src/hotspot/share/gc/g1/heapRegionManager.cpp

/*
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 * 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
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 *
 * 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.
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#include "precompiled.hpp"
#include "gc/g1/g1Arguments.hpp"
#include "gc/g1/g1CollectedHeap.inline.hpp"
#include "gc/g1/g1ConcurrentRefine.hpp"
#include "gc/g1/g1CommittedRegionMap.inline.hpp"
#include "gc/g1/g1NUMAStats.hpp"
#include "gc/g1/heapRegion.hpp"
#include "gc/g1/heapRegionManager.inline.hpp"
#include "gc/g1/heapRegionSet.inline.hpp"
#include "jfr/jfrEvents.hpp"
#include "logging/logStream.hpp"
#include "memory/allocation.hpp"
#include "runtime/atomic.hpp"
#include "runtime/mutexLocker.hpp"
#include "runtime/orderAccess.hpp"
#include "utilities/bitMap.inline.hpp"

class MasterFreeRegionListChecker : public HeapRegionSetChecker {
public:
  void check_mt_safety() {
    // Master Free List MT safety protocol:
    // (a) If we're at a safepoint, operations on the master free list
    // should be invoked by either the VM thread (which will serialize
    // them) or by the GC workers while holding the
    // FreeList_lock.
    // (b) If we're not at a safepoint, operations on the master free
    // list should be invoked while holding the Heap_lock.

    if (SafepointSynchronize::is_at_safepoint()) {
      guarantee(Thread::current()->is_VM_thread() ||
                FreeList_lock->owned_by_self(), "master free list MT safety protocol at a safepoint");
    } else {
      guarantee(Heap_lock->owned_by_self(), "master free list MT safety protocol outside a safepoint");
    }
  }
  bool is_correct_type(HeapRegion* hr) { return hr->is_free(); }
  const char* get_description() { return "Free Regions"; }
};

HeapRegionManager::HeapRegionManager() :
  _bot_mapper(nullptr),
  _cardtable_mapper(nullptr),
  _committed_map(),
  _allocated_heapregions_length(0),
  _regions(), _heap_mapper(nullptr),
  _bitmap_mapper(nullptr),
  _free_list("Free list", new MasterFreeRegionListChecker())
{ }

void HeapRegionManager::initialize(G1RegionToSpaceMapper* heap_storage,
                                   G1RegionToSpaceMapper* bitmap,
                                   G1RegionToSpaceMapper* bot,
                                   G1RegionToSpaceMapper* cardtable) {
  _allocated_heapregions_length = 0;

  _heap_mapper = heap_storage;

  _bitmap_mapper = bitmap;

  _bot_mapper = bot;
  _cardtable_mapper = cardtable;

  _regions.initialize(heap_storage->reserved(), HeapRegion::GrainBytes);

  _committed_map.initialize(reserved_length());
}

HeapRegion* HeapRegionManager::allocate_free_region(HeapRegionType type, uint requested_node_index) {
  HeapRegion* hr = nullptr;
  bool from_head = !type.is_young();
  G1NUMA* numa = G1NUMA::numa();

  if (requested_node_index != G1NUMA::AnyNodeIndex && numa->is_enabled()) {
    // Try to allocate with requested node index.
    hr = _free_list.remove_region_with_node_index(from_head, requested_node_index);
  }

  if (hr == nullptr) {
    // If there's a single active node or we did not get a region from our requested node,
    // try without requested node index.
    hr = _free_list.remove_region(from_head);
  }

  if (hr != nullptr) {
    assert(hr->next() == nullptr, "Single region should not have next");
    assert(is_available(hr->hrm_index()), "Must be committed");

    if (numa->is_enabled() && hr->node_index() < numa->num_active_nodes()) {
      numa->update_statistics(G1NUMAStats::NewRegionAlloc, requested_node_index, hr->node_index());
    }
  }

  return hr;
}

HeapRegion* HeapRegionManager::allocate_humongous_from_free_list(uint num_regions) {
  uint candidate = find_contiguous_in_free_list(num_regions);
  if (candidate == G1_NO_HRM_INDEX) {
    return nullptr;
  }
  return allocate_free_regions_starting_at(candidate, num_regions);
}

HeapRegion* HeapRegionManager::allocate_humongous_allow_expand(uint num_regions) {
  uint candidate = find_contiguous_allow_expand(num_regions);
  if (candidate == G1_NO_HRM_INDEX) {
    return nullptr;
  }
  expand_exact(candidate, num_regions, G1CollectedHeap::heap()->workers());
  return allocate_free_regions_starting_at(candidate, num_regions);
}

HeapRegion* HeapRegionManager::allocate_humongous(uint num_regions) {
  // Special case a single region to avoid expensive search.
  if (num_regions == 1) {
    return allocate_free_region(HeapRegionType::Humongous, G1NUMA::AnyNodeIndex);
  }
  return allocate_humongous_from_free_list(num_regions);
}

HeapRegion* HeapRegionManager::expand_and_allocate_humongous(uint num_regions) {
  return allocate_humongous_allow_expand(num_regions);
}

#ifdef ASSERT
bool HeapRegionManager::is_free(HeapRegion* hr) const {
  return _free_list.contains(hr);
}
#endif

HeapRegion* HeapRegionManager::new_heap_region(uint hrm_index) {
  G1CollectedHeap* g1h = G1CollectedHeap::heap();
  HeapWord* bottom = g1h->bottom_addr_for_region(hrm_index);
  MemRegion mr(bottom, bottom + HeapRegion::GrainWords);
  assert(reserved().contains(mr), "invariant");
  return g1h->new_heap_region(hrm_index, mr);
}

void HeapRegionManager::expand(uint start, uint num_regions, WorkerThreads* pretouch_workers) {
  commit_regions(start, num_regions, pretouch_workers);
  for (uint i = start; i < start + num_regions; i++) {
    HeapRegion* hr = _regions.get_by_index(i);
    if (hr == nullptr) {
      hr = new_heap_region(i);
      OrderAccess::storestore();
      _regions.set_by_index(i, hr);
      _allocated_heapregions_length = MAX2(_allocated_heapregions_length, i + 1);
    }
    G1CollectedHeap::heap()->hr_printer()->commit(hr);
  }
  activate_regions(start, num_regions);
}

void HeapRegionManager::commit_regions(uint index, size_t num_regions, WorkerThreads* pretouch_workers) {
  guarantee(num_regions > 0, "Must commit more than zero regions");
  guarantee(num_regions <= available(),
            "Cannot commit more than the maximum amount of regions");

  _heap_mapper->commit_regions(index, num_regions, pretouch_workers);

  // Also commit auxiliary data
  _bitmap_mapper->commit_regions(index, num_regions, pretouch_workers);

  _bot_mapper->commit_regions(index, num_regions, pretouch_workers);
  _cardtable_mapper->commit_regions(index, num_regions, pretouch_workers);
}

void HeapRegionManager::uncommit_regions(uint start, uint num_regions) {
  guarantee(num_regions > 0, "No point in calling this for zero regions");

  uint end = start + num_regions;
  G1HRPrinter* printer = G1CollectedHeap::heap()->hr_printer();
  if (printer->is_active()) {
    for (uint i = start; i < end; i++) {
      // Can't use at() here since region is no longer marked available.
      HeapRegion* hr = _regions.get_by_index(i);
      assert(hr != nullptr, "Region should still be present");
      printer->uncommit(hr);
    }
  }

  // Uncommit heap memory
  _heap_mapper->uncommit_regions(start, num_regions);

  // Also uncommit auxiliary data
  _bitmap_mapper->uncommit_regions(start, num_regions);

  _bot_mapper->uncommit_regions(start, num_regions);
  _cardtable_mapper->uncommit_regions(start, num_regions);

  _committed_map.uncommit(start, end);
}

void HeapRegionManager::initialize_regions(uint start, uint num_regions) {
  for (uint i = start; i < start + num_regions; i++) {
    assert(is_available(i), "Just made region %u available but is apparently not.", i);
    HeapRegion* hr = at(i);

    hr->initialize();
    hr->set_node_index(G1NUMA::numa()->index_for_region(hr));
    insert_into_free_list(hr);
    G1CollectedHeap::heap()->hr_printer()->active(hr);
  }
}

void HeapRegionManager::activate_regions(uint start, uint num_regions) {
  _committed_map.activate(start, start + num_regions);
  initialize_regions(start, num_regions);
}

void HeapRegionManager::reactivate_regions(uint start, uint num_regions) {
  assert(num_regions > 0, "No point in calling this for zero regions");

  clear_auxiliary_data_structures(start, num_regions);

  _committed_map.reactivate(start, start + num_regions);
  initialize_regions(start, num_regions);
}

void HeapRegionManager::deactivate_regions(uint start, uint num_regions) {
  assert(num_regions > 0, "Need to specify at least one region to uncommit, tried to uncommit zero regions at %u", start);
  assert(length() >= num_regions, "pre-condition");

  // Reset NUMA index to and print state change.
  uint end = start + num_regions;
  for (uint i = start; i < end; i++) {
    HeapRegion* hr = at(i);
    hr->set_node_index(G1NUMA::UnknownNodeIndex);
    G1CollectedHeap::heap()->hr_printer()->inactive(hr);
  }

  _committed_map.deactivate(start, end);
}

void HeapRegionManager::clear_auxiliary_data_structures(uint start, uint num_regions) {
  // Signal marking bitmaps to clear the given regions.
  _bitmap_mapper->signal_mapping_changed(start, num_regions);
  // Signal G1BlockOffsetTable to clear the given regions.
  _bot_mapper->signal_mapping_changed(start, num_regions);
  // Signal G1CardTable to clear the given regions.
  _cardtable_mapper->signal_mapping_changed(start, num_regions);
}

MemoryUsage HeapRegionManager::get_auxiliary_data_memory_usage() const {
  size_t used_sz =
    _bitmap_mapper->committed_size() +
    _bot_mapper->committed_size() +
    _cardtable_mapper->committed_size();

  size_t committed_sz =
    _bitmap_mapper->reserved_size() +
    _bot_mapper->reserved_size() +
    _cardtable_mapper->reserved_size();

  return MemoryUsage(0, used_sz, committed_sz, committed_sz);
}

bool HeapRegionManager::has_inactive_regions() const {
  return _committed_map.num_inactive() > 0;
}

uint HeapRegionManager::uncommit_inactive_regions(uint limit) {
  assert(limit > 0, "Need to specify at least one region to uncommit");

  uint uncommitted = 0;
  uint offset = 0;
  do {
    MutexLocker uc(Uncommit_lock, Mutex::_no_safepoint_check_flag);
    HeapRegionRange range = _committed_map.next_inactive_range(offset);
    // No more regions available for uncommit. Return the number of regions
    // already uncommitted or 0 if there were no longer any inactive regions.
    if (range.length() == 0) {
      return uncommitted;
    }

    uint start = range.start();
    uint num_regions = MIN2(range.length(), limit - uncommitted);
    uncommitted += num_regions;
    uncommit_regions(start, num_regions);
  } while (uncommitted < limit);

  assert(uncommitted == limit, "Invariant");
  return uncommitted;
}

uint HeapRegionManager::expand_inactive(uint num_regions) {
  uint offset = 0;
  uint expanded = 0;

  do {
    HeapRegionRange regions = _committed_map.next_inactive_range(offset);
    if (regions.length() == 0) {
      // No more unavailable regions.
      break;
    }

    uint to_expand = MIN2(num_regions - expanded, regions.length());
    reactivate_regions(regions.start(), to_expand);
    expanded += to_expand;
    offset = regions.end();
  } while (expanded < num_regions);

  return expanded;
}

uint HeapRegionManager::expand_any(uint num_regions, WorkerThreads* pretouch_workers) {
  assert(num_regions > 0, "Must expand at least 1 region");

  uint offset = 0;
  uint expanded = 0;

  do {
    HeapRegionRange regions = _committed_map.next_committable_range(offset);
    if (regions.length() == 0) {
      // No more unavailable regions.
      break;
    }

    uint to_expand = MIN2(num_regions - expanded, regions.length());
    expand(regions.start(), to_expand, pretouch_workers);
    expanded += to_expand;
    offset = regions.end();
  } while (expanded < num_regions);

  return expanded;
}

uint HeapRegionManager::expand_by(uint num_regions, WorkerThreads* pretouch_workers) {
  assert(num_regions > 0, "Must expand at least 1 region");

  // First "undo" any requests to uncommit memory concurrently by
  // reverting such regions to being available.
  uint expanded = expand_inactive(num_regions);

  // Commit more regions if needed.
  if (expanded < num_regions) {
    expanded += expand_any(num_regions - expanded, pretouch_workers);
  }

  verify_optional();
  return expanded;
}

void HeapRegionManager::expand_exact(uint start, uint num_regions, WorkerThreads* pretouch_workers) {
  assert(num_regions != 0, "Need to request at least one region");
  uint end = start + num_regions;

  for (uint i = start; i < end; i++) {
    // First check inactive. If the regions is inactive, try to reactivate it
    // before it get uncommitted by the G1SeriveThread.
    if (_committed_map.inactive(i)) {
      // Need to grab the lock since this can be called by a java thread
      // doing humongous allocations.
      MutexLocker uc(Uncommit_lock, Mutex::_no_safepoint_check_flag);
      // State might change while getting the lock.
      if (_committed_map.inactive(i)) {
        reactivate_regions(i, 1);
      }
    }
    // Not else-if to catch the case where the inactive region was uncommitted
    // while waiting to get the lock.
    if (!_committed_map.active(i)) {
      expand(i, 1, pretouch_workers);
    }

    assert(at(i)->is_free(), "Region must be free at this point");
  }

  verify_optional();
}

uint HeapRegionManager::expand_on_preferred_node(uint preferred_index) {
  uint expand_candidate = UINT_MAX;

  if (available() >= 1) {
    for (uint i = 0; i < reserved_length(); i++) {
      if (is_available(i)) {
        // Already in use continue
        continue;
      }
      // Always save the candidate so we can expand later on.
      expand_candidate = i;
      if (is_on_preferred_index(expand_candidate, preferred_index)) {
        // We have found a candidate on the preferred node, break.
        break;
      }
    }
  }

  if (expand_candidate == UINT_MAX) {
     // No regions left, expand failed.
    return 0;
  }

  expand_exact(expand_candidate, 1, nullptr);
  return 1;
}

bool HeapRegionManager::is_on_preferred_index(uint region_index, uint preferred_node_index) {
  uint region_node_index = G1NUMA::numa()->preferred_node_index_for_index(region_index);
  return region_node_index == preferred_node_index;
}

#ifdef ASSERT
void HeapRegionManager::assert_contiguous_range(uint start, uint num_regions) {
  // General sanity check, regions found should either be available and empty
  // or not available so that we can make them available and use them.
  for (uint i = start; i < (start + num_regions); i++) {
    HeapRegion* hr = _regions.get_by_index(i);
    assert(!is_available(i) || hr->is_free(),
           "Found region sequence starting at " UINT32_FORMAT ", length " UINT32_FORMAT
           " that is not free at " UINT32_FORMAT ". Hr is " PTR_FORMAT ", type is %s",
           start, num_regions, i, p2i(hr), hr->get_type_str());
  }
}
#endif

uint HeapRegionManager::find_contiguous_in_range(uint start, uint end, uint num_regions) {
  assert(start <= end, "precondition");
  assert(num_regions >= 1, "precondition");
  uint candidate = start;       // First region in candidate sequence.
  uint unchecked = candidate;   // First unchecked region in candidate.
  // While the candidate sequence fits in the range...
  while (num_regions <= (end - candidate)) {
    // Walk backward over the regions for the current candidate.
    for (uint i = candidate + num_regions - 1; true; --i) {
      if (is_available(i) && !at(i)->is_free()) {
        // Region i can't be used, so restart with i+1 as the start
        // of a new candidate sequence, and with the region after the
        // old candidate sequence being the first unchecked region.
        unchecked = candidate + num_regions;
        candidate = i + 1;
        break;
      } else if (i == unchecked) {
        // All regions of candidate sequence have passed check.
        assert_contiguous_range(candidate, num_regions);
        return candidate;
      }
    }
  }
  return G1_NO_HRM_INDEX;
}

uint HeapRegionManager::find_contiguous_in_free_list(uint num_regions) {
  uint candidate = G1_NO_HRM_INDEX;
  HeapRegionRange range(0,0);

  do {
    range = _committed_map.next_active_range(range.end());
    candidate = find_contiguous_in_range(range.start(), range.end(), num_regions);
  } while (candidate == G1_NO_HRM_INDEX && range.end() < reserved_length());

  return candidate;
}

uint HeapRegionManager::find_contiguous_allow_expand(uint num_regions) {
  // Check if we can actually satisfy the allocation.
  if (num_regions > available()) {
    return G1_NO_HRM_INDEX;
  }
  // Find any candidate.
  return find_contiguous_in_range(0, reserved_length(), num_regions);
}

HeapRegion* HeapRegionManager::next_region_in_heap(const HeapRegion* r) const {
  guarantee(r != nullptr, "Start region must be a valid region");
  guarantee(is_available(r->hrm_index()), "Trying to iterate starting from region %u which is not in the heap", r->hrm_index());
  for (uint i = r->hrm_index() + 1; i < _allocated_heapregions_length; i++) {
    HeapRegion* hr = _regions.get_by_index(i);
    if (is_available(i)) {
      return hr;
    }
  }
  return nullptr;
}

void HeapRegionManager::iterate(HeapRegionClosure* blk) const {
  uint len = reserved_length();

  for (uint i = 0; i < len; i++) {
    if (!is_available(i)) {
      continue;
    }
    guarantee(at(i) != nullptr, "Tried to access region %u that has a null HeapRegion*", i);
    bool res = blk->do_heap_region(at(i));
    if (res) {
      blk->set_incomplete();
      return;
    }
  }
}

void HeapRegionManager::iterate(HeapRegionIndexClosure* blk) const {
  uint len = reserved_length();

  for (uint i = 0; i < len; i++) {
    if (!is_available(i)) {
      continue;
    }
    bool res = blk->do_heap_region_index(i);
    if (res) {
      blk->set_incomplete();
      return;
    }
  }
}

uint HeapRegionManager::find_highest_free(bool* expanded) {
  // Loop downwards from the highest region index, looking for an
  // entry which is either free or not yet committed.  If not yet
  // committed, expand at that index.
  for (uint curr = reserved_length(); curr-- > 0;) {
    HeapRegion *hr = _regions.get_by_index(curr);
    if (hr == nullptr || !is_available(curr)) {
      // Found uncommitted and free region, expand to make it available for use.
      expand_exact(curr, 1, nullptr);
      assert(at(curr)->is_free(), "Region (%u) must be available and free after expand", curr);

      *expanded = true;
      return curr;
    }
    if (hr->is_free()) {
      *expanded = false;
      return curr;
    }
  }
  return G1_NO_HRM_INDEX;
}

bool HeapRegionManager::allocate_containing_regions(MemRegion range, size_t* commit_count, WorkerThreads* pretouch_workers) {
  size_t commits = 0;
  uint start_index = (uint)_regions.get_index_by_address(range.start());
  uint last_index = (uint)_regions.get_index_by_address(range.last());

  // Ensure that each G1 region in the range is free, returning false if not.
  // Commit those that are not yet available, and keep count.
  for (uint curr_index = start_index; curr_index <= last_index; curr_index++) {
    if (!is_available(curr_index)) {
      commits++;
      expand_exact(curr_index, 1, pretouch_workers);
    }
    HeapRegion* curr_region  = _regions.get_by_index(curr_index);
    if (!curr_region->is_free()) {
      return false;
    }
  }

  allocate_free_regions_starting_at(start_index, (last_index - start_index) + 1);
  *commit_count = commits;
  return true;
}

void HeapRegionManager::par_iterate(HeapRegionClosure* blk, HeapRegionClaimer* hrclaimer, const uint start_index) const {
  // Every worker will actually look at all regions, skipping over regions that
  // are currently not committed.
  // This also (potentially) iterates over regions newly allocated during GC. This
  // is no problem except for some extra work.
  const uint n_regions = hrclaimer->n_regions();
  for (uint count = 0; count < n_regions; count++) {
    const uint index = (start_index + count) % n_regions;
    assert(index < n_regions, "sanity");
    // Skip over unavailable regions
    if (!is_available(index)) {
      continue;
    }
    HeapRegion* r = _regions.get_by_index(index);
    // We'll ignore regions already claimed.
    if (hrclaimer->is_region_claimed(index)) {
      continue;
    }
    // OK, try to claim it
    if (!hrclaimer->claim_region(index)) {
      continue;
    }
    bool res = blk->do_heap_region(r);
    if (res) {
      return;
    }
  }
}

uint HeapRegionManager::shrink_by(uint num_regions_to_remove) {
  assert(length() > 0, "the region sequence should not be empty");
  assert(length() <= _allocated_heapregions_length, "invariant");
  assert(_allocated_heapregions_length > 0, "we should have at least one region committed");
  assert(num_regions_to_remove < length(), "We should never remove all regions");

  if (num_regions_to_remove == 0) {
    return 0;
  }

  uint removed = 0;
  uint cur = _allocated_heapregions_length - 1;
  uint idx_last_found = 0;
  uint num_last_found = 0;

  while ((removed < num_regions_to_remove) &&
      (num_last_found = find_empty_from_idx_reverse(cur, &idx_last_found)) > 0) {
    uint to_remove = MIN2(num_regions_to_remove - removed, num_last_found);

    shrink_at(idx_last_found + num_last_found - to_remove, to_remove);

    cur = idx_last_found;
    removed += to_remove;
  }

  verify_optional();

  return removed;
}

void HeapRegionManager::shrink_at(uint index, size_t num_regions) {
#ifdef ASSERT
  for (uint i = index; i < (index + num_regions); i++) {
    assert(is_available(i), "Expected available region at index %u", i);
    assert(at(i)->is_empty(), "Expected empty region at index %u", i);
    assert(at(i)->is_free(), "Expected free region at index %u", i);
  }
#endif
  // Mark regions as inactive making them ready for uncommit.
  deactivate_regions(index, (uint) num_regions);
}

uint HeapRegionManager::find_empty_from_idx_reverse(uint start_idx, uint* res_idx) const {
  guarantee(start_idx < _allocated_heapregions_length, "checking");
  guarantee(res_idx != nullptr, "checking");

  uint num_regions_found = 0;

  jlong cur = start_idx;
  while (cur != -1 && !(is_available(cur) && at(cur)->is_empty())) {
    cur--;
  }
  if (cur == -1) {
    return num_regions_found;
  }
  jlong old_cur = cur;
  // cur indexes the first empty region
  while (cur != -1 && is_available(cur) && at(cur)->is_empty()) {
    cur--;
  }
  *res_idx = cur + 1;
  num_regions_found = old_cur - cur;

#ifdef ASSERT
  for (uint i = *res_idx; i < (*res_idx + num_regions_found); i++) {
    assert(at(i)->is_empty(), "just checking");
  }
#endif
  return num_regions_found;
}

void HeapRegionManager::verify() {
  guarantee(length() <= _allocated_heapregions_length,
            "invariant: _length: %u _allocated_length: %u",
            length(), _allocated_heapregions_length);
  guarantee(_allocated_heapregions_length <= reserved_length(),
            "invariant: _allocated_length: %u _max_length: %u",
            _allocated_heapregions_length, reserved_length());
  guarantee(length() <= max_length(),
            "invariant: committed regions: %u max_regions: %u",
            length(), max_length());

  bool prev_committed = true;
  uint num_committed = 0;
  HeapWord* prev_end = heap_bottom();
  for (uint i = 0; i < _allocated_heapregions_length; i++) {
    if (!is_available(i)) {
      prev_committed = false;
      continue;
    }
    num_committed++;
    HeapRegion* hr = _regions.get_by_index(i);
    guarantee(hr != nullptr, "invariant: i: %u", i);
    guarantee(!prev_committed || hr->bottom() == prev_end,
              "invariant i: %u " HR_FORMAT " prev_end: " PTR_FORMAT,
              i, HR_FORMAT_PARAMS(hr), p2i(prev_end));
    guarantee(hr->hrm_index() == i,
              "invariant: i: %u hrm_index(): %u", i, hr->hrm_index());
    // Asserts will fire if i is >= _length
    HeapWord* addr = hr->bottom();
    guarantee(addr_to_region(addr) == hr, "sanity");
    // We cannot check whether the region is part of a particular set: at the time
    // this method may be called, we have only completed allocation of the regions,
    // but not put into a region set.
    prev_committed = true;
    prev_end = hr->end();
  }
  for (uint i = _allocated_heapregions_length; i < reserved_length(); i++) {
    guarantee(_regions.get_by_index(i) == nullptr, "invariant i: %u", i);
  }

  guarantee(num_committed == length(), "Found %u committed regions, but should be %u", num_committed, length());
  _free_list.verify();
}

#ifndef PRODUCT
void HeapRegionManager::verify_optional() {
  verify();
}
#endif // PRODUCT

HeapRegionClaimer::HeapRegionClaimer(uint n_workers) :
    _n_workers(n_workers), _n_regions(G1CollectedHeap::heap()->_hrm._allocated_heapregions_length), _claims(nullptr) {
  uint* new_claims = NEW_C_HEAP_ARRAY(uint, _n_regions, mtGC);
  memset(new_claims, Unclaimed, sizeof(*_claims) * _n_regions);
  _claims = new_claims;
}

HeapRegionClaimer::~HeapRegionClaimer() {
  FREE_C_HEAP_ARRAY(uint, _claims);
}

uint HeapRegionClaimer::offset_for_worker(uint worker_id) const {
  assert(_n_workers > 0, "must be set");
  assert(worker_id < _n_workers, "Invalid worker_id.");
  return _n_regions * worker_id / _n_workers;
}

bool HeapRegionClaimer::is_region_claimed(uint region_index) const {
  assert(region_index < _n_regions, "Invalid index.");
  return _claims[region_index] == Claimed;
}

bool HeapRegionClaimer::claim_region(uint region_index) {
  assert(region_index < _n_regions, "Invalid index.");
  uint old_val = Atomic::cmpxchg(&_claims[region_index], Unclaimed, Claimed);
  return old_val == Unclaimed;
}

class G1RebuildFreeListTask : public WorkerTask {
  HeapRegionManager* _hrm;
  FreeRegionList*    _worker_freelists;
  uint               _worker_chunk_size;
  uint               _num_workers;

public:
  G1RebuildFreeListTask(HeapRegionManager* hrm, uint num_workers) :
      WorkerTask("G1 Rebuild Free List Task"),
      _hrm(hrm),
      _worker_freelists(NEW_C_HEAP_ARRAY(FreeRegionList, num_workers, mtGC)),
      _worker_chunk_size((_hrm->reserved_length() + num_workers - 1) / num_workers),
      _num_workers(num_workers) {
    for (uint worker = 0; worker < _num_workers; worker++) {
      ::new (&_worker_freelists[worker]) FreeRegionList("Appendable Worker Free List");
    }
  }

  ~G1RebuildFreeListTask() {
    for (uint worker = 0; worker < _num_workers; worker++) {
      _worker_freelists[worker].~FreeRegionList();
    }
    FREE_C_HEAP_ARRAY(FreeRegionList, _worker_freelists);
  }

  FreeRegionList* worker_freelist(uint worker) {
    return &_worker_freelists[worker];
  }

  // Each worker creates a free list for a chunk of the heap. The chunks won't
  // be overlapping so we don't need to do any claiming.
  void work(uint worker_id) {
    Ticks start_time = Ticks::now();
    EventGCPhaseParallel event;

    uint start = worker_id * _worker_chunk_size;
    uint end = MIN2(start + _worker_chunk_size, _hrm->reserved_length());

    // If start is outside the heap, this worker has nothing to do.
    if (start > end) {
      return;
    }

    FreeRegionList *free_list = worker_freelist(worker_id);
    for (uint i = start; i < end; i++) {
      HeapRegion *region = _hrm->at_or_null(i);
      if (region != nullptr && region->is_free()) {
        // Need to clear old links to allow to be added to new freelist.
        region->unlink_from_list();
        free_list->add_to_tail(region);
      }
    }

    event.commit(GCId::current(), worker_id, G1GCPhaseTimes::phase_name(G1GCPhaseTimes::RebuildFreeList));
    G1CollectedHeap::heap()->phase_times()->record_time_secs(G1GCPhaseTimes::RebuildFreeList, worker_id, (Ticks::now() - start_time).seconds());
  }
};

void HeapRegionManager::rebuild_free_list(WorkerThreads* workers) {
  // Abandon current free list to allow a rebuild.
  _free_list.abandon();

  uint const num_workers = clamp(max_length(), 1u, workers->active_workers());
  G1RebuildFreeListTask task(this, num_workers);

  log_debug(gc, ergo)("Running %s using %u workers for rebuilding free list of regions",
                      task.name(), num_workers);
  workers->run_task(&task, num_workers);

  // Link the partial free lists together.
  Ticks serial_time = Ticks::now();
  for (uint worker = 0; worker < num_workers; worker++) {
    _free_list.append_ordered(task.worker_freelist(worker));
  }
  G1CollectedHeap::heap()->phase_times()->record_serial_rebuild_freelist_time_ms((Ticks::now() - serial_time).seconds() * 1000.0);
}