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

 /*

  * Copyright 2001-2009 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/_heapRegion.cpp.incl"

 int HeapRegion::LogOfHRGrainBytes = 0;

 int HeapRegion::LogOfHRGrainWords = 0;

 int HeapRegion::GrainBytes        = 0;

 int HeapRegion::GrainWords        = 0;

 int HeapRegion::CardsPerRegion    = 0;

 HeapRegionDCTOC::HeapRegionDCTOC(G1CollectedHeap* g1,

                                  HeapRegion* hr, OopClosure* cl,

                                  CardTableModRefBS::PrecisionStyle precision,

                                  FilterKind fk) :

   ContiguousSpaceDCTOC(hr, cl, precision, NULL),

   _hr(hr), _fk(fk), _g1(g1)

 {}

 FilterOutOfRegionClosure::FilterOutOfRegionClosure(HeapRegion* r,

                                                    OopClosure* oc) :

   _r_bottom(r->bottom()), _r_end(r->end()),

   _oc(oc), _out_of_region(0)

 {}

 class VerifyLiveClosure: public OopClosure {

 private:

   G1CollectedHeap* _g1h;

   CardTableModRefBS* _bs;

   oop _containing_obj;

   bool _failures;

   int _n_failures;

   bool _use_prev_marking;

 public:

   // use_prev_marking == true  -> use "prev" marking information,

   // use_prev_marking == false -> use "next" marking information

   VerifyLiveClosure(G1CollectedHeap* g1h, bool use_prev_marking) :

     _g1h(g1h), _bs(NULL), _containing_obj(NULL),

     _failures(false), _n_failures(0), _use_prev_marking(use_prev_marking)

   {

     BarrierSet* bs = _g1h->barrier_set();

     if (bs->is_a(BarrierSet::CardTableModRef))

       _bs = (CardTableModRefBS*)bs;

   }

   void set_containing_obj(oop obj) {

     _containing_obj = obj;

   }

   bool failures() { return _failures; }

   int n_failures() { return _n_failures; }

   virtual void do_oop(narrowOop* p) { do_oop_work(p); }

   virtual void do_oop(      oop* p) { do_oop_work(p); }

   template <class T> void do_oop_work(T* p) {

     assert(_containing_obj != NULL, "Precondition");

     assert(!_g1h->is_obj_dead_cond(_containing_obj, _use_prev_marking),

            "Precondition");

     T heap_oop = oopDesc::load_heap_oop(p);

     if (!oopDesc::is_null(heap_oop)) {

       oop obj = oopDesc::decode_heap_oop_not_null(heap_oop);

       bool failed = false;

       if (!_g1h->is_in_closed_subset(obj) ||

           _g1h->is_obj_dead_cond(obj, _use_prev_marking)) {

         if (!_failures) {

           gclog_or_tty->print_cr("");

           gclog_or_tty->print_cr("----------");

         }

         if (!_g1h->is_in_closed_subset(obj)) {

           gclog_or_tty->print_cr("Field "PTR_FORMAT

                         " of live obj "PTR_FORMAT

                         " points to obj "PTR_FORMAT

                         " not in the heap.",

                         p, (void*) _containing_obj, (void*) obj);

         } else {

           gclog_or_tty->print_cr("Field "PTR_FORMAT

                         " of live obj "PTR_FORMAT

                         " points to dead obj "PTR_FORMAT".",

                         p, (void*) _containing_obj, (void*) obj);

         }

         gclog_or_tty->print_cr("Live obj:");

         _containing_obj->print_on(gclog_or_tty);

         gclog_or_tty->print_cr("Bad referent:");

         obj->print_on(gclog_or_tty);

         gclog_or_tty->print_cr("----------");

         _failures = true;

         failed = true;

         _n_failures++;

       }

       if (!_g1h->full_collection()) {

         HeapRegion* from = _g1h->heap_region_containing((HeapWord*)p);

         HeapRegion* to   = _g1h->heap_region_containing(obj);

         if (from != NULL && to != NULL &&

             from != to &&

             !to->isHumongous()) {

           jbyte cv_obj = *_bs->byte_for_const(_containing_obj);

           jbyte cv_field = *_bs->byte_for_const(p);

           const jbyte dirty = CardTableModRefBS::dirty_card_val();

           bool is_bad = !(from->is_young()

                           || to->rem_set()->contains_reference(p)

                           || !G1HRRSFlushLogBuffersOnVerify && // buffers were not flushed

                               (_containing_obj->is_objArray() ?

                                   cv_field == dirty

                                : cv_obj == dirty || cv_field == dirty));

           if (is_bad) {

             if (!_failures) {

               gclog_or_tty->print_cr("");

               gclog_or_tty->print_cr("----------");

             }

             gclog_or_tty->print_cr("Missing rem set entry:");

             gclog_or_tty->print_cr("Field "PTR_FORMAT

                           " of obj "PTR_FORMAT

                           ", in region %d ["PTR_FORMAT

                           ", "PTR_FORMAT"),",

                           p, (void*) _containing_obj,

                           from->hrs_index(),

                           from->bottom(),

                           from->end());

             _containing_obj->print_on(gclog_or_tty);

             gclog_or_tty->print_cr("points to obj "PTR_FORMAT

                           " in region %d ["PTR_FORMAT

                           ", "PTR_FORMAT").",

                           (void*) obj, to->hrs_index(),

                           to->bottom(), to->end());

             obj->print_on(gclog_or_tty);

             gclog_or_tty->print_cr("Obj head CTE = %d, field CTE = %d.",

                           cv_obj, cv_field);

             gclog_or_tty->print_cr("----------");

             _failures = true;

             if (!failed) _n_failures++;

           }

         }

       }

     }

   }

 };

 template<class ClosureType>

 HeapWord* walk_mem_region_loop(ClosureType* cl, G1CollectedHeap* g1h,

                                HeapRegion* hr,

                                HeapWord* cur, HeapWord* top) {

   oop cur_oop = oop(cur);

   int oop_size = cur_oop->size();

   HeapWord* next_obj = cur + oop_size;

   while (next_obj < top) {

     // Keep filtering the remembered set.

     if (!g1h->is_obj_dead(cur_oop, hr)) {

       // Bottom lies entirely below top, so we can call the

       // non-memRegion version of oop_iterate below.

       cur_oop->oop_iterate(cl);

     }

     cur = next_obj;

     cur_oop = oop(cur);

     oop_size = cur_oop->size();

     next_obj = cur + oop_size;

   }

   return cur;

 }

 void HeapRegionDCTOC::walk_mem_region_with_cl(MemRegion mr,

                                               HeapWord* bottom,

                                               HeapWord* top,

                                               OopClosure* cl) {

   G1CollectedHeap* g1h = _g1;

   int oop_size;

   OopClosure* cl2 = cl;

   FilterIntoCSClosure intoCSFilt(this, g1h, cl);

   FilterOutOfRegionClosure outOfRegionFilt(_hr, cl);

   switch (_fk) {

   case IntoCSFilterKind:      cl2 = &intoCSFilt; break;

   case OutOfRegionFilterKind: cl2 = &outOfRegionFilt; break;

   }

   // Start filtering what we add to the remembered set. If the object is

   // not considered dead, either because it is marked (in the mark bitmap)

   // or it was allocated after marking finished, then we add it. Otherwise

   // we can safely ignore the object.

   if (!g1h->is_obj_dead(oop(bottom), _hr)) {

     oop_size = oop(bottom)->oop_iterate(cl2, mr);

   } else {

     oop_size = oop(bottom)->size();

   }

   bottom += oop_size;

   if (bottom < top) {

     // We replicate the loop below for several kinds of possible filters.

     switch (_fk) {

     case NoFilterKind:

       bottom = walk_mem_region_loop(cl, g1h, _hr, bottom, top);

       break;

     case IntoCSFilterKind: {

       FilterIntoCSClosure filt(this, g1h, cl);

       bottom = walk_mem_region_loop(&filt, g1h, _hr, bottom, top);

       break;

     }

     case OutOfRegionFilterKind: {

       FilterOutOfRegionClosure filt(_hr, cl);

       bottom = walk_mem_region_loop(&filt, g1h, _hr, bottom, top);

       break;

     }

     default:

       ShouldNotReachHere();

     }

     // Last object. Need to do dead-obj filtering here too.

     if (!g1h->is_obj_dead(oop(bottom), _hr)) {

       oop(bottom)->oop_iterate(cl2, mr);

     }

   }

 }

 // Minimum region size; we won't go lower than that.

 // We might want to decrease this in the future, to deal with small

 // heaps a bit more efficiently.

 #define MIN_REGION_SIZE  (      1024 * 1024 )

 // Maximum region size; we don't go higher than that. There's a good

 // reason for having an upper bound. We don't want regions to get too

 // large, otherwise cleanup's effectiveness would decrease as there

 // will be fewer opportunities to find totally empty regions after

 // marking.

 #define MAX_REGION_SIZE  ( 32 * 1024 * 1024 )

 // The automatic region size calculation will try to have around this

 // many regions in the heap (based on the min heap size).

 #define TARGET_REGION_NUMBER          2048

 void HeapRegion::setup_heap_region_size(uintx min_heap_size) {

   // region_size in bytes

   uintx region_size = G1HeapRegionSize;

   if (FLAG_IS_DEFAULT(G1HeapRegionSize)) {

     // We base the automatic calculation on the min heap size. This

     // can be problematic if the spread between min and max is quite

     // wide, imagine -Xms128m -Xmx32g. But, if we decided it based on

     // the max size, the region size might be way too large for the

     // min size. Either way, some users might have to set the region

     // size manually for some -Xms / -Xmx combos.

     region_size = MAX2(min_heap_size / TARGET_REGION_NUMBER,

                        (uintx) MIN_REGION_SIZE);

   }

   int region_size_log = log2_long((jlong) region_size);

   // Recalculate the region size to make sure it's a power of

   // 2. This means that region_size is the largest power of 2 that's

   // <= what we've calculated so far.

   region_size = 1 << region_size_log;

   // Now make sure that we don't go over or under our limits.

   if (region_size < MIN_REGION_SIZE) {

     region_size = MIN_REGION_SIZE;

   } else if (region_size > MAX_REGION_SIZE) {

     region_size = MAX_REGION_SIZE;

   }

   // And recalculate the log.

   region_size_log = log2_long((jlong) region_size);

   // Now, set up the globals.

   guarantee(LogOfHRGrainBytes == 0, "we should only set it once");

   LogOfHRGrainBytes = region_size_log;

   guarantee(LogOfHRGrainWords == 0, "we should only set it once");

   LogOfHRGrainWords = LogOfHRGrainBytes - LogHeapWordSize;

   guarantee(GrainBytes == 0, "we should only set it once");

   // The cast to int is safe, given that we've bounded region_size by

   // MIN_REGION_SIZE and MAX_REGION_SIZE.

   GrainBytes = (int) region_size;

   guarantee(GrainWords == 0, "we should only set it once");

   GrainWords = GrainBytes >> LogHeapWordSize;

   guarantee(1 << LogOfHRGrainWords == GrainWords, "sanity");

   guarantee(CardsPerRegion == 0, "we should only set it once");

   CardsPerRegion = GrainBytes >> CardTableModRefBS::card_shift;

 }

 void HeapRegion::reset_after_compaction() {

   G1OffsetTableContigSpace::reset_after_compaction();

   // After a compaction the mark bitmap is invalid, so we must

   // treat all objects as being inside the unmarked area.

   zero_marked_bytes();

   init_top_at_mark_start();

 }

 DirtyCardToOopClosure*

 HeapRegion::new_dcto_closure(OopClosure* cl,

                              CardTableModRefBS::PrecisionStyle precision,

                              HeapRegionDCTOC::FilterKind fk) {

   return new HeapRegionDCTOC(G1CollectedHeap::heap(),

                              this, cl, precision, fk);

 }

 void HeapRegion::hr_clear(bool par, bool clear_space) {

   _humongous_type = NotHumongous;

   _humongous_start_region = NULL;

   _in_collection_set = false;

   _is_gc_alloc_region = false;

   // Age stuff (if parallel, this will be done separately, since it needs

   // to be sequential).

   G1CollectedHeap* g1h = G1CollectedHeap::heap();

   set_young_index_in_cset(-1);

   uninstall_surv_rate_group();

   set_young_type(NotYoung);

   // In case it had been the start of a humongous sequence, reset its end.

   set_end(_orig_end);

   if (!par) {

     // If this is parallel, this will be done later.

     HeapRegionRemSet* hrrs = rem_set();

     if (hrrs != NULL) hrrs->clear();

     _claimed = InitialClaimValue;

   }

   zero_marked_bytes();

   set_sort_index(-1);

   _offsets.resize(HeapRegion::GrainWords);

   init_top_at_mark_start();

   if (clear_space) clear(SpaceDecorator::Mangle);

 }

 // <PREDICTION>

 void HeapRegion::calc_gc_efficiency() {

   G1CollectedHeap* g1h = G1CollectedHeap::heap();

   _gc_efficiency = (double) garbage_bytes() /

                             g1h->predict_region_elapsed_time_ms(this, false);

 }

 // </PREDICTION>

 void HeapRegion::set_startsHumongous() {

   _humongous_type = StartsHumongous;

   _humongous_start_region = this;

   assert(end() == _orig_end, "Should be normal before alloc.");

 }

 bool HeapRegion::claimHeapRegion(jint claimValue) {

   jint current = _claimed;

   if (current != claimValue) {

     jint res = Atomic::cmpxchg(claimValue, &_claimed, current);

     if (res == current) {

       return true;

     }

   }

   return false;

 }

 HeapWord* HeapRegion::next_block_start_careful(HeapWord* addr) {

   HeapWord* low = addr;

   HeapWord* high = end();

   while (low < high) {

     size_t diff = pointer_delta(high, low);

     // Must add one below to bias toward the high amount.  Otherwise, if

   // "high" were at the desired value, and "low" were one less, we

     // would not converge on "high".  This is not symmetric, because

     // we set "high" to a block start, which might be the right one,

     // which we don't do for "low".

     HeapWord* middle = low + (diff+1)/2;

     if (middle == high) return high;

     HeapWord* mid_bs = block_start_careful(middle);

     if (mid_bs < addr) {

       low = middle;

     } else {

       high = mid_bs;

     }

   }

   assert(low == high && low >= addr, "Didn't work.");

   return low;

 }

 void HeapRegion::set_next_on_unclean_list(HeapRegion* r) {

   assert(r == NULL || r->is_on_unclean_list(), "Malformed unclean list.");

   _next_in_special_set = r;

 }

 void HeapRegion::set_on_unclean_list(bool b) {

   _is_on_unclean_list = b;

 }

 void HeapRegion::initialize(MemRegion mr, bool clear_space, bool mangle_space) {

   G1OffsetTableContigSpace::initialize(mr, false, mangle_space);

   hr_clear(false/*par*/, clear_space);

 }

 #ifdef _MSC_VER // the use of 'this' below gets a warning, make it go away

 #pragma warning( disable:4355 ) // 'this' : used in base member initializer list

 #endif // _MSC_VER

 HeapRegion::

 HeapRegion(G1BlockOffsetSharedArray* sharedOffsetArray,

                      MemRegion mr, bool is_zeroed)

   : G1OffsetTableContigSpace(sharedOffsetArray, mr, is_zeroed),

     _next_fk(HeapRegionDCTOC::NoFilterKind),

     _hrs_index(-1),

     _humongous_type(NotHumongous), _humongous_start_region(NULL),

     _in_collection_set(false), _is_gc_alloc_region(false),

     _is_on_free_list(false), _is_on_unclean_list(false),

     _next_in_special_set(NULL), _orig_end(NULL),

     _claimed(InitialClaimValue), _evacuation_failed(false),

     _prev_marked_bytes(0), _next_marked_bytes(0), _sort_index(-1),

     _young_type(NotYoung), _next_young_region(NULL),

     _next_dirty_cards_region(NULL),

     _young_index_in_cset(-1), _surv_rate_group(NULL), _age_index(-1),

     _rem_set(NULL), _zfs(NotZeroFilled)

 {

   _orig_end = mr.end();

   // Note that initialize() will set the start of the unmarked area of the

   // region.

   this->initialize(mr, !is_zeroed, SpaceDecorator::Mangle);

   set_top(bottom());

   set_saved_mark();

   _rem_set =  new HeapRegionRemSet(sharedOffsetArray, this);

   assert(HeapRegionRemSet::num_par_rem_sets() > 0, "Invariant.");

   // In case the region is allocated during a pause, note the top.

   // We haven't done any counting on a brand new region.

   _top_at_conc_mark_count = bottom();

 }

 class NextCompactionHeapRegionClosure: public HeapRegionClosure {

   const HeapRegion* _target;

   bool _target_seen;

   HeapRegion* _last;

   CompactibleSpace* _res;

 public:

   NextCompactionHeapRegionClosure(const HeapRegion* target) :

     _target(target), _target_seen(false), _res(NULL) {}

   bool doHeapRegion(HeapRegion* cur) {

     if (_target_seen) {

       if (!cur->isHumongous()) {

         _res = cur;

         return true;

       }

     } else if (cur == _target) {

       _target_seen = true;

     }

     return false;

   }

   CompactibleSpace* result() { return _res; }

 };

 CompactibleSpace* HeapRegion::next_compaction_space() const {

   G1CollectedHeap* g1h = G1CollectedHeap::heap();

   // cast away const-ness

   HeapRegion* r = (HeapRegion*) this;

   NextCompactionHeapRegionClosure blk(r);

   g1h->heap_region_iterate_from(r, &blk);

   return blk.result();

 }

 void HeapRegion::set_continuesHumongous(HeapRegion* start) {

   // The order is important here.

   start->add_continuingHumongousRegion(this);

   _humongous_type = ContinuesHumongous;

   _humongous_start_region = start;

 }

 void HeapRegion::add_continuingHumongousRegion(HeapRegion* cont) {

   // Must join the blocks of the current H region seq with the block of the

   // added region.

   offsets()->join_blocks(bottom(), cont->bottom());

   arrayOop obj = (arrayOop)(bottom());

   obj->set_length((int) (obj->length() + cont->capacity()/jintSize));

   set_end(cont->end());

   set_top(cont->end());

 }

 void HeapRegion::save_marks() {

   set_saved_mark();

 }

 void HeapRegion::oops_in_mr_iterate(MemRegion mr, OopClosure* cl) {

   HeapWord* p = mr.start();

   HeapWord* e = mr.end();

   oop obj;

   while (p < e) {

     obj = oop(p);

     p += obj->oop_iterate(cl);

   }

   assert(p == e, "bad memregion: doesn't end on obj boundary");

 }

 #define HeapRegion_OOP_SINCE_SAVE_MARKS_DEFN(OopClosureType, nv_suffix) \

 void HeapRegion::oop_since_save_marks_iterate##nv_suffix(OopClosureType* cl) { \

   ContiguousSpace::oop_since_save_marks_iterate##nv_suffix(cl);              \

 }

 SPECIALIZED_SINCE_SAVE_MARKS_CLOSURES(HeapRegion_OOP_SINCE_SAVE_MARKS_DEFN)

 void HeapRegion::oop_before_save_marks_iterate(OopClosure* cl) {

   oops_in_mr_iterate(MemRegion(bottom(), saved_mark_word()), cl);

 }

 #ifdef DEBUG

 HeapWord* HeapRegion::allocate(size_t size) {

   jint state = zero_fill_state();

   assert(!G1CollectedHeap::heap()->allocs_are_zero_filled() ||

          zero_fill_is_allocated(),

          "When ZF is on, only alloc in ZF'd regions");

   return G1OffsetTableContigSpace::allocate(size);

 }

 #endif

 void HeapRegion::set_zero_fill_state_work(ZeroFillState zfs) {

   assert(top() == bottom() || zfs == Allocated,

          "Region must be empty, or we must be setting it to allocated.");

   assert(ZF_mon->owned_by_self() ||

          Universe::heap()->is_gc_active(),

          "Must hold the lock or be a full GC to modify.");

   _zfs = zfs;

 }

 void HeapRegion::set_zero_fill_complete() {

   set_zero_fill_state_work(ZeroFilled);

   if (ZF_mon->owned_by_self()) {

     ZF_mon->notify_all();

   }

 }

 void HeapRegion::ensure_zero_filled() {

   MutexLockerEx x(ZF_mon, Mutex::_no_safepoint_check_flag);

   ensure_zero_filled_locked();

 }

 void HeapRegion::ensure_zero_filled_locked() {

   assert(ZF_mon->owned_by_self(), "Precondition");

   bool should_ignore_zf = SafepointSynchronize::is_at_safepoint();

   assert(should_ignore_zf || Heap_lock->is_locked(),

          "Either we're in a GC or we're allocating a region.");

   switch (zero_fill_state()) {

   case HeapRegion::NotZeroFilled:

     set_zero_fill_in_progress(Thread::current());

     {

       ZF_mon->unlock();

       Copy::fill_to_words(bottom(), capacity()/HeapWordSize);

       ZF_mon->lock_without_safepoint_check();

     }

     // A trap.

     guarantee(zero_fill_state() == HeapRegion::ZeroFilling

               && zero_filler() == Thread::current(),

               "AHA!  Tell Dave D if you see this...");

     set_zero_fill_complete();

     // gclog_or_tty->print_cr("Did sync ZF.");

     ConcurrentZFThread::note_sync_zfs();

     break;

   case HeapRegion::ZeroFilling:

     if (should_ignore_zf) {

       // We can "break" the lock and take over the work.

       Copy::fill_to_words(bottom(), capacity()/HeapWordSize);

       set_zero_fill_complete();

       ConcurrentZFThread::note_sync_zfs();

       break;

     } else {

       ConcurrentZFThread::wait_for_ZF_completed(this);

     }

   case HeapRegion::ZeroFilled:

     // Nothing to do.

     break;

   case HeapRegion::Allocated:

     guarantee(false, "Should not call on allocated regions.");

   }

   assert(zero_fill_state() == HeapRegion::ZeroFilled, "Post");

 }

 HeapWord*

 HeapRegion::object_iterate_mem_careful(MemRegion mr,

                                                  ObjectClosure* cl) {

   G1CollectedHeap* g1h = G1CollectedHeap::heap();

   // We used to use "block_start_careful" here.  But we're actually happy

   // to update the BOT while we do this...

   HeapWord* cur = block_start(mr.start());

   mr = mr.intersection(used_region());

   if (mr.is_empty()) return NULL;

   // Otherwise, find the obj that extends onto mr.start().

   assert(cur <= mr.start()

          && (oop(cur)->klass_or_null() == NULL ||

              cur + oop(cur)->size() > mr.start()),

          "postcondition of block_start");

   oop obj;

   while (cur < mr.end()) {

     obj = oop(cur);

     if (obj->klass_or_null() == NULL) {

       // Ran into an unparseable point.

       return cur;

     } else if (!g1h->is_obj_dead(obj)) {

       cl->do_object(obj);

     }

     if (cl->abort()) return cur;

     // The check above must occur before the operation below, since an

     // abort might invalidate the "size" operation.

     cur += obj->size();

   }

   return NULL;

 }

 HeapWord*

 HeapRegion::

 oops_on_card_seq_iterate_careful(MemRegion mr,

                                      FilterOutOfRegionClosure* cl) {

   G1CollectedHeap* g1h = G1CollectedHeap::heap();

   // If we're within a stop-world GC, then we might look at a card in a

   // GC alloc region that extends onto a GC LAB, which may not be

   // parseable.  Stop such at the "saved_mark" of the region.

   if (G1CollectedHeap::heap()->is_gc_active()) {

     mr = mr.intersection(used_region_at_save_marks());

   } else {

     mr = mr.intersection(used_region());

   }

   if (mr.is_empty()) return NULL;

   // Otherwise, find the obj that extends onto mr.start().

   // We used to use "block_start_careful" here.  But we're actually happy

   // to update the BOT while we do this...

   HeapWord* cur = block_start(mr.start());

   assert(cur <= mr.start(), "Postcondition");

   while (cur <= mr.start()) {

     if (oop(cur)->klass_or_null() == NULL) {

       // Ran into an unparseable point.

       return cur;

     }

     // Otherwise...

     int sz = oop(cur)->size();

     if (cur + sz > mr.start()) break;

     // Otherwise, go on.

     cur = cur + sz;

   }

   oop obj;

   obj = oop(cur);

   // If we finish this loop...

   assert(cur <= mr.start()

          && obj->klass_or_null() != NULL

          && cur + obj->size() > mr.start(),

          "Loop postcondition");

   if (!g1h->is_obj_dead(obj)) {

     obj->oop_iterate(cl, mr);

   }

   HeapWord* next;

   while (cur < mr.end()) {

     obj = oop(cur);

     if (obj->klass_or_null() == NULL) {

       // Ran into an unparseable point.

       return cur;

     };

     // Otherwise:

     next = (cur + obj->size());

     if (!g1h->is_obj_dead(obj)) {

       if (next < mr.end()) {

         obj->oop_iterate(cl);

       } else {

         // this obj spans the boundary.  If it's an array, stop at the

         // boundary.

         if (obj->is_objArray()) {

           obj->oop_iterate(cl, mr);

         } else {

           obj->oop_iterate(cl);

         }

       }

     }

     cur = next;

   }

   return NULL;

 }

 void HeapRegion::print() const { print_on(gclog_or_tty); }

 void HeapRegion::print_on(outputStream* st) const {

   if (isHumongous()) {

     if (startsHumongous())

       st->print(" HS");

     else

       st->print(" HC");

   } else {

     st->print("   ");

   }

   if (in_collection_set())

     st->print(" CS");

   else if (is_gc_alloc_region())

     st->print(" A ");

   else

     st->print("   ");

   if (is_young())

     st->print(is_scan_only() ? " SO" : (is_survivor() ? " SU" : " Y "));

   else

     st->print("   ");

   if (is_empty())

     st->print(" F");

   else

     st->print("  ");

   st->print(" %d", _gc_time_stamp);

   G1OffsetTableContigSpace::print_on(st);

 }

 void HeapRegion::verify(bool allow_dirty) const {

   verify(allow_dirty, /* use_prev_marking */ true);

 }

 #define OBJ_SAMPLE_INTERVAL 0

 #define BLOCK_SAMPLE_INTERVAL 100

 // This really ought to be commoned up into OffsetTableContigSpace somehow.

 // We would need a mechanism to make that code skip dead objects.

 void HeapRegion::verify(bool allow_dirty, bool use_prev_marking) const {

   G1CollectedHeap* g1 = G1CollectedHeap::heap();

   HeapWord* p = bottom();

   HeapWord* prev_p = NULL;

   int objs = 0;

   int blocks = 0;

   VerifyLiveClosure vl_cl(g1, use_prev_marking);

   while (p < top()) {

     size_t size = oop(p)->size();

     if (blocks == BLOCK_SAMPLE_INTERVAL) {

       guarantee(p == block_start_const(p + (size/2)),

                 "check offset computation");

       blocks = 0;

     } else {

       blocks++;

     }

     if (objs == OBJ_SAMPLE_INTERVAL) {

       oop obj = oop(p);

       if (!g1->is_obj_dead_cond(obj, this, use_prev_marking)) {

         obj->verify();

         vl_cl.set_containing_obj(obj);

         obj->oop_iterate(&vl_cl);

         if (G1MaxVerifyFailures >= 0

             && vl_cl.n_failures() >= G1MaxVerifyFailures) break;

       }

       objs = 0;

     } else {

       objs++;

     }

     prev_p = p;

     p += size;

   }

   HeapWord* rend = end();

   HeapWord* rtop = top();

   if (rtop < rend) {

     guarantee(block_start_const(rtop + (rend - rtop) / 2) == rtop,

               "check offset computation");

   }

   if (vl_cl.failures()) {

     gclog_or_tty->print_cr("Heap:");

     G1CollectedHeap::heap()->print_on(gclog_or_tty, true /* extended */);

     gclog_or_tty->print_cr("");

   }

   if (VerifyDuringGC &&

       G1VerifyConcMarkPrintReachable &&

       vl_cl.failures()) {

     g1->concurrent_mark()->print_prev_bitmap_reachable();

   }

   guarantee(!vl_cl.failures(), "region verification failed");

   guarantee(p == top(), "end of last object must match end of space");

 }

 // G1OffsetTableContigSpace code; copied from space.cpp.  Hope this can go

 // away eventually.

 void G1OffsetTableContigSpace::initialize(MemRegion mr, bool clear_space, bool mangle_space) {

   // false ==> we'll do the clearing if there's clearing to be done.

   ContiguousSpace::initialize(mr, false, mangle_space);

   _offsets.zero_bottom_entry();

   _offsets.initialize_threshold();

   if (clear_space) clear(mangle_space);

 }

 void G1OffsetTableContigSpace::clear(bool mangle_space) {

   ContiguousSpace::clear(mangle_space);

   _offsets.zero_bottom_entry();

   _offsets.initialize_threshold();

 }

 void G1OffsetTableContigSpace::set_bottom(HeapWord* new_bottom) {

   Space::set_bottom(new_bottom);

   _offsets.set_bottom(new_bottom);

 }

 void G1OffsetTableContigSpace::set_end(HeapWord* new_end) {

   Space::set_end(new_end);

   _offsets.resize(new_end - bottom());

 }

 void G1OffsetTableContigSpace::print() const {

   print_short();

   gclog_or_tty->print_cr(" [" INTPTR_FORMAT ", " INTPTR_FORMAT ", "

                 INTPTR_FORMAT ", " INTPTR_FORMAT ")",

                 bottom(), top(), _offsets.threshold(), end());

 }

 HeapWord* G1OffsetTableContigSpace::initialize_threshold() {

   return _offsets.initialize_threshold();

 }

 HeapWord* G1OffsetTableContigSpace::cross_threshold(HeapWord* start,

                                                     HeapWord* end) {

   _offsets.alloc_block(start, end);

   return _offsets.threshold();

 }

 HeapWord* G1OffsetTableContigSpace::saved_mark_word() const {

   G1CollectedHeap* g1h = G1CollectedHeap::heap();

   assert( _gc_time_stamp <= g1h->get_gc_time_stamp(), "invariant" );

   if (_gc_time_stamp < g1h->get_gc_time_stamp())

     return top();

   else

     return ContiguousSpace::saved_mark_word();

 }

 void G1OffsetTableContigSpace::set_saved_mark() {

   G1CollectedHeap* g1h = G1CollectedHeap::heap();

   unsigned curr_gc_time_stamp = g1h->get_gc_time_stamp();

   if (_gc_time_stamp < curr_gc_time_stamp) {

     // The order of these is important, as another thread might be

     // about to start scanning this region. If it does so after

     // set_saved_mark and before _gc_time_stamp = ..., then the latter

     // will be false, and it will pick up top() as the high water mark

     // of region. If it does so after _gc_time_stamp = ..., then it

     // will pick up the right saved_mark_word() as the high water mark

     // of the region. Either way, the behaviour will be correct.

     ContiguousSpace::set_saved_mark();

     OrderAccess::storestore();

     _gc_time_stamp = curr_gc_time_stamp;

     // The following fence is to force a flush of the writes above, but

     // is strictly not needed because when an allocating worker thread

     // calls set_saved_mark() it does so under the ParGCRareEvent_lock;

     // when the lock is released, the write will be flushed.

     // OrderAccess::fence();

   }

 }

 G1OffsetTableContigSpace::

 G1OffsetTableContigSpace(G1BlockOffsetSharedArray* sharedOffsetArray,

                          MemRegion mr, bool is_zeroed) :

   _offsets(sharedOffsetArray, mr),

   _par_alloc_lock(Mutex::leaf, "OffsetTableContigSpace par alloc lock", true),

   _gc_time_stamp(0)

 {

   _offsets.set_space(this);

   initialize(mr, !is_zeroed, SpaceDecorator::Mangle);

 }

 size_t RegionList::length() {

   size_t len = 0;

   HeapRegion* cur = hd();

   DEBUG_ONLY(HeapRegion* last = NULL);

   while (cur != NULL) {

     len++;

     DEBUG_ONLY(last = cur);

     cur = get_next(cur);

   }

   assert(last == tl(), "Invariant");

   return len;

 }

 void RegionList::insert_before_head(HeapRegion* r) {

   assert(well_formed(), "Inv");

   set_next(r, hd());

   _hd = r;

   _sz++;

   if (tl() == NULL) _tl = r;

   assert(well_formed(), "Inv");

 }

 void RegionList::prepend_list(RegionList* new_list) {

   assert(well_formed(), "Precondition");

   assert(new_list->well_formed(), "Precondition");

   HeapRegion* new_tl = new_list->tl();

   if (new_tl != NULL) {

     set_next(new_tl, hd());

     _hd = new_list->hd();

     _sz += new_list->sz();

     if (tl() == NULL) _tl = new_list->tl();

   } else {

     assert(new_list->hd() == NULL && new_list->sz() == 0, "Inv");

   }

   assert(well_formed(), "Inv");

 }

 void RegionList::delete_after(HeapRegion* r) {

   assert(well_formed(), "Precondition");

   HeapRegion* next = get_next(r);

   assert(r != NULL, "Precondition");

   HeapRegion* next_tl = get_next(next);

   set_next(r, next_tl);

   dec_sz();

   if (next == tl()) {

     assert(next_tl == NULL, "Inv");

     _tl = r;

   }

   assert(well_formed(), "Inv");

 }

 HeapRegion* RegionList::pop() {

   assert(well_formed(), "Inv");

   HeapRegion* res = hd();

   if (res != NULL) {

     _hd = get_next(res);

     _sz--;

     set_next(res, NULL);

     if (sz() == 0) _tl = NULL;

   }

   assert(well_formed(), "Inv");

   return res;

 }