view src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp @ 2534:6747fd0512e0

7004681: G1: Extend marking verification to Full GCs Summary: Perform a heap verification after the first phase of G1's full GC using objects' mark words to determine liveness. The third parameter of the heap verification routines, which was used in G1 to determine which marking bitmap to use in liveness calculations, has been changed from a boolean to an enum with values defined for using the mark word, and the 'prev' and 'next' bitmaps. Reviewed-by: tonyp, ysr
author johnc
date Tue, 14 Jun 2011 11:01:10 -0700
parents c3f1170908be
children c9ca3f51cf41
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.
 *
 */

#ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1COLLECTEDHEAP_HPP
#define SHARE_VM_GC_IMPLEMENTATION_G1_G1COLLECTEDHEAP_HPP

#include "gc_implementation/g1/concurrentMark.hpp"
#include "gc_implementation/g1/g1AllocRegion.hpp"
#include "gc_implementation/g1/g1RemSet.hpp"
#include "gc_implementation/g1/g1MonitoringSupport.hpp"
#include "gc_implementation/g1/heapRegionSeq.hpp"
#include "gc_implementation/g1/heapRegionSets.hpp"
#include "gc_implementation/shared/hSpaceCounters.hpp"
#include "gc_implementation/parNew/parGCAllocBuffer.hpp"
#include "memory/barrierSet.hpp"
#include "memory/memRegion.hpp"
#include "memory/sharedHeap.hpp"

// A "G1CollectedHeap" is an implementation of a java heap for HotSpot.
// It uses the "Garbage First" heap organization and algorithm, which
// may combine concurrent marking with parallel, incremental compaction of
// heap subsets that will yield large amounts of garbage.

class HeapRegion;
class HRRSCleanupTask;
class PermanentGenerationSpec;
class GenerationSpec;
class OopsInHeapRegionClosure;
class G1ScanHeapEvacClosure;
class ObjectClosure;
class SpaceClosure;
class CompactibleSpaceClosure;
class Space;
class G1CollectorPolicy;
class GenRemSet;
class G1RemSet;
class HeapRegionRemSetIterator;
class ConcurrentMark;
class ConcurrentMarkThread;
class ConcurrentG1Refine;
class GenerationCounters;

typedef OverflowTaskQueue<StarTask>         RefToScanQueue;
typedef GenericTaskQueueSet<RefToScanQueue> RefToScanQueueSet;

typedef int RegionIdx_t;   // needs to hold [ 0..max_regions() )
typedef int CardIdx_t;     // needs to hold [ 0..CardsPerRegion )

enum GCAllocPurpose {
  GCAllocForTenured,
  GCAllocForSurvived,
  GCAllocPurposeCount
};

class YoungList : public CHeapObj {
private:
  G1CollectedHeap* _g1h;

  HeapRegion* _head;

  HeapRegion* _survivor_head;
  HeapRegion* _survivor_tail;

  HeapRegion* _curr;

  size_t      _length;
  size_t      _survivor_length;

  size_t      _last_sampled_rs_lengths;
  size_t      _sampled_rs_lengths;

  void         empty_list(HeapRegion* list);

public:
  YoungList(G1CollectedHeap* g1h);

  void         push_region(HeapRegion* hr);
  void         add_survivor_region(HeapRegion* hr);

  void         empty_list();
  bool         is_empty() { return _length == 0; }
  size_t       length() { return _length; }
  size_t       survivor_length() { return _survivor_length; }

  // Currently we do not keep track of the used byte sum for the
  // young list and the survivors and it'd be quite a lot of work to
  // do so. When we'll eventually replace the young list with
  // instances of HeapRegionLinkedList we'll get that for free. So,
  // we'll report the more accurate information then.
  size_t       eden_used_bytes() {
    assert(length() >= survivor_length(), "invariant");
    return (length() - survivor_length()) * HeapRegion::GrainBytes;
  }
  size_t       survivor_used_bytes() {
    return survivor_length() * HeapRegion::GrainBytes;
  }

  void rs_length_sampling_init();
  bool rs_length_sampling_more();
  void rs_length_sampling_next();

  void reset_sampled_info() {
    _last_sampled_rs_lengths =   0;
  }
  size_t sampled_rs_lengths() { return _last_sampled_rs_lengths; }

  // for development purposes
  void reset_auxilary_lists();
  void clear() { _head = NULL; _length = 0; }

  void clear_survivors() {
    _survivor_head    = NULL;
    _survivor_tail    = NULL;
    _survivor_length  = 0;
  }

  HeapRegion* first_region() { return _head; }
  HeapRegion* first_survivor_region() { return _survivor_head; }
  HeapRegion* last_survivor_region() { return _survivor_tail; }

  // debugging
  bool          check_list_well_formed();
  bool          check_list_empty(bool check_sample = true);
  void          print();
};

class MutatorAllocRegion : public G1AllocRegion {
protected:
  virtual HeapRegion* allocate_new_region(size_t word_size, bool force);
  virtual void retire_region(HeapRegion* alloc_region, size_t allocated_bytes);
public:
  MutatorAllocRegion()
    : G1AllocRegion("Mutator Alloc Region", false /* bot_updates */) { }
};

class RefineCardTableEntryClosure;
class G1CollectedHeap : public SharedHeap {
  friend class VM_G1CollectForAllocation;
  friend class VM_GenCollectForPermanentAllocation;
  friend class VM_G1CollectFull;
  friend class VM_G1IncCollectionPause;
  friend class VMStructs;
  friend class MutatorAllocRegion;

  // Closures used in implementation.
  friend class G1ParCopyHelper;
  friend class G1IsAliveClosure;
  friend class G1EvacuateFollowersClosure;
  friend class G1ParScanThreadState;
  friend class G1ParScanClosureSuper;
  friend class G1ParEvacuateFollowersClosure;
  friend class G1ParTask;
  friend class G1FreeGarbageRegionClosure;
  friend class RefineCardTableEntryClosure;
  friend class G1PrepareCompactClosure;
  friend class RegionSorter;
  friend class RegionResetter;
  friend class CountRCClosure;
  friend class EvacPopObjClosure;
  friend class G1ParCleanupCTTask;

  // Other related classes.
  friend class G1MarkSweep;

private:
  // The one and only G1CollectedHeap, so static functions can find it.
  static G1CollectedHeap* _g1h;

  static size_t _humongous_object_threshold_in_words;

  // Storage for the G1 heap (excludes the permanent generation).
  VirtualSpace _g1_storage;
  MemRegion    _g1_reserved;

  // The part of _g1_storage that is currently committed.
  MemRegion _g1_committed;

  // The master free list. It will satisfy all new region allocations.
  MasterFreeRegionList      _free_list;

  // The secondary free list which contains regions that have been
  // freed up during the cleanup process. This will be appended to the
  // master free list when appropriate.
  SecondaryFreeRegionList   _secondary_free_list;

  // It keeps track of the humongous regions.
  MasterHumongousRegionSet  _humongous_set;

  // The number of regions we could create by expansion.
  size_t _expansion_regions;

  // The block offset table for the G1 heap.
  G1BlockOffsetSharedArray* _bot_shared;

  // Move all of the regions off the free lists, then rebuild those free
  // lists, before and after full GC.
  void tear_down_region_lists();
  void rebuild_region_lists();

  // The sequence of all heap regions in the heap.
  HeapRegionSeq _hrs;

  // Alloc region used to satisfy mutator allocation requests.
  MutatorAllocRegion _mutator_alloc_region;

  // It resets the mutator alloc region before new allocations can take place.
  void init_mutator_alloc_region();

  // It releases the mutator alloc region.
  void release_mutator_alloc_region();

  void abandon_gc_alloc_regions();

  // The to-space memory regions into which objects are being copied during
  // a GC.
  HeapRegion* _gc_alloc_regions[GCAllocPurposeCount];
  size_t _gc_alloc_region_counts[GCAllocPurposeCount];
  // These are the regions, one per GCAllocPurpose, that are half-full
  // at the end of a collection and that we want to reuse during the
  // next collection.
  HeapRegion* _retained_gc_alloc_regions[GCAllocPurposeCount];
  // This specifies whether we will keep the last half-full region at
  // the end of a collection so that it can be reused during the next
  // collection (this is specified per GCAllocPurpose)
  bool _retain_gc_alloc_region[GCAllocPurposeCount];

  // A list of the regions that have been set to be alloc regions in the
  // current collection.
  HeapRegion* _gc_alloc_region_list;

  // Helper for monitoring and management support.
  G1MonitoringSupport* _g1mm;

  // Determines PLAB size for a particular allocation purpose.
  static size_t desired_plab_sz(GCAllocPurpose purpose);

  // When called by par thread, requires the FreeList_lock to be held.
  void push_gc_alloc_region(HeapRegion* hr);

  // This should only be called single-threaded.  Undeclares all GC alloc
  // regions.
  void forget_alloc_region_list();

  // Should be used to set an alloc region, because there's other
  // associated bookkeeping.
  void set_gc_alloc_region(int purpose, HeapRegion* r);

  // Check well-formedness of alloc region list.
  bool check_gc_alloc_regions();

  // Outside of GC pauses, the number of bytes used in all regions other
  // than the current allocation region.
  size_t _summary_bytes_used;

  // This is used for a quick test on whether a reference points into
  // the collection set or not. Basically, we have an array, with one
  // byte per region, and that byte denotes whether the corresponding
  // region is in the collection set or not. The entry corresponding
  // the bottom of the heap, i.e., region 0, is pointed to by
  // _in_cset_fast_test_base.  The _in_cset_fast_test field has been
  // biased so that it actually points to address 0 of the address
  // space, to make the test as fast as possible (we can simply shift
  // the address to address into it, instead of having to subtract the
  // bottom of the heap from the address before shifting it; basically
  // it works in the same way the card table works).
  bool* _in_cset_fast_test;

  // The allocated array used for the fast test on whether a reference
  // points into the collection set or not. This field is also used to
  // free the array.
  bool* _in_cset_fast_test_base;

  // The length of the _in_cset_fast_test_base array.
  size_t _in_cset_fast_test_length;

  volatile unsigned _gc_time_stamp;

  size_t* _surviving_young_words;

  void setup_surviving_young_words();
  void update_surviving_young_words(size_t* surv_young_words);
  void cleanup_surviving_young_words();

  // It decides whether an explicit GC should start a concurrent cycle
  // instead of doing a STW GC. Currently, a concurrent cycle is
  // explicitly started if:
  // (a) cause == _gc_locker and +GCLockerInvokesConcurrent, or
  // (b) cause == _java_lang_system_gc and +ExplicitGCInvokesConcurrent.
  bool should_do_concurrent_full_gc(GCCause::Cause cause);

  // Keeps track of how many "full collections" (i.e., Full GCs or
  // concurrent cycles) we have completed. The number of them we have
  // started is maintained in _total_full_collections in CollectedHeap.
  volatile unsigned int _full_collections_completed;

  // This is a non-product method that is helpful for testing. It is
  // called at the end of a GC and artificially expands the heap by
  // allocating a number of dead regions. This way we can induce very
  // frequent marking cycles and stress the cleanup / concurrent
  // cleanup code more (as all the regions that will be allocated by
  // this method will be found dead by the marking cycle).
  void allocate_dummy_regions() PRODUCT_RETURN;

  // These are macros so that, if the assert fires, we get the correct
  // line number, file, etc.

#define heap_locking_asserts_err_msg(_extra_message_)                         \
  err_msg("%s : Heap_lock locked: %s, at safepoint: %s, is VM thread: %s",    \
          (_extra_message_),                                                  \
          BOOL_TO_STR(Heap_lock->owned_by_self()),                            \
          BOOL_TO_STR(SafepointSynchronize::is_at_safepoint()),               \
          BOOL_TO_STR(Thread::current()->is_VM_thread()))

#define assert_heap_locked()                                                  \
  do {                                                                        \
    assert(Heap_lock->owned_by_self(),                                        \
           heap_locking_asserts_err_msg("should be holding the Heap_lock"));  \
  } while (0)

#define assert_heap_locked_or_at_safepoint(_should_be_vm_thread_)             \
  do {                                                                        \
    assert(Heap_lock->owned_by_self() ||                                      \
           (SafepointSynchronize::is_at_safepoint() &&                        \
             ((_should_be_vm_thread_) == Thread::current()->is_VM_thread())), \
           heap_locking_asserts_err_msg("should be holding the Heap_lock or " \
                                        "should be at a safepoint"));         \
  } while (0)

#define assert_heap_locked_and_not_at_safepoint()                             \
  do {                                                                        \
    assert(Heap_lock->owned_by_self() &&                                      \
                                    !SafepointSynchronize::is_at_safepoint(), \
          heap_locking_asserts_err_msg("should be holding the Heap_lock and " \
                                       "should not be at a safepoint"));      \
  } while (0)

#define assert_heap_not_locked()                                              \
  do {                                                                        \
    assert(!Heap_lock->owned_by_self(),                                       \
        heap_locking_asserts_err_msg("should not be holding the Heap_lock")); \
  } while (0)

#define assert_heap_not_locked_and_not_at_safepoint()                         \
  do {                                                                        \
    assert(!Heap_lock->owned_by_self() &&                                     \
                                    !SafepointSynchronize::is_at_safepoint(), \
      heap_locking_asserts_err_msg("should not be holding the Heap_lock and " \
                                   "should not be at a safepoint"));          \
  } while (0)

#define assert_at_safepoint(_should_be_vm_thread_)                            \
  do {                                                                        \
    assert(SafepointSynchronize::is_at_safepoint() &&                         \
              ((_should_be_vm_thread_) == Thread::current()->is_VM_thread()), \
           heap_locking_asserts_err_msg("should be at a safepoint"));         \
  } while (0)

#define assert_not_at_safepoint()                                             \
  do {                                                                        \
    assert(!SafepointSynchronize::is_at_safepoint(),                          \
           heap_locking_asserts_err_msg("should not be at a safepoint"));     \
  } while (0)

protected:

  // Returns "true" iff none of the gc alloc regions have any allocations
  // since the last call to "save_marks".
  bool all_alloc_regions_no_allocs_since_save_marks();
  // Perform finalization stuff on all allocation regions.
  void retire_all_alloc_regions();

  // The number of regions allocated to hold humongous objects.
  int         _num_humongous_regions;
  YoungList*  _young_list;

  // The current policy object for the collector.
  G1CollectorPolicy* _g1_policy;

  // This is the second level of trying to allocate a new region. If
  // new_region() didn't find a region on the free_list, this call will
  // check whether there's anything available on the
  // secondary_free_list and/or wait for more regions to appear on
  // that list, if _free_regions_coming is set.
  HeapRegion* new_region_try_secondary_free_list();

  // Try to allocate a single non-humongous HeapRegion sufficient for
  // an allocation of the given word_size. If do_expand is true,
  // attempt to expand the heap if necessary to satisfy the allocation
  // request.
  HeapRegion* new_region(size_t word_size, bool do_expand);

  // Try to allocate a new region to be used for allocation by
  // a GC thread. It will try to expand the heap if no region is
  // available.
  HeapRegion* new_gc_alloc_region(int purpose, size_t word_size);

  // Attempt to satisfy a humongous allocation request of the given
  // size by finding a contiguous set of free regions of num_regions
  // length and remove them from the master free list. Return the
  // index of the first region or G1_NULL_HRS_INDEX if the search
  // was unsuccessful.
  size_t humongous_obj_allocate_find_first(size_t num_regions,
                                           size_t word_size);

  // Initialize a contiguous set of free regions of length num_regions
  // and starting at index first so that they appear as a single
  // humongous region.
  HeapWord* humongous_obj_allocate_initialize_regions(size_t first,
                                                      size_t num_regions,
                                                      size_t word_size);

  // Attempt to allocate a humongous object of the given size. Return
  // NULL if unsuccessful.
  HeapWord* humongous_obj_allocate(size_t word_size);

  // The following two methods, allocate_new_tlab() and
  // mem_allocate(), are the two main entry points from the runtime
  // into the G1's allocation routines. They have the following
  // assumptions:
  //
  // * They should both be called outside safepoints.
  //
  // * They should both be called without holding the Heap_lock.
  //
  // * All allocation requests for new TLABs should go to
  //   allocate_new_tlab().
  //
  // * All non-TLAB allocation requests should go to mem_allocate()
  //   and mem_allocate() should never be called with is_tlab == true.
  //
  // * If either call cannot satisfy the allocation request using the
  //   current allocating region, they will try to get a new one. If
  //   this fails, they will attempt to do an evacuation pause and
  //   retry the allocation.
  //
  // * If all allocation attempts fail, even after trying to schedule
  //   an evacuation pause, allocate_new_tlab() will return NULL,
  //   whereas mem_allocate() will attempt a heap expansion and/or
  //   schedule a Full GC.
  //
  // * We do not allow humongous-sized TLABs. So, allocate_new_tlab
  //   should never be called with word_size being humongous. All
  //   humongous allocation requests should go to mem_allocate() which
  //   will satisfy them with a special path.

  virtual HeapWord* allocate_new_tlab(size_t word_size);

  virtual HeapWord* mem_allocate(size_t word_size,
                                 bool   is_noref,
                                 bool   is_tlab, /* expected to be false */
                                 bool*  gc_overhead_limit_was_exceeded);

  // The following three methods take a gc_count_before_ret
  // parameter which is used to return the GC count if the method
  // returns NULL. Given that we are required to read the GC count
  // while holding the Heap_lock, and these paths will take the
  // Heap_lock at some point, it's easier to get them to read the GC
  // count while holding the Heap_lock before they return NULL instead
  // of the caller (namely: mem_allocate()) having to also take the
  // Heap_lock just to read the GC count.

  // First-level mutator allocation attempt: try to allocate out of
  // the mutator alloc region without taking the Heap_lock. This
  // should only be used for non-humongous allocations.
  inline HeapWord* attempt_allocation(size_t word_size,
                                      unsigned int* gc_count_before_ret);

  // Second-level mutator allocation attempt: take the Heap_lock and
  // retry the allocation attempt, potentially scheduling a GC
  // pause. This should only be used for non-humongous allocations.
  HeapWord* attempt_allocation_slow(size_t word_size,
                                    unsigned int* gc_count_before_ret);

  // Takes the Heap_lock and attempts a humongous allocation. It can
  // potentially schedule a GC pause.
  HeapWord* attempt_allocation_humongous(size_t word_size,
                                         unsigned int* gc_count_before_ret);

  // Allocation attempt that should be called during safepoints (e.g.,
  // at the end of a successful GC). expect_null_mutator_alloc_region
  // specifies whether the mutator alloc region is expected to be NULL
  // or not.
  HeapWord* attempt_allocation_at_safepoint(size_t word_size,
                                       bool expect_null_mutator_alloc_region);

  // It dirties the cards that cover the block so that so that the post
  // write barrier never queues anything when updating objects on this
  // block. It is assumed (and in fact we assert) that the block
  // belongs to a young region.
  inline void dirty_young_block(HeapWord* start, size_t word_size);

  // Allocate blocks during garbage collection. Will ensure an
  // allocation region, either by picking one or expanding the
  // heap, and then allocate a block of the given size. The block
  // may not be a humongous - it must fit into a single heap region.
  HeapWord* par_allocate_during_gc(GCAllocPurpose purpose, size_t word_size);

  HeapWord* allocate_during_gc_slow(GCAllocPurpose purpose,
                                    HeapRegion*    alloc_region,
                                    bool           par,
                                    size_t         word_size);

  // Ensure that no further allocations can happen in "r", bearing in mind
  // that parallel threads might be attempting allocations.
  void par_allocate_remaining_space(HeapRegion* r);

  // Retires an allocation region when it is full or at the end of a
  // GC pause.
  void  retire_alloc_region(HeapRegion* alloc_region, bool par);

  // These two methods are the "callbacks" from the G1AllocRegion class.

  HeapRegion* new_mutator_alloc_region(size_t word_size, bool force);
  void retire_mutator_alloc_region(HeapRegion* alloc_region,
                                   size_t allocated_bytes);

  // - if explicit_gc is true, the GC is for a System.gc() or a heap
  //   inspection request and should collect the entire heap
  // - if clear_all_soft_refs is true, all soft references should be
  //   cleared during the GC
  // - if explicit_gc is false, word_size describes the allocation that
  //   the GC should attempt (at least) to satisfy
  // - it returns false if it is unable to do the collection due to the
  //   GC locker being active, true otherwise
  bool do_collection(bool explicit_gc,
                     bool clear_all_soft_refs,
                     size_t word_size);

  // Callback from VM_G1CollectFull operation.
  // Perform a full collection.
  void do_full_collection(bool clear_all_soft_refs);

  // Resize the heap if necessary after a full collection.  If this is
  // after a collect-for allocation, "word_size" is the allocation size,
  // and will be considered part of the used portion of the heap.
  void resize_if_necessary_after_full_collection(size_t word_size);

  // Callback from VM_G1CollectForAllocation operation.
  // This function does everything necessary/possible to satisfy a
  // failed allocation request (including collection, expansion, etc.)
  HeapWord* satisfy_failed_allocation(size_t word_size, bool* succeeded);

  // Attempting to expand the heap sufficiently
  // to support an allocation of the given "word_size".  If
  // successful, perform the allocation and return the address of the
  // allocated block, or else "NULL".
  HeapWord* expand_and_allocate(size_t word_size);

public:

  G1MonitoringSupport* g1mm() { return _g1mm; }

  // Expand the garbage-first heap by at least the given size (in bytes!).
  // Returns true if the heap was expanded by the requested amount;
  // false otherwise.
  // (Rounds up to a HeapRegion boundary.)
  bool expand(size_t expand_bytes);

  // Do anything common to GC's.
  virtual void gc_prologue(bool full);
  virtual void gc_epilogue(bool full);

  // We register a region with the fast "in collection set" test. We
  // simply set to true the array slot corresponding to this region.
  void register_region_with_in_cset_fast_test(HeapRegion* r) {
    assert(_in_cset_fast_test_base != NULL, "sanity");
    assert(r->in_collection_set(), "invariant");
    size_t index = r->hrs_index();
    assert(index < _in_cset_fast_test_length, "invariant");
    assert(!_in_cset_fast_test_base[index], "invariant");
    _in_cset_fast_test_base[index] = true;
  }

  // This is a fast test on whether a reference points into the
  // collection set or not. It does not assume that the reference
  // points into the heap; if it doesn't, it will return false.
  bool in_cset_fast_test(oop obj) {
    assert(_in_cset_fast_test != NULL, "sanity");
    if (_g1_committed.contains((HeapWord*) obj)) {
      // no need to subtract the bottom of the heap from obj,
      // _in_cset_fast_test is biased
      size_t index = ((size_t) obj) >> HeapRegion::LogOfHRGrainBytes;
      bool ret = _in_cset_fast_test[index];
      // let's make sure the result is consistent with what the slower
      // test returns
      assert( ret || !obj_in_cs(obj), "sanity");
      assert(!ret ||  obj_in_cs(obj), "sanity");
      return ret;
    } else {
      return false;
    }
  }

  void clear_cset_fast_test() {
    assert(_in_cset_fast_test_base != NULL, "sanity");
    memset(_in_cset_fast_test_base, false,
        _in_cset_fast_test_length * sizeof(bool));
  }

  // This is called at the end of either a concurrent cycle or a Full
  // GC to update the number of full collections completed. Those two
  // can happen in a nested fashion, i.e., we start a concurrent
  // cycle, a Full GC happens half-way through it which ends first,
  // and then the cycle notices that a Full GC happened and ends
  // too. The concurrent parameter is a boolean to help us do a bit
  // tighter consistency checking in the method. If concurrent is
  // false, the caller is the inner caller in the nesting (i.e., the
  // Full GC). If concurrent is true, the caller is the outer caller
  // in this nesting (i.e., the concurrent cycle). Further nesting is
  // not currently supported. The end of the this call also notifies
  // the FullGCCount_lock in case a Java thread is waiting for a full
  // GC to happen (e.g., it called System.gc() with
  // +ExplicitGCInvokesConcurrent).
  void increment_full_collections_completed(bool concurrent);

  unsigned int full_collections_completed() {
    return _full_collections_completed;
  }

protected:

  // Shrink the garbage-first heap by at most the given size (in bytes!).
  // (Rounds down to a HeapRegion boundary.)
  virtual void shrink(size_t expand_bytes);
  void shrink_helper(size_t expand_bytes);

  #if TASKQUEUE_STATS
  static void print_taskqueue_stats_hdr(outputStream* const st = gclog_or_tty);
  void print_taskqueue_stats(outputStream* const st = gclog_or_tty) const;
  void reset_taskqueue_stats();
  #endif // TASKQUEUE_STATS

  // Schedule the VM operation that will do an evacuation pause to
  // satisfy an allocation request of word_size. *succeeded will
  // return whether the VM operation was successful (it did do an
  // evacuation pause) or not (another thread beat us to it or the GC
  // locker was active). Given that we should not be holding the
  // Heap_lock when we enter this method, we will pass the
  // gc_count_before (i.e., total_collections()) as a parameter since
  // it has to be read while holding the Heap_lock. Currently, both
  // methods that call do_collection_pause() release the Heap_lock
  // before the call, so it's easy to read gc_count_before just before.
  HeapWord* do_collection_pause(size_t       word_size,
                                unsigned int gc_count_before,
                                bool*        succeeded);

  // The guts of the incremental collection pause, executed by the vm
  // thread. It returns false if it is unable to do the collection due
  // to the GC locker being active, true otherwise
  bool do_collection_pause_at_safepoint(double target_pause_time_ms);

  // Actually do the work of evacuating the collection set.
  void evacuate_collection_set();

  // The g1 remembered set of the heap.
  G1RemSet* _g1_rem_set;
  // And it's mod ref barrier set, used to track updates for the above.
  ModRefBarrierSet* _mr_bs;

  // A set of cards that cover the objects for which the Rsets should be updated
  // concurrently after the collection.
  DirtyCardQueueSet _dirty_card_queue_set;

  // The Heap Region Rem Set Iterator.
  HeapRegionRemSetIterator** _rem_set_iterator;

  // The closure used to refine a single card.
  RefineCardTableEntryClosure* _refine_cte_cl;

  // A function to check the consistency of dirty card logs.
  void check_ct_logs_at_safepoint();

  // A DirtyCardQueueSet that is used to hold cards that contain
  // references into the current collection set. This is used to
  // update the remembered sets of the regions in the collection
  // set in the event of an evacuation failure.
  DirtyCardQueueSet _into_cset_dirty_card_queue_set;

  // After a collection pause, make the regions in the CS into free
  // regions.
  void free_collection_set(HeapRegion* cs_head);

  // Abandon the current collection set without recording policy
  // statistics or updating free lists.
  void abandon_collection_set(HeapRegion* cs_head);

  // Applies "scan_non_heap_roots" to roots outside the heap,
  // "scan_rs" to roots inside the heap (having done "set_region" to
  // indicate the region in which the root resides), and does "scan_perm"
  // (setting the generation to the perm generation.)  If "scan_rs" is
  // NULL, then this step is skipped.  The "worker_i"
  // param is for use with parallel roots processing, and should be
  // the "i" of the calling parallel worker thread's work(i) function.
  // In the sequential case this param will be ignored.
  void g1_process_strong_roots(bool collecting_perm_gen,
                               SharedHeap::ScanningOption so,
                               OopClosure* scan_non_heap_roots,
                               OopsInHeapRegionClosure* scan_rs,
                               OopsInGenClosure* scan_perm,
                               int worker_i);

  // Apply "blk" to all the weak roots of the system.  These include
  // JNI weak roots, the code cache, system dictionary, symbol table,
  // string table, and referents of reachable weak refs.
  void g1_process_weak_roots(OopClosure* root_closure,
                             OopClosure* non_root_closure);

  // Invoke "save_marks" on all heap regions.
  void save_marks();

  // Frees a non-humongous region by initializing its contents and
  // adding it to the free list that's passed as a parameter (this is
  // usually a local list which will be appended to the master free
  // list later). The used bytes of freed regions are accumulated in
  // pre_used. If par is true, the region's RSet will not be freed
  // up. The assumption is that this will be done later.
  void free_region(HeapRegion* hr,
                   size_t* pre_used,
                   FreeRegionList* free_list,
                   bool par);

  // Frees a humongous region by collapsing it into individual regions
  // and calling free_region() for each of them. The freed regions
  // will be added to the free list that's passed as a parameter (this
  // is usually a local list which will be appended to the master free
  // list later). The used bytes of freed regions are accumulated in
  // pre_used. If par is true, the region's RSet will not be freed
  // up. The assumption is that this will be done later.
  void free_humongous_region(HeapRegion* hr,
                             size_t* pre_used,
                             FreeRegionList* free_list,
                             HumongousRegionSet* humongous_proxy_set,
                             bool par);

  // Notifies all the necessary spaces that the committed space has
  // been updated (either expanded or shrunk). It should be called
  // after _g1_storage is updated.
  void update_committed_space(HeapWord* old_end, HeapWord* new_end);

  // The concurrent marker (and the thread it runs in.)
  ConcurrentMark* _cm;
  ConcurrentMarkThread* _cmThread;
  bool _mark_in_progress;

  // The concurrent refiner.
  ConcurrentG1Refine* _cg1r;

  // The parallel task queues
  RefToScanQueueSet *_task_queues;

  // True iff a evacuation has failed in the current collection.
  bool _evacuation_failed;

  // Set the attribute indicating whether evacuation has failed in the
  // current collection.
  void set_evacuation_failed(bool b) { _evacuation_failed = b; }

  // Failed evacuations cause some logical from-space objects to have
  // forwarding pointers to themselves.  Reset them.
  void remove_self_forwarding_pointers();

  // When one is non-null, so is the other.  Together, they each pair is
  // an object with a preserved mark, and its mark value.
  GrowableArray<oop>*     _objs_with_preserved_marks;
  GrowableArray<markOop>* _preserved_marks_of_objs;

  // Preserve the mark of "obj", if necessary, in preparation for its mark
  // word being overwritten with a self-forwarding-pointer.
  void preserve_mark_if_necessary(oop obj, markOop m);

  // The stack of evac-failure objects left to be scanned.
  GrowableArray<oop>*    _evac_failure_scan_stack;
  // The closure to apply to evac-failure objects.

  OopsInHeapRegionClosure* _evac_failure_closure;
  // Set the field above.
  void
  set_evac_failure_closure(OopsInHeapRegionClosure* evac_failure_closure) {
    _evac_failure_closure = evac_failure_closure;
  }

  // Push "obj" on the scan stack.
  void push_on_evac_failure_scan_stack(oop obj);
  // Process scan stack entries until the stack is empty.
  void drain_evac_failure_scan_stack();
  // True iff an invocation of "drain_scan_stack" is in progress; to
  // prevent unnecessary recursion.
  bool _drain_in_progress;

  // Do any necessary initialization for evacuation-failure handling.
  // "cl" is the closure that will be used to process evac-failure
  // objects.
  void init_for_evac_failure(OopsInHeapRegionClosure* cl);
  // Do any necessary cleanup for evacuation-failure handling data
  // structures.
  void finalize_for_evac_failure();

  // An attempt to evacuate "obj" has failed; take necessary steps.
  oop handle_evacuation_failure_par(OopsInHeapRegionClosure* cl, oop obj);
  void handle_evacuation_failure_common(oop obj, markOop m);

  // Ensure that the relevant gc_alloc regions are set.
  void get_gc_alloc_regions();
  // We're done with GC alloc regions. We are going to tear down the
  // gc alloc list and remove the gc alloc tag from all the regions on
  // that list. However, we will also retain the last (i.e., the one
  // that is half-full) GC alloc region, per GCAllocPurpose, for
  // possible reuse during the next collection, provided
  // _retain_gc_alloc_region[] indicates that it should be the
  // case. Said regions are kept in the _retained_gc_alloc_regions[]
  // array. If the parameter totally is set, we will not retain any
  // regions, irrespective of what _retain_gc_alloc_region[]
  // indicates.
  void release_gc_alloc_regions(bool totally);
#ifndef PRODUCT
  // Useful for debugging.
  void print_gc_alloc_regions();
#endif // !PRODUCT

  // Instance of the concurrent mark is_alive closure for embedding
  // into the reference processor as the is_alive_non_header. This
  // prevents unnecessary additions to the discovered lists during
  // concurrent discovery.
  G1CMIsAliveClosure _is_alive_closure;

  // ("Weak") Reference processing support
  ReferenceProcessor* _ref_processor;

  enum G1H_process_strong_roots_tasks {
    G1H_PS_mark_stack_oops_do,
    G1H_PS_refProcessor_oops_do,
    // Leave this one last.
    G1H_PS_NumElements
  };

  SubTasksDone* _process_strong_tasks;

  volatile bool _free_regions_coming;

public:

  SubTasksDone* process_strong_tasks() { return _process_strong_tasks; }

  void set_refine_cte_cl_concurrency(bool concurrent);

  RefToScanQueue *task_queue(int i) const;

  // A set of cards where updates happened during the GC
  DirtyCardQueueSet& dirty_card_queue_set() { return _dirty_card_queue_set; }

  // A DirtyCardQueueSet that is used to hold cards that contain
  // references into the current collection set. This is used to
  // update the remembered sets of the regions in the collection
  // set in the event of an evacuation failure.
  DirtyCardQueueSet& into_cset_dirty_card_queue_set()
        { return _into_cset_dirty_card_queue_set; }

  // Create a G1CollectedHeap with the specified policy.
  // Must call the initialize method afterwards.
  // May not return if something goes wrong.
  G1CollectedHeap(G1CollectorPolicy* policy);

  // Initialize the G1CollectedHeap to have the initial and
  // maximum sizes, permanent generation, and remembered and barrier sets
  // specified by the policy object.
  jint initialize();

  virtual void ref_processing_init();

  void set_par_threads(int t) {
    SharedHeap::set_par_threads(t);
    _process_strong_tasks->set_n_threads(t);
  }

  virtual CollectedHeap::Name kind() const {
    return CollectedHeap::G1CollectedHeap;
  }

  // The current policy object for the collector.
  G1CollectorPolicy* g1_policy() const { return _g1_policy; }

  // Adaptive size policy.  No such thing for g1.
  virtual AdaptiveSizePolicy* size_policy() { return NULL; }

  // The rem set and barrier set.
  G1RemSet* g1_rem_set() const { return _g1_rem_set; }
  ModRefBarrierSet* mr_bs() const { return _mr_bs; }

  // The rem set iterator.
  HeapRegionRemSetIterator* rem_set_iterator(int i) {
    return _rem_set_iterator[i];
  }

  HeapRegionRemSetIterator* rem_set_iterator() {
    return _rem_set_iterator[0];
  }

  unsigned get_gc_time_stamp() {
    return _gc_time_stamp;
  }

  void reset_gc_time_stamp() {
    _gc_time_stamp = 0;
    OrderAccess::fence();
  }

  void increment_gc_time_stamp() {
    ++_gc_time_stamp;
    OrderAccess::fence();
  }

  void iterate_dirty_card_closure(CardTableEntryClosure* cl,
                                  DirtyCardQueue* into_cset_dcq,
                                  bool concurrent, int worker_i);

  // The shared block offset table array.
  G1BlockOffsetSharedArray* bot_shared() const { return _bot_shared; }

  // Reference Processing accessor
  ReferenceProcessor* ref_processor() { return _ref_processor; }

  virtual size_t capacity() const;
  virtual size_t used() const;
  // This should be called when we're not holding the heap lock. The
  // result might be a bit inaccurate.
  size_t used_unlocked() const;
  size_t recalculate_used() const;
#ifndef PRODUCT
  size_t recalculate_used_regions() const;
#endif // PRODUCT

  // These virtual functions do the actual allocation.
  // Some heaps may offer a contiguous region for shared non-blocking
  // allocation, via inlined code (by exporting the address of the top and
  // end fields defining the extent of the contiguous allocation region.)
  // But G1CollectedHeap doesn't yet support this.

  // Return an estimate of the maximum allocation that could be performed
  // without triggering any collection or expansion activity.  In a
  // generational collector, for example, this is probably the largest
  // allocation that could be supported (without expansion) in the youngest
  // generation.  It is "unsafe" because no locks are taken; the result
  // should be treated as an approximation, not a guarantee, for use in
  // heuristic resizing decisions.
  virtual size_t unsafe_max_alloc();

  virtual bool is_maximal_no_gc() const {
    return _g1_storage.uncommitted_size() == 0;
  }

  // The total number of regions in the heap.
  size_t n_regions() { return _hrs.length(); }

  // The max number of regions in the heap.
  size_t max_regions() { return _hrs.max_length(); }

  // The number of regions that are completely free.
  size_t free_regions() { return _free_list.length(); }

  // The number of regions that are not completely free.
  size_t used_regions() { return n_regions() - free_regions(); }

  // The number of regions available for "regular" expansion.
  size_t expansion_regions() { return _expansion_regions; }

  // Factory method for HeapRegion instances. It will return NULL if
  // the allocation fails.
  HeapRegion* new_heap_region(size_t hrs_index, HeapWord* bottom);

  void verify_not_dirty_region(HeapRegion* hr) PRODUCT_RETURN;
  void verify_dirty_region(HeapRegion* hr) PRODUCT_RETURN;
  void verify_dirty_young_list(HeapRegion* head) PRODUCT_RETURN;
  void verify_dirty_young_regions() PRODUCT_RETURN;

  // verify_region_sets() performs verification over the region
  // lists. It will be compiled in the product code to be used when
  // necessary (i.e., during heap verification).
  void verify_region_sets();

  // verify_region_sets_optional() is planted in the code for
  // list verification in non-product builds (and it can be enabled in
  // product builds by definning HEAP_REGION_SET_FORCE_VERIFY to be 1).
#if HEAP_REGION_SET_FORCE_VERIFY
  void verify_region_sets_optional() {
    verify_region_sets();
  }
#else // HEAP_REGION_SET_FORCE_VERIFY
  void verify_region_sets_optional() { }
#endif // HEAP_REGION_SET_FORCE_VERIFY

#ifdef ASSERT
  bool is_on_master_free_list(HeapRegion* hr) {
    return hr->containing_set() == &_free_list;
  }

  bool is_in_humongous_set(HeapRegion* hr) {
    return hr->containing_set() == &_humongous_set;
  }
#endif // ASSERT

  // Wrapper for the region list operations that can be called from
  // methods outside this class.

  void secondary_free_list_add_as_tail(FreeRegionList* list) {
    _secondary_free_list.add_as_tail(list);
  }

  void append_secondary_free_list() {
    _free_list.add_as_head(&_secondary_free_list);
  }

  void append_secondary_free_list_if_not_empty_with_lock() {
    // If the secondary free list looks empty there's no reason to
    // take the lock and then try to append it.
    if (!_secondary_free_list.is_empty()) {
      MutexLockerEx x(SecondaryFreeList_lock, Mutex::_no_safepoint_check_flag);
      append_secondary_free_list();
    }
  }

  void set_free_regions_coming();
  void reset_free_regions_coming();
  bool free_regions_coming() { return _free_regions_coming; }
  void wait_while_free_regions_coming();

  // Perform a collection of the heap; intended for use in implementing
  // "System.gc".  This probably implies as full a collection as the
  // "CollectedHeap" supports.
  virtual void collect(GCCause::Cause cause);

  // The same as above but assume that the caller holds the Heap_lock.
  void collect_locked(GCCause::Cause cause);

  // This interface assumes that it's being called by the
  // vm thread. It collects the heap assuming that the
  // heap lock is already held and that we are executing in
  // the context of the vm thread.
  virtual void collect_as_vm_thread(GCCause::Cause cause);

  // True iff a evacuation has failed in the most-recent collection.
  bool evacuation_failed() { return _evacuation_failed; }

  // It will free a region if it has allocated objects in it that are
  // all dead. It calls either free_region() or
  // free_humongous_region() depending on the type of the region that
  // is passed to it.
  void free_region_if_empty(HeapRegion* hr,
                            size_t* pre_used,
                            FreeRegionList* free_list,
                            HumongousRegionSet* humongous_proxy_set,
                            HRRSCleanupTask* hrrs_cleanup_task,
                            bool par);

  // It appends the free list to the master free list and updates the
  // master humongous list according to the contents of the proxy
  // list. It also adjusts the total used bytes according to pre_used
  // (if par is true, it will do so by taking the ParGCRareEvent_lock).
  void update_sets_after_freeing_regions(size_t pre_used,
                                       FreeRegionList* free_list,
                                       HumongousRegionSet* humongous_proxy_set,
                                       bool par);

  // Returns "TRUE" iff "p" points into the allocated area of the heap.
  virtual bool is_in(const void* p) const;

  // Return "TRUE" iff the given object address is within the collection
  // set.
  inline bool obj_in_cs(oop obj);

  // Return "TRUE" iff the given object address is in the reserved
  // region of g1 (excluding the permanent generation).
  bool is_in_g1_reserved(const void* p) const {
    return _g1_reserved.contains(p);
  }

  // Returns a MemRegion that corresponds to the space that has been
  // reserved for the heap
  MemRegion g1_reserved() {
    return _g1_reserved;
  }

  // Returns a MemRegion that corresponds to the space that has been
  // committed in the heap
  MemRegion g1_committed() {
    return _g1_committed;
  }

  virtual bool is_in_closed_subset(const void* p) const;

  // Dirty card table entries covering a list of young regions.
  void dirtyCardsForYoungRegions(CardTableModRefBS* ct_bs, HeapRegion* list);

  // This resets the card table to all zeros.  It is used after
  // a collection pause which used the card table to claim cards.
  void cleanUpCardTable();

  // Iteration functions.

  // Iterate over all the ref-containing fields of all objects, calling
  // "cl.do_oop" on each.
  virtual void oop_iterate(OopClosure* cl) {
    oop_iterate(cl, true);
  }
  void oop_iterate(OopClosure* cl, bool do_perm);

  // Same as above, restricted to a memory region.
  virtual void oop_iterate(MemRegion mr, OopClosure* cl) {
    oop_iterate(mr, cl, true);
  }
  void oop_iterate(MemRegion mr, OopClosure* cl, bool do_perm);

  // Iterate over all objects, calling "cl.do_object" on each.
  virtual void object_iterate(ObjectClosure* cl) {
    object_iterate(cl, true);
  }
  virtual void safe_object_iterate(ObjectClosure* cl) {
    object_iterate(cl, true);
  }
  void object_iterate(ObjectClosure* cl, bool do_perm);

  // Iterate over all objects allocated since the last collection, calling
  // "cl.do_object" on each.  The heap must have been initialized properly
  // to support this function, or else this call will fail.
  virtual void object_iterate_since_last_GC(ObjectClosure* cl);

  // Iterate over all spaces in use in the heap, in ascending address order.
  virtual void space_iterate(SpaceClosure* cl);

  // Iterate over heap regions, in address order, terminating the
  // iteration early if the "doHeapRegion" method returns "true".
  void heap_region_iterate(HeapRegionClosure* blk) const;

  // Iterate over heap regions starting with r (or the first region if "r"
  // is NULL), in address order, terminating early if the "doHeapRegion"
  // method returns "true".
  void heap_region_iterate_from(HeapRegion* r, HeapRegionClosure* blk) const;

  // Return the region with the given index. It assumes the index is valid.
  HeapRegion* region_at(size_t index) const { return _hrs.at(index); }

  // Divide the heap region sequence into "chunks" of some size (the number
  // of regions divided by the number of parallel threads times some
  // overpartition factor, currently 4).  Assumes that this will be called
  // in parallel by ParallelGCThreads worker threads with discinct worker
  // ids in the range [0..max(ParallelGCThreads-1, 1)], that all parallel
  // calls will use the same "claim_value", and that that claim value is
  // different from the claim_value of any heap region before the start of
  // the iteration.  Applies "blk->doHeapRegion" to each of the regions, by
  // attempting to claim the first region in each chunk, and, if
  // successful, applying the closure to each region in the chunk (and
  // setting the claim value of the second and subsequent regions of the
  // chunk.)  For now requires that "doHeapRegion" always returns "false",
  // i.e., that a closure never attempt to abort a traversal.
  void heap_region_par_iterate_chunked(HeapRegionClosure* blk,
                                       int worker,
                                       jint claim_value);

  // It resets all the region claim values to the default.
  void reset_heap_region_claim_values();

#ifdef ASSERT
  bool check_heap_region_claim_values(jint claim_value);
#endif // ASSERT

  // Iterate over the regions (if any) in the current collection set.
  void collection_set_iterate(HeapRegionClosure* blk);

  // As above but starting from region r
  void collection_set_iterate_from(HeapRegion* r, HeapRegionClosure *blk);

  // Returns the first (lowest address) compactible space in the heap.
  virtual CompactibleSpace* first_compactible_space();

  // A CollectedHeap will contain some number of spaces.  This finds the
  // space containing a given address, or else returns NULL.
  virtual Space* space_containing(const void* addr) const;

  // A G1CollectedHeap will contain some number of heap regions.  This
  // finds the region containing a given address, or else returns NULL.
  template <class T>
  inline HeapRegion* heap_region_containing(const T addr) const;

  // Like the above, but requires "addr" to be in the heap (to avoid a
  // null-check), and unlike the above, may return an continuing humongous
  // region.
  template <class T>
  inline HeapRegion* heap_region_containing_raw(const T addr) const;

  // A CollectedHeap is divided into a dense sequence of "blocks"; that is,
  // each address in the (reserved) heap is a member of exactly
  // one block.  The defining characteristic of a block is that it is
  // possible to find its size, and thus to progress forward to the next
  // block.  (Blocks may be of different sizes.)  Thus, blocks may
  // represent Java objects, or they might be free blocks in a
  // free-list-based heap (or subheap), as long as the two kinds are
  // distinguishable and the size of each is determinable.

  // Returns the address of the start of the "block" that contains the
  // address "addr".  We say "blocks" instead of "object" since some heaps
  // may not pack objects densely; a chunk may either be an object or a
  // non-object.
  virtual HeapWord* block_start(const void* addr) const;

  // Requires "addr" to be the start of a chunk, and returns its size.
  // "addr + size" is required to be the start of a new chunk, or the end
  // of the active area of the heap.
  virtual size_t block_size(const HeapWord* addr) const;

  // Requires "addr" to be the start of a block, and returns "TRUE" iff
  // the block is an object.
  virtual bool block_is_obj(const HeapWord* addr) const;

  // Does this heap support heap inspection? (+PrintClassHistogram)
  virtual bool supports_heap_inspection() const { return true; }

  // Section on thread-local allocation buffers (TLABs)
  // See CollectedHeap for semantics.

  virtual bool supports_tlab_allocation() const;
  virtual size_t tlab_capacity(Thread* thr) const;
  virtual size_t unsafe_max_tlab_alloc(Thread* thr) const;

  // Can a compiler initialize a new object without store barriers?
  // This permission only extends from the creation of a new object
  // via a TLAB up to the first subsequent safepoint. If such permission
  // is granted for this heap type, the compiler promises to call
  // defer_store_barrier() below on any slow path allocation of
  // a new object for which such initializing store barriers will
  // have been elided. G1, like CMS, allows this, but should be
  // ready to provide a compensating write barrier as necessary
  // if that storage came out of a non-young region. The efficiency
  // of this implementation depends crucially on being able to
  // answer very efficiently in constant time whether a piece of
  // storage in the heap comes from a young region or not.
  // See ReduceInitialCardMarks.
  virtual bool can_elide_tlab_store_barriers() const {
    // 6920090: Temporarily disabled, because of lingering
    // instabilities related to RICM with G1. In the
    // interim, the option ReduceInitialCardMarksForG1
    // below is left solely as a debugging device at least
    // until 6920109 fixes the instabilities.
    return ReduceInitialCardMarksForG1;
  }

  virtual bool card_mark_must_follow_store() const {
    return true;
  }

  bool is_in_young(const oop obj) {
    HeapRegion* hr = heap_region_containing(obj);
    return hr != NULL && hr->is_young();
  }

#ifdef ASSERT
  virtual bool is_in_partial_collection(const void* p);
#endif

  virtual bool is_scavengable(const void* addr);

  // We don't need barriers for initializing stores to objects
  // in the young gen: for the SATB pre-barrier, there is no
  // pre-value that needs to be remembered; for the remembered-set
  // update logging post-barrier, we don't maintain remembered set
  // information for young gen objects. Note that non-generational
  // G1 does not have any "young" objects, should not elide
  // the rs logging barrier and so should always answer false below.
  // However, non-generational G1 (-XX:-G1Gen) appears to have
  // bit-rotted so was not tested below.
  virtual bool can_elide_initializing_store_barrier(oop new_obj) {
    // Re 6920090, 6920109 above.
    assert(ReduceInitialCardMarksForG1, "Else cannot be here");
    assert(G1Gen || !is_in_young(new_obj),
           "Non-generational G1 should never return true below");
    return is_in_young(new_obj);
  }

  // Can a compiler elide a store barrier when it writes
  // a permanent oop into the heap?  Applies when the compiler
  // is storing x to the heap, where x->is_perm() is true.
  virtual bool can_elide_permanent_oop_store_barriers() const {
    // At least until perm gen collection is also G1-ified, at
    // which point this should return false.
    return true;
  }

  // The boundary between a "large" and "small" array of primitives, in
  // words.
  virtual size_t large_typearray_limit();

  // Returns "true" iff the given word_size is "very large".
  static bool isHumongous(size_t word_size) {
    // Note this has to be strictly greater-than as the TLABs
    // are capped at the humongous thresold and we want to
    // ensure that we don't try to allocate a TLAB as
    // humongous and that we don't allocate a humongous
    // object in a TLAB.
    return word_size > _humongous_object_threshold_in_words;
  }

  // Update mod union table with the set of dirty cards.
  void updateModUnion();

  // Set the mod union bits corresponding to the given memRegion.  Note
  // that this is always a safe operation, since it doesn't clear any
  // bits.
  void markModUnionRange(MemRegion mr);

  // Records the fact that a marking phase is no longer in progress.
  void set_marking_complete() {
    _mark_in_progress = false;
  }
  void set_marking_started() {
    _mark_in_progress = true;
  }
  bool mark_in_progress() {
    return _mark_in_progress;
  }

  // Print the maximum heap capacity.
  virtual size_t max_capacity() const;

  virtual jlong millis_since_last_gc();

  // Perform any cleanup actions necessary before allowing a verification.
  virtual void prepare_for_verify();

  // Perform verification.

  // vo == UsePrevMarking  -> use "prev" marking information,
  // vo == UseNextMarking -> use "next" marking information
  // vo == UseMarkWord    -> use the mark word in the object header
  //
  // NOTE: Only the "prev" marking information is guaranteed to be
  // consistent most of the time, so most calls to this should use
  // vo == UsePrevMarking.
  // Currently, there is only one case where this is called with
  // vo == UseNextMarking, which is to verify the "next" marking
  // information at the end of remark.
  // Currently there is only one place where this is called with
  // vo == UseMarkWord, which is to verify the marking during a
  // full GC.
  void verify(bool allow_dirty, bool silent, VerifyOption vo);

  // Override; it uses the "prev" marking information
  virtual void verify(bool allow_dirty, bool silent);
  // Default behavior by calling print(tty);
  virtual void print() const;
  // This calls print_on(st, PrintHeapAtGCExtended).
  virtual void print_on(outputStream* st) const;
  // If extended is true, it will print out information for all
  // regions in the heap by calling print_on_extended(st).
  virtual void print_on(outputStream* st, bool extended) const;
  virtual void print_on_extended(outputStream* st) const;

  virtual void print_gc_threads_on(outputStream* st) const;
  virtual void gc_threads_do(ThreadClosure* tc) const;

  // Override
  void print_tracing_info() const;

  // Convenience function to be used in situations where the heap type can be
  // asserted to be this type.
  static G1CollectedHeap* heap();

  void empty_young_list();

  void set_region_short_lived_locked(HeapRegion* hr);
  // add appropriate methods for any other surv rate groups

  YoungList* young_list() { return _young_list; }

  // debugging
  bool check_young_list_well_formed() {
    return _young_list->check_list_well_formed();
  }

  bool check_young_list_empty(bool check_heap,
                              bool check_sample = true);

  // *** Stuff related to concurrent marking.  It's not clear to me that so
  // many of these need to be public.

  // The functions below are helper functions that a subclass of
  // "CollectedHeap" can use in the implementation of its virtual
  // functions.
  // This performs a concurrent marking of the live objects in a
  // bitmap off to the side.
  void doConcurrentMark();

  // Do a full concurrent marking, synchronously.
  void do_sync_mark();

  bool isMarkedPrev(oop obj) const;
  bool isMarkedNext(oop obj) const;

  // vo == UsePrevMarking -> use "prev" marking information,
  // vo == UseNextMarking -> use "next" marking information,
  // vo == UseMarkWord    -> use mark word from object header
  bool is_obj_dead_cond(const oop obj,
                        const HeapRegion* hr,
                        const VerifyOption vo) const {

    switch (vo) {
      case VerifyOption_G1UsePrevMarking:
        return is_obj_dead(obj, hr);
      case VerifyOption_G1UseNextMarking:
        return is_obj_ill(obj, hr);
      default:
        assert(vo == VerifyOption_G1UseMarkWord, "must be");
        return !obj->is_gc_marked();
    }
  }

  // Determine if an object is dead, given the object and also
  // the region to which the object belongs. An object is dead
  // iff a) it was not allocated since the last mark and b) it
  // is not marked.

  bool is_obj_dead(const oop obj, const HeapRegion* hr) const {
    return
      !hr->obj_allocated_since_prev_marking(obj) &&
      !isMarkedPrev(obj);
  }

  // This is used when copying an object to survivor space.
  // If the object is marked live, then we mark the copy live.
  // If the object is allocated since the start of this mark
  // cycle, then we mark the copy live.
  // If the object has been around since the previous mark
  // phase, and hasn't been marked yet during this phase,
  // then we don't mark it, we just wait for the
  // current marking cycle to get to it.

  // This function returns true when an object has been
  // around since the previous marking and hasn't yet
  // been marked during this marking.

  bool is_obj_ill(const oop obj, const HeapRegion* hr) const {
    return
      !hr->obj_allocated_since_next_marking(obj) &&
      !isMarkedNext(obj);
  }

  // Determine if an object is dead, given only the object itself.
  // This will find the region to which the object belongs and
  // then call the region version of the same function.

  // Added if it is in permanent gen it isn't dead.
  // Added if it is NULL it isn't dead.

  // vo == UsePrevMarking -> use "prev" marking information,
  // vo == UseNextMarking -> use "next" marking information,
  // vo == UseMarkWord    -> use mark word from object header
  bool is_obj_dead_cond(const oop obj,
                        const VerifyOption vo) const {

    switch (vo) {
      case VerifyOption_G1UsePrevMarking:
        return is_obj_dead(obj);
      case VerifyOption_G1UseNextMarking:
        return is_obj_ill(obj);
      default:
        assert(vo == VerifyOption_G1UseMarkWord, "must be");
        return !obj->is_gc_marked();
    }
  }

  bool is_obj_dead(const oop obj) const {
    const HeapRegion* hr = heap_region_containing(obj);
    if (hr == NULL) {
      if (Universe::heap()->is_in_permanent(obj))
        return false;
      else if (obj == NULL) return false;
      else return true;
    }
    else return is_obj_dead(obj, hr);
  }

  bool is_obj_ill(const oop obj) const {
    const HeapRegion* hr = heap_region_containing(obj);
    if (hr == NULL) {
      if (Universe::heap()->is_in_permanent(obj))
        return false;
      else if (obj == NULL) return false;
      else return true;
    }
    else return is_obj_ill(obj, hr);
  }

  // The following is just to alert the verification code
  // that a full collection has occurred and that the
  // remembered sets are no longer up to date.
  bool _full_collection;
  void set_full_collection() { _full_collection = true;}
  void clear_full_collection() {_full_collection = false;}
  bool full_collection() {return _full_collection;}

  ConcurrentMark* concurrent_mark() const { return _cm; }
  ConcurrentG1Refine* concurrent_g1_refine() const { return _cg1r; }

  // The dirty cards region list is used to record a subset of regions
  // whose cards need clearing. The list if populated during the
  // remembered set scanning and drained during the card table
  // cleanup. Although the methods are reentrant, population/draining
  // phases must not overlap. For synchronization purposes the last
  // element on the list points to itself.
  HeapRegion* _dirty_cards_region_list;
  void push_dirty_cards_region(HeapRegion* hr);
  HeapRegion* pop_dirty_cards_region();

public:
  void stop_conc_gc_threads();

  // <NEW PREDICTION>

  double predict_region_elapsed_time_ms(HeapRegion* hr, bool young);
  void check_if_region_is_too_expensive(double predicted_time_ms);
  size_t pending_card_num();
  size_t max_pending_card_num();
  size_t cards_scanned();

  // </NEW PREDICTION>

protected:
  size_t _max_heap_capacity;
};

#define use_local_bitmaps         1
#define verify_local_bitmaps      0
#define oop_buffer_length       256

#ifndef PRODUCT
class GCLabBitMap;
class GCLabBitMapClosure: public BitMapClosure {
private:
  ConcurrentMark* _cm;
  GCLabBitMap*    _bitmap;

public:
  GCLabBitMapClosure(ConcurrentMark* cm,
                     GCLabBitMap* bitmap) {
    _cm     = cm;
    _bitmap = bitmap;
  }

  virtual bool do_bit(size_t offset);
};
#endif // !PRODUCT

class GCLabBitMap: public BitMap {
private:
  ConcurrentMark* _cm;

  int       _shifter;
  size_t    _bitmap_word_covers_words;

  // beginning of the heap
  HeapWord* _heap_start;

  // this is the actual start of the GCLab
  HeapWord* _real_start_word;

  // this is the actual end of the GCLab
  HeapWord* _real_end_word;

  // this is the first word, possibly located before the actual start
  // of the GCLab, that corresponds to the first bit of the bitmap
  HeapWord* _start_word;

  // size of a GCLab in words
  size_t _gclab_word_size;

  static int shifter() {
    return MinObjAlignment - 1;
  }

  // how many heap words does a single bitmap word corresponds to?
  static size_t bitmap_word_covers_words() {
    return BitsPerWord << shifter();
  }

  size_t gclab_word_size() const {
    return _gclab_word_size;
  }

  // Calculates actual GCLab size in words
  size_t gclab_real_word_size() const {
    return bitmap_size_in_bits(pointer_delta(_real_end_word, _start_word))
           / BitsPerWord;
  }

  static size_t bitmap_size_in_bits(size_t gclab_word_size) {
    size_t bits_in_bitmap = gclab_word_size >> shifter();
    // We are going to ensure that the beginning of a word in this
    // bitmap also corresponds to the beginning of a word in the
    // global marking bitmap. To handle the case where a GCLab
    // starts from the middle of the bitmap, we need to add enough
    // space (i.e. up to a bitmap word) to ensure that we have
    // enough bits in the bitmap.
    return bits_in_bitmap + BitsPerWord - 1;
  }
public:
  GCLabBitMap(HeapWord* heap_start, size_t gclab_word_size)
    : BitMap(bitmap_size_in_bits(gclab_word_size)),
      _cm(G1CollectedHeap::heap()->concurrent_mark()),
      _shifter(shifter()),
      _bitmap_word_covers_words(bitmap_word_covers_words()),
      _heap_start(heap_start),
      _gclab_word_size(gclab_word_size),
      _real_start_word(NULL),
      _real_end_word(NULL),
      _start_word(NULL)
  {
    guarantee( size_in_words() >= bitmap_size_in_words(),
               "just making sure");
  }

  inline unsigned heapWordToOffset(HeapWord* addr) {
    unsigned offset = (unsigned) pointer_delta(addr, _start_word) >> _shifter;
    assert(offset < size(), "offset should be within bounds");
    return offset;
  }

  inline HeapWord* offsetToHeapWord(size_t offset) {
    HeapWord* addr =  _start_word + (offset << _shifter);
    assert(_real_start_word <= addr && addr < _real_end_word, "invariant");
    return addr;
  }

  bool fields_well_formed() {
    bool ret1 = (_real_start_word == NULL) &&
                (_real_end_word == NULL) &&
                (_start_word == NULL);
    if (ret1)
      return true;

    bool ret2 = _real_start_word >= _start_word &&
      _start_word < _real_end_word &&
      (_real_start_word + _gclab_word_size) == _real_end_word &&
      (_start_word + _gclab_word_size + _bitmap_word_covers_words)
                                                              > _real_end_word;
    return ret2;
  }

  inline bool mark(HeapWord* addr) {
    guarantee(use_local_bitmaps, "invariant");
    assert(fields_well_formed(), "invariant");

    if (addr >= _real_start_word && addr < _real_end_word) {
      assert(!isMarked(addr), "should not have already been marked");

      // first mark it on the bitmap
      at_put(heapWordToOffset(addr), true);

      return true;
    } else {
      return false;
    }
  }

  inline bool isMarked(HeapWord* addr) {
    guarantee(use_local_bitmaps, "invariant");
    assert(fields_well_formed(), "invariant");

    return at(heapWordToOffset(addr));
  }

  void set_buffer(HeapWord* start) {
    guarantee(use_local_bitmaps, "invariant");
    clear();

    assert(start != NULL, "invariant");
    _real_start_word = start;
    _real_end_word   = start + _gclab_word_size;

    size_t diff =
      pointer_delta(start, _heap_start) % _bitmap_word_covers_words;
    _start_word = start - diff;

    assert(fields_well_formed(), "invariant");
  }

#ifndef PRODUCT
  void verify() {
    // verify that the marks have been propagated
    GCLabBitMapClosure cl(_cm, this);
    iterate(&cl);
  }
#endif // PRODUCT

  void retire() {
    guarantee(use_local_bitmaps, "invariant");
    assert(fields_well_formed(), "invariant");

    if (_start_word != NULL) {
      CMBitMap*       mark_bitmap = _cm->nextMarkBitMap();

      // this means that the bitmap was set up for the GCLab
      assert(_real_start_word != NULL && _real_end_word != NULL, "invariant");

      mark_bitmap->mostly_disjoint_range_union(this,
                                0, // always start from the start of the bitmap
                                _start_word,
                                gclab_real_word_size());
      _cm->grayRegionIfNecessary(MemRegion(_real_start_word, _real_end_word));

#ifndef PRODUCT
      if (use_local_bitmaps && verify_local_bitmaps)
        verify();
#endif // PRODUCT
    } else {
      assert(_real_start_word == NULL && _real_end_word == NULL, "invariant");
    }
  }

  size_t bitmap_size_in_words() const {
    return (bitmap_size_in_bits(gclab_word_size()) + BitsPerWord - 1) / BitsPerWord;
  }

};

class G1ParGCAllocBuffer: public ParGCAllocBuffer {
private:
  bool        _retired;
  bool        _during_marking;
  GCLabBitMap _bitmap;

public:
  G1ParGCAllocBuffer(size_t gclab_word_size) :
    ParGCAllocBuffer(gclab_word_size),
    _during_marking(G1CollectedHeap::heap()->mark_in_progress()),
    _bitmap(G1CollectedHeap::heap()->reserved_region().start(), gclab_word_size),
    _retired(false)
  { }

  inline bool mark(HeapWord* addr) {
    guarantee(use_local_bitmaps, "invariant");
    assert(_during_marking, "invariant");
    return _bitmap.mark(addr);
  }

  inline void set_buf(HeapWord* buf) {
    if (use_local_bitmaps && _during_marking)
      _bitmap.set_buffer(buf);
    ParGCAllocBuffer::set_buf(buf);
    _retired = false;
  }

  inline void retire(bool end_of_gc, bool retain) {
    if (_retired)
      return;
    if (use_local_bitmaps && _during_marking) {
      _bitmap.retire();
    }
    ParGCAllocBuffer::retire(end_of_gc, retain);
    _retired = true;
  }
};

class G1ParScanThreadState : public StackObj {
protected:
  G1CollectedHeap* _g1h;
  RefToScanQueue*  _refs;
  DirtyCardQueue   _dcq;
  CardTableModRefBS* _ct_bs;
  G1RemSet* _g1_rem;

  G1ParGCAllocBuffer  _surviving_alloc_buffer;
  G1ParGCAllocBuffer  _tenured_alloc_buffer;
  G1ParGCAllocBuffer* _alloc_buffers[GCAllocPurposeCount];
  ageTable            _age_table;

  size_t           _alloc_buffer_waste;
  size_t           _undo_waste;

  OopsInHeapRegionClosure*      _evac_failure_cl;
  G1ParScanHeapEvacClosure*     _evac_cl;
  G1ParScanPartialArrayClosure* _partial_scan_cl;

  int _hash_seed;
  int _queue_num;

  size_t _term_attempts;

  double _start;
  double _start_strong_roots;
  double _strong_roots_time;
  double _start_term;
  double _term_time;

  // Map from young-age-index (0 == not young, 1 is youngest) to
  // surviving words. base is what we get back from the malloc call
  size_t* _surviving_young_words_base;
  // this points into the array, as we use the first few entries for padding
  size_t* _surviving_young_words;

#define PADDING_ELEM_NUM (DEFAULT_CACHE_LINE_SIZE / sizeof(size_t))

  void   add_to_alloc_buffer_waste(size_t waste) { _alloc_buffer_waste += waste; }

  void   add_to_undo_waste(size_t waste)         { _undo_waste += waste; }

  DirtyCardQueue& dirty_card_queue()             { return _dcq;  }
  CardTableModRefBS* ctbs()                      { return _ct_bs; }

  template <class T> void immediate_rs_update(HeapRegion* from, T* p, int tid) {
    if (!from->is_survivor()) {
      _g1_rem->par_write_ref(from, p, tid);
    }
  }

  template <class T> void deferred_rs_update(HeapRegion* from, T* p, int tid) {
    // If the new value of the field points to the same region or
    // is the to-space, we don't need to include it in the Rset updates.
    if (!from->is_in_reserved(oopDesc::load_decode_heap_oop(p)) && !from->is_survivor()) {
      size_t card_index = ctbs()->index_for(p);
      // If the card hasn't been added to the buffer, do it.
      if (ctbs()->mark_card_deferred(card_index)) {
        dirty_card_queue().enqueue((jbyte*)ctbs()->byte_for_index(card_index));
      }
    }
  }

public:
  G1ParScanThreadState(G1CollectedHeap* g1h, int queue_num);

  ~G1ParScanThreadState() {
    FREE_C_HEAP_ARRAY(size_t, _surviving_young_words_base);
  }

  RefToScanQueue*   refs()            { return _refs;             }
  ageTable*         age_table()       { return &_age_table;       }

  G1ParGCAllocBuffer* alloc_buffer(GCAllocPurpose purpose) {
    return _alloc_buffers[purpose];
  }

  size_t alloc_buffer_waste() const              { return _alloc_buffer_waste; }
  size_t undo_waste() const                      { return _undo_waste; }

#ifdef ASSERT
  bool verify_ref(narrowOop* ref) const;
  bool verify_ref(oop* ref) const;
  bool verify_task(StarTask ref) const;
#endif // ASSERT

  template <class T> void push_on_queue(T* ref) {
    assert(verify_ref(ref), "sanity");
    refs()->push(ref);
  }

  template <class T> void update_rs(HeapRegion* from, T* p, int tid) {
    if (G1DeferredRSUpdate) {
      deferred_rs_update(from, p, tid);
    } else {
      immediate_rs_update(from, p, tid);
    }
  }

  HeapWord* allocate_slow(GCAllocPurpose purpose, size_t word_sz) {

    HeapWord* obj = NULL;
    size_t gclab_word_size = _g1h->desired_plab_sz(purpose);
    if (word_sz * 100 < gclab_word_size * ParallelGCBufferWastePct) {
      G1ParGCAllocBuffer* alloc_buf = alloc_buffer(purpose);
      assert(gclab_word_size == alloc_buf->word_sz(),
             "dynamic resizing is not supported");
      add_to_alloc_buffer_waste(alloc_buf->words_remaining());
      alloc_buf->retire(false, false);

      HeapWord* buf = _g1h->par_allocate_during_gc(purpose, gclab_word_size);
      if (buf == NULL) return NULL; // Let caller handle allocation failure.
      // Otherwise.
      alloc_buf->set_buf(buf);

      obj = alloc_buf->allocate(word_sz);
      assert(obj != NULL, "buffer was definitely big enough...");
    } else {
      obj = _g1h->par_allocate_during_gc(purpose, word_sz);
    }
    return obj;
  }

  HeapWord* allocate(GCAllocPurpose purpose, size_t word_sz) {
    HeapWord* obj = alloc_buffer(purpose)->allocate(word_sz);
    if (obj != NULL) return obj;
    return allocate_slow(purpose, word_sz);
  }

  void undo_allocation(GCAllocPurpose purpose, HeapWord* obj, size_t word_sz) {
    if (alloc_buffer(purpose)->contains(obj)) {
      assert(alloc_buffer(purpose)->contains(obj + word_sz - 1),
             "should contain whole object");
      alloc_buffer(purpose)->undo_allocation(obj, word_sz);
    } else {
      CollectedHeap::fill_with_object(obj, word_sz);
      add_to_undo_waste(word_sz);
    }
  }

  void set_evac_failure_closure(OopsInHeapRegionClosure* evac_failure_cl) {
    _evac_failure_cl = evac_failure_cl;
  }
  OopsInHeapRegionClosure* evac_failure_closure() {
    return _evac_failure_cl;
  }

  void set_evac_closure(G1ParScanHeapEvacClosure* evac_cl) {
    _evac_cl = evac_cl;
  }

  void set_partial_scan_closure(G1ParScanPartialArrayClosure* partial_scan_cl) {
    _partial_scan_cl = partial_scan_cl;
  }

  int* hash_seed() { return &_hash_seed; }
  int  queue_num() { return _queue_num; }

  size_t term_attempts() const  { return _term_attempts; }
  void note_term_attempt() { _term_attempts++; }

  void start_strong_roots() {
    _start_strong_roots = os::elapsedTime();
  }
  void end_strong_roots() {
    _strong_roots_time += (os::elapsedTime() - _start_strong_roots);
  }
  double strong_roots_time() const { return _strong_roots_time; }

  void start_term_time() {
    note_term_attempt();
    _start_term = os::elapsedTime();
  }
  void end_term_time() {
    _term_time += (os::elapsedTime() - _start_term);
  }
  double term_time() const { return _term_time; }

  double elapsed_time() const {
    return os::elapsedTime() - _start;
  }

  static void
    print_termination_stats_hdr(outputStream* const st = gclog_or_tty);
  void
    print_termination_stats(int i, outputStream* const st = gclog_or_tty) const;

  size_t* surviving_young_words() {
    // We add on to hide entry 0 which accumulates surviving words for
    // age -1 regions (i.e. non-young ones)
    return _surviving_young_words;
  }

  void retire_alloc_buffers() {
    for (int ap = 0; ap < GCAllocPurposeCount; ++ap) {
      size_t waste = _alloc_buffers[ap]->words_remaining();
      add_to_alloc_buffer_waste(waste);
      _alloc_buffers[ap]->retire(true, false);
    }
  }

  template <class T> void deal_with_reference(T* ref_to_scan) {
    if (has_partial_array_mask(ref_to_scan)) {
      _partial_scan_cl->do_oop_nv(ref_to_scan);
    } else {
      // Note: we can use "raw" versions of "region_containing" because
      // "obj_to_scan" is definitely in the heap, and is not in a
      // humongous region.
      HeapRegion* r = _g1h->heap_region_containing_raw(ref_to_scan);
      _evac_cl->set_region(r);
      _evac_cl->do_oop_nv(ref_to_scan);
    }
  }

  void deal_with_reference(StarTask ref) {
    assert(verify_task(ref), "sanity");
    if (ref.is_narrow()) {
      deal_with_reference((narrowOop*)ref);
    } else {
      deal_with_reference((oop*)ref);
    }
  }

public:
  void trim_queue();
};

#endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1COLLECTEDHEAP_HPP