view src/share/vm/memory/referenceProcessor.cpp @ 2740:4dfb2df418f2

6484982: G1: process references during evacuation pauses Summary: G1 now uses two reference processors - one is used by concurrent marking and the other is used by STW GCs (both full and incremental evacuation pauses). In an evacuation pause, the reference processor is embedded into the closures used to scan objects. Doing so causes causes reference objects to be 'discovered' by the reference processor. At the end of the evacuation pause, these discovered reference objects are processed - preserving (and copying) referent objects (and their reachable graphs) as appropriate. Reviewed-by: ysr, jwilhelm, brutisso, stefank, tonyp
author johnc
date Thu, 22 Sep 2011 10:57:37 -0700
parents eca1193ca245
children d1bdeef3e3e2
line wrap: on
line source
/*
 * Copyright (c) 2001, 2011, Oracle and/or its affiliates. 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

#include "precompiled.hpp"
#include "classfile/javaClasses.hpp"
#include "classfile/systemDictionary.hpp"
#include "gc_interface/collectedHeap.hpp"
#include "gc_interface/collectedHeap.inline.hpp"
#include "memory/referencePolicy.hpp"
#include "memory/referenceProcessor.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/java.hpp"
#include "runtime/jniHandles.hpp"

ReferencePolicy* ReferenceProcessor::_always_clear_soft_ref_policy = NULL;
ReferencePolicy* ReferenceProcessor::_default_soft_ref_policy      = NULL;
bool             ReferenceProcessor::_pending_list_uses_discovered_field = false;

void referenceProcessor_init() {
  ReferenceProcessor::init_statics();
}

void ReferenceProcessor::init_statics() {
  // Initialize the master soft ref clock.
  java_lang_ref_SoftReference::set_clock(os::javaTimeMillis());

  _always_clear_soft_ref_policy = new AlwaysClearPolicy();
  _default_soft_ref_policy      = new COMPILER2_PRESENT(LRUMaxHeapPolicy())
                                      NOT_COMPILER2(LRUCurrentHeapPolicy());
  if (_always_clear_soft_ref_policy == NULL || _default_soft_ref_policy == NULL) {
    vm_exit_during_initialization("Could not allocate reference policy object");
  }
  guarantee(RefDiscoveryPolicy == ReferenceBasedDiscovery ||
            RefDiscoveryPolicy == ReferentBasedDiscovery,
            "Unrecongnized RefDiscoveryPolicy");
  _pending_list_uses_discovered_field = JDK_Version::current().pending_list_uses_discovered_field();
}

ReferenceProcessor::ReferenceProcessor(MemRegion span,
                                       bool      mt_processing,
                                       int       mt_processing_degree,
                                       bool      mt_discovery,
                                       int       mt_discovery_degree,
                                       bool      atomic_discovery,
                                       BoolObjectClosure* is_alive_non_header,
                                       bool      discovered_list_needs_barrier)  :
  _discovering_refs(false),
  _enqueuing_is_done(false),
  _is_alive_non_header(is_alive_non_header),
  _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               = MAX2(1, mt_processing_degree);
  _max_num_q           = MAX2(_num_q, mt_discovery_degree);
  _discoveredSoftRefs  = NEW_C_HEAP_ARRAY(DiscoveredList,
                                          _max_num_q * number_of_subclasses_of_ref());
  if (_discoveredSoftRefs == NULL) {
    vm_exit_during_initialization("Could not allocated RefProc Array");
  }
  _discoveredWeakRefs    = &_discoveredSoftRefs[_max_num_q];
  _discoveredFinalRefs   = &_discoveredWeakRefs[_max_num_q];
  _discoveredPhantomRefs = &_discoveredFinalRefs[_max_num_q];
  // Initialized all entries to NULL
  for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
    _discoveredSoftRefs[i].set_head(NULL);
    _discoveredSoftRefs[i].set_length(0);
  }
  // If we do barriers, cache a copy of the barrier set.
  if (discovered_list_needs_barrier) {
    _bs = Universe::heap()->barrier_set();
  }
  setup_policy(false /* default soft ref policy */);
}

#ifndef PRODUCT
void ReferenceProcessor::verify_no_references_recorded() {
  guarantee(!_discovering_refs, "Discovering refs?");
  for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
    guarantee(_discoveredSoftRefs[i].is_empty(),
              "Found non-empty discovered list");
  }
}
#endif

void ReferenceProcessor::weak_oops_do(OopClosure* f) {
  for (int i = 0; i < _max_num_q * number_of_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::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(
  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, _current_soft_ref_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);
  complete_gc->do_void();
}


template <class T>
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), self-loop their "next" field
  // thus distinguishing them from active References, then
  // prepend them to the pending list.
  // BKWRD COMPATIBILITY NOTE: For older JDKs (prior to the fix for 4956777),
  // the "next" field is used to chain the pending list, not the discovered
  // field.

  if (TraceReferenceGC && PrintGCDetails) {
    gclog_or_tty->print_cr("ReferenceProcessor::enqueue_discovered_reflist list "
                           INTPTR_FORMAT, (address)refs_list.head());
  }

  oop obj = NULL;
  oop next_d = refs_list.head();
  if (pending_list_uses_discovered_field()) { // New behaviour
    // Walk down the list, self-looping the next field
    // so that the References are not considered active.
    while (obj != next_d) {
      obj = next_d;
      assert(obj->is_instanceRef(), "should be reference object");
      next_d = java_lang_ref_Reference::discovered(obj);
      if (TraceReferenceGC && PrintGCDetails) {
        gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
                               obj, next_d);
      }
      assert(java_lang_ref_Reference::next(obj) == NULL,
             "Reference not active; should not be discovered");
      // Self-loop next, so as to make Ref not active.
      java_lang_ref_Reference::set_next(obj, obj);
      if (next_d == obj) {  // obj is last
        // Swap refs_list into pendling_list_addr and
        // set obj's discovered 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.
        java_lang_ref_Reference::set_discovered(obj, old); // old may be NULL
      }
    }
  } else { // Old behaviour
    // Walk down the list, copying the discovered field into
    // the next field and clearing the discovered field.
    while (obj != next_d) {
      obj = next_d;
      assert(obj->is_instanceRef(), "should be reference object");
      next_d = java_lang_ref_Reference::discovered(obj);
      if (TraceReferenceGC && PrintGCDetails) {
        gclog_or_tty->print_cr("        obj " INTPTR_FORMAT "/next_d " INTPTR_FORMAT,
                               obj, next_d);
      }
      assert(java_lang_ref_Reference::next(obj) == NULL,
             "The reference should not be enqueued");
      if (next_d == obj) {  // 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_d);
      }
      java_lang_ref_Reference::set_discovered(obj, (oop) NULL);
    }
  }
}

// Parallel enqueue task
class RefProcEnqueueTask: public AbstractRefProcTaskExecutor::EnqueueTask {
public:
  RefProcEnqueueTask(ReferenceProcessor& ref_processor,
                     DiscoveredList      discovered_refs[],
                     HeapWord*           pending_list_addr,
                     int                 n_queues)
    : EnqueueTask(ref_processor, discovered_refs,
                  pending_list_addr, n_queues)
  { }

  virtual void work(unsigned int work_id) {
    assert(work_id < (unsigned int)_ref_processor.max_num_q(), "Index out-of-bounds");
    // Simplest first cut: static partitioning.
    int index = work_id;
    // The increment on "index" must correspond to the maximum number of queues
    // (n_queues) with which that ReferenceProcessor was created.  That
    // is because of the "clever" way the discovered references lists were
    // allocated and are indexed into.
    assert(_n_queues == (int) _ref_processor.max_num_q(), "Different number not expected");
    for (int j = 0;
         j < ReferenceProcessor::number_of_subclasses_of_ref();
         j++, index += _n_queues) {
      _ref_processor.enqueue_discovered_reflist(
        _refs_lists[index], _pending_list_addr);
      _refs_lists[index].set_head(NULL);
      _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, _max_num_q);
    task_executor->execute(tsk);
  } else {
    // Serial code: call the parent class's implementation
    for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
      enqueue_discovered_reflist(_discoveredSoftRefs[i], pending_list_addr);
      _discoveredSoftRefs[i].set_head(NULL);
      _discoveredSoftRefs[i].set_length(0);
    }
  }
}

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");
}

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).
  oop new_next;
  if (_next == _ref) {
    // At the end of the list, we should make _prev point to itself.
    // If _ref is the first ref, then _prev_next will be in the DiscoveredList,
    // and _prev will be NULL.
    new_next = _prev;
  } else {
    new_next = _next;
  }

  if (UseCompressedOops) {
    // Remove Reference object from list.
    oopDesc::encode_store_heap_oop((narrowOop*)_prev_next, new_next);
  } else {
    // Remove Reference object from list.
    oopDesc::store_heap_oop((oop*)_prev_next, new_next);
  }
  NOT_PRODUCT(_removed++);
  _refs_list.dec_length(1);
}

// Make the Reference object active again.
void DiscoveredListIterator::make_active() {
  // For G1 we don't want to use set_next - it
  // will dirty the card for the next field of
  // the reference object and will fail
  // CT verification.
  if (UseG1GC) {
    BarrierSet* bs = oopDesc::bs();
    HeapWord* next_addr = java_lang_ref_Reference::next_addr(_ref);

    if (UseCompressedOops) {
      bs->write_ref_field_pre((narrowOop*)next_addr, NULL);
    } else {
      bs->write_ref_field_pre((oop*)next_addr, NULL);
    }
    java_lang_ref_Reference::set_next_raw(_ref, NULL);
  } else {
    java_lang_ref_Reference::set_next(_ref, NULL);
  }
}

void DiscoveredListIterator::clear_referent() {
  oop_store_raw(_referent_addr, NULL);
}

// 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());
      }
      // Remove Reference object from list
      iter.remove();
      // Make the Reference object active again
      iter.make_active();
      // keep the referent around
      iter.make_referent_alive();
      iter.move_to_next();
    } else {
      iter.next();
    }
  }
  // Close the reachable set
  complete_gc->do_void();
  NOT_PRODUCT(
    if (PrintGCDetails && TraceReferenceGC) {
      gclog_or_tty->print_cr(" Dropped %d dead Refs out of %d "
        "discovered Refs by policy, from list " INTPTR_FORMAT,
        iter.removed(), iter.processed(), (address)refs_list.head());
    }
  )
}

// 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.
      // Remove Reference object from list.
      iter.remove();
      // 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();
      iter.move_to_next();
    } else {
      iter.next();
    }
  }
  NOT_PRODUCT(
    if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
      gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
        "Refs in discovered list " INTPTR_FORMAT,
        iter.removed(), iter.processed(), (address)refs_list.head());
    }
  )
}

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);
      }
      iter.move_to_next();
    } else {
      iter.next();
    }
  }
  // Now close the newly reachable set
  complete_gc->do_void();
  NOT_PRODUCT(
    if (PrintGCDetails && TraceReferenceGC && (iter.processed() > 0)) {
      gclog_or_tty->print_cr(" Dropped %d active Refs out of %d "
        "Refs in discovered list " INTPTR_FORMAT,
        iter.removed(), iter.processed(), (address)refs_list.head());
    }
  )
}

// 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) {
  ResourceMark rm;
  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");
    iter.next();
  }
  // Remember to update the next pointer of the last ref.
  iter.update_discovered();
  // Close the reachable set
  complete_gc->do_void();
}

void
ReferenceProcessor::clear_discovered_references(DiscoveredList& refs_list) {
  oop obj = NULL;
  oop next = refs_list.head();
  while (next != obj) {
    obj = next;
    next = java_lang_ref_Reference::discovered(obj);
    java_lang_ref_Reference::set_discovered_raw(obj, NULL);
  }
  refs_list.set_head(NULL);
  refs_list.set_length(0);
}

void
ReferenceProcessor::abandon_partial_discovered_list(DiscoveredList& refs_list) {
  clear_discovered_references(refs_list);
}

void ReferenceProcessor::abandon_partial_discovery() {
  // loop over the lists
  for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
    if (TraceReferenceGC && PrintGCDetails && ((i % _max_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)
  {
    Thread* thr = Thread::current();
    int refs_list_index = ((WorkerThread*)thr)->id();
    _ref_processor.process_phase1(_refs_lists[refs_list_index], _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)
  {
    // Don't use "refs_list_index" calculated in this way because
    // balance_queues() has moved the Ref's into the first n queues.
    // Thread* thr = Thread::current();
    // int refs_list_index = ((WorkerThread*)thr)->id();
    // _ref_processor.process_phase3(_refs_lists[refs_list_index], _clear_referent,
    _ref_processor.process_phase3(_refs_lists[i], _clear_referent,
                                  &is_alive, &keep_alive, &complete_gc);
  }
private:
  bool _clear_referent;
};

void ReferenceProcessor::set_discovered(oop ref, oop value) {
  if (_discovered_list_needs_barrier) {
    java_lang_ref_Reference::set_discovered(ref, value);
  } else {
    java_lang_ref_Reference::set_discovered_raw(ref, value);
  }
}

// Balances reference queues.
// Move entries from all queues[0, 1, ..., _max_num_q-1] to
// queues[0, 1, ..., _num_q-1] because only the first _num_q
// corresponding to the active workers will be processed.
void ReferenceProcessor::balance_queues(DiscoveredList ref_lists[])
{
  // calculate total length
  size_t total_refs = 0;
  if (TraceReferenceGC && PrintGCDetails) {
    gclog_or_tty->print_cr("\nBalance ref_lists ");
  }

  for (int i = 0; i < _max_num_q; ++i) {
    total_refs += ref_lists[i].length();
    if (TraceReferenceGC && PrintGCDetails) {
      gclog_or_tty->print("%d ", ref_lists[i].length());
    }
  }
  if (TraceReferenceGC && PrintGCDetails) {
    gclog_or_tty->print_cr(" = %d", total_refs);
  }
  size_t avg_refs = total_refs / _num_q + 1;
  int to_idx = 0;
  for (int from_idx = 0; from_idx < _max_num_q; from_idx++) {
    bool move_all = false;
    if (from_idx >= _num_q) {
      move_all = ref_lists[from_idx].length() > 0;
    }
    while ((ref_lists[from_idx].length() > avg_refs) ||
           move_all) {
      assert(to_idx < _num_q, "Sanity Check!");
      if (ref_lists[to_idx].length() < avg_refs) {
        // move superfluous refs
        size_t refs_to_move;
        // Move all the Ref's if the from queue will not be processed.
        if (move_all) {
          refs_to_move = MIN2(ref_lists[from_idx].length(),
                              avg_refs - ref_lists[to_idx].length());
        } else {
          refs_to_move = MIN2(ref_lists[from_idx].length() - avg_refs,
                              avg_refs - ref_lists[to_idx].length());
        }

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

        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);
        }

        // Add the chain to the to list.
        if (ref_lists[to_idx].head() == NULL) {
          // to list is empty. Make a loop at the end.
          set_discovered(move_tail, move_tail);
        } else {
          set_discovered(move_tail, ref_lists[to_idx].head());
        }
        ref_lists[to_idx].set_head(move_head);
        ref_lists[to_idx].inc_length(refs_to_move);

        // Remove the chain from the from list.
        if (move_tail == new_head) {
          // We found the end of the from list.
          ref_lists[from_idx].set_head(NULL);
        } else {
          ref_lists[from_idx].set_head(new_head);
        }
        ref_lists[from_idx].dec_length(refs_to_move);
        if (ref_lists[from_idx].length() == 0) {
          break;
        }
      } else {
        to_idx = (to_idx + 1) % _num_q;
      }
    }
  }
#ifdef ASSERT
  size_t balanced_total_refs = 0;
  for (int i = 0; i < _max_num_q; ++i) {
    balanced_total_refs += ref_lists[i].length();
    if (TraceReferenceGC && PrintGCDetails) {
      gclog_or_tty->print("%d ", ref_lists[i].length());
    }
  }
  if (TraceReferenceGC && PrintGCDetails) {
    gclog_or_tty->print_cr(" = %d", balanced_total_refs);
    gclog_or_tty->flush();
  }
  assert(total_refs == balanced_total_refs, "Balancing was incomplete");
#endif
}

void ReferenceProcessor::balance_all_queues() {
  balance_queues(_discoveredSoftRefs);
  balance_queues(_discoveredWeakRefs);
  balance_queues(_discoveredFinalRefs);
  balance_queues(_discoveredPhantomRefs);
}

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_processing = task_executor != NULL && _processing_is_mt;
  // If discovery used MT and a dynamic number of GC threads, then
  // the queues must be balanced for correctness if fewer than the
  // maximum number of queues were used.  The number of queue used
  // during discovery may be different than the number to be used
  // for processing so don't depend of _num_q < _max_num_q as part
  // of the test.
  bool must_balance = _discovery_is_mt;

  if ((mt_processing && ParallelRefProcBalancingEnabled) ||
      must_balance) {
    balance_queues(refs_lists);
  }
  if (PrintReferenceGC && PrintGCDetails) {
    size_t total = 0;
    for (int i = 0; i < _max_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_processing) {
      RefProcPhase1Task phase1(*this, refs_lists, policy, true /*marks_oops_alive*/);
      task_executor->execute(phase1);
    } else {
      for (int i = 0; i < _max_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_processing) {
    RefProcPhase2Task phase2(*this, refs_lists, !discovery_is_atomic() /*marks_oops_alive*/);
    task_executor->execute(phase2);
  } else {
    for (int i = 0; i < _max_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_processing) {
    RefProcPhase3Task phase3(*this, refs_lists, clear_referent, true /*marks_oops_alive*/);
    task_executor->execute(phase3);
  } else {
    for (int i = 0; i < _max_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 < _max_num_q * number_of_subclasses_of_ref(); i++) {
    if (TraceReferenceGC && PrintGCDetails && ((i % _max_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);
  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();
      iter.move_to_next();
    } else {
      iter.next();
    }
  }
  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();
    id = thr->as_Worker_thread()->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 < _max_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();
  }
  if (TraceReferenceGC && PrintGCDetails) {
    gclog_or_tty->print_cr("Thread %d gets list " INTPTR_FORMAT, id, list);
  }
  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();
  // The last ref must have its discovered field pointing to itself.
  oop next_discovered = (current_head != NULL) ? current_head : obj;

  // Note: In the case of G1, this specific pre-barrier is strictly
  // not necessary because the only case we are interested in
  // here is when *discovered_addr is NULL (see the CAS further below),
  // so this will expand to nothing. As a result, we have manually
  // elided this out for G1, but left in the test for some future
  // collector that might have need for a pre-barrier here, e.g.:-
  // _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
  assert(!_discovered_list_needs_barrier || UseG1GC,
         "Need to check non-G1 collector: "
         "may need a pre-write-barrier for CAS from NULL below");
  oop retest = oopDesc::atomic_compare_exchange_oop(next_discovered, 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.inc_length(1);
    if (_discovered_list_needs_barrier) {
      _bs->write_ref_field((void*)discovered_addr, next_discovered);
    }

    if (TraceReferenceGC) {
      gclog_or_tty->print_cr("Discovered reference (mt) (" INTPTR_FORMAT ": %s)",
                             obj, obj->blueprint()->internal_name());
    }
  } 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 discovered reference (" INTPTR_FORMAT ": %s)",
                             obj, obj->blueprint()->internal_name());
    }
  }
}

#ifndef PRODUCT
// Non-atomic (i.e. concurrent) discovery might allow us
// to observe j.l.References with NULL referents, being those
// cleared concurrently by mutators during (or after) discovery.
void ReferenceProcessor::verify_referent(oop obj) {
  bool da = discovery_is_atomic();
  oop referent = java_lang_ref_Reference::referent(obj);
  assert(da ? referent->is_oop() : referent->is_oop_or_null(),
         err_msg("Bad referent " INTPTR_FORMAT " found in Reference "
                 INTPTR_FORMAT " during %satomic discovery ",
                 (intptr_t)referent, (intptr_t)obj, da ? "" : "non-"));
}
#endif

// 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 discovered 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) {
  // Make sure we are discovering refs (rather than processing discovered refs).
  if (!_discovering_refs || !RegisterReferences) {
    return false;
  }
  // We only discover active references.
  oop next = java_lang_ref_Reference::next(obj);
  if (next != NULL) {   // Ref is no longer active
    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 discover references whose referents are not (yet)
  // known to be strongly reachable.
  if (is_alive_non_header() != NULL) {
    verify_referent(obj);
    if (is_alive_non_header()->do_object_b(java_lang_ref_Reference::referent(obj))) {
      return false;  // referent is reachable
    }
  }
  if (rt == REF_SOFT) {
    // For soft refs we can decide now if these are not
    // current candidates for clearing, in which case we
    // can mark through them now, rather than delaying that
    // to the reference-processing phase. Since all current
    // time-stamp policies advance the soft-ref clock only
    // at a major collection cycle, this is always currently
    // accurate.
    if (!_current_soft_ref_policy->should_clear_reference(obj)) {
      return false;
    }
  }

  ResourceMark rm;      // Needed for tracing.

  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 discovered 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 || UseG1GC,
             "Only possible with a concurrent marking collector");
      return true;
    }
  }

  if (RefDiscoveryPolicy == ReferentBasedDiscovery) {
    verify_referent(obj);
    // Discover 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(java_lang_ref_Reference::referent(obj)))) {
      // 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();
    // The last ref must have its discovered field pointing to itself.
    oop next_discovered = (current_head != NULL) ? current_head : obj;

    // 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.
    // e.g.:- _bs->write_ref_field_pre((oop* or narrowOop*)discovered_addr, next_discovered);
    assert(discovered == NULL, "control point invariant");
    assert(!_discovered_list_needs_barrier || UseG1GC,
           "For non-G1 collector, may need a pre-write-barrier for CAS from NULL below");
    oop_store_raw(discovered_addr, next_discovered);
    if (_discovered_list_needs_barrier) {
      _bs->write_ref_field((void*)discovered_addr, next_discovered);
    }
    list->set_head(obj);
    list->inc_length(1);

    if (TraceReferenceGC) {
      gclog_or_tty->print_cr("Discovered reference (" INTPTR_FORMAT ": %s)",
                                obj, obj->blueprint()->internal_name());
    }
  }
  assert(obj->is_oop(), "Discovered a bad reference");
  verify_referent(obj);
  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,
  bool should_unload_classes) {

  NOT_PRODUCT(verify_ok_to_handle_reflists());

#ifdef ASSERT
  bool must_remember_klasses = ClassUnloading && !UseConcMarkSweepGC ||
                               CMSClassUnloadingEnabled && UseConcMarkSweepGC ||
                               ExplicitGCInvokesConcurrentAndUnloadsClasses &&
                                 UseConcMarkSweepGC && should_unload_classes;
  RememberKlassesChecker mx(must_remember_klasses);
#endif
  // Soft references
  {
    TraceTime tt("Preclean SoftReferences", PrintGCDetails && PrintReferenceGC,
              false, gclog_or_tty);
    for (int i = 0; i < _max_num_q; i++) {
      if (yield->should_return()) {
        return;
      }
      preclean_discovered_reflist(_discoveredSoftRefs[i], is_alive,
                                  keep_alive, complete_gc, yield);
    }
  }

  // Weak references
  {
    TraceTime tt("Preclean WeakReferences", PrintGCDetails && PrintReferenceGC,
              false, gclog_or_tty);
    for (int i = 0; i < _max_num_q; i++) {
      if (yield->should_return()) {
        return;
      }
      preclean_discovered_reflist(_discoveredWeakRefs[i], is_alive,
                                  keep_alive, complete_gc, yield);
    }
  }

  // Final references
  {
    TraceTime tt("Preclean FinalReferences", PrintGCDetails && PrintReferenceGC,
              false, gclog_or_tty);
    for (int i = 0; i < _max_num_q; i++) {
      if (yield->should_return()) {
        return;
      }
      preclean_discovered_reflist(_discoveredFinalRefs[i], is_alive,
                                  keep_alive, complete_gc, yield);
    }
  }

  // Phantom references
  {
    TraceTime tt("Preclean PhantomReferences", PrintGCDetails && PrintReferenceGC,
              false, gclog_or_tty);
    for (int i = 0; i < _max_num_q; i++) {
      if (yield->should_return()) {
        return;
      }
      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: When we are
// thus precleaning the ref lists (which happens single-threaded today),
// we do not disable refs discovery to honour the correct semantics of
// java.lang.Reference. As a result, we need to be careful below
// that ref removal steps interleave safely with ref discovery steps
// (in this thread).
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);
  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();
      // 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);
      }
      iter.move_to_next();
    } else {
      iter.next();
    }
  }
  // Close the reachable set
  complete_gc->do_void();

  NOT_PRODUCT(
    if (PrintGCDetails && PrintReferenceGC && (iter.processed() > 0)) {
      gclog_or_tty->print_cr(" Dropped %d Refs out of %d "
        "Refs in discovered list " INTPTR_FORMAT,
        iter.removed(), iter.processed(), (address)refs_list.head());
    }
  )
}

const char* ReferenceProcessor::list_name(int i) {
   assert(i >= 0 && i <= _max_num_q * number_of_subclasses_of_ref(),
          "Out of bounds index");

   int j = i / _max_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

#ifndef PRODUCT
void ReferenceProcessor::clear_discovered_references() {
  guarantee(!_discovering_refs, "Discovering refs?");
  for (int i = 0; i < _max_num_q * number_of_subclasses_of_ref(); i++) {
    clear_discovered_references(_discoveredSoftRefs[i]);
  }
}

#endif // PRODUCT