Java example source code file (g1BlockOffsetTable.cpp)
The g1BlockOffsetTable.cpp Java example source code
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
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* 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_implementation/g1/g1BlockOffsetTable.inline.hpp"
#include "memory/space.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/java.hpp"
#include "services/memTracker.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");
}
MemTracker::record_virtual_memory_type((address)rs.base(), mtGC);
_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, OOM_MMAP_ERROR, "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,
err_msg("Should be in logarithmic region - "
"entry: " UINT32_FORMAT ", "
"_array->offset_array(c): " UINT32_FORMAT ", "
"N_words: " UINT32_FORMAT,
entry, _array->offset_array(c), N_words));
}
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,
err_msg("Monotonicity - landing_card offset: " UINT32_FORMAT ", "
"entry: " UINT32_FORMAT,
_array->offset_array(landing_card), entry));
} else {
guarantee(landing_card == start_card - 1, "Tautology");
// Note that N_words is the maximum offset value
guarantee(_array->offset_array(landing_card) <= N_words,
err_msg("landing card offset: " UINT32_FORMAT ", "
"N_words: " UINT32_FORMAT,
_array->offset_array(landing_card), N_words));
}
}
}
// 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 n_index = _array->index_for(n);
size_t next_index = _array->index_for(n) + !_array->is_card_boundary(n);
// Calculate a consistent next boundary. If "n" is not at the boundary
// already, step to the boundary.
HeapWord* next_boundary = _array->address_for_index(n_index) +
(n_index == next_index ? 0 : N_words);
assert(next_boundary <= _array->_end,
err_msg("next_boundary is beyond the end of the covered region "
" next_boundary " PTR_FORMAT " _array->_end " PTR_FORMAT,
next_boundary, _array->_end));
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),
err_msg("offset array should have been set - "
"orig_index offset: " UINT32_FORMAT ", "
"blk_start: " PTR_FORMAT ", "
"boundary: " PTR_FORMAT,
_array->offset_array(orig_index),
blk_start, boundary));
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),
err_msg("offset array should have been set - "
UINT32_FORMAT " not > 0 OR "
UINT32_FORMAT " not <= " UINT32_FORMAT,
_array->offset_array(j),
_array->offset_array(j),
(u_char) (N_words+BlockOffsetArray::N_powers-1)));
}
#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
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