xref: /openjdk9/hotspot/src/share/vm/gc/g1/g1RemSet.cpp

/*
 * Copyright (c) 2001, 2016, 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 "gc/g1/concurrentG1Refine.hpp"
#include "gc/g1/dirtyCardQueue.hpp"
#include "gc/g1/g1BlockOffsetTable.inline.hpp"
#include "gc/g1/g1CollectedHeap.inline.hpp"
#include "gc/g1/g1FromCardCache.hpp"
#include "gc/g1/g1GCPhaseTimes.hpp"
#include "gc/g1/g1HotCardCache.hpp"
#include "gc/g1/g1OopClosures.inline.hpp"
#include "gc/g1/g1RemSet.inline.hpp"
#include "gc/g1/g1SATBCardTableModRefBS.inline.hpp"
#include "gc/g1/heapRegion.inline.hpp"
#include "gc/g1/heapRegionManager.inline.hpp"
#include "gc/g1/heapRegionRemSet.hpp"
#include "gc/shared/gcTraceTime.inline.hpp"
#include "memory/iterator.hpp"
#include "memory/resourceArea.hpp"
#include "oops/oop.inline.hpp"
#include "utilities/globalDefinitions.hpp"
#include "utilities/intHisto.hpp"
#include "utilities/stack.inline.hpp"

// Collects information about the overall remembered set scan progress during an evacuation.
class G1RemSetScanState : public CHeapObj<mtGC> {
private:
  class G1ClearCardTableTask : public AbstractGangTask {
    G1CollectedHeap* _g1h;
    uint* _dirty_region_list;
    size_t _num_dirty_regions;
    size_t _chunk_length;

    size_t volatile _cur_dirty_regions;
  public:
    G1ClearCardTableTask(G1CollectedHeap* g1h,
                         uint* dirty_region_list,
                         size_t num_dirty_regions,
                         size_t chunk_length) :
      AbstractGangTask("G1 Clear Card Table Task"),
      _g1h(g1h),
      _dirty_region_list(dirty_region_list),
      _num_dirty_regions(num_dirty_regions),
      _chunk_length(chunk_length),
      _cur_dirty_regions(0) {

      assert(chunk_length > 0, "must be");
    }

    static size_t chunk_size() { return M; }

    void work(uint worker_id) {
      G1SATBCardTableModRefBS* ct_bs = _g1h->g1_barrier_set();

      while (_cur_dirty_regions < _num_dirty_regions) {
        size_t next = Atomic::add(_chunk_length, &_cur_dirty_regions) - _chunk_length;
        size_t max = MIN2(next + _chunk_length, _num_dirty_regions);

        for (size_t i = next; i < max; i++) {
          HeapRegion* r = _g1h->region_at(_dirty_region_list[i]);
          if (!r->is_survivor()) {
            ct_bs->clear(MemRegion(r->bottom(), r->end()));
          }
        }
      }
    }
  };

  size_t _max_regions;

  // Scan progress for the remembered set of a single region. Transitions from
  // Unclaimed -> Claimed -> Complete.
  // At each of the transitions the thread that does the transition needs to perform
  // some special action once. This is the reason for the extra "Claimed" state.
  typedef jint G1RemsetIterState;

  static const G1RemsetIterState Unclaimed = 0; // The remembered set has not been scanned yet.
  static const G1RemsetIterState Claimed = 1;   // The remembered set is currently being scanned.
  static const G1RemsetIterState Complete = 2;  // The remembered set has been completely scanned.

  G1RemsetIterState volatile* _iter_states;
  // The current location where the next thread should continue scanning in a region's
  // remembered set.
  size_t volatile* _iter_claims;

  // Temporary buffer holding the regions we used to store remembered set scan duplicate
  // information. These are also called "dirty". Valid entries are from [0.._cur_dirty_region)
  uint* _dirty_region_buffer;

  typedef jbyte IsDirtyRegionState;
  static const IsDirtyRegionState Clean = 0;
  static const IsDirtyRegionState Dirty = 1;
  // Holds a flag for every region whether it is in the _dirty_region_buffer already
  // to avoid duplicates. Uses jbyte since there are no atomic instructions for bools.
  IsDirtyRegionState* _in_dirty_region_buffer;
  size_t _cur_dirty_region;
public:
  G1RemSetScanState() :
    _max_regions(0),
    _iter_states(NULL),
    _iter_claims(NULL),
    _dirty_region_buffer(NULL),
    _in_dirty_region_buffer(NULL),
    _cur_dirty_region(0) {

  }

  ~G1RemSetScanState() {
    if (_iter_states != NULL) {
      FREE_C_HEAP_ARRAY(G1RemsetIterState, _iter_states);
    }
    if (_iter_claims != NULL) {
      FREE_C_HEAP_ARRAY(size_t, _iter_claims);
    }
    if (_dirty_region_buffer != NULL) {
      FREE_C_HEAP_ARRAY(uint, _dirty_region_buffer);
    }
    if (_in_dirty_region_buffer != NULL) {
      FREE_C_HEAP_ARRAY(IsDirtyRegionState, _in_dirty_region_buffer);
    }
  }

  void initialize(uint max_regions) {
    assert(_iter_states == NULL, "Must not be initialized twice");
    assert(_iter_claims == NULL, "Must not be initialized twice");
    _max_regions = max_regions;
    _iter_states = NEW_C_HEAP_ARRAY(G1RemsetIterState, max_regions, mtGC);
    _iter_claims = NEW_C_HEAP_ARRAY(size_t, max_regions, mtGC);
    _dirty_region_buffer = NEW_C_HEAP_ARRAY(uint, max_regions, mtGC);
    _in_dirty_region_buffer = NEW_C_HEAP_ARRAY(IsDirtyRegionState, max_regions, mtGC);
  }

  void reset() {
    for (uint i = 0; i < _max_regions; i++) {
      _iter_states[i] = Unclaimed;
    }
    memset((void*)_iter_claims, 0, _max_regions * sizeof(size_t));
    memset(_in_dirty_region_buffer, Clean, _max_regions * sizeof(IsDirtyRegionState));
    _cur_dirty_region = 0;
  }

  // Attempt to claim the remembered set of the region for iteration. Returns true
  // if this call caused the transition from Unclaimed to Claimed.
  inline bool claim_iter(uint region) {
    assert(region < _max_regions, "Tried to access invalid region %u", region);
    if (_iter_states[region] != Unclaimed) {
      return false;
    }
    jint res = Atomic::cmpxchg(Claimed, (jint*)(&_iter_states[region]), Unclaimed);
    return (res == Unclaimed);
  }

  // Try to atomically sets the iteration state to "complete". Returns true for the
  // thread that caused the transition.
  inline bool set_iter_complete(uint region) {
    if (iter_is_complete(region)) {
      return false;
    }
    jint res = Atomic::cmpxchg(Complete, (jint*)(&_iter_states[region]), Claimed);
    return (res == Claimed);
  }

  // Returns true if the region's iteration is complete.
  inline bool iter_is_complete(uint region) const {
    assert(region < _max_regions, "Tried to access invalid region %u", region);
    return _iter_states[region] == Complete;
  }

  // The current position within the remembered set of the given region.
  inline size_t iter_claimed(uint region) const {
    assert(region < _max_regions, "Tried to access invalid region %u", region);
    return _iter_claims[region];
  }

  // Claim the next block of cards within the remembered set of the region with
  // step size.
  inline size_t iter_claimed_next(uint region, size_t step) {
    return Atomic::add(step, &_iter_claims[region]) - step;
  }

  void add_dirty_region(uint region) {
    if (_in_dirty_region_buffer[region] == Dirty) {
      return;
    }

    bool marked_as_dirty = Atomic::cmpxchg(Dirty, &_in_dirty_region_buffer[region], Clean) == Clean;
    if (marked_as_dirty) {
      size_t allocated = Atomic::add(1, &_cur_dirty_region) - 1;
      _dirty_region_buffer[allocated] = region;
    }
  }

  // Clear the card table of "dirty" regions.
  void clear_card_table(WorkGang* workers) {
    if (_cur_dirty_region == 0) {
      return;
    }

    size_t const num_chunks = align_size_up(_cur_dirty_region * HeapRegion::CardsPerRegion, G1ClearCardTableTask::chunk_size()) / G1ClearCardTableTask::chunk_size();
    uint const num_workers = (uint)MIN2(num_chunks, (size_t)workers->active_workers());
    size_t const chunk_length = G1ClearCardTableTask::chunk_size() / HeapRegion::CardsPerRegion;

    // Iterate over the dirty cards region list.
    G1ClearCardTableTask cl(G1CollectedHeap::heap(), _dirty_region_buffer, _cur_dirty_region, chunk_length);

    log_debug(gc, ergo)("Running %s using %u workers for " SIZE_FORMAT " "
                        "units of work for " SIZE_FORMAT " regions.",
                        cl.name(), num_workers, num_chunks, _cur_dirty_region);
    workers->run_task(&cl, num_workers);

#ifndef PRODUCT
    // Need to synchronize with concurrent cleanup since it needs to
    // finish its card table clearing before we can verify.
    G1CollectedHeap::heap()->wait_while_free_regions_coming();
    G1CollectedHeap::heap()->verifier()->verify_card_table_cleanup();
#endif
  }
};

G1RemSet::G1RemSet(G1CollectedHeap* g1,
                   CardTableModRefBS* ct_bs,
                   G1HotCardCache* hot_card_cache) :
  _g1(g1),
  _scan_state(new G1RemSetScanState()),
  _conc_refine_cards(0),
  _ct_bs(ct_bs),
  _g1p(_g1->g1_policy()),
  _hot_card_cache(hot_card_cache),
  _prev_period_summary(),
  _into_cset_dirty_card_queue_set(false)
{
  if (log_is_enabled(Trace, gc, remset)) {
    _prev_period_summary.initialize(this);
  }
  // Initialize the card queue set used to hold cards containing
  // references into the collection set.
  _into_cset_dirty_card_queue_set.initialize(NULL, // Should never be called by the Java code
                                             DirtyCardQ_CBL_mon,
                                             DirtyCardQ_FL_lock,
                                             -1, // never trigger processing
                                             -1, // no limit on length
                                             Shared_DirtyCardQ_lock,
                                             &JavaThread::dirty_card_queue_set());
}

G1RemSet::~G1RemSet() {
  if (_scan_state != NULL) {
    delete _scan_state;
  }
}

uint G1RemSet::num_par_rem_sets() {
  return MAX2(DirtyCardQueueSet::num_par_ids() + ConcurrentG1Refine::thread_num(), ParallelGCThreads);
}

void G1RemSet::initialize(size_t capacity, uint max_regions) {
  G1FromCardCache::initialize(num_par_rem_sets(), max_regions);
  _scan_state->initialize(max_regions);
  {
    GCTraceTime(Debug, gc, marking)("Initialize Card Live Data");
    _card_live_data.initialize(capacity, max_regions);
  }
  if (G1PretouchAuxiliaryMemory) {
    GCTraceTime(Debug, gc, marking)("Pre-Touch Card Live Data");
    _card_live_data.pretouch();
  }
}

G1ScanRSClosure::G1ScanRSClosure(G1RemSetScanState* scan_state,
                                 G1ParPushHeapRSClosure* push_heap_cl,
                                 CodeBlobClosure* code_root_cl,
                                 uint worker_i) :
  _scan_state(scan_state),
  _push_heap_cl(push_heap_cl),
  _code_root_cl(code_root_cl),
  _strong_code_root_scan_time_sec(0.0),
  _cards(0),
  _cards_done(0),
  _worker_i(worker_i) {
  _g1h = G1CollectedHeap::heap();
  _bot = _g1h->bot();
  _ct_bs = _g1h->g1_barrier_set();
  _block_size = MAX2<size_t>(G1RSetScanBlockSize, 1);
}

void G1ScanRSClosure::scan_card(size_t index, HeapRegion *r) {
  // Stack allocate the DirtyCardToOopClosure instance
  HeapRegionDCTOC cl(_g1h, r, _push_heap_cl, CardTableModRefBS::Precise);

  // Set the "from" region in the closure.
  _push_heap_cl->set_region(r);
  MemRegion card_region(_bot->address_for_index(index), BOTConstants::N_words);
  MemRegion pre_gc_allocated(r->bottom(), r->scan_top());
  MemRegion mr = pre_gc_allocated.intersection(card_region);
  if (!mr.is_empty() && !_ct_bs->is_card_claimed(index)) {
    // We make the card as "claimed" lazily (so races are possible
    // but they're benign), which reduces the number of duplicate
    // scans (the rsets of the regions in the cset can intersect).
    _ct_bs->set_card_claimed(index);
    _cards_done++;
    cl.do_MemRegion(mr);
  }
}

void G1ScanRSClosure::scan_strong_code_roots(HeapRegion* r) {
  double scan_start = os::elapsedTime();
  r->strong_code_roots_do(_code_root_cl);
  _strong_code_root_scan_time_sec += (os::elapsedTime() - scan_start);
}

bool G1ScanRSClosure::doHeapRegion(HeapRegion* r) {
  assert(r->in_collection_set(), "should only be called on elements of CS.");
  uint region_idx = r->hrm_index();

  if (_scan_state->iter_is_complete(region_idx)) {
    return false;
  }
  if (_scan_state->claim_iter(region_idx)) {
    // If we ever free the collection set concurrently, we should also
    // clear the card table concurrently therefore we won't need to
    // add regions of the collection set to the dirty cards region.
    _scan_state->add_dirty_region(region_idx);
  }

  HeapRegionRemSetIterator iter(r->rem_set());
  size_t card_index;

  // We claim cards in block so as to reduce the contention. The block size is determined by
  // the G1RSetScanBlockSize parameter.
  size_t claimed_card_block = _scan_state->iter_claimed_next(region_idx, _block_size);
  for (size_t current_card = 0; iter.has_next(card_index); current_card++) {
    if (current_card >= claimed_card_block + _block_size) {
      claimed_card_block = _scan_state->iter_claimed_next(region_idx, _block_size);
    }
    if (current_card < claimed_card_block) {
      continue;
    }
    HeapWord* card_start = _g1h->bot()->address_for_index(card_index);

    HeapRegion* card_region = _g1h->heap_region_containing(card_start);
    _cards++;

    _scan_state->add_dirty_region(card_region->hrm_index());

    // If the card is dirty, then we will scan it during updateRS.
    if (!card_region->in_collection_set() &&
        !_ct_bs->is_card_dirty(card_index)) {
      scan_card(card_index, card_region);
    }
  }
  if (_scan_state->set_iter_complete(region_idx)) {
    // Scan the strong code root list attached to the current region
    scan_strong_code_roots(r);
  }
  return false;
}

size_t G1RemSet::scan_rem_set(G1ParPushHeapRSClosure* oops_in_heap_closure,
                              CodeBlobClosure* heap_region_codeblobs,
                              uint worker_i) {
  double rs_time_start = os::elapsedTime();

  G1ScanRSClosure cl(_scan_state, oops_in_heap_closure, heap_region_codeblobs, worker_i);
  _g1->collection_set_iterate_from(&cl, worker_i);

   double scan_rs_time_sec = (os::elapsedTime() - rs_time_start) -
                              cl.strong_code_root_scan_time_sec();

  _g1p->phase_times()->record_time_secs(G1GCPhaseTimes::ScanRS, worker_i, scan_rs_time_sec);
  _g1p->phase_times()->record_time_secs(G1GCPhaseTimes::CodeRoots, worker_i, cl.strong_code_root_scan_time_sec());

  return cl.cards_done();
}

// Closure used for updating RSets and recording references that
// point into the collection set. Only called during an
// evacuation pause.

class RefineRecordRefsIntoCSCardTableEntryClosure: public CardTableEntryClosure {
  G1RemSet* _g1rs;
  DirtyCardQueue* _into_cset_dcq;
  G1ParPushHeapRSClosure* _cl;
public:
  RefineRecordRefsIntoCSCardTableEntryClosure(G1CollectedHeap* g1h,
                                              DirtyCardQueue* into_cset_dcq,
                                              G1ParPushHeapRSClosure* cl) :
    _g1rs(g1h->g1_rem_set()), _into_cset_dcq(into_cset_dcq), _cl(cl)
  {}

  bool do_card_ptr(jbyte* card_ptr, uint worker_i) {
    // The only time we care about recording cards that
    // contain references that point into the collection set
    // is during RSet updating within an evacuation pause.
    // In this case worker_i should be the id of a GC worker thread.
    assert(SafepointSynchronize::is_at_safepoint(), "not during an evacuation pause");
    assert(worker_i < ParallelGCThreads, "should be a GC worker");

    if (_g1rs->refine_card(card_ptr, worker_i, _cl)) {
      // 'card_ptr' contains references that point into the collection
      // set. We need to record the card in the DCQS
      // (_into_cset_dirty_card_queue_set)
      // that's used for that purpose.
      //
      // Enqueue the card
      _into_cset_dcq->enqueue(card_ptr);
    }
    return true;
  }
};

void G1RemSet::update_rem_set(DirtyCardQueue* into_cset_dcq,
                              G1ParPushHeapRSClosure* oops_in_heap_closure,
                              uint worker_i) {
  RefineRecordRefsIntoCSCardTableEntryClosure into_cset_update_rs_cl(_g1, into_cset_dcq, oops_in_heap_closure);

  G1GCParPhaseTimesTracker x(_g1p->phase_times(), G1GCPhaseTimes::UpdateRS, worker_i);
  if (G1HotCardCache::default_use_cache()) {
    // Apply the closure to the entries of the hot card cache.
    G1GCParPhaseTimesTracker y(_g1p->phase_times(), G1GCPhaseTimes::ScanHCC, worker_i);
    _g1->iterate_hcc_closure(&into_cset_update_rs_cl, worker_i);
  }
  // Apply the closure to all remaining log entries.
  _g1->iterate_dirty_card_closure(&into_cset_update_rs_cl, worker_i);
}

void G1RemSet::cleanupHRRS() {
  HeapRegionRemSet::cleanup();
}

size_t G1RemSet::oops_into_collection_set_do(G1ParPushHeapRSClosure* cl,
                                             CodeBlobClosure* heap_region_codeblobs,
                                             uint worker_i) {
  // A DirtyCardQueue that is used to hold cards containing references
  // that point into the collection set. This DCQ is associated with a
  // special DirtyCardQueueSet (see g1CollectedHeap.hpp).  Under normal
  // circumstances (i.e. the pause successfully completes), these cards
  // are just discarded (there's no need to update the RSets of regions
  // that were in the collection set - after the pause these regions
  // are wholly 'free' of live objects. In the event of an evacuation
  // failure the cards/buffers in this queue set are passed to the
  // DirtyCardQueueSet that is used to manage RSet updates
  DirtyCardQueue into_cset_dcq(&_into_cset_dirty_card_queue_set);

  update_rem_set(&into_cset_dcq, cl, worker_i);
  return scan_rem_set(cl, heap_region_codeblobs, worker_i);;
}

void G1RemSet::prepare_for_oops_into_collection_set_do() {
  _g1->set_refine_cte_cl_concurrency(false);
  DirtyCardQueueSet& dcqs = JavaThread::dirty_card_queue_set();
  dcqs.concatenate_logs();

  _scan_state->reset();
}

void G1RemSet::cleanup_after_oops_into_collection_set_do() {
  G1GCPhaseTimes* phase_times = _g1->g1_policy()->phase_times();
  // Cleanup after copy
  _g1->set_refine_cte_cl_concurrency(true);

  // Set all cards back to clean.
  double start = os::elapsedTime();
  _scan_state->clear_card_table(_g1->workers());
  phase_times->record_clear_ct_time((os::elapsedTime() - start) * 1000.0);

  DirtyCardQueueSet& into_cset_dcqs = _into_cset_dirty_card_queue_set;

  if (_g1->evacuation_failed()) {
    double restore_remembered_set_start = os::elapsedTime();

    // Restore remembered sets for the regions pointing into the collection set.
    // We just need to transfer the completed buffers from the DirtyCardQueueSet
    // used to hold cards that contain references that point into the collection set
    // to the DCQS used to hold the deferred RS updates.
    _g1->dirty_card_queue_set().merge_bufferlists(&into_cset_dcqs);
    phase_times->record_evac_fail_restore_remsets((os::elapsedTime() - restore_remembered_set_start) * 1000.0);
  }

  // Free any completed buffers in the DirtyCardQueueSet used to hold cards
  // which contain references that point into the collection.
  _into_cset_dirty_card_queue_set.clear();
  assert(_into_cset_dirty_card_queue_set.completed_buffers_num() == 0,
         "all buffers should be freed");
  _into_cset_dirty_card_queue_set.clear_n_completed_buffers();
}

class G1ScrubRSClosure: public HeapRegionClosure {
  G1CollectedHeap* _g1h;
  G1CardLiveData* _live_data;
public:
  G1ScrubRSClosure(G1CardLiveData* live_data) :
    _g1h(G1CollectedHeap::heap()),
    _live_data(live_data) { }

  bool doHeapRegion(HeapRegion* r) {
    if (!r->is_continues_humongous()) {
      r->rem_set()->scrub(_live_data);
    }
    return false;
  }
};

void G1RemSet::scrub(uint worker_num, HeapRegionClaimer *hrclaimer) {
  G1ScrubRSClosure scrub_cl(&_card_live_data);
  _g1->heap_region_par_iterate(&scrub_cl, worker_num, hrclaimer);
}

G1TriggerClosure::G1TriggerClosure() :
  _triggered(false) { }

G1InvokeIfNotTriggeredClosure::G1InvokeIfNotTriggeredClosure(G1TriggerClosure* t_cl,
                                                             OopClosure* oop_cl)  :
  _trigger_cl(t_cl), _oop_cl(oop_cl) { }

G1Mux2Closure::G1Mux2Closure(OopClosure *c1, OopClosure *c2) :
  _c1(c1), _c2(c2) { }

G1UpdateRSOrPushRefOopClosure::
G1UpdateRSOrPushRefOopClosure(G1CollectedHeap* g1h,
                              G1RemSet* rs,
                              G1ParPushHeapRSClosure* push_ref_cl,
                              bool record_refs_into_cset,
                              uint worker_i) :
  _g1(g1h), _g1_rem_set(rs), _from(NULL),
  _record_refs_into_cset(record_refs_into_cset),
  _push_ref_cl(push_ref_cl), _worker_i(worker_i) { }

// Returns true if the given card contains references that point
// into the collection set, if we're checking for such references;
// false otherwise.

bool G1RemSet::refine_card(jbyte* card_ptr,
                           uint worker_i,
                           G1ParPushHeapRSClosure*  oops_in_heap_closure) {
  assert(_g1->is_in_exact(_ct_bs->addr_for(card_ptr)),
         "Card at " PTR_FORMAT " index " SIZE_FORMAT " representing heap at " PTR_FORMAT " (%u) must be in committed heap",
         p2i(card_ptr),
         _ct_bs->index_for(_ct_bs->addr_for(card_ptr)),
         p2i(_ct_bs->addr_for(card_ptr)),
         _g1->addr_to_region(_ct_bs->addr_for(card_ptr)));

  bool check_for_refs_into_cset = oops_in_heap_closure != NULL;

  // If the card is no longer dirty, nothing to do.
  if (*card_ptr != CardTableModRefBS::dirty_card_val()) {
    // No need to return that this card contains refs that point
    // into the collection set.
    return false;
  }

  // Construct the region representing the card.
  HeapWord* start = _ct_bs->addr_for(card_ptr);
  // And find the region containing it.
  HeapRegion* r = _g1->heap_region_containing(start);

  // This check is needed for some uncommon cases where we should
  // ignore the card.
  //
  // The region could be young.  Cards for young regions are
  // distinctly marked (set to g1_young_gen), so the post-barrier will
  // filter them out.  However, that marking is performed
  // concurrently.  A write to a young object could occur before the
  // card has been marked young, slipping past the filter.
  //
  // The card could be stale, because the region has been freed since
  // the card was recorded. In this case the region type could be
  // anything.  If (still) free or (reallocated) young, just ignore
  // it.  If (reallocated) old or humongous, the later card trimming
  // and additional checks in iteration may detect staleness.  At
  // worst, we end up processing a stale card unnecessarily.
  //
  // In the normal (non-stale) case, the synchronization between the
  // enqueueing of the card and processing it here will have ensured
  // we see the up-to-date region type here.
  if (!r->is_old_or_humongous()) {
    return false;
  }

  // While we are processing RSet buffers during the collection, we
  // actually don't want to scan any cards on the collection set,
  // since we don't want to update remembered sets with entries that
  // point into the collection set, given that live objects from the
  // collection set are about to move and such entries will be stale
  // very soon. This change also deals with a reliability issue which
  // involves scanning a card in the collection set and coming across
  // an array that was being chunked and looking malformed. Note,
  // however, that if evacuation fails, we have to scan any objects
  // that were not moved and create any missing entries.
  if (r->in_collection_set()) {
    return false;
  }

  // The result from the hot card cache insert call is either:
  //   * pointer to the current card
  //     (implying that the current card is not 'hot'),
  //   * null
  //     (meaning we had inserted the card ptr into the "hot" card cache,
  //     which had some headroom),
  //   * a pointer to a "hot" card that was evicted from the "hot" cache.
  //

  if (_hot_card_cache->use_cache()) {
    assert(!check_for_refs_into_cset, "sanity");
    assert(!SafepointSynchronize::is_at_safepoint(), "sanity");

    const jbyte* orig_card_ptr = card_ptr;
    card_ptr = _hot_card_cache->insert(card_ptr);
    if (card_ptr == NULL) {
      // There was no eviction. Nothing to do.
      return false;
    } else if (card_ptr != orig_card_ptr) {
      // Original card was inserted and an old card was evicted.
      start = _ct_bs->addr_for(card_ptr);
      r = _g1->heap_region_containing(start);

      // Check whether the region formerly in the cache should be
      // ignored, as discussed earlier for the original card.  The
      // region could have been freed while in the cache.  The cset is
      // not relevant here, since we're in concurrent phase.
      if (!r->is_old_or_humongous()) {
        return false;
      }
    } // Else we still have the original card.
  }

  // Trim the region designated by the card to what's been allocated
  // in the region.  The card could be stale, or the card could cover
  // (part of) an object at the end of the allocated space and extend
  // beyond the end of allocation.
  HeapWord* scan_limit;
  if (_g1->is_gc_active()) {
    // If we're in a STW GC, then a card might be in a GC alloc region
    // and extend onto a GC LAB, which may not be parsable.  Stop such
    // at the "scan_top" of the region.
    scan_limit = r->scan_top();
  } else {
    // Non-humongous objects are only allocated in the old-gen during
    // GC, so if region is old then top is stable.  Humongous object
    // allocation sets top last; if top has not yet been set, this is
    // a stale card and we'll end up with an empty intersection.  If
    // this is not a stale card, the synchronization between the
    // enqueuing of the card and processing it here will have ensured
    // we see the up-to-date top here.
    scan_limit = r->top();
  }
  if (scan_limit <= start) {
    // If the trimmed region is empty, the card must be stale.
    return false;
  }

  // Okay to clean and process the card now.  There are still some
  // stale card cases that may be detected by iteration and dealt with
  // as iteration failure.
  *const_cast<volatile jbyte*>(card_ptr) = CardTableModRefBS::clean_card_val();

  // This fence serves two purposes.  First, the card must be cleaned
  // before processing the contents.  Second, we can't proceed with
  // processing until after the read of top, for synchronization with
  // possibly concurrent humongous object allocation.  It's okay that
  // reading top and reading type were racy wrto each other.  We need
  // both set, in any order, to proceed.
  OrderAccess::fence();

  // Don't use addr_for(card_ptr + 1) which can ask for
  // a card beyond the heap.
  HeapWord* end = start + CardTableModRefBS::card_size_in_words;
  MemRegion dirty_region(start, MIN2(scan_limit, end));
  assert(!dirty_region.is_empty(), "sanity");

  G1UpdateRSOrPushRefOopClosure update_rs_oop_cl(_g1,
                                                 _g1->g1_rem_set(),
                                                 oops_in_heap_closure,
                                                 check_for_refs_into_cset,
                                                 worker_i);
  update_rs_oop_cl.set_from(r);

  G1TriggerClosure trigger_cl;
  FilterIntoCSClosure into_cs_cl(_g1, &trigger_cl);
  G1InvokeIfNotTriggeredClosure invoke_cl(&trigger_cl, &into_cs_cl);
  G1Mux2Closure mux(&invoke_cl, &update_rs_oop_cl);

  FilterOutOfRegionClosure filter_then_update_rs_oop_cl(r,
                        (check_for_refs_into_cset ?
                                (OopClosure*)&mux :
                                (OopClosure*)&update_rs_oop_cl));

  bool card_processed =
    r->oops_on_card_seq_iterate_careful(dirty_region,
                                        &filter_then_update_rs_oop_cl);

  // If unable to process the card then we encountered an unparsable
  // part of the heap (e.g. a partially allocated object) while
  // processing a stale card.  Despite the card being stale, redirty
  // and re-enqueue, because we've already cleaned the card.  Without
  // this we could incorrectly discard a non-stale card.
  if (!card_processed) {
    assert(!_g1->is_gc_active(), "Unparsable heap during GC");
    // The card might have gotten re-dirtied and re-enqueued while we
    // worked.  (In fact, it's pretty likely.)
    if (*card_ptr != CardTableModRefBS::dirty_card_val()) {
      *card_ptr = CardTableModRefBS::dirty_card_val();
      MutexLockerEx x(Shared_DirtyCardQ_lock,
                      Mutex::_no_safepoint_check_flag);
      DirtyCardQueue* sdcq =
        JavaThread::dirty_card_queue_set().shared_dirty_card_queue();
      sdcq->enqueue(card_ptr);
    }
  } else {
    _conc_refine_cards++;
  }

  // This gets set to true if the card being refined has
  // references that point into the collection set.
  bool has_refs_into_cset = trigger_cl.triggered();

  // We should only be detecting that the card contains references
  // that point into the collection set if the current thread is
  // a GC worker thread.
  assert(!has_refs_into_cset || SafepointSynchronize::is_at_safepoint(),
           "invalid result at non safepoint");

  return has_refs_into_cset;
}

void G1RemSet::print_periodic_summary_info(const char* header, uint period_count) {
  if ((G1SummarizeRSetStatsPeriod > 0) && log_is_enabled(Trace, gc, remset) &&
      (period_count % G1SummarizeRSetStatsPeriod == 0)) {

    if (!_prev_period_summary.initialized()) {
      _prev_period_summary.initialize(this);
    }

    G1RemSetSummary current;
    current.initialize(this);
    _prev_period_summary.subtract_from(&current);

    Log(gc, remset) log;
    log.trace("%s", header);
    ResourceMark rm;
    _prev_period_summary.print_on(log.trace_stream());

    _prev_period_summary.set(&current);
  }
}

void G1RemSet::print_summary_info() {
  Log(gc, remset, exit) log;
  if (log.is_trace()) {
    log.trace(" Cumulative RS summary");
    G1RemSetSummary current;
    current.initialize(this);
    ResourceMark rm;
    current.print_on(log.trace_stream());
  }
}

void G1RemSet::prepare_for_verify() {
  if (G1HRRSFlushLogBuffersOnVerify &&
      (VerifyBeforeGC || VerifyAfterGC)
      &&  (!_g1->collector_state()->full_collection() || G1VerifyRSetsDuringFullGC)) {
    cleanupHRRS();
    _g1->set_refine_cte_cl_concurrency(false);
    if (SafepointSynchronize::is_at_safepoint()) {
      DirtyCardQueueSet& dcqs = JavaThread::dirty_card_queue_set();
      dcqs.concatenate_logs();
    }

    bool use_hot_card_cache = _hot_card_cache->use_cache();
    _hot_card_cache->set_use_cache(false);

    DirtyCardQueue into_cset_dcq(&_into_cset_dirty_card_queue_set);
    update_rem_set(&into_cset_dcq, NULL, 0);
    _into_cset_dirty_card_queue_set.clear();

    _hot_card_cache->set_use_cache(use_hot_card_cache);
    assert(JavaThread::dirty_card_queue_set().completed_buffers_num() == 0, "All should be consumed");
  }
}

void G1RemSet::create_card_live_data(WorkGang* workers, G1CMBitMap* mark_bitmap) {
  _card_live_data.create(workers, mark_bitmap);
}

void G1RemSet::finalize_card_live_data(WorkGang* workers, G1CMBitMap* mark_bitmap) {
  _card_live_data.finalize(workers, mark_bitmap);
}

void G1RemSet::verify_card_live_data(WorkGang* workers, G1CMBitMap* bitmap) {
  _card_live_data.verify(workers, bitmap);
}

void G1RemSet::clear_card_live_data(WorkGang* workers) {
  _card_live_data.clear(workers);
}

#ifdef ASSERT
void G1RemSet::verify_card_live_data_is_clear() {
  _card_live_data.verify_is_clear();
}
#endif