Mercurial > jdk8-mips64-public > hotspot / file revision

 /*

  * Copyright 2001-2007 Sun Microsystems, Inc.  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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

  * CA 95054 USA or visit www.sun.com if you need additional information or

  * have any questions.

  *

  */

 #include "incls/_precompiled.incl"

 #include "incls/_g1BlockOffsetTable.cpp.incl"

 //////////////////////////////////////////////////////////////////////

 // 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 - udpate 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);

 }

 void G1BlockOffsetArray::join_blocks(HeapWord* blk1, HeapWord* blk2) {

   HeapWord* blk1_start = Universe::heap()->block_start(blk1);

   HeapWord* blk2_start = Universe::heap()->block_start(blk2);

   assert(blk1 == blk1_start && blk2 == blk2_start,

          "Must be block starts.");

   assert(blk1 + _sp->block_size(blk1) == blk2, "Must be contiguous.");

   size_t blk1_start_index = _array->index_for(blk1);

   size_t blk2_start_index = _array->index_for(blk2);

   assert(blk1_start_index <= blk2_start_index, "sanity");

   HeapWord* blk2_card_start = _array->address_for_index(blk2_start_index);

   if (blk2 == blk2_card_start) {

     // blk2 starts a card.  Does blk1 start on the prevous card, or futher

     // back?

     assert(blk1_start_index < blk2_start_index, "must be lower card.");

     if (blk1_start_index + 1 == blk2_start_index) {

       // previous card; new value for blk2 card is size of blk1.

       _array->set_offset_array(blk2_start_index, (u_char) _sp->block_size(blk1));

     } else {

       // Earlier card; go back a card.

       _array->set_offset_array(blk2_start_index, N_words);

     }

   } else {

     // blk2 does not start a card.  Does it cross a card?  If not, nothing

     // to do.

     size_t blk2_end_index =

       _array->index_for(blk2 + _sp->block_size(blk2) - 1);

     assert(blk2_end_index >= blk2_start_index, "sanity");

     if (blk2_end_index > blk2_start_index) {

       // Yes, it crosses a card.  The value for the next card must change.

       if (blk1_start_index + 1 == blk2_start_index) {

         // previous card; new value for second blk2 card is size of blk1.

         _array->set_offset_array(blk2_start_index + 1,

                                  (u_char) _sp->block_size(blk1));

       } else {

         // Earlier card; go back a card.

         _array->set_offset_array(blk2_start_index + 1, N_words);

       }

     }

   }

 }

 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.

   size_t next_index = _array->index_for(n) + 1;

   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() == 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() == 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

 }

 //////////////////////////////////////////////////////////////////////

 // 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);

 }