying a predicate to

   484 // each of its elements and possibly removing that element from the

   485 // list and applying some further closures to that element.

   486 // We should consider the possibility of replacing these

   487 // process_phase*() methods by abstracting them into

   488 // a single general iterator invocation that receives appropriate

   489 // closures that accomplish this work.

   490 

   491 // (SoftReferences only) Traverse the list and remove any SoftReferences whose

   492 // referents are not alive, but that should be kept alive for policy reasons.

   493 // Keep alive the transitive closure of all such referents.

   494 void

   495 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,

   496                                    ReferencePolicy*   policy,

   497                                    BoolObjectClosure* is_alive,

   498                                    OopClosure*        keep_alive,

   499                                    VoidClosure*       complete_gc) {

   500   assert(policy != NULL, "Must have a non-NULL policy");

   501   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

   502   // Decide which softly reachable refs should be kept alive.

   503   while (iter.has_next()) {

   504     iter.load_ptrs(DEBUG_ONLY(!discovery_is_atomic() /* allow_null_referent */));

   505     bool referent_is_dead = (iter.referent() != NULL) && !iter.is_referent_alive();

   506     if (referent_is_dead &&

   507         !policy->should_clear_reference(iter.obj(), _soft_ref_timestamp_clock)) {

   508       if (TraceReferenceGC) {

   509         gclog_or_tty->print_cr("Dropping reference (" INTPTR_FORMAT ": %s"  ") by policy",

   510                                p2i(iter.obj()), iter.obj()->klass()->internal_name());

   511       }

   512       // Remove Reference object from list

   513       iter.remove();

   514       // keep the referent around

   515       iter.make_referent_alive();

   516       iter.move_to_next();

   517     } else {

   518       iter.next();

   519     }

   520   }

   521   // Close the reachable set

   522   complete_gc->do_void();

   523   NOT_PRODUCT(

   524     if (PrintGCDetails && TraceReferenceGC) {

   525       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " dead Refs out of " SIZE_FORMAT

   526         " discovered Refs by policy, from list " INTPTR_FORMAT,

   527         iter.removed(), iter.processed(), p2i(refs_list.head()));

   528     }

   529   )

   530 }

   531 

   532 // Traverse the list and remove any Refs that are not active, or

   533 // whose referents are either alive or NULL.

   534 void

   535 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,

   536                              BoolObjectClosure* is_alive,

   537                              OopClosure*        keep_alive) {

   538   assert(discovery_is_atomic(), "Error");

   539   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

   540   while (iter.has_next()) {

   541     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));

   542     DEBUG_ONLY(oop next = java_lang_ref_Reference::next(iter.obj());)

   543     assert(next == NULL, "Should not discover inactive Reference");

   544     if (iter.is_referent_alive()) {

   545       if (TraceReferenceGC) {

   546         gclog_or_tty->print_cr("Dropping strongly reachable reference (" INTPTR_FORMAT ": %s)",

   547                                p2i(iter.obj()), iter.obj()->klass()->internal_name());

   548       }

   549       // The referent is reachable after all.

   550       // Remove Reference object from list.

   551       iter.remove();

   552       // Update the referent pointer as necessary: Note that this

   553       // should not entail any recursive marking because the

   554       // referent must already have been traversed.

   555       iter.make_referent_alive();

   556       iter.move_to_next();

   557     } else {

   558       iter.next();

   559     }

   560   }

   561   NOT_PRODUCT(

   562     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {

   563       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT

   564         " Refs in discovered list " INTPTR_FORMAT,

   565         iter.removed(), iter.processed(), p2i(refs_list.head()));

   566     }

   567   )

   568 }

   569 

   570 void

   571 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,

   572                                                   BoolObjectClosure* is_alive,

   573                                                   OopClosure*        keep_alive,

   574                                                   VoidClosure*       complete_gc) {

   575   assert(!discovery_is_atomic(), "Error");

   576   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

   577   while (iter.has_next()) {

   578     iter.load_ptrs(DEBUG_ONLY(true /* allow_null_referent */));

   579     HeapWord* next_addr = java_lang_ref_Reference::next_addr(iter.obj());

   580     oop next = java_lang_ref_Reference::next(iter.obj());

   581     if ((iter.referent() == NULL || iter.is_referent_alive() ||

   582          next != NULL)) {

   583       assert(next->is_oop_or_null(), err_msg("Expected an oop or NULL for next field at " PTR_FORMAT, p2i(next)));

   584       // Remove Reference object from list

   585       iter.remove();

   586       // Trace the cohorts

   587       iter.make_referent_alive();

   588       if (UseCompressedOops) {

   589         keep_alive->do_oop((narrowOop*)next_addr);

   590       } else {

   591         keep_alive->do_oop((oop*)next_addr);

   592       }

   593       iter.move_to_next();

   594     } else {

   595       iter.next();

   596     }

   597   }

   598   // Now close the newly reachable set

   599   complete_gc->do_void();

   600   NOT_PRODUCT(

   601     if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {

   602       gclog_or_tty->print_cr(" Dropped " SIZE_FORMAT " active Refs out of " SIZE_FORMAT

   603         " Refs in discovered list " INTPTR_FORMAT,

   604         iter.removed(), iter.processed(), p2i(refs_list.head()));

   605     }

   606   )

   607 }

   608 

   609 // Traverse the list and process the referents, by either

   610 // clearing them or keeping them (and their reachable

   611 // closure) alive.

   612 void

   613 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,

   614                                    bool               clear_referent,

   615                                    BoolObjectClosure* is_alive,

   616                                    OopClosure*        keep_alive,

   617                                    VoidClosure*       complete_gc) {

   618   ResourceMark rm;

   619   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

   620   while (iter.has_next()) {

   621     iter.load_ptrs(DEBUG_ONLY(false /* allow_null_referent */));

   622     if (clear_referent) {

   623       // NULL out referent pointer

   624       iter.clear_referent();

   625     } else {

   626       // keep the referent around

   627       iter.make_referent_alive();

   628     }

   629     if (TraceReferenceGC) {

   630       gclog_or_tty->print_cr("Adding %sreference (" INTPTR_FORMAT ": %s) as pending",

   631                              clear_referent ? "cleared " : "",

   632                              p2i(iter.obj()), iter.obj()->klass()->internal_name());

   633     }

   634     assert(iter.obj()->is_oop(UseConcMarkSweepGC), "Adding a bad reference");

   635     iter.next();

   636   }

   637   // Close the reachable set

   638   complete_gc->do_void();

   639 }

   640 

   641 void

   642 ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {

   643   oop obj = NULL;

   644   oop next = refs_list.head();

   645   while (next != obj) {

   646     obj = next;

   647     next = java_lang_ref_Reference::discovered(obj);

   648     java_lang_ref_Reference::set_discovered_raw(obj, NULL);

   649   }

   650   refs_list.set_head(NULL);

   651   refs_list.set_length(0);

   652 }

   653 

   654 void

   655 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {

   656   clear_discovered_references(refs_list);

   657 }

   658 

   659 void ReferenceProcessor::abandon_partial_discovery() {

   660   // loop over the lists

   661   for (uint i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {

   662     if (TraceReferenceGC && PrintGCDetails && ((i % _max_num_q) == 0)) {

   663       gclog_or_tty->print_cr("\nAbandoning %s discovered list", list_name(i));

   664     }

   665     abandon_partial_discovered_list(_discovered_refs[i]);

   666   }

   667 }

   668 

   669 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {

   670 public:

   671   RefProcPhase1Task(ReferenceProcessor& ref_processor,

   672                     DiscoveredList      refs_lists[],

   673                     ReferencePolicy*    policy,

   674                     bool                marks_oops_alive)

   675     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),

   676       _policy(policy)

   677   { }

   678   virtual void work(unsigned int i, BoolObjectClosure& is_alive,

   679                     OopClosure& keep_alive,

   680                     VoidClosure& complete_gc)

   681   {

   682     Thread* thr = Thread::current();

   683     int refs_list_index = ((WorkerThread*)thr)->id();

   684     _ref_processor.process_phase1(_refs_lists[refs_list_index], _policy,

   685                                   &is_alive, &keep_alive, &complete_gc);

   686   }

   687 private:

   688   ReferencePolicy* _policy;

   689 };

   690 

   691 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {

   692 public:

   693   RefProcPhase2Task(ReferenceProcessor& ref_processor,

   694                     DiscoveredList      refs_lists[],

   695                     bool                marks_oops_alive)

   696     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)

   697   { }

   698   virtual void work(unsigned int i, BoolObjectClosure& is_alive,

   699                     OopClosure& keep_alive,

   700                     VoidClosure& complete_gc)

   701   {

   702     _ref_processor.process_phase2(_refs_lists[i],

   703                                   &is_alive, &keep_alive, &complete_gc);

   704   }

   705 };

   706 

   707 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {

   708 public:

   709   RefProcPhase3Task(ReferenceProcessor& ref_processor,

   710                     DiscoveredList      refs_lists[],

   711                     bool                clear_referent,

   712                     bool                marks_oops_alive)

   713     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),

   714       _clear_referent(clear_referent)

   715   { }

   716   virtual void work(unsigned int i, BoolObjectClosure& is_alive,

   717                     OopClosure& keep_alive,

   718                     VoidClosure& complete_gc)

   719   {

   720     // Don't use "refs_list_index" calculated in this way because

   721     // balance_queues() has moved the Ref's into the first n queues.

   722     // Thread* thr = Thread::current();

   723     // int refs_list_index = ((WorkerThread*)thr)->id();

   724     // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,

   725     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,

   726                                   &is_alive, &keep_alive, &complete_gc);

   727   }

   728 private:

   729   bool _clear_referent;

   730 };

   731 

   732 // Balances reference queues.

   733 // Move entries from all queues[0, 1, ..., _max_num_q-1] to

   734 // queues[0, 1, ..., _num_q-1] because only the first _num_q

   735 // corresponding to the active workers will be processed.

   736 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])

   737 {

   738   // calculate total length

   739   size_t total_refs = 0;

   740   if (TraceReferenceGC && PrintGCDetails) {

   741     gclog_or_tty->print_cr("\nBalance ref_lists ");

   742   }

   743 

   744   for (uint i = 0; i < _max_num_q; ++i) {

   745     total_refs += ref_lists[i].length();

   746     if (TraceReferenceGC && PrintGCDetails) {

   747       gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());

   748     }

   749   }

   750   if (TraceReferenceGC && PrintGCDetails) {

   751     gclog_or_tty->print_cr(" = " SIZE_FORMAT, total_refs);

   752   }

   753   size_t avg_refs = total_refs / _num_q + 1;

   754   uint to_idx = 0;

   755   for (uint from_idx = 0; from_idx < _max_num_q; from_idx++) {

   756     bool move_all = false;

   757     if (from_idx >= _num_q) {

   758       move_all = ref_lists[from_idx].length() > 0;

   759     }

   760     while ((ref_lists[from_idx].length() > avg_refs) ||

   761            move_all) {

   762       assert(to_idx < _num_q, "Sanity Check!");

   763       if (ref_lists[to_idx].length() < avg_refs) {

   764         // move superfluous refs

   765         size_t refs_to_move;

   766         // Move all the Ref's if the from queue will not be processed.

   767         if (move_all) {

   768           refs_to_move = MIN2(ref_lists[from_idx].length(),

   769                               avg_refs - ref_lists[to_idx].length());

   770         } else {

   771           refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,

   772                               avg_refs - ref_lists[to_idx].length());

   773         }

   774 

   775         assert(refs_to_move > 0, "otherwise the code below will fail");

   776 

   777         oop move_head = ref_lists[from_idx].head();

   778         oop move_tail = move_head;

   779         oop new_head  = move_head;

   780         // find an element to split the list on

   781         for (size_t j = 0; j < refs_to_move; ++j) {

   782           move_tail = new_head;

   783           new_head = java_lang_ref_Reference::discovered(new_head);

   784         }

   785 

   786         // Add the chain to the to list.

   787         if (ref_lists[to_idx].head() == NULL) {

   788           // to list is empty. Make a loop at the end.

   789           java_lang_ref_Reference::set_discovered_raw(move_tail, move_tail);

   790         } else {

   791           java_lang_ref_Reference::set_discovered_raw(move_tail, ref_lists[to_idx].head());

   792         }

   793         ref_lists[to_idx].set_head(move_head);

   794         ref_lists[to_idx].inc_length(refs_to_move);

   795 

   796         // Remove the chain from the from list.

   797         if (move_tail == new_head) {

   798           // We found the end of the from list.

   799           ref_lists[from_idx].set_head(NULL);

   800         } else {

   801           ref_lists[from_idx].set_head(new_head);

   802         }

   803         ref_lists[from_idx].dec_length(refs_to_move);

   804         if (ref_lists[from_idx].length() == 0) {

   805           break;

   806         }

   807       } else {

   808         to_idx = (to_idx + 1) % _num_q;

   809       }

   810     }

   811   }

   812 #ifdef ASSERT

   813   size_t balanced_total_refs = 0;

   814   for (uint i = 0; i < _max_num_q; ++i) {

   815     balanced_total_refs += ref_lists[i].length();

   816     if (TraceReferenceGC && PrintGCDetails) {

   817       gclog_or_tty->print(SIZE_FORMAT " ", ref_lists[i].length());

   818     }

   819   }

   820   if (TraceReferenceGC && PrintGCDetails) {

   821     gclog_or_tty->print_cr(" = " SIZE_FORMAT, balanced_total_refs);

   822     gclog_or_tty->flush();

   823   }

   824   assert(total_refs == balanced_total_refs, "Balancing was incomplete");

   825 #endif

   826 }

   827 

   828 void ReferenceProcessor::balance_all_queues() {

   829   balance_queues(_discoveredSoftRefs);

   830   balance_queues(_discoveredWeakRefs);

   831   balance_queues(_discoveredFinalRefs);

   832   balance_queues(_discoveredPhantomRefs);

   833   balance_queues(_discoveredCleanerRefs);

   834 }

   835 

   836 size_t

   837 ReferenceProcessor::process_discovered_reflist(

   838   DiscoveredList               refs_lists[],

   839   ReferencePolicy*             policy,

   840   bool                         clear_referent,

   841   BoolObjectClosure*           is_alive,

   842   OopClosure*                  keep_alive,

   843   VoidClosure*                 complete_gc,

   844   AbstractRefProcTaskExecutor* task_executor)

   845 {

   846   bool mt_processing = task_executor != NULL && _processing_is_mt;

   847   // If discovery used MT and a dynamic number of GC threads, then

   848   // the queues must be balanced for correctness if fewer than the

   849   // maximum number of queues were used.  The number of queue used

   850   // during discovery may be different than the number to be used

   851   // for processing so don't depend of _num_q < _max_num_q as part

   852   // of the test.

   853   bool must_balance = _discovery_is_mt;

   854 

   855   if ((mt_processing && ParallelRefProcBalancingEnabled) ||

   856       must_balance) {

   857     balance_queues(refs_lists);

   858   }

   859 

   860   size_t total_list_count = total_count(refs_lists);

   861 

   862   if (PrintReferenceGC && PrintGCDetails) {

   863     gclog_or_tty->print(", " SIZE_FORMAT " refs", total_list_count);

   864   }

   865 

   866   // Phase 1 (soft refs only):

   867   // . Traverse the list and remove any SoftReferences whose

   868   //   referents are not alive, but that should be kept alive for

   869   //   policy reasons. Keep alive the transitive closure of all

   870   //   such referents.

   871   if (policy != NULL) {

   872     if (mt_processing) {

   873       RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);

   874       task_executor->execute(phase1);

   875     } else {

   876       for (uint i = 0; i < _max_num_q; i++) {

   877         process_phase1(refs_lists[i], policy,

   878                        is_alive, keep_alive, complete_gc);

   879       }

   880     }

   881   } else { // policy == NULL

   882     assert(refs_lists != _discoveredSoftRefs,

   883            "Policy must be specified for soft references.");

   884   }

   885 

   886   // Phase 2:

   887   // . Traverse the list and remove any refs whose referents are alive.

   888   if (mt_processing) {

   889     RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);

   890     task_executor->execute(phase2);

   891   } else {

   892     for (uint i = 0; i < _max_num_q; i++) {

   893       process_phase2(refs_lists[i], is_alive, keep_alive, complete_gc);

   894     }

   895   }

   896 

   897   // Phase 3:

   898   // . Traverse the list and process referents as appropriate.

   899   if (mt_processing) {

   900     RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);

   901     task_executor->execute(phase3);

   902   } else {

   903     for (uint i = 0; i < _max_num_q; i++) {

   904       process_phase3(refs_lists[i], clear_referent,

   905                      is_alive, keep_alive, complete_gc);

   906     }

   907   }

   908 

   909   return total_list_count;

   910 }

   911 

   912 inline DiscoveredList* ReferenceProcessor::get_discovered_list(ReferenceType rt) {

   913   uint id = 0;

   914   // Determine the queue index to use for this object.

   915   if (_discovery_is_mt) {

   916     // During a multi-threaded discovery phase,

   917     // each thread saves to its "own" list.

   918     Thread* thr = Thread::current();

   919     id = thr->as_Worker_thread()->id();

   920   } else {

   921     // single-threaded discovery, we save in round-robin

   922     // fashion to each of the lists.

   923     if (_processing_is_mt) {

   924       id = next_id();

   925     }

   926   }

   927   assert(id < _max_num_q, "Id is out-of-bounds (call Freud?)");

   928 

   929   // Get the discovered queue to which we will add

   930   DiscoveredList* list = NULL;

   931   switch (rt) {

   932     case REF_OTHER:

   933       // Unknown reference type, no special treatment

   934       break;

   935     case REF_SOFT:

   936       list = &_discoveredSoftRefs[id];

   937       break;

   938     case REF_WEAK:

   939       list = &_discoveredWeakRefs[id];

   940       break;

   941     case REF_FINAL:

   942       list = &_discoveredFinalRefs[id];

   943       break;

   944     case REF_PHANTOM:

   945       list = &_discoveredPhantomRefs[id];

   946       break;

   947     case REF_CLEANER:

   948       list = &_discoveredCleanerRefs[id];

   949       break;

   950     case REF_NONE:

   951       // we should not reach here if we are an InstanceRefKlass

   952     default:

   953       ShouldNotReachHere();

   954   }

   955   if (TraceReferenceGC && PrintGCDetails) {

   956     gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, p2i(list));

   957   }

   958   return list;

   959 }

   960 

   961 inline void

   962 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,

   963                                               oop             obj,

   964                                               HeapWord*       discovered_addr) {

   965   assert(_discovery_is_mt, "!_discovery_is_mt should have been handled by caller");

   966   // First we must make sure this object is only enqueued once. CAS in a non null

   967   // discovered_addr.

   968   oop current_head = refs_list.head();

   969   // The last ref must have its discovered field pointing to itself.

   970   oop next_discovered = (current_head != NULL) ? current_head : obj;

   971 

   972   oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, discovered_addr,

   973                                                     NULL);

   974   if (retest == NULL) {

   975     // This thread just won the right to enqueue the object.

   976     // We have separate lists for enqueueing, so no synchronization

   977     // is necessary.

   978     refs_list.set_head(obj);

   979     refs_list.inc_length(1);

   980 

   981     if (TraceReferenceGC) {

   982       gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",

   983                              p2i(obj), obj->klass()->internal_name());

   984     }

   985   } else {

   986     // If retest was non NULL, another thread beat us to it:

   987     // The reference has already been discovered...

   988     if (TraceReferenceGC) {

   989       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",

   990                              p2i(obj), obj->klass()->internal_name());

   991     }

   992   }

   993 }

   994 

   995 #ifndef PRODUCT

   996 // Non-atomic (i.e. concurrent) discovery might allow us

   997 // to observe j.l.References with NULL referents, being those

   998 // cleared concurrently by mutators during (or after) discovery.

   999 void ReferenceProcessor::verify_referent(oop obj) {

  1000   bool da = discovery_is_atomic();

  1001   oop referent = java_lang_ref_Reference::referent(obj);

  1002   assert(da ? referent->is_oop() : referent->is_oop_or_null(),

  1003          err_msg("Bad referent " INTPTR_FORMAT " found in Reference "

  1004                  INTPTR_FORMAT " during %satomic discovery ",

  1005                  p2i(referent), p2i(obj), da ? "" : "non-"));

  1006 }

  1007 #endif

  1008 

  1009 // We mention two of several possible choices here:

  1010 // #0: if the reference object is not in the "originating generation"

  1011 //     (or part of the heap being collected, indicated by our "span"

  1012 //     we don't treat it specially (i.e. we scan it as we would

  1013 //     a normal oop, treating its references as strong references).

  1014 //     This means that references can't be discovered unless their

  1015 //     referent is also in the same span. This is the simplest,

  1016 //     most "local" and most conservative approach, albeit one

  1017 //     that may cause weak references to be enqueued least promptly.

  1018 //     We call this choice the "ReferenceBasedDiscovery" policy.

  1019 // #1: the reference object may be in any generation (span), but if

  1020 //     the referent is in the generation (span) being currently collected

  1021 //     then we can discover the reference object, provided

  1022 //     the object has not already been discovered by

  1023 //     a different concurrently running collector (as may be the

  1024 //     case, for instance, if the reference object is in CMS and

  1025 //     the referent in DefNewGeneration), and provided the processing

  1026 //     of this reference object by the current collector will

  1027 //     appear atomic to every other collector in the system.

  1028 //     (Thus, for instance, a concurrent collector may not

  1029 //     discover references in other generations even if the

  1030 //     referent is in its own generation). This policy may,

  1031 //     in certain cases, enqueue references somewhat sooner than

  1032 //     might Policy #0 above, but at marginally increased cost

  1033 //     and complexity in processing these references.

  1034 //     We call this choice the "RefeferentBasedDiscovery" policy.

  1035 bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {

  1036   // Make sure we are discovering refs (rather than processing discovered refs).

  1037   if (!_discovering_refs || !RegisterReferences) {

  1038     return false;

  1039   }

  1040   // We only discover active references.

  1041   oop next = java_lang_ref_Reference::next(obj);

  1042   if (next != NULL) {   // Ref is no longer active

  1043     return false;

  1044   }

  1045 

  1046   HeapWord* obj_addr = (HeapWord*)obj;

  1047   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&

  1048       !_span.contains(obj_addr)) {

  1049     // Reference is not in the originating generation;

  1050     // don't treat it specially (i.e. we want to scan it as a normal

  1051     // object with strong references).

  1052     return false;

  1053   }

  1054 

  1055   // We only discover references whose referents are not (yet)

  1056   // known to be strongly reachable.

  1057   if (is_alive_non_header() != NULL) {

  1058     verify_referent(obj);

  1059     if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {

  1060       return false;  // referent is reachable

  1061     }

  1062   }

  1063   if (rt == REF_SOFT) {

  1064     // For soft refs we can decide now if these are not

  1065     // current candidates for clearing, in which case we

  1066     // can mark through them now, rather than delaying that

  1067     // to the reference-processing phase. Since all current

  1068     // time-stamp policies advance the soft-ref clock only

  1069     // at a major collection cycle, this is always currently

  1070     // accurate.

  1071     if (!_current_soft_ref_policy->should_clear_reference(obj, _soft_ref_timestamp_clock)) {

  1072       return false;

  1073     }

  1074   }

  1075 

  1076   ResourceMark rm;      // Needed for tracing.

  1077 

  1078   HeapWord* const discovered_addr = java_lang_ref_Reference::discovered_addr(obj);

  1079   const oop  discovered = java_lang_ref_Reference::discovered(obj);

  1080   assert(discovered->is_oop_or_null(), err_msg("Expected an oop or NULL for discovered field at " PTR_FORMAT, p2i(discovered)));

  1081   if (discovered != NULL) {

  1082     // The reference has already been discovered...

  1083     if (TraceReferenceGC) {

  1084       gclog_or_tty->print_cr("Already discovered reference (" INTPTR_FORMAT ": %s)",

  1085                              p2i(obj), obj->klass()->internal_name());

  1086     }

  1087     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {

  1088       // assumes that an object is not processed twice;

  1089       // if it's been already discovered it must be on another

  1090       // generation's discovered list; so we won't discover it.

  1091       return false;

  1092     } else {

  1093       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,

  1094              "Unrecognized policy");

  1095       // Check assumption that an object is not potentially

  1096       // discovered twice except by concurrent collectors that potentially

  1097       // trace the same Reference object twice.

  1098       assert(UseConcMarkSweepGC || UseG1GC,

  1099              "Only possible with a concurrent marking collector");

  1100       return true;

  1101     }

  1102   }

  1103 

  1104   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {

  1105     verify_referent(obj);

  1106     // Discover if and only if EITHER:

  1107     // .. reference is in our span, OR

  1108     // .. we are an atomic collector and referent is in our span

  1109     if (_span.contains(obj_addr) ||

  1110         (discovery_is_atomic() &&

  1111          _span.contains(java_lang_ref_Reference::referent(obj)))) {

  1112       // should_enqueue = true;

  1113     } else {

  1114       return false;

  1115     }

  1116   } else {

  1117     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&

  1118            _span.contains(obj_addr), "code inconsistency");

  1119   }

  1120 

  1121   // Get the right type of discovered queue head.

  1122   DiscoveredList* list = get_discovered_list(rt);

  1123   if (list == NULL) {

  1124     return false;   // nothing special needs to be done

  1125   }

  1126 

  1127   if (_discovery_is_mt) {

  1128     add_to_discovered_list_mt(*list, obj, discovered_addr);

  1129   } else {

  1130     // We do a raw store here: the field will be visited later when processing

  1131     // the discovered references.

  1132     oop current_head = list->head();

  1133     // The last ref must have its discovered field pointing to itself.

  1134     oop next_discovered = (current_head != NULL) ? current_head : obj;