view src/hotspot/share/gc/g1/heapRegion.hpp @ 50441:523c2a73a3dc

8204081: Mismatch in rebuild policy and collection set chooser causes remembered sets to be kept errorneously Summary: Due to mismatch in which region's remembered sets should be rebuilt and the ones that are looked at in the collection set chooser superfluous remembered sets might be built and kept alive until the next marking. Reviewed-by: sjohanss, kbarrett
author tschatzl
date Thu, 07 Jun 2018 11:20:30 +0200
parents e79bbf1635da
children 1906adbef2dc
line wrap: on
line source
 * Copyright (c) 2001, 2018, Oracle and/or its affiliates. All rights reserved.
 * 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 if you need additional information or have any
 * questions.


#include "gc/g1/g1BlockOffsetTable.hpp"
#include "gc/g1/g1HeapRegionTraceType.hpp"
#include "gc/g1/heapRegionTracer.hpp"
#include "gc/g1/heapRegionType.hpp"
#include "gc/g1/survRateGroup.hpp"
#include "gc/shared/ageTable.hpp"
#include "gc/shared/cardTable.hpp"
#include "gc/shared/spaceDecorator.hpp"
#include "utilities/macros.hpp"

// A HeapRegion is the smallest piece of a G1CollectedHeap that
// can be collected independently.

// NOTE: Although a HeapRegion is a Space, its
// Space::initDirtyCardClosure method must not be called.
// The problem is that the existence of this method breaks
// the independence of barrier sets from remembered sets.
// The solution is to remove this method from the definition
// of a Space.

// Each heap region is self contained. top() and end() can never
// be set beyond the end of the region. For humongous objects,
// the first region is a StartsHumongous region. If the humongous
// object is larger than a heap region, the following regions will
// be of type ContinuesHumongous. In this case the top() of the
// StartHumongous region and all ContinuesHumongous regions except
// the last will point to their own end. The last ContinuesHumongous
// region may have top() equal the end of object if there isn't
// room for filler objects to pad out to the end of the region.

class G1CollectedHeap;
class G1CMBitMap;
class G1IsAliveAndApplyClosure;
class HeapRegionRemSet;
class HeapRegionRemSetIterator;
class HeapRegion;
class HeapRegionSetBase;
class nmethod;

#define HR_FORMAT "%u:(%s)[" PTR_FORMAT "," PTR_FORMAT "," PTR_FORMAT "]"
#define HR_FORMAT_PARAMS(_hr_) \
                (_hr_)->hrm_index(), \
                (_hr_)->get_short_type_str(), \
                p2i((_hr_)->bottom()), p2i((_hr_)->top()), p2i((_hr_)->end())

// sentinel value for hrm_index
#define G1_NO_HRM_INDEX ((uint) -1)

// The complicating factor is that BlockOffsetTable diverged
// significantly, and we need functionality that is only in the G1 version.
// So I copied that code, which led to an alternate G1 version of
// OffsetTableContigSpace.  If the two versions of BlockOffsetTable could
// be reconciled, then G1OffsetTableContigSpace could go away.

// The idea behind time stamps is the following. We want to keep track of
// the highest address where it's safe to scan objects for each region.
// This is only relevant for current GC alloc regions so we keep a time stamp
// per region to determine if the region has been allocated during the current
// GC or not. If the time stamp is current we report a scan_top value which
// was saved at the end of the previous GC for retained alloc regions and which is
// equal to the bottom for all other regions.
// There is a race between card scanners and allocating gc workers where we must ensure
// that card scanners do not read the memory allocated by the gc workers.
// In order to enforce that, we must not return a value of _top which is more recent than the
// time stamp. This is due to the fact that a region may become a gc alloc region at
// some point after we've read the timestamp value as being < the current time stamp.
// The time stamps are re-initialized to zero at cleanup and at Full GCs.
// The current scheme that uses sequential unsigned ints will fail only if we have 4b
// evacuation pauses between two cleanups, which is _highly_ unlikely.
class G1ContiguousSpace: public CompactibleSpace {
  friend class VMStructs;
  HeapWord* volatile _top;
  G1BlockOffsetTablePart _bot_part;
  Mutex _par_alloc_lock;
  // When we need to retire an allocation region, while other threads
  // are also concurrently trying to allocate into it, we typically
  // allocate a dummy object at the end of the region to ensure that
  // no more allocations can take place in it. However, sometimes we
  // want to know where the end of the last "real" object we allocated
  // into the region was and this is what this keeps track.
  HeapWord* _pre_dummy_top;

  G1ContiguousSpace(G1BlockOffsetTable* bot);

  void set_top(HeapWord* value) { _top = value; }
  HeapWord* top() const { return _top; }

  // Reset the G1ContiguousSpace.
  virtual void initialize(MemRegion mr, bool clear_space, bool mangle_space);

  HeapWord* volatile* top_addr() { return &_top; }
  // Try to allocate at least min_word_size and up to desired_size from this Space.
  // Returns NULL if not possible, otherwise sets actual_word_size to the amount of
  // space allocated.
  // This version assumes that all allocation requests to this Space are properly
  // synchronized.
  inline HeapWord* allocate_impl(size_t min_word_size, size_t desired_word_size, size_t* actual_word_size);
  // Try to allocate at least min_word_size and up to desired_size from this Space.
  // Returns NULL if not possible, otherwise sets actual_word_size to the amount of
  // space allocated.
  // This version synchronizes with other calls to par_allocate_impl().
  inline HeapWord* par_allocate_impl(size_t min_word_size, size_t desired_word_size, size_t* actual_word_size);

  void reset_after_compaction() { set_top(compaction_top()); }

  size_t used() const { return byte_size(bottom(), top()); }
  size_t free() const { return byte_size(top(), end()); }
  bool is_free_block(const HeapWord* p) const { return p >= top(); }

  MemRegion used_region() const { return MemRegion(bottom(), top()); }

  void object_iterate(ObjectClosure* blk);
  void safe_object_iterate(ObjectClosure* blk);

  void mangle_unused_area() PRODUCT_RETURN;
  void mangle_unused_area_complete() PRODUCT_RETURN;

  // See the comment above in the declaration of _pre_dummy_top for an
  // explanation of what it is.
  void set_pre_dummy_top(HeapWord* pre_dummy_top) {
    assert(is_in(pre_dummy_top) && pre_dummy_top <= top(), "pre-condition");
    _pre_dummy_top = pre_dummy_top;
  HeapWord* pre_dummy_top() {
    return (_pre_dummy_top == NULL) ? top() : _pre_dummy_top;
  void reset_pre_dummy_top() { _pre_dummy_top = NULL; }

  virtual void clear(bool mangle_space);

  HeapWord* block_start(const void* p);
  HeapWord* block_start_const(const void* p) const;

  // Allocation (return NULL if full).  Assumes the caller has established
  // mutually exclusive access to the space.
  HeapWord* allocate(size_t min_word_size, size_t desired_word_size, size_t* actual_word_size);
  // Allocation (return NULL if full).  Enforces mutual exclusion internally.
  HeapWord* par_allocate(size_t min_word_size, size_t desired_word_size, size_t* actual_word_size);

  virtual HeapWord* allocate(size_t word_size);
  virtual HeapWord* par_allocate(size_t word_size);

  HeapWord* saved_mark_word() const { ShouldNotReachHere(); return NULL; }

  // MarkSweep support phase3
  virtual HeapWord* initialize_threshold();
  virtual HeapWord* cross_threshold(HeapWord* start, HeapWord* end);

  virtual void print() const;

  void reset_bot() {

  void print_bot_on(outputStream* out) {

class HeapRegion: public G1ContiguousSpace {
  friend class VMStructs;
  // Allow scan_and_forward to call (private) overrides for auxiliary functions on this class
  template <typename SpaceType>
  friend void CompactibleSpace::scan_and_forward(SpaceType* space, CompactPoint* cp);

  // The remembered set for this region.
  // (Might want to make this "inline" later, to avoid some alloc failure
  // issues.)
  HeapRegionRemSet* _rem_set;

  // Auxiliary functions for scan_and_forward support.
  // See comments for CompactibleSpace for more information.
  inline HeapWord* scan_limit() const {
    return top();

  inline bool scanned_block_is_obj(const HeapWord* addr) const {
    return true; // Always true, since scan_limit is top

  inline size_t scanned_block_size(const HeapWord* addr) const {
    return HeapRegion::block_size(addr); // Avoid virtual call

  void report_region_type_change(G1HeapRegionTraceType::Type to);

  // Returns whether the given object address refers to a dead object, and either the
  // size of the object (if live) or the size of the block (if dead) in size.
  // May
  // - only called with obj < top()
  // - not called on humongous objects or archive regions
  inline bool is_obj_dead_with_size(const oop obj, const G1CMBitMap* const prev_bitmap, size_t* size) const;

  // The index of this region in the heap region sequence.
  uint  _hrm_index;

  HeapRegionType _type;

  // For a humongous region, region in which it starts.
  HeapRegion* _humongous_start_region;

  // True iff an attempt to evacuate an object in the region failed.
  bool _evacuation_failed;

  // Fields used by the HeapRegionSetBase class and subclasses.
  HeapRegion* _next;
  HeapRegion* _prev;
#ifdef ASSERT
  HeapRegionSetBase* _containing_set;
#endif // ASSERT

  // We use concurrent marking to determine the amount of live data
  // in each heap region.
  size_t _prev_marked_bytes;    // Bytes known to be live via last completed marking.
  size_t _next_marked_bytes;    // Bytes known to be live via in-progress marking.

  // The calculated GC efficiency of the region.
  double _gc_efficiency;

  int  _young_index_in_cset;
  SurvRateGroup* _surv_rate_group;
  int  _age_index;

  // The start of the unmarked area. The unmarked area extends from this
  // word until the top and/or end of the region, and is the part
  // of the region for which no marking was done, i.e. objects may
  // have been allocated in this part since the last mark phase.
  // "prev" is the top at the start of the last completed marking.
  // "next" is the top at the start of the in-progress marking (if any.)
  HeapWord* _prev_top_at_mark_start;
  HeapWord* _next_top_at_mark_start;
  // If a collection pause is in progress, this is the top at the start
  // of that pause.

  void init_top_at_mark_start() {
    assert(_prev_marked_bytes == 0 &&
           _next_marked_bytes == 0,
           "Must be called after zero_marked_bytes.");
    HeapWord* bot = bottom();
    _prev_top_at_mark_start = bot;
    _next_top_at_mark_start = bot;

  // Cached attributes used in the collection set policy information

  // The RSet length that was added to the total value
  // for the collection set.
  size_t _recorded_rs_length;

  // The predicted elapsed time that was added to total value
  // for the collection set.
  double _predicted_elapsed_time_ms;

  // Iterate over the references in a humongous objects and apply the given closure
  // to them.
  // Humongous objects are allocated directly in the old-gen. So we need special
  // handling for concurrent processing encountering an in-progress allocation.
  template <class Closure, bool is_gc_active>
  inline bool do_oops_on_card_in_humongous(MemRegion mr,
                                           Closure* cl,
                                           G1CollectedHeap* g1h);

  // Returns the block size of the given (dead, potentially having its class unloaded) object
  // starting at p extending to at most the prev TAMS using the given mark bitmap.
  inline size_t block_size_using_bitmap(const HeapWord* p, const G1CMBitMap* const prev_bitmap) const;
  HeapRegion(uint hrm_index,
             G1BlockOffsetTable* bot,
             MemRegion mr);

  // Initializing the HeapRegion not only resets the data structure, but also
  // resets the BOT for that heap region.
  // The default values for clear_space means that we will do the clearing if
  // there's clearing to be done ourselves. We also always mangle the space.
  virtual void initialize(MemRegion mr, bool clear_space = false, bool mangle_space = SpaceDecorator::Mangle);

  static int    LogOfHRGrainBytes;
  static int    LogOfHRGrainWords;

  static size_t GrainBytes;
  static size_t GrainWords;
  static size_t CardsPerRegion;

  static size_t align_up_to_region_byte_size(size_t sz) {
    return (sz + (size_t) GrainBytes - 1) &
                                      ~((1 << (size_t) LogOfHRGrainBytes) - 1);

  // Returns whether a field is in the same region as the obj it points to.
  template <typename T>
  static bool is_in_same_region(T* p, oop obj) {
    assert(p != NULL, "p can't be NULL");
    assert(obj != NULL, "obj can't be NULL");
    return (((uintptr_t) p ^ cast_from_oop<uintptr_t>(obj)) >> LogOfHRGrainBytes) == 0;

  static size_t max_region_size();
  static size_t min_region_size_in_words();

  // It sets up the heap region size (GrainBytes / GrainWords), as
  // well as other related fields that are based on the heap region
  // size (LogOfHRGrainBytes / LogOfHRGrainWords /
  // CardsPerRegion). All those fields are considered constant
  // throughout the JVM's execution, therefore they should only be set
  // up once during initialization time.
  static void setup_heap_region_size(size_t initial_heap_size, size_t max_heap_size);

  // All allocated blocks are occupied by objects in a HeapRegion
  bool block_is_obj(const HeapWord* p) const;

  // Returns whether the given object is dead based on TAMS and bitmap.
  bool is_obj_dead(const oop obj, const G1CMBitMap* const prev_bitmap) const;

  // Returns the object size for all valid block starts
  // and the amount of unallocated words if called on top()
  size_t block_size(const HeapWord* p) const;

  // Scans through the region using the bitmap to determine what
  // objects to call size_t ApplyToMarkedClosure::apply(oop) for.
  template<typename ApplyToMarkedClosure>
  inline void apply_to_marked_objects(G1CMBitMap* bitmap, ApplyToMarkedClosure* closure);
  // Override for scan_and_forward support.
  void prepare_for_compaction(CompactPoint* cp);
  // Update heap region to be consistent after compaction.
  void complete_compaction();

  inline HeapWord* par_allocate_no_bot_updates(size_t min_word_size, size_t desired_word_size, size_t* word_size);
  inline HeapWord* allocate_no_bot_updates(size_t word_size);
  inline HeapWord* allocate_no_bot_updates(size_t min_word_size, size_t desired_word_size, size_t* actual_size);

  // If this region is a member of a HeapRegionManager, the index in that
  // sequence, otherwise -1.
  uint hrm_index() const { return _hrm_index; }

  // The number of bytes marked live in the region in the last marking phase.
  size_t marked_bytes()    { return _prev_marked_bytes; }
  size_t live_bytes() {
    return (top() - prev_top_at_mark_start()) * HeapWordSize + marked_bytes();

  // The number of bytes counted in the next marking.
  size_t next_marked_bytes() { return _next_marked_bytes; }
  // The number of bytes live wrt the next marking.
  size_t next_live_bytes() {
      (top() - next_top_at_mark_start()) * HeapWordSize + next_marked_bytes();

  // A lower bound on the amount of garbage bytes in the region.
  size_t garbage_bytes() {
    size_t used_at_mark_start_bytes =
      (prev_top_at_mark_start() - bottom()) * HeapWordSize;
    return used_at_mark_start_bytes - marked_bytes();

  // Return the amount of bytes we'll reclaim if we collect this
  // region. This includes not only the known garbage bytes in the
  // region but also any unallocated space in it, i.e., [top, end),
  // since it will also be reclaimed if we collect the region.
  size_t reclaimable_bytes() {
    size_t known_live_bytes = live_bytes();
    assert(known_live_bytes <= capacity(), "sanity");
    return capacity() - known_live_bytes;

  // An upper bound on the number of live bytes in the region.
  size_t max_live_bytes() { return used() - garbage_bytes(); }

  void add_to_marked_bytes(size_t incr_bytes) {
    _next_marked_bytes = _next_marked_bytes + incr_bytes;

  void zero_marked_bytes()      {
    _prev_marked_bytes = _next_marked_bytes = 0;

  const char* get_type_str() const { return _type.get_str(); }
  const char* get_short_type_str() const { return _type.get_short_str(); }
  G1HeapRegionTraceType::Type get_trace_type() { return _type.get_trace_type(); }

  bool is_free() const { return _type.is_free(); }

  bool is_young()    const { return _type.is_young();    }
  bool is_eden()     const { return _type.is_eden();     }
  bool is_survivor() const { return _type.is_survivor(); }

  bool is_humongous() const { return _type.is_humongous(); }
  bool is_starts_humongous() const { return _type.is_starts_humongous(); }
  bool is_continues_humongous() const { return _type.is_continues_humongous();   }

  bool is_old() const { return _type.is_old(); }

  bool is_old_or_humongous() const { return _type.is_old_or_humongous(); }

  // A pinned region contains objects which are not moved by garbage collections.
  // Humongous regions and archive regions are pinned.
  bool is_pinned() const { return _type.is_pinned(); }

  // An archive region is a pinned region, also tagged as old, which
  // should not be marked during mark/sweep. This allows the address
  // space to be shared by JVM instances.
  bool is_archive()        const { return _type.is_archive(); }
  bool is_open_archive()   const { return _type.is_open_archive(); }
  bool is_closed_archive() const { return _type.is_closed_archive(); }

  // For a humongous region, region in which it starts.
  HeapRegion* humongous_start_region() const {
    return _humongous_start_region;

  // Makes the current region be a "starts humongous" region, i.e.,
  // the first region in a series of one or more contiguous regions
  // that will contain a single "humongous" object.
  // obj_top : points to the top of the humongous object.
  // fill_size : size of the filler object at the end of the region series.
  void set_starts_humongous(HeapWord* obj_top, size_t fill_size);

  // Makes the current region be a "continues humongous'
  // region. first_hr is the "start humongous" region of the series
  // which this region will be part of.
  void set_continues_humongous(HeapRegion* first_hr);

  // Unsets the humongous-related fields on the region.
  void clear_humongous();

  // If the region has a remembered set, return a pointer to it.
  HeapRegionRemSet* rem_set() const {
    return _rem_set;

  inline bool in_collection_set() const;

  // Methods used by the HeapRegionSetBase class and subclasses.

  // Getter and setter for the next and prev fields used to link regions into
  // linked lists.
  HeapRegion* next()              { return _next; }
  HeapRegion* prev()              { return _prev; }

  void set_next(HeapRegion* next) { _next = next; }
  void set_prev(HeapRegion* prev) { _prev = prev; }

  // Every region added to a set is tagged with a reference to that
  // set. This is used for doing consistency checking to make sure that
  // the contents of a set are as they should be and it's only
  // available in non-product builds.
#ifdef ASSERT
  void set_containing_set(HeapRegionSetBase* containing_set) {
    assert((containing_set == NULL && _containing_set != NULL) ||
           (containing_set != NULL && _containing_set == NULL),
           "containing_set: " PTR_FORMAT " "
           "_containing_set: " PTR_FORMAT,
           p2i(containing_set), p2i(_containing_set));

    _containing_set = containing_set;

  HeapRegionSetBase* containing_set() { return _containing_set; }
#else // ASSERT
  void set_containing_set(HeapRegionSetBase* containing_set) { }

  // containing_set() is only used in asserts so there's no reason
  // to provide a dummy version of it.
#endif // ASSERT

  // Reset the HeapRegion to default values.
  // If skip_remset is true, do not clear the remembered set.
  // If clear_space is true, clear the HeapRegion's memory.
  // If locked is true, assume we are the only thread doing this operation.
  void hr_clear(bool skip_remset, bool clear_space, bool locked = false);
  // Clear the card table corresponding to this region.
  void clear_cardtable();

  // Get the start of the unmarked area in this region.
  HeapWord* prev_top_at_mark_start() const { return _prev_top_at_mark_start; }
  HeapWord* next_top_at_mark_start() const { return _next_top_at_mark_start; }

  // Note the start or end of marking. This tells the heap region
  // that the collector is about to start or has finished (concurrently)
  // marking the heap.

  // Notify the region that concurrent marking is starting. Initialize
  // all fields related to the next marking info.
  inline void note_start_of_marking();

  // Notify the region that concurrent marking has finished. Copy the
  // (now finalized) next marking info fields into the prev marking
  // info fields.
  inline void note_end_of_marking();

  // Notify the region that it will be used as to-space during a GC
  // and we are about to start copying objects into it.
  inline void note_start_of_copying(bool during_initial_mark);

  // Notify the region that it ceases being to-space during a GC and
  // we will not copy objects into it any more.
  inline void note_end_of_copying(bool during_initial_mark);

  // Notify the region that we are about to start processing
  // self-forwarded objects during evac failure handling.
  void note_self_forwarding_removal_start(bool during_initial_mark,
                                          bool during_conc_mark);

  // Notify the region that we have finished processing self-forwarded
  // objects during evac failure handling.
  void note_self_forwarding_removal_end(size_t marked_bytes);

  void reset_during_compaction() {
           "should only be called for humongous regions");


  void calc_gc_efficiency(void);
  double gc_efficiency() { return _gc_efficiency;}

  int  young_index_in_cset() const { return _young_index_in_cset; }
  void set_young_index_in_cset(int index) {
    assert( (index == -1) || is_young(), "pre-condition" );
    _young_index_in_cset = index;

  int age_in_surv_rate_group() {
    assert( _surv_rate_group != NULL, "pre-condition" );
    assert( _age_index > -1, "pre-condition" );
    return _surv_rate_group->age_in_group(_age_index);

  void record_surv_words_in_group(size_t words_survived) {
    assert( _surv_rate_group != NULL, "pre-condition" );
    assert( _age_index > -1, "pre-condition" );
    int age_in_group = age_in_surv_rate_group();
    _surv_rate_group->record_surviving_words(age_in_group, words_survived);

  int age_in_surv_rate_group_cond() {
    if (_surv_rate_group != NULL)
      return age_in_surv_rate_group();
      return -1;

  SurvRateGroup* surv_rate_group() {
    return _surv_rate_group;

  void install_surv_rate_group(SurvRateGroup* surv_rate_group) {
    assert( surv_rate_group != NULL, "pre-condition" );
    assert( _surv_rate_group == NULL, "pre-condition" );
    assert( is_young(), "pre-condition" );

    _surv_rate_group = surv_rate_group;
    _age_index = surv_rate_group->next_age_index();

  void uninstall_surv_rate_group() {
    if (_surv_rate_group != NULL) {
      assert( _age_index > -1, "pre-condition" );
      assert( is_young(), "pre-condition" );

      _surv_rate_group = NULL;
      _age_index = -1;
    } else {
      assert( _age_index == -1, "pre-condition" );

  void set_free();

  void set_eden();
  void set_eden_pre_gc();
  void set_survivor();

  void move_to_old();
  void set_old();

  void set_open_archive();
  void set_closed_archive();

  // Determine if an object has been allocated since the last
  // mark performed by the collector. This returns true iff the object
  // is within the unmarked area of the region.
  bool obj_allocated_since_prev_marking(oop obj) const {
    return (HeapWord *) obj >= prev_top_at_mark_start();
  bool obj_allocated_since_next_marking(oop obj) const {
    return (HeapWord *) obj >= next_top_at_mark_start();

  // Returns the "evacuation_failed" property of the region.
  bool evacuation_failed() { return _evacuation_failed; }

  // Sets the "evacuation_failed" property of the region.
  void set_evacuation_failed(bool b) {
    _evacuation_failed = b;

    if (b) {
      _next_marked_bytes = 0;

  // Iterate over the objects overlapping part of a card, applying cl
  // to all references in the region.  This is a helper for
  // G1RemSet::refine_card*, and is tightly coupled with them.
  // mr is the memory region covered by the card, trimmed to the
  // allocated space for this region.  Must not be empty.
  // This region must be old or humongous.
  // Returns true if the designated objects were successfully
  // processed, false if an unparsable part of the heap was
  // encountered; that only happens when invoked concurrently with the
  // mutator.
  template <bool is_gc_active, class Closure>
  inline bool oops_on_card_seq_iterate_careful(MemRegion mr, Closure* cl);

  size_t recorded_rs_length() const        { return _recorded_rs_length; }
  double predicted_elapsed_time_ms() const { return _predicted_elapsed_time_ms; }

  void set_recorded_rs_length(size_t rs_length) {
    _recorded_rs_length = rs_length;

  void set_predicted_elapsed_time_ms(double ms) {
    _predicted_elapsed_time_ms = ms;

  // Routines for managing a list of code roots (attached to the
  // this region's RSet) that point into this heap region.
  void add_strong_code_root(nmethod* nm);
  void add_strong_code_root_locked(nmethod* nm);
  void remove_strong_code_root(nmethod* nm);

  // Applies blk->do_code_blob() to each of the entries in
  // the strong code roots list for this region
  void strong_code_roots_do(CodeBlobClosure* blk) const;

  // Verify that the entries on the strong code root list for this
  // region are live and include at least one pointer into this region.
  void verify_strong_code_roots(VerifyOption vo, bool* failures) const;

  void print() const;
  void print_on(outputStream* st) const;

  // vo == UsePrevMarking -> use "prev" marking information,
  // vo == UseNextMarking -> use "next" marking information
  // vo == UseFullMarking -> use "next" marking bitmap but no TAMS
  // 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 == UseFullMarking, which is to verify the marking during a
  // full GC.
  void verify(VerifyOption vo, bool *failures) const;

  // Override; it uses the "prev" marking information
  virtual void verify() const;

  void verify_rem_set(VerifyOption vo, bool *failures) const;
  void verify_rem_set() const;

// HeapRegionClosure is used for iterating over regions.
// Terminates the iteration when the "do_heap_region" method returns "true".
class HeapRegionClosure : public StackObj {
  friend class HeapRegionManager;
  friend class G1CollectionSet;
  friend class CollectionSetChooser;

  bool _is_complete;
  void set_incomplete() { _is_complete = false; }

  HeapRegionClosure(): _is_complete(true) {}

  // Typically called on each region until it returns true.
  virtual bool do_heap_region(HeapRegion* r) = 0;

  // True after iteration if the closure was applied to all heap regions
  // and returned "false" in all cases.
  bool is_complete() { return _is_complete; }