view src/share/vm/gc_implementation/g1/g1BlockOffsetTable.cpp @ 2034:7e37af9d69ef

7011379: G1: overly long concurrent marking cycles Summary: This changeset introduces filtering of SATB buffers at the point when they are about to be enqueued. If this filtering clears enough entries on each buffer, the buffer can then be re-used and not enqueued. This cuts down the number of SATB buffers that need to be processed by the concurrent marking threads. Reviewed-by: johnc, ysr
author tonyp
date Wed, 19 Jan 2011 09:35:17 -0500
parents f95d63e2154a
children d2a62e0f25eb
line wrap: on
line source
/*
 * Copyright (c) 2001, 2011, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

#include "precompiled.hpp"
#include "gc_implementation/g1/g1BlockOffsetTable.inline.hpp"
#include "memory/space.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/java.hpp"

//////////////////////////////////////////////////////////////////////
// G1BlockOffsetSharedArray
//////////////////////////////////////////////////////////////////////

G1BlockOffsetSharedArray::G1BlockOffsetSharedArray(MemRegion reserved,
                                                   size_t init_word_size) :
  _reserved(reserved), _end(NULL)
{
  size_t size = compute_size(reserved.word_size());
  ReservedSpace rs(ReservedSpace::allocation_align_size_up(size));
  if (!rs.is_reserved()) {
    vm_exit_during_initialization("Could not reserve enough space for heap offset array");
  }
  if (!_vs.initialize(rs, 0)) {
    vm_exit_during_initialization("Could not reserve enough space for heap offset array");
  }
  _offset_array = (u_char*)_vs.low_boundary();
  resize(init_word_size);
  if (TraceBlockOffsetTable) {
    gclog_or_tty->print_cr("G1BlockOffsetSharedArray::G1BlockOffsetSharedArray: ");
    gclog_or_tty->print_cr("  "
                  "  rs.base(): " INTPTR_FORMAT
                  "  rs.size(): " INTPTR_FORMAT
                  "  rs end(): " INTPTR_FORMAT,
                  rs.base(), rs.size(), rs.base() + rs.size());
    gclog_or_tty->print_cr("  "
                  "  _vs.low_boundary(): " INTPTR_FORMAT
                  "  _vs.high_boundary(): " INTPTR_FORMAT,
                  _vs.low_boundary(),
                  _vs.high_boundary());
  }
}

void G1BlockOffsetSharedArray::resize(size_t new_word_size) {
  assert(new_word_size <= _reserved.word_size(), "Resize larger than reserved");
  size_t new_size = compute_size(new_word_size);
  size_t old_size = _vs.committed_size();
  size_t delta;
  char* high = _vs.high();
  _end = _reserved.start() + new_word_size;
  if (new_size > old_size) {
    delta = ReservedSpace::page_align_size_up(new_size - old_size);
    assert(delta > 0, "just checking");
    if (!_vs.expand_by(delta)) {
      // Do better than this for Merlin
      vm_exit_out_of_memory(delta, "offset table expansion");
    }
    assert(_vs.high() == high + delta, "invalid expansion");
    // Initialization of the contents is left to the
    // G1BlockOffsetArray that uses it.
  } else {
    delta = ReservedSpace::page_align_size_down(old_size - new_size);
    if (delta == 0) return;
    _vs.shrink_by(delta);
    assert(_vs.high() == high - delta, "invalid expansion");
  }
}

bool G1BlockOffsetSharedArray::is_card_boundary(HeapWord* p) const {
  assert(p >= _reserved.start(), "just checking");
  size_t delta = pointer_delta(p, _reserved.start());
  return (delta & right_n_bits(LogN_words)) == (size_t)NoBits;
}


//////////////////////////////////////////////////////////////////////
// G1BlockOffsetArray
//////////////////////////////////////////////////////////////////////

G1BlockOffsetArray::G1BlockOffsetArray(G1BlockOffsetSharedArray* array,
                                       MemRegion mr, bool init_to_zero) :
  G1BlockOffsetTable(mr.start(), mr.end()),
  _unallocated_block(_bottom),
  _array(array), _csp(NULL),
  _init_to_zero(init_to_zero) {
  assert(_bottom <= _end, "arguments out of order");
  if (!_init_to_zero) {
    // initialize cards to point back to mr.start()
    set_remainder_to_point_to_start(mr.start() + N_words, mr.end());
    _array->set_offset_array(0, 0);  // set first card to 0
  }
}

void G1BlockOffsetArray::set_space(Space* sp) {
  _sp = sp;
  _csp = sp->toContiguousSpace();
}

// The arguments follow the normal convention of denoting
// a right-open interval: [start, end)
void
G1BlockOffsetArray:: set_remainder_to_point_to_start(HeapWord* start, HeapWord* end) {

  if (start >= end) {
    // The start address is equal to the end address (or to
    // the right of the end address) so there are not cards
    // that need to be updated..
    return;
  }

  // Write the backskip value for each region.
  //
  //    offset
  //    card             2nd                       3rd
  //     | +- 1st        |                         |
  //     v v             v                         v
  //    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+-+-+-
  //    |x|0|0|0|0|0|0|0|1|1|1|1|1|1| ... |1|1|1|1|2|2|2|2|2|2| ...
  //    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+-+-+-
  //    11              19                        75
  //      12
  //
  //    offset card is the card that points to the start of an object
  //      x - offset value of offset card
  //    1st - start of first logarithmic region
  //      0 corresponds to logarithmic value N_words + 0 and 2**(3 * 0) = 1
  //    2nd - start of second logarithmic region
  //      1 corresponds to logarithmic value N_words + 1 and 2**(3 * 1) = 8
  //    3rd - start of third logarithmic region
  //      2 corresponds to logarithmic value N_words + 2 and 2**(3 * 2) = 64
  //
  //    integer below the block offset entry is an example of
  //    the index of the entry
  //
  //    Given an address,
  //      Find the index for the address
  //      Find the block offset table entry
  //      Convert the entry to a back slide
  //        (e.g., with today's, offset = 0x81 =>
  //          back slip = 2**(3*(0x81 - N_words)) = 2**3) = 8
  //      Move back N (e.g., 8) entries and repeat with the
  //        value of the new entry
  //
  size_t start_card = _array->index_for(start);
  size_t end_card = _array->index_for(end-1);
  assert(start ==_array->address_for_index(start_card), "Precondition");
  assert(end ==_array->address_for_index(end_card)+N_words, "Precondition");
  set_remainder_to_point_to_start_incl(start_card, end_card); // closed interval
}

// Unlike the normal convention in this code, the argument here denotes
// a closed, inclusive interval: [start_card, end_card], cf set_remainder_to_point_to_start()
// above.
void
G1BlockOffsetArray::set_remainder_to_point_to_start_incl(size_t start_card, size_t end_card) {
  if (start_card > end_card) {
    return;
  }
  assert(start_card > _array->index_for(_bottom), "Cannot be first card");
  assert(_array->offset_array(start_card-1) <= N_words,
         "Offset card has an unexpected value");
  size_t start_card_for_region = start_card;
  u_char offset = max_jubyte;
  for (int i = 0; i < BlockOffsetArray::N_powers; i++) {
    // -1 so that the the card with the actual offset is counted.  Another -1
    // so that the reach ends in this region and not at the start
    // of the next.
    size_t reach = start_card - 1 + (BlockOffsetArray::power_to_cards_back(i+1) - 1);
    offset = N_words + i;
    if (reach >= end_card) {
      _array->set_offset_array(start_card_for_region, end_card, offset);
      start_card_for_region = reach + 1;
      break;
    }
    _array->set_offset_array(start_card_for_region, reach, offset);
    start_card_for_region = reach + 1;
  }
  assert(start_card_for_region > end_card, "Sanity check");
  DEBUG_ONLY(check_all_cards(start_card, end_card);)
}

// The block [blk_start, blk_end) has been allocated;
// adjust the block offset table to represent this information;
// right-open interval: [blk_start, blk_end)
void
G1BlockOffsetArray::alloc_block(HeapWord* blk_start, HeapWord* blk_end) {
  mark_block(blk_start, blk_end);
  allocated(blk_start, blk_end);
}

// Adjust BOT to show that a previously whole block has been split
// into two.
void G1BlockOffsetArray::split_block(HeapWord* blk, size_t blk_size,
                                     size_t left_blk_size) {
  // Verify that the BOT shows [blk, blk + blk_size) to be one block.
  verify_single_block(blk, blk_size);
  // Update the BOT to indicate that [blk + left_blk_size, blk + blk_size)
  // is one single block.
  mark_block(blk + left_blk_size, blk + blk_size);
}


// Action_mark - update the BOT for the block [blk_start, blk_end).
//               Current typical use is for splitting a block.
// Action_single - update the BOT for an allocation.
// Action_verify - BOT verification.
void G1BlockOffsetArray::do_block_internal(HeapWord* blk_start,
                                           HeapWord* blk_end,
                                           Action action) {
  assert(Universe::heap()->is_in_reserved(blk_start),
         "reference must be into the heap");
  assert(Universe::heap()->is_in_reserved(blk_end-1),
         "limit must be within the heap");
  // This is optimized to make the test fast, assuming we only rarely
  // cross boundaries.
  uintptr_t end_ui = (uintptr_t)(blk_end - 1);
  uintptr_t start_ui = (uintptr_t)blk_start;
  // Calculate the last card boundary preceding end of blk
  intptr_t boundary_before_end = (intptr_t)end_ui;
  clear_bits(boundary_before_end, right_n_bits(LogN));
  if (start_ui <= (uintptr_t)boundary_before_end) {
    // blk starts at or crosses a boundary
    // Calculate index of card on which blk begins
    size_t    start_index = _array->index_for(blk_start);
    // Index of card on which blk ends
    size_t    end_index   = _array->index_for(blk_end - 1);
    // Start address of card on which blk begins
    HeapWord* boundary    = _array->address_for_index(start_index);
    assert(boundary <= blk_start, "blk should start at or after boundary");
    if (blk_start != boundary) {
      // blk starts strictly after boundary
      // adjust card boundary and start_index forward to next card
      boundary += N_words;
      start_index++;
    }
    assert(start_index <= end_index, "monotonicity of index_for()");
    assert(boundary <= (HeapWord*)boundary_before_end, "tautology");
    switch (action) {
      case Action_mark: {
        if (init_to_zero()) {
          _array->set_offset_array(start_index, boundary, blk_start);
          break;
        } // Else fall through to the next case
      }
      case Action_single: {
        _array->set_offset_array(start_index, boundary, blk_start);
        // We have finished marking the "offset card". We need to now
        // mark the subsequent cards that this blk spans.
        if (start_index < end_index) {
          HeapWord* rem_st = _array->address_for_index(start_index) + N_words;
          HeapWord* rem_end = _array->address_for_index(end_index) + N_words;
          set_remainder_to_point_to_start(rem_st, rem_end);
        }
        break;
      }
      case Action_check: {
        _array->check_offset_array(start_index, boundary, blk_start);
        // We have finished checking the "offset card". We need to now
        // check the subsequent cards that this blk spans.
        check_all_cards(start_index + 1, end_index);
        break;
      }
      default:
        ShouldNotReachHere();
    }
  }
}

// The card-interval [start_card, end_card] is a closed interval; this
// is an expensive check -- use with care and only under protection of
// suitable flag.
void G1BlockOffsetArray::check_all_cards(size_t start_card, size_t end_card) const {

  if (end_card < start_card) {
    return;
  }
  guarantee(_array->offset_array(start_card) == N_words, "Wrong value in second card");
  for (size_t c = start_card + 1; c <= end_card; c++ /* yeah! */) {
    u_char entry = _array->offset_array(c);
    if (c - start_card > BlockOffsetArray::power_to_cards_back(1)) {
      guarantee(entry > N_words, "Should be in logarithmic region");
    }
    size_t backskip = BlockOffsetArray::entry_to_cards_back(entry);
    size_t landing_card = c - backskip;
    guarantee(landing_card >= (start_card - 1), "Inv");
    if (landing_card >= start_card) {
      guarantee(_array->offset_array(landing_card) <= entry, "monotonicity");
    } else {
      guarantee(landing_card == start_card - 1, "Tautology");
      guarantee(_array->offset_array(landing_card) <= N_words, "Offset value");
    }
  }
}

// The range [blk_start, blk_end) represents a single contiguous block
// of storage; modify the block offset table to represent this
// information; Right-open interval: [blk_start, blk_end)
// NOTE: this method does _not_ adjust _unallocated_block.
void
G1BlockOffsetArray::single_block(HeapWord* blk_start, HeapWord* blk_end) {
  do_block_internal(blk_start, blk_end, Action_single);
}

// Mark the BOT such that if [blk_start, blk_end) straddles a card
// boundary, the card following the first such boundary is marked
// with the appropriate offset.
// NOTE: this method does _not_ adjust _unallocated_block or
// any cards subsequent to the first one.
void
G1BlockOffsetArray::mark_block(HeapWord* blk_start, HeapWord* blk_end) {
  do_block_internal(blk_start, blk_end, Action_mark);
}

HeapWord* G1BlockOffsetArray::block_start_unsafe(const void* addr) {
  assert(_bottom <= addr && addr < _end,
         "addr must be covered by this Array");
  // Must read this exactly once because it can be modified by parallel
  // allocation.
  HeapWord* ub = _unallocated_block;
  if (BlockOffsetArrayUseUnallocatedBlock && addr >= ub) {
    assert(ub < _end, "tautology (see above)");
    return ub;
  }
  // Otherwise, find the block start using the table.
  HeapWord* q = block_at_or_preceding(addr, false, 0);
  return forward_to_block_containing_addr(q, addr);
}

// This duplicates a little code from the above: unavoidable.
HeapWord*
G1BlockOffsetArray::block_start_unsafe_const(const void* addr) const {
  assert(_bottom <= addr && addr < _end,
         "addr must be covered by this Array");
  // Must read this exactly once because it can be modified by parallel
  // allocation.
  HeapWord* ub = _unallocated_block;
  if (BlockOffsetArrayUseUnallocatedBlock && addr >= ub) {
    assert(ub < _end, "tautology (see above)");
    return ub;
  }
  // Otherwise, find the block start using the table.
  HeapWord* q = block_at_or_preceding(addr, false, 0);
  HeapWord* n = q + _sp->block_size(q);
  return forward_to_block_containing_addr_const(q, n, addr);
}


HeapWord*
G1BlockOffsetArray::forward_to_block_containing_addr_slow(HeapWord* q,
                                                          HeapWord* n,
                                                          const void* addr) {
  // We're not in the normal case.  We need to handle an important subcase
  // here: LAB allocation.  An allocation previously recorded in the
  // offset table was actually a lab allocation, and was divided into
  // several objects subsequently.  Fix this situation as we answer the
  // query, by updating entries as we cross them.

  // If the fist object's end q is at the card boundary. Start refining
  // with the corresponding card (the value of the entry will be basically
  // set to 0). If the object crosses the boundary -- start from the next card.
  size_t next_index = _array->index_for(n) + !_array->is_card_boundary(n);
  HeapWord* next_boundary = _array->address_for_index(next_index);
  if (csp() != NULL) {
    if (addr >= csp()->top()) return csp()->top();
    while (next_boundary < addr) {
      while (n <= next_boundary) {
        q = n;
        oop obj = oop(q);
        if (obj->klass_or_null() == NULL) return q;
        n += obj->size();
      }
      assert(q <= next_boundary && n > next_boundary, "Consequence of loop");
      // [q, n) is the block that crosses the boundary.
      alloc_block_work2(&next_boundary, &next_index, q, n);
    }
  } else {
    while (next_boundary < addr) {
      while (n <= next_boundary) {
        q = n;
        oop obj = oop(q);
        if (obj->klass_or_null() == NULL) return q;
        n += _sp->block_size(q);
      }
      assert(q <= next_boundary && n > next_boundary, "Consequence of loop");
      // [q, n) is the block that crosses the boundary.
      alloc_block_work2(&next_boundary, &next_index, q, n);
    }
  }
  return forward_to_block_containing_addr_const(q, n, addr);
}

HeapWord* G1BlockOffsetArray::block_start_careful(const void* addr) const {
  assert(_array->offset_array(0) == 0, "objects can't cross covered areas");

  assert(_bottom <= addr && addr < _end,
         "addr must be covered by this Array");
  // Must read this exactly once because it can be modified by parallel
  // allocation.
  HeapWord* ub = _unallocated_block;
  if (BlockOffsetArrayUseUnallocatedBlock && addr >= ub) {
    assert(ub < _end, "tautology (see above)");
    return ub;
  }

  // Otherwise, find the block start using the table, but taking
  // care (cf block_start_unsafe() above) not to parse any objects/blocks
  // on the cards themsleves.
  size_t index = _array->index_for(addr);
  assert(_array->address_for_index(index) == addr,
         "arg should be start of card");

  HeapWord* q = (HeapWord*)addr;
  uint offset;
  do {
    offset = _array->offset_array(index--);
    q -= offset;
  } while (offset == N_words);
  assert(q <= addr, "block start should be to left of arg");
  return q;
}

// Note that the committed size of the covered space may have changed,
// so the table size might also wish to change.
void G1BlockOffsetArray::resize(size_t new_word_size) {
  HeapWord* new_end = _bottom + new_word_size;
  if (_end < new_end && !init_to_zero()) {
    // verify that the old and new boundaries are also card boundaries
    assert(_array->is_card_boundary(_end),
           "_end not a card boundary");
    assert(_array->is_card_boundary(new_end),
           "new _end would not be a card boundary");
    // set all the newly added cards
    _array->set_offset_array(_end, new_end, N_words);
  }
  _end = new_end;  // update _end
}

void G1BlockOffsetArray::set_region(MemRegion mr) {
  _bottom = mr.start();
  _end = mr.end();
}

//
//              threshold_
//              |   _index_
//              v   v
//      +-------+-------+-------+-------+-------+
//      | i-1   |   i   | i+1   | i+2   | i+3   |
//      +-------+-------+-------+-------+-------+
//       ( ^    ]
//         block-start
//
void G1BlockOffsetArray::alloc_block_work2(HeapWord** threshold_, size_t* index_,
                                           HeapWord* blk_start, HeapWord* blk_end) {
  // For efficiency, do copy-in/copy-out.
  HeapWord* threshold = *threshold_;
  size_t    index = *index_;

  assert(blk_start != NULL && blk_end > blk_start,
         "phantom block");
  assert(blk_end > threshold, "should be past threshold");
  assert(blk_start <= threshold, "blk_start should be at or before threshold");
  assert(pointer_delta(threshold, blk_start) <= N_words,
         "offset should be <= BlockOffsetSharedArray::N");
  assert(Universe::heap()->is_in_reserved(blk_start),
         "reference must be into the heap");
  assert(Universe::heap()->is_in_reserved(blk_end-1),
         "limit must be within the heap");
  assert(threshold == _array->_reserved.start() + index*N_words,
         "index must agree with threshold");

  DEBUG_ONLY(size_t orig_index = index;)

  // Mark the card that holds the offset into the block.  Note
  // that _next_offset_index and _next_offset_threshold are not
  // updated until the end of this method.
  _array->set_offset_array(index, threshold, blk_start);

  // We need to now mark the subsequent cards that this blk spans.

  // Index of card on which blk ends.
  size_t end_index   = _array->index_for(blk_end - 1);

  // Are there more cards left to be updated?
  if (index + 1 <= end_index) {
    HeapWord* rem_st  = _array->address_for_index(index + 1);
    // Calculate rem_end this way because end_index
    // may be the last valid index in the covered region.
    HeapWord* rem_end = _array->address_for_index(end_index) +  N_words;
    set_remainder_to_point_to_start(rem_st, rem_end);
  }

  index = end_index + 1;
  // Calculate threshold_ this way because end_index
  // may be the last valid index in the covered region.
  threshold = _array->address_for_index(end_index) + N_words;
  assert(threshold >= blk_end, "Incorrect offset threshold");

  // index_ and threshold_ updated here.
  *threshold_ = threshold;
  *index_ = index;

#ifdef ASSERT
  // The offset can be 0 if the block starts on a boundary.  That
  // is checked by an assertion above.
  size_t start_index = _array->index_for(blk_start);
  HeapWord* boundary    = _array->address_for_index(start_index);
  assert((_array->offset_array(orig_index) == 0 &&
          blk_start == boundary) ||
          (_array->offset_array(orig_index) > 0 &&
         _array->offset_array(orig_index) <= N_words),
         "offset array should have been set");
  for (size_t j = orig_index + 1; j <= end_index; j++) {
    assert(_array->offset_array(j) > 0 &&
           _array->offset_array(j) <=
             (u_char) (N_words+BlockOffsetArray::N_powers-1),
           "offset array should have been set");
  }
#endif
}

bool
G1BlockOffsetArray::verify_for_object(HeapWord* obj_start,
                                      size_t word_size) const {
  size_t first_card = _array->index_for(obj_start);
  size_t last_card = _array->index_for(obj_start + word_size - 1);
  if (!_array->is_card_boundary(obj_start)) {
    // If the object is not on a card boundary the BOT entry of the
    // first card should point to another object so we should not
    // check that one.
    first_card += 1;
  }
  for (size_t card = first_card; card <= last_card; card += 1) {
    HeapWord* card_addr = _array->address_for_index(card);
    HeapWord* block_start = block_start_const(card_addr);
    if (block_start != obj_start) {
      gclog_or_tty->print_cr("block start: "PTR_FORMAT" is incorrect - "
                             "card index: "SIZE_FORMAT" "
                             "card addr: "PTR_FORMAT" BOT entry: %u "
                             "obj: "PTR_FORMAT" word size: "SIZE_FORMAT" "
                             "cards: ["SIZE_FORMAT","SIZE_FORMAT"]",
                             block_start, card, card_addr,
                             _array->offset_array(card),
                             obj_start, word_size, first_card, last_card);
      return false;
    }
  }
  return true;
}

#ifndef PRODUCT
void
G1BlockOffsetArray::print_on(outputStream* out) {
  size_t from_index = _array->index_for(_bottom);
  size_t to_index = _array->index_for(_end);
  out->print_cr(">> BOT for area ["PTR_FORMAT","PTR_FORMAT") "
                "cards ["SIZE_FORMAT","SIZE_FORMAT")",
                _bottom, _end, from_index, to_index);
  for (size_t i = from_index; i < to_index; ++i) {
    out->print_cr("  entry "SIZE_FORMAT_W(8)" | "PTR_FORMAT" : %3u",
                  i, _array->address_for_index(i),
                  (uint) _array->offset_array(i));
  }
}
#endif // !PRODUCT

//////////////////////////////////////////////////////////////////////
// G1BlockOffsetArrayContigSpace
//////////////////////////////////////////////////////////////////////

HeapWord*
G1BlockOffsetArrayContigSpace::block_start_unsafe(const void* addr) {
  assert(_bottom <= addr && addr < _end,
         "addr must be covered by this Array");
  HeapWord* q = block_at_or_preceding(addr, true, _next_offset_index-1);
  return forward_to_block_containing_addr(q, addr);
}

HeapWord*
G1BlockOffsetArrayContigSpace::
block_start_unsafe_const(const void* addr) const {
  assert(_bottom <= addr && addr < _end,
         "addr must be covered by this Array");
  HeapWord* q = block_at_or_preceding(addr, true, _next_offset_index-1);
  HeapWord* n = q + _sp->block_size(q);
  return forward_to_block_containing_addr_const(q, n, addr);
}

G1BlockOffsetArrayContigSpace::
G1BlockOffsetArrayContigSpace(G1BlockOffsetSharedArray* array,
                              MemRegion mr) :
  G1BlockOffsetArray(array, mr, true)
{
  _next_offset_threshold = NULL;
  _next_offset_index = 0;
}

HeapWord* G1BlockOffsetArrayContigSpace::initialize_threshold() {
  assert(!Universe::heap()->is_in_reserved(_array->_offset_array),
         "just checking");
  _next_offset_index = _array->index_for(_bottom);
  _next_offset_index++;
  _next_offset_threshold =
    _array->address_for_index(_next_offset_index);
  return _next_offset_threshold;
}

void G1BlockOffsetArrayContigSpace::zero_bottom_entry() {
  assert(!Universe::heap()->is_in_reserved(_array->_offset_array),
         "just checking");
  size_t bottom_index = _array->index_for(_bottom);
  assert(_array->address_for_index(bottom_index) == _bottom,
         "Precondition of call");
  _array->set_offset_array(bottom_index, 0);
}

void
G1BlockOffsetArrayContigSpace::set_for_starts_humongous(HeapWord* new_top) {
  assert(new_top <= _end, "_end should have already been updated");

  // The first BOT entry should have offset 0.
  zero_bottom_entry();
  initialize_threshold();
  alloc_block(_bottom, new_top);
 }

#ifndef PRODUCT
void
G1BlockOffsetArrayContigSpace::print_on(outputStream* out) {
  G1BlockOffsetArray::print_on(out);
  out->print_cr("  next offset threshold: "PTR_FORMAT, _next_offset_threshold);
  out->print_cr("  next offset index:     "SIZE_FORMAT, _next_offset_index);
}
#endif // !PRODUCT