Mercurial > jdk8-mips64-public > hotspot / file revision

 /*

  * Copyright 2001-2008 Sun Microsystems, Inc.  All Rights Reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

  * CA 95054 USA or visit www.sun.com if you need additional information or

  * have any questions.

  *

  */

 # include "incls/_precompiled.incl"

 # include "incls/_referenceProcessor.cpp.incl"

 // List of discovered references.

 class DiscoveredList {

 public:

   DiscoveredList() : _len(0), _compressed_head(0), _oop_head(NULL) { }

   oop head() const     {

      return UseCompressedOops ?  oopDesc::decode_heap_oop_not_null(_compressed_head) :

                                 _oop_head;

   }

   HeapWord* adr_head() {

     return UseCompressedOops ? (HeapWord*)&_compressed_head :

                                (HeapWord*)&_oop_head;

   }

   void   set_head(oop o) {

     if (UseCompressedOops) {

       // Must compress the head ptr.

       _compressed_head = oopDesc::encode_heap_oop_not_null(o);

     } else {

       _oop_head = o;

     }

   }

   bool   empty() const          { return head() == ReferenceProcessor::sentinel_ref(); }

   size_t length()               { return _len; }

   void   set_length(size_t len) { _len = len; }

 private:

   // Set value depending on UseCompressedOops. This could be a template class

   // but then we have to fix all the instantiations and declarations that use this class.

   oop       _oop_head;

   narrowOop _compressed_head;

   size_t _len;

 };

 oop  ReferenceProcessor::_sentinelRef = NULL;

 const int subclasses_of_ref = REF_PHANTOM - REF_OTHER;

 void referenceProcessor_init() {

   ReferenceProcessor::init_statics();

 }

 void ReferenceProcessor::init_statics() {

   assert(_sentinelRef == NULL, "should be initialized precisely once");

   EXCEPTION_MARK;

   _sentinelRef = instanceKlass::cast(

                     SystemDictionary::reference_klass())->

                       allocate_permanent_instance(THREAD);

   // Initialize the master soft ref clock.

   java_lang_ref_SoftReference::set_clock(os::javaTimeMillis());

   if (HAS_PENDING_EXCEPTION) {

       Handle ex(THREAD, PENDING_EXCEPTION);

       vm_exit_during_initialization(ex);

   }

   assert(_sentinelRef != NULL && _sentinelRef->is_oop(),

          "Just constructed it!");

   guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||

             RefDiscoveryPolicy == ReferentBasedDiscovery,

             "Unrecongnized RefDiscoveryPolicy");

 }

 ReferenceProcessor*

 ReferenceProcessor::create_ref_processor(MemRegion          span,

                                          bool               atomic_discovery,

                                          bool               mt_discovery,

                                          BoolObjectClosure* is_alive_non_header,

                                          int                parallel_gc_threads,

                                          bool               mt_processing,

                                          bool               dl_needs_barrier) {

   int mt_degree = 1;

   if (parallel_gc_threads > 1) {

     mt_degree = parallel_gc_threads;

   }

   ReferenceProcessor* rp =

     new ReferenceProcessor(span, atomic_discovery,

                            mt_discovery, mt_degree,

                            mt_processing && (parallel_gc_threads > 0),

                            dl_needs_barrier);

   if (rp == NULL) {

     vm_exit_during_initialization("Could not allocate ReferenceProcessor object");

   }

   rp->set_is_alive_non_header(is_alive_non_header);

   return rp;

 }

 ReferenceProcessor::ReferenceProcessor(MemRegion span,

                                        bool      atomic_discovery,

                                        bool      mt_discovery,

                                        int       mt_degree,

                                        bool      mt_processing,

                                        bool      discovered_list_needs_barrier)  :

   _discovering_refs(false),

   _enqueuing_is_done(false),

   _is_alive_non_header(NULL),

   _discovered_list_needs_barrier(discovered_list_needs_barrier),

   _bs(NULL),

   _processing_is_mt(mt_processing),

   _next_id(0)

 {

   _span = span;

   _discovery_is_atomic = atomic_discovery;

   _discovery_is_mt     = mt_discovery;

   _num_q               = mt_degree;

   _discoveredSoftRefs  = NEW_C_HEAP_ARRAY(DiscoveredList, _num_q * subclasses_of_ref);

   if (_discoveredSoftRefs == NULL) {

     vm_exit_during_initialization("Could not allocated RefProc Array");

   }

   _discoveredWeakRefs    = &_discoveredSoftRefs[_num_q];

   _discoveredFinalRefs   = &_discoveredWeakRefs[_num_q];

   _discoveredPhantomRefs = &_discoveredFinalRefs[_num_q];

   assert(sentinel_ref() != NULL, "_sentinelRef is NULL");

   // Initialized all entries to _sentinelRef

   for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

         _discoveredSoftRefs[i].set_head(sentinel_ref());

     _discoveredSoftRefs[i].set_length(0);

   }

   // If we do barreirs, cache a copy of the barrier set.

   if (discovered_list_needs_barrier) {

     _bs = Universe::heap()->barrier_set();

   }

 }

 #ifndef PRODUCT

 void ReferenceProcessor::verify_no_references_recorded() {

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

   for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

     guarantee(_discoveredSoftRefs[i].empty(),

               "Found non-empty discovered list");

   }

 }

 #endif

 void ReferenceProcessor::weak_oops_do(OopClosure* f) {

   for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

     if (UseCompressedOops) {

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

     } else {

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

     }

   }

 }

 void ReferenceProcessor::oops_do(OopClosure* f) {

   f->do_oop(adr_sentinel_ref());

 }

 void ReferenceProcessor::update_soft_ref_master_clock() {

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

   // after processing the soft ref list.

   jlong now = os::javaTimeMillis();

   jlong clock = java_lang_ref_SoftReference::clock();

   NOT_PRODUCT(

   if (now < clock) {

     warning("time warp: %d to %d", clock, now);

   }

   )

   // In product mode, protect ourselves from system time being adjusted

   // externally and going backward; see note in the implementation of

   // GenCollectedHeap::time_since_last_gc() for the right way to fix

   // this uniformly throughout the VM; see bug-id 4741166. XXX

   if (now > clock) {

     java_lang_ref_SoftReference::set_clock(now);

   }

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

   // past clock value.

 }

 void ReferenceProcessor::process_discovered_references(

   ReferencePolicy*             policy,

   BoolObjectClosure*           is_alive,

   OopClosure*                  keep_alive,

   VoidClosure*                 complete_gc,

   AbstractRefProcTaskExecutor* task_executor) {

   NOT_PRODUCT(verify_ok_to_handle_reflists());

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

   // Stop treating discovered references specially.

   disable_discovery();

   bool trace_time = PrintGCDetails && PrintReferenceGC;

   // Soft references

   {

     TraceTime tt("SoftReference", trace_time, false, gclog_or_tty);

     process_discovered_reflist(_discoveredSoftRefs, policy, true,

                                is_alive, keep_alive, complete_gc, task_executor);

   }

   update_soft_ref_master_clock();

   // Weak references

   {

     TraceTime tt("WeakReference", trace_time, false, gclog_or_tty);

     process_discovered_reflist(_discoveredWeakRefs, NULL, true,

                                is_alive, keep_alive, complete_gc, task_executor);

   }

   // Final references

   {

     TraceTime tt("FinalReference", trace_time, false, gclog_or_tty);

     process_discovered_reflist(_discoveredFinalRefs, NULL, false,

                                is_alive, keep_alive, complete_gc, task_executor);

   }

   // Phantom references

   {

     TraceTime tt("PhantomReference", trace_time, false, gclog_or_tty);

     process_discovered_reflist(_discoveredPhantomRefs, NULL, false,

                                is_alive, keep_alive, complete_gc, task_executor);

   }

   // Weak global JNI references. It would make more sense (semantically) to

   // traverse these simultaneously with the regular weak references above, but

   // that is not how the JDK1.2 specification is. See #4126360. Native code can

   // thus use JNI weak references to circumvent the phantom references and

   // resurrect a "post-mortem" object.

   {

     TraceTime tt("JNI Weak Reference", trace_time, false, gclog_or_tty);

     if (task_executor != NULL) {

       task_executor->set_single_threaded_mode();

     }

     process_phaseJNI(is_alive, keep_alive, complete_gc);

   }

 }

 #ifndef PRODUCT

 // Calculate the number of jni handles.

 uint ReferenceProcessor::count_jni_refs() {

   class AlwaysAliveClosure: public BoolObjectClosure {

   public:

     virtual bool do_object_b(oop obj) { return true; }

     virtual void do_object(oop obj) { assert(false, "Don't call"); }

   };

   class CountHandleClosure: public OopClosure {

   private:

     int _count;

   public:

     CountHandleClosure(): _count(0) {}

     void do_oop(oop* unused)       { _count++; }

     void do_oop(narrowOop* unused) { ShouldNotReachHere(); }

     int count() { return _count; }

   };

   CountHandleClosure global_handle_count;

   AlwaysAliveClosure always_alive;

   JNIHandles::weak_oops_do(&always_alive, &global_handle_count);

   return global_handle_count.count();

 }

 #endif

 void ReferenceProcessor::process_phaseJNI(BoolObjectClosure* is_alive,

                                           OopClosure*        keep_alive,

                                           VoidClosure*       complete_gc) {

 #ifndef PRODUCT

   if (PrintGCDetails && PrintReferenceGC) {

     unsigned int count = count_jni_refs();

     gclog_or_tty->print(", %u refs", count);

   }

 #endif

   JNIHandles::weak_oops_do(is_alive, keep_alive);

   // Finally remember to keep sentinel around

   keep_alive->do_oop(adr_sentinel_ref());

   complete_gc->do_void();

 }

 template <class T>

 static bool enqueue_discovered_ref_helper(ReferenceProcessor* ref,

                                           AbstractRefProcTaskExecutor* task_executor) {

   // Remember old value of pending references list

   T* pending_list_addr = (T*)java_lang_ref_Reference::pending_list_addr();

   T old_pending_list_value = *pending_list_addr;

   // Enqueue references that are not made active again, and

   // clear the decks for the next collection (cycle).

   ref->enqueue_discovered_reflists((HeapWord*)pending_list_addr, task_executor);

   // Do the oop-check on pending_list_addr missed in

   // enqueue_discovered_reflist. We should probably

   // do a raw oop_check so that future such idempotent

   // oop_stores relying on the oop-check side-effect

   // may be elided automatically and safely without

   // affecting correctness.

   oop_store(pending_list_addr, oopDesc::load_decode_heap_oop(pending_list_addr));

   // Stop treating discovered references specially.

   ref->disable_discovery();

   // Return true if new pending references were added

   return old_pending_list_value != *pending_list_addr;

 }

 bool ReferenceProcessor::enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor) {

   NOT_PRODUCT(verify_ok_to_handle_reflists());

   if (UseCompressedOops) {

     return enqueue_discovered_ref_helper<narrowOop>(this, task_executor);

   } else {

     return enqueue_discovered_ref_helper<oop>(this, task_executor);

   }

 }

 void ReferenceProcessor::enqueue_discovered_reflist(DiscoveredList& refs_list,

                                                     HeapWord* pending_list_addr) {

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

   // (java.lang.ref.Reference.discovered) chain them through the

   // "next" field (java.lang.ref.Reference.next) and prepend

   // to the pending list.

   if (TraceReferenceGC && PrintGCDetails) {

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

                            INTPTR_FORMAT, (address)refs_list.head());

   }

   oop obj = refs_list.head();

   // Walk down the list, copying the discovered field into

   // the next field and clearing it (except for the last

   // non-sentinel object which is treated specially to avoid

   // confusion with an active reference).

   while (obj != sentinel_ref()) {

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

     oop next = java_lang_ref_Reference::discovered(obj);

     if (TraceReferenceGC && PrintGCDetails) {

       gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next " INTPTR_FORMAT,

                              obj, next);

     }

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

            "The reference should not be enqueued");

     if (next == sentinel_ref()) {  // obj is last

       // Swap refs_list into pendling_list_addr and

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

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

       // Need oop_check on pending_list_addr above;

       // see special oop-check code at the end of

       // enqueue_discovered_reflists() further below.

       if (old == NULL) {

         // obj should be made to point to itself, since

         // pending list was empty.

         java_lang_ref_Reference::set_next(obj, obj);

       } else {

         java_lang_ref_Reference::set_next(obj, old);

       }

     } else {

       java_lang_ref_Reference::set_next(obj, next);

     }

     java_lang_ref_Reference::set_discovered(obj, (oop) NULL);

     obj = next;

   }

 }

 // Parallel enqueue task

 class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {

 public:

   RefProcEnqueueTask(ReferenceProcessor& ref_processor,

                      DiscoveredList      discovered_refs[],

                      HeapWord*           pending_list_addr,

                      oop                 sentinel_ref,

                      int                 n_queues)

     : EnqueueTask(ref_processor, discovered_refs,

                   pending_list_addr, sentinel_ref, n_queues)

   { }

   virtual void work(unsigned int work_id) {

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

     // Simplest first cut: static partitioning.

     int index = work_id;

     for (int j = 0; j < subclasses_of_ref; j++, index += _n_queues) {

       _ref_processor.enqueue_discovered_reflist(

         _refs_lists[index], _pending_list_addr);

       _refs_lists[index].set_head(_sentinel_ref);

       _refs_lists[index].set_length(0);

     }

   }

 };

 // Enqueue references that are not made active again

 void ReferenceProcessor::enqueue_discovered_reflists(HeapWord* pending_list_addr,

   AbstractRefProcTaskExecutor* task_executor) {

   if (_processing_is_mt && task_executor != NULL) {

     // Parallel code

     RefProcEnqueueTask tsk(*this, _discoveredSoftRefs,

                            pending_list_addr, sentinel_ref(), _num_q);

     task_executor->execute(tsk);

   } else {

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

     for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

       enqueue_discovered_reflist(_discoveredSoftRefs[i], pending_list_addr);

       _discoveredSoftRefs[i].set_head(sentinel_ref());

       _discoveredSoftRefs[i].set_length(0);

     }

   }

 }

 // Iterator for the list of discovered references.

 class DiscoveredListIterator {

 public:

   inline DiscoveredListIterator(DiscoveredList&    refs_list,

                                 OopClosure*        keep_alive,

                                 BoolObjectClosure* is_alive);

   // End Of List.

   inline bool has_next() const { return _next != ReferenceProcessor::sentinel_ref(); }

   // Get oop to the Reference object.

   inline oop obj() const { return _ref; }

   // Get oop to the referent object.

   inline oop referent() const { return _referent; }

   // Returns true if referent is alive.

   inline bool is_referent_alive() const;

   // Loads data for the current reference.

   // The "allow_null_referent" argument tells us to allow for the possibility

   // of a NULL referent in the discovered Reference object. This typically

   // happens in the case of concurrent collectors that may have done the

   // discovery concurrently or interleaved with mutator execution.

   inline void load_ptrs(DEBUG_ONLY(bool allow_null_referent));

   // Move to the next discovered reference.

   inline void next();

   // Remove the current reference from the list and move to the next.

   inline void remove();

   // Make the Reference object active again.

   inline void make_active() { java_lang_ref_Reference::set_next(_ref, NULL); }

   // Make the referent alive.

   inline void make_referent_alive() {

     if (UseCompressedOops) {

       _keep_alive->do_oop((narrowOop*)_referent_addr);

     } else {

       _keep_alive->do_oop((oop*)_referent_addr);

     }

   }

   // Update the discovered field.

   inline void update_discovered() {

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

     if (UseCompressedOops) {

       _keep_alive->do_oop((narrowOop*)_prev_next);

     } else {

       _keep_alive->do_oop((oop*)_prev_next);

     }

   }

   // NULL out referent pointer.

   inline void clear_referent() { oop_store_raw(_referent_addr, NULL); }

   // Statistics

   NOT_PRODUCT(

   inline size_t processed() const { return _processed; }

   inline size_t removed() const   { return _removed; }

   )

 private:

   inline void move_to_next();

 private:

   DiscoveredList&    _refs_list;

   HeapWord*          _prev_next;

   oop                _ref;

   HeapWord*          _discovered_addr;

   oop                _next;

   HeapWord*          _referent_addr;

   oop                _referent;

   OopClosure*        _keep_alive;

   BoolObjectClosure* _is_alive;

   DEBUG_ONLY(

   oop                _first_seen; // cyclic linked list check

   )

   NOT_PRODUCT(

   size_t             _processed;

   size_t             _removed;

   )

 };

 inline DiscoveredListIterator::DiscoveredListIterator(DiscoveredList&    refs_list,

                                                       OopClosure*        keep_alive,

                                                       BoolObjectClosure* is_alive)

   : _refs_list(refs_list),

     _prev_next(refs_list.adr_head()),

     _ref(refs_list.head()),

 #ifdef ASSERT

     _first_seen(refs_list.head()),

 #endif

 #ifndef PRODUCT

     _processed(0),

     _removed(0),

 #endif

     _next(refs_list.head()),

     _keep_alive(keep_alive),

     _is_alive(is_alive)

 { }

 inline bool DiscoveredListIterator::is_referent_alive() const {

   return _is_alive->do_object_b(_referent);

 }

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

   _discovered_addr = java_lang_ref_Reference::discovered_addr(_ref);

   oop discovered = java_lang_ref_Reference::discovered(_ref);

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

          "discovered field is bad");

   _next = discovered;

   _referent_addr = java_lang_ref_Reference::referent_addr(_ref);

   _referent = java_lang_ref_Reference::referent(_ref);

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

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

   assert(allow_null_referent ?

              _referent->is_oop_or_null()

            : _referent->is_oop(),

          "bad referent");

 }

 inline void DiscoveredListIterator::next() {

   _prev_next = _discovered_addr;

   move_to_next();

 }

 inline void DiscoveredListIterator::remove() {

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

   oop_store_raw(_discovered_addr, NULL);

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

   if (UseCompressedOops) {

     // Remove Reference object from list.

     oopDesc::encode_store_heap_oop_not_null((narrowOop*)_prev_next, _next);

   } else {

     // Remove Reference object from list.

     oopDesc::store_heap_oop((oop*)_prev_next, _next);

   }

   NOT_PRODUCT(_removed++);

   move_to_next();

 }

 inline void DiscoveredListIterator::move_to_next() {

   _ref = _next;

   assert(_ref != _first_seen, "cyclic ref_list found");

   NOT_PRODUCT(_processed++);

 }

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

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

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

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

 // We should consider the possibility of replacing these

 // process_phase*() methods by abstracting them into

 // a single general iterator invocation that receives appropriate

 // closures that accomplish this work.

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

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

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

 void

 ReferenceProcessor::process_phase1(DiscoveredList&    refs_list,

                                    ReferencePolicy*   policy,

                                    BoolObjectClosure* is_alive,

                                    OopClosure*        keep_alive,

                                    VoidClosure*       complete_gc) {

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

   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

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

   while (iter.has_next()) {

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

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

     if (referent_is_dead && !policy->should_clear_reference(iter.obj())) {

       if (TraceReferenceGC) {

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

                                iter.obj(), iter.obj()->blueprint()->internal_name());

       }

       // Make the Reference object active again

       iter.make_active();

       // keep the referent around

       iter.make_referent_alive();

       // Remove Reference object from list

       iter.remove();

     } else {

       iter.next();

     }

   }

   // Close the reachable set

   complete_gc->do_void();

   NOT_PRODUCT(

     if (PrintGCDetails && TraceReferenceGC) {

       gclog_or_tty->print(" Dropped %d dead Refs out of %d "

         "discovered Refs by policy ", iter.removed(), iter.processed());

     }

   )

 }

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

 // whose referents are either alive or NULL.

 void

 ReferenceProcessor::pp2_work(DiscoveredList&    refs_list,

                              BoolObjectClosure* is_alive,

                              OopClosure*        keep_alive) {

   assert(discovery_is_atomic(), "Error");

   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

   while (iter.has_next()) {

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

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

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

     if (iter.is_referent_alive()) {

       if (TraceReferenceGC) {

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

                                iter.obj(), iter.obj()->blueprint()->internal_name());

       }

       // The referent is reachable after all.

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

       // should not entail any recursive marking because the

       // referent must already have been traversed.

       iter.make_referent_alive();

       // Remove Reference object from list

       iter.remove();

     } else {

       iter.next();

     }

   }

   NOT_PRODUCT(

     if (PrintGCDetails && TraceReferenceGC) {

       gclog_or_tty->print(" Dropped %d active Refs out of %d "

         "Refs in discovered list ", iter.removed(), iter.processed());

     }

   )

 }

 void

 ReferenceProcessor::pp2_work_concurrent_discovery(DiscoveredList&    refs_list,

                                                   BoolObjectClosure* is_alive,

                                                   OopClosure*        keep_alive,

                                                   VoidClosure*       complete_gc) {

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

   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

   while (iter.has_next()) {

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

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

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

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

          next != NULL)) {

       assert(next->is_oop_or_null(), "bad next field");

       // Remove Reference object from list

       iter.remove();

       // Trace the cohorts

       iter.make_referent_alive();

       if (UseCompressedOops) {

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

       } else {

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

       }

     } else {

       iter.next();

     }

   }

   // Now close the newly reachable set

   complete_gc->do_void();

   NOT_PRODUCT(

     if (PrintGCDetails && TraceReferenceGC) {

       gclog_or_tty->print(" Dropped %d active Refs out of %d "

         "Refs in discovered list ", iter.removed(), iter.processed());

     }

   )

 }

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

 // clearing them or keeping them (and their reachable

 // closure) alive.

 void

 ReferenceProcessor::process_phase3(DiscoveredList&    refs_list,

                                    bool               clear_referent,

                                    BoolObjectClosure* is_alive,

                                    OopClosure*        keep_alive,

                                    VoidClosure*       complete_gc) {

   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

   while (iter.has_next()) {

     iter.update_discovered();

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

     if (clear_referent) {

       // NULL out referent pointer

       iter.clear_referent();

     } else {

       // keep the referent around

       iter.make_referent_alive();

     }

     if (TraceReferenceGC) {

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

                              clear_referent ? "cleared " : "",

                              iter.obj(), iter.obj()->blueprint()->internal_name());

     }

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

     // If discovery is concurrent, we may have objects with null referents,

     // being those that were concurrently cleared after they were discovered

     // (and not subsequently precleaned).

     assert(   (discovery_is_atomic() && iter.referent()->is_oop())

            || (!discovery_is_atomic() && iter.referent()->is_oop_or_null(UseConcMarkSweepGC)),

            "Adding a bad referent");

     iter.next();

   }

   // Remember to keep sentinel pointer around

   iter.update_discovered();

   // Close the reachable set

   complete_gc->do_void();

 }

 void

 ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {

   oop obj = refs_list.head();

   while (obj != sentinel_ref()) {

     oop discovered = java_lang_ref_Reference::discovered(obj);

     java_lang_ref_Reference::set_discovered_raw(obj, NULL);

     obj = discovered;

   }

   refs_list.set_head(sentinel_ref());

   refs_list.set_length(0);

 }

 void ReferenceProcessor::abandon_partial_discovery() {

   // loop over the lists

   for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

     if (TraceReferenceGC && PrintGCDetails && ((i % _num_q) == 0)) {

       gclog_or_tty->print_cr(

         "\nAbandoning %s discovered list",

         list_name(i));

     }

     abandon_partial_discovered_list(_discoveredSoftRefs[i]);

   }

 }

 class RefProcPhase1Task: public AbstractRefProcTaskExecutor::ProcessTask {

 public:

   RefProcPhase1Task(ReferenceProcessor& ref_processor,

                     DiscoveredList      refs_lists[],

                     ReferencePolicy*    policy,

                     bool                marks_oops_alive)

     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),

       _policy(policy)

   { }

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

                     OopClosure& keep_alive,

                     VoidClosure& complete_gc)

   {

     _ref_processor.process_phase1(_refs_lists[i], _policy,

                                   &is_alive, &keep_alive, &complete_gc);

   }

 private:

   ReferencePolicy* _policy;

 };

 class RefProcPhase2Task: public AbstractRefProcTaskExecutor::ProcessTask {

 public:

   RefProcPhase2Task(ReferenceProcessor& ref_processor,

                     DiscoveredList      refs_lists[],

                     bool                marks_oops_alive)

     : ProcessTask(ref_processor, refs_lists, marks_oops_alive)

   { }

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

                     OopClosure& keep_alive,

                     VoidClosure& complete_gc)

   {

     _ref_processor.process_phase2(_refs_lists[i],

                                   &is_alive, &keep_alive, &complete_gc);

   }

 };

 class RefProcPhase3Task: public AbstractRefProcTaskExecutor::ProcessTask {

 public:

   RefProcPhase3Task(ReferenceProcessor& ref_processor,

                     DiscoveredList      refs_lists[],

                     bool                clear_referent,

                     bool                marks_oops_alive)

     : ProcessTask(ref_processor, refs_lists, marks_oops_alive),

       _clear_referent(clear_referent)

   { }

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

                     OopClosure& keep_alive,

                     VoidClosure& complete_gc)

   {

     _ref_processor.process_phase3(_refs_lists[i], _clear_referent,

                                   &is_alive, &keep_alive, &complete_gc);

   }

 private:

   bool _clear_referent;

 };

 // Balances reference queues.

 void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])

 {

   // calculate total length

   size_t total_refs = 0;

   for (int i = 0; i < _num_q; ++i) {

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

   }

   size_t avg_refs = total_refs / _num_q + 1;

   int to_idx = 0;

   for (int from_idx = 0; from_idx < _num_q; from_idx++) {

     while (ref_lists[from_idx].length() > avg_refs) {

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

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

         // move superfluous refs

         size_t refs_to_move =

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

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

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

         oop move_tail = move_head;

         oop new_head  = move_head;

         // find an element to split the list on

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

           move_tail = new_head;

           new_head = java_lang_ref_Reference::discovered(new_head);

         }

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

         ref_lists[to_idx].set_head(move_head);

         ref_lists[to_idx].set_length(ref_lists[to_idx].length() + refs_to_move);

         ref_lists[from_idx].set_head(new_head);

         ref_lists[from_idx].set_length(ref_lists[from_idx].length() - refs_to_move);

       } else {

         ++to_idx;

       }

     }

   }

 }

 void

 ReferenceProcessor::process_discovered_reflist(

   DiscoveredList               refs_lists[],

   ReferencePolicy*             policy,

   bool                         clear_referent,

   BoolObjectClosure*           is_alive,

   OopClosure*                  keep_alive,

   VoidClosure*                 complete_gc,

   AbstractRefProcTaskExecutor* task_executor)

 {

   bool mt = task_executor != NULL && _processing_is_mt;

   if (mt && ParallelRefProcBalancingEnabled) {

     balance_queues(refs_lists);

   }

   if (PrintReferenceGC && PrintGCDetails) {

     size_t total = 0;

     for (int i = 0; i < _num_q; ++i) {

       total += refs_lists[i].length();

     }

     gclog_or_tty->print(", %u refs", total);

   }

   // Phase 1 (soft refs only):

   // . Traverse the list and remove any SoftReferences whose

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

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

   //   such referents.

   if (policy != NULL) {

     if (mt) {

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

       task_executor->execute(phase1);

     } else {

       for (int i = 0; i < _num_q; i++) {

         process_phase1(refs_lists[i], policy,

                        is_alive, keep_alive, complete_gc);

       }

     }

   } else { // policy == NULL

     assert(refs_lists != _discoveredSoftRefs,

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

   }

   // Phase 2:

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

   if (mt) {

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

     task_executor->execute(phase2);

   } else {

     for (int i = 0; i < _num_q; i++) {

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

     }

   }

   // Phase 3:

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

   if (mt) {

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

     task_executor->execute(phase3);

   } else {

     for (int i = 0; i < _num_q; i++) {

       process_phase3(refs_lists[i], clear_referent,

                      is_alive, keep_alive, complete_gc);

     }

   }

 }

 void ReferenceProcessor::clean_up_discovered_references() {

   // loop over the lists

   for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

     if (TraceReferenceGC && PrintGCDetails && ((i % _num_q) == 0)) {

       gclog_or_tty->print_cr(

         "\nScrubbing %s discovered list of Null referents",

         list_name(i));

     }

     clean_up_discovered_reflist(_discoveredSoftRefs[i]);

   }

 }

 void ReferenceProcessor::clean_up_discovered_reflist(DiscoveredList& refs_list) {

   assert(!discovery_is_atomic(), "Else why call this method?");

   DiscoveredListIterator iter(refs_list, NULL, NULL);

   size_t length = refs_list.length();

   while (iter.has_next()) {

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

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

     assert(next->is_oop_or_null(), "bad next field");

     // If referent has been cleared or Reference is not active,

     // drop it.

     if (iter.referent() == NULL || next != NULL) {

       debug_only(

         if (PrintGCDetails && TraceReferenceGC) {

           gclog_or_tty->print_cr("clean_up_discovered_list: Dropping Reference: "

             INTPTR_FORMAT " with next field: " INTPTR_FORMAT

             " and referent: " INTPTR_FORMAT,

             iter.obj(), next, iter.referent());

         }

       )

       // Remove Reference object from list

       iter.remove();

       --length;

     } else {

       iter.next();

     }

   }

   refs_list.set_length(length);

   NOT_PRODUCT(

     if (PrintGCDetails && TraceReferenceGC) {

       gclog_or_tty->print(

         " Removed %d Refs with NULL referents out of %d discovered Refs",

         iter.removed(), iter.processed());

     }

   )

 }

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

   int id = 0;

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

   if (_discovery_is_mt) {

     // During a multi-threaded discovery phase,

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

     Thread* thr = Thread::current();

     assert(thr->is_GC_task_thread(),

            "Dubious cast from Thread* to WorkerThread*?");

     id = ((WorkerThread*)thr)->id();

   } else {

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

     // fashion to each of the lists.

     if (_processing_is_mt) {

       id = next_id();

     }

   }

   assert(0 <= id && id < _num_q, "Id is out-of-bounds (call Freud?)");

   // Get the discovered queue to which we will add

   DiscoveredList* list = NULL;

   switch (rt) {

     case REF_OTHER:

       // Unknown reference type, no special treatment

       break;

     case REF_SOFT:

       list = &_discoveredSoftRefs[id];

       break;

     case REF_WEAK:

       list = &_discoveredWeakRefs[id];

       break;

     case REF_FINAL:

       list = &_discoveredFinalRefs[id];

       break;

     case REF_PHANTOM:

       list = &_discoveredPhantomRefs[id];

       break;

     case REF_NONE:

       // we should not reach here if we are an instanceRefKlass

     default:

       ShouldNotReachHere();

   }

   return list;

 }

 inline void

 ReferenceProcessor::add_to_discovered_list_mt(DiscoveredList& refs_list,

                                               oop             obj,

                                               HeapWord*       discovered_addr) {

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

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

   // discovered_addr.

   oop current_head = refs_list.head();

   // Note: In the case of G1, this pre-barrier is strictly

   // not necessary because the only case we are interested in

   // here is when *discovered_addr is NULL, so this will expand to

   // nothing. As a result, I am just manually eliding this out for G1.

   if (_discovered_list_needs_barrier && !UseG1GC) {

     _bs->write_ref_field_pre((void*)discovered_addr, current_head); guarantee(false, "Needs to be fixed: YSR");

   }

   oop retest = oopDesc::atomic_compare_exchange_oop(current_head, discovered_addr,

                                                     NULL);

   if (retest == NULL) {

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

     // We have separate lists for enqueueing so no synchronization

     // is necessary.

     refs_list.set_head(obj);

     refs_list.set_length(refs_list.length() + 1);

     if (_discovered_list_needs_barrier) {

       _bs->write_ref_field((void*)discovered_addr, current_head); guarantee(false, "Needs to be fixed: YSR");

     }

   } else {

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

     // The reference has already been discovered...

     if (TraceReferenceGC) {

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

                              obj, obj->blueprint()->internal_name());

     }

   }

 }

 // We mention two of several possible choices here:

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

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

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

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

 //     This means that references can't be enqueued unless their

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

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

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

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

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

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

 //     then we can discover the reference object, provided

 //     the object has not already been discovered by

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

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

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

 //     of this reference object by the current collector will

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

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

 //     discover references in other generations even if the

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

 //     in certain cases, enqueue references somewhat sooner than

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

 //     and complexity in processing these references.

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

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

   // We enqueue references only if we are discovering refs

   // (rather than processing discovered refs).

   if (!_discovering_refs || !RegisterReferences) {

     return false;

   }

   // We only enqueue active references.

   oop next = java_lang_ref_Reference::next(obj);

   if (next != NULL) {

     return false;

   }

   HeapWord* obj_addr = (HeapWord*)obj;

   if (RefDiscoveryPolicy == ReferenceBasedDiscovery &&

       !_span.contains(obj_addr)) {

     // Reference is not in the originating generation;

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

     // object with strong references).

     return false;

   }

   // We only enqueue references whose referents are not (yet) strongly

   // reachable.

   if (is_alive_non_header() != NULL) {

     oop referent = java_lang_ref_Reference::referent(obj);

     // We'd like to assert the following:

     // assert(referent != NULL, "Refs with null referents already filtered");

     // However, since this code may be executed concurrently with

     // mutators, which can clear() the referent, it is not

     // guaranteed that the referent is non-NULL.

     if (is_alive_non_header()->do_object_b(referent)) {

       return false;  // referent is reachable

     }

   }

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

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

   assert(discovered->is_oop_or_null(), "bad discovered field");

   if (discovered != NULL) {

     // The reference has already been discovered...

     if (TraceReferenceGC) {

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

                              obj, obj->blueprint()->internal_name());

     }

     if (RefDiscoveryPolicy == ReferentBasedDiscovery) {

       // assumes that an object is not processed twice;

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

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

       return false;

     } else {

       assert(RefDiscoveryPolicy == ReferenceBasedDiscovery,

              "Unrecognized policy");

       // Check assumption that an object is not potentially

       // discovered twice except by concurrent collectors that potentially

       // trace the same Reference object twice.

       assert(UseConcMarkSweepGC,

              "Only possible with an incremental-update concurrent collector");

       return true;

     }

   }

   if (RefDiscoveryPolicy == ReferentBasedDiscovery) {

     oop referent = java_lang_ref_Reference::referent(obj);

     assert(referent->is_oop(), "bad referent");

     // enqueue if and only if either:

     // reference is in our span or

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

     if (_span.contains(obj_addr) ||

         (discovery_is_atomic() && _span.contains(referent))) {

       // should_enqueue = true;

     } else {

       return false;

     }

   } else {

     assert(RefDiscoveryPolicy == ReferenceBasedDiscovery &&

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

   }

   // Get the right type of discovered queue head.

   DiscoveredList* list = get_discovered_list(rt);

   if (list == NULL) {

     return false;   // nothing special needs to be done

   }

   if (_discovery_is_mt) {

     add_to_discovered_list_mt(*list, obj, discovered_addr);

   } else {

     // If "_discovered_list_needs_barrier", we do write barriers when

     // updating the discovered reference list.  Otherwise, we do a raw store

     // here: the field will be visited later when processing the discovered

     // references.

     oop current_head = list->head();

     // As in the case further above, since we are over-writing a NULL

     // pre-value, we can safely elide the pre-barrier here for the case of G1.

     assert(discovered == NULL, "control point invariant");

     if (_discovered_list_needs_barrier && !UseG1GC) { // safe to elide for G1

       _bs->write_ref_field_pre((oop*)discovered_addr, current_head);

     }

     oop_store_raw(discovered_addr, current_head);

     if (_discovered_list_needs_barrier) {

       _bs->write_ref_field((oop*)discovered_addr, current_head);

     }

     list->set_head(obj);

     list->set_length(list->length() + 1);

   }

   // In the MT discovery case, it is currently possible to see

   // the following message multiple times if several threads

   // discover a reference about the same time. Only one will

   // however have actually added it to the disocvered queue.

   // One could let add_to_discovered_list_mt() return an

   // indication for success in queueing (by 1 thread) or

   // failure (by all other threads), but I decided the extra

   // code was not worth the effort for something that is

   // only used for debugging support.

   if (TraceReferenceGC) {

     oop referent = java_lang_ref_Reference::referent(obj);

     if (PrintGCDetails) {

       gclog_or_tty->print_cr("Enqueued reference (" INTPTR_FORMAT ": %s)",

                              obj, obj->blueprint()->internal_name());

     }

     assert(referent->is_oop(), "Enqueued a bad referent");

   }

   assert(obj->is_oop(), "Enqueued a bad reference");

   return true;

 }

 // Preclean the discovered references by removing those

 // whose referents are alive, and by marking from those that

 // are not active. These lists can be handled here

 // in any order and, indeed, concurrently.

 void ReferenceProcessor::preclean_discovered_references(

   BoolObjectClosure* is_alive,

   OopClosure* keep_alive,

   VoidClosure* complete_gc,

   YieldClosure* yield) {

   NOT_PRODUCT(verify_ok_to_handle_reflists());

   // Soft references

   {

     TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,

               false, gclog_or_tty);

     for (int i = 0; i < _num_q; i++) {

       preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,

                                   keep_alive, complete_gc, yield);

     }

   }

   if (yield->should_return()) {

     return;

   }

   // Weak references

   {

     TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,

               false, gclog_or_tty);

     for (int i = 0; i < _num_q; i++) {

       preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,

                                   keep_alive, complete_gc, yield);

     }

   }

   if (yield->should_return()) {

     return;

   }

   // Final references

   {

     TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,

               false, gclog_or_tty);

     for (int i = 0; i < _num_q; i++) {

       preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,

                                   keep_alive, complete_gc, yield);

     }

   }

   if (yield->should_return()) {

     return;

   }

   // Phantom references

   {

     TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,

               false, gclog_or_tty);

     for (int i = 0; i < _num_q; i++) {

       preclean_discovered_reflist(_discoveredPhantomRefs[i], is_alive,

                                   keep_alive, complete_gc, yield);

     }

   }

 }

 // Walk the given discovered ref list, and remove all reference objects

 // whose referents are still alive, whose referents are NULL or which

 // are not active (have a non-NULL next field). NOTE: For this to work

 // correctly, refs discovery can not be happening concurrently with this

 // step.

 void

 ReferenceProcessor::preclean_discovered_reflist(DiscoveredList&    refs_list,

                                                 BoolObjectClosure* is_alive,

                                                 OopClosure*        keep_alive,

                                                 VoidClosure*       complete_gc,

                                                 YieldClosure*      yield) {

   DiscoveredListIterator iter(refs_list, keep_alive, is_alive);

   size_t length = refs_list.length();

   while (iter.has_next()) {

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

     oop obj = iter.obj();

     oop next = java_lang_ref_Reference::next(obj);

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

         next != NULL) {

       // The referent has been cleared, or is alive, or the Reference is not

       // active; we need to trace and mark its cohort.

       if (TraceReferenceGC) {

         gclog_or_tty->print_cr("Precleaning Reference (" INTPTR_FORMAT ": %s)",

                                iter.obj(), iter.obj()->blueprint()->internal_name());

       }

       // Remove Reference object from list

       iter.remove();

       --length;

       // Keep alive its cohort.

       iter.make_referent_alive();

       if (UseCompressedOops) {

         narrowOop* next_addr = (narrowOop*)java_lang_ref_Reference::next_addr(obj);

         keep_alive->do_oop(next_addr);

       } else {

         oop* next_addr = (oop*)java_lang_ref_Reference::next_addr(obj);

         keep_alive->do_oop(next_addr);

       }

     } else {

       iter.next();

     }

   }

   refs_list.set_length(length);

   // Close the reachable set

   complete_gc->do_void();

   NOT_PRODUCT(

     if (PrintGCDetails && PrintReferenceGC) {

       gclog_or_tty->print(" Dropped %d Refs out of %d "

         "Refs in discovered list ", iter.removed(), iter.processed());

     }

   )

 }

 const char* ReferenceProcessor::list_name(int i) {

    assert(i >= 0 && i <= _num_q * subclasses_of_ref, "Out of bounds index");

    int j = i / _num_q;

    switch (j) {

      case 0: return "SoftRef";

      case 1: return "WeakRef";

      case 2: return "FinalRef";

      case 3: return "PhantomRef";

    }

    ShouldNotReachHere();

    return NULL;

 }

 #ifndef PRODUCT

 void ReferenceProcessor::verify_ok_to_handle_reflists() {

   // empty for now

 }

 #endif

 void ReferenceProcessor::verify() {

   guarantee(sentinel_ref() != NULL && sentinel_ref()->is_oop(), "Lost _sentinelRef");

 }

 #ifndef PRODUCT

 void ReferenceProcessor::clear_discovered_references() {

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

   for (int i = 0; i < _num_q * subclasses_of_ref; i++) {

     oop obj = _discoveredSoftRefs[i].head();

     while (obj != sentinel_ref()) {

       oop next = java_lang_ref_Reference::discovered(obj);

       java_lang_ref_Reference::set_discovered(obj, (oop) NULL);

       obj = next;

     }

     _discoveredSoftRefs[i].set_head(sentinel_ref());

     _discoveredSoftRefs[i].set_length(0);

   }

 }

 #endif // PRODUCT