src/hotspot/share/gc/shenandoah/shenandoahHeap.inline.hpp

/*
 * Copyright (c) 2015, 2020, Red Hat, Inc. All rights reserved.
 * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
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#ifndef SHARE_GC_SHENANDOAH_SHENANDOAHHEAP_INLINE_HPP
#define SHARE_GC_SHENANDOAH_SHENANDOAHHEAP_INLINE_HPP

#include "gc/shenandoah/shenandoahHeap.hpp"

#include "classfile/javaClasses.inline.hpp"
#include "gc/shared/continuationGCSupport.inline.hpp"
#include "gc/shared/markBitMap.inline.hpp"
#include "gc/shared/suspendibleThreadSet.hpp"
#include "gc/shared/threadLocalAllocBuffer.inline.hpp"
#include "gc/shared/tlab_globals.hpp"
#include "gc/shenandoah/mode/shenandoahMode.hpp"
#include "gc/shenandoah/shenandoahAsserts.hpp"
#include "gc/shenandoah/shenandoahBarrierSet.inline.hpp"
#include "gc/shenandoah/shenandoahCollectionSet.inline.hpp"
#include "gc/shenandoah/shenandoahForwarding.inline.hpp"
#include "gc/shenandoah/shenandoahGeneration.hpp"
#include "gc/shenandoah/shenandoahHeapRegion.inline.hpp"
#include "gc/shenandoah/shenandoahHeapRegionSet.inline.hpp"
#include "gc/shenandoah/shenandoahMarkingContext.inline.hpp"
#include "gc/shenandoah/shenandoahThreadLocalData.hpp"
#include "gc/shenandoah/shenandoahWorkGroup.hpp"
#include "oops/compressedOops.inline.hpp"
#include "oops/oop.inline.hpp"
#include "runtime/atomic.hpp"
#include "runtime/javaThread.hpp"
#include "runtime/objectMonitor.inline.hpp"
#include "runtime/prefetch.inline.hpp"
#include "utilities/copy.hpp"
#include "utilities/globalDefinitions.hpp"

inline ShenandoahHeap* ShenandoahHeap::heap() {
  return named_heap<ShenandoahHeap>(CollectedHeap::Shenandoah);
}

inline ShenandoahHeapRegion* ShenandoahRegionIterator::next() {
  size_t new_index = Atomic::add(&_index, (size_t) 1, memory_order_relaxed);
  // get_region() provides the bounds-check and returns null on OOB.
  return _heap->get_region(new_index - 1);
}

inline WorkerThreads* ShenandoahHeap::workers() const {
  return _workers;
}

inline WorkerThreads* ShenandoahHeap::safepoint_workers() {
  return _safepoint_workers;
}

inline void ShenandoahHeap::notify_gc_progress() {
  Atomic::store(&_gc_no_progress_count, (size_t) 0);

}
inline void ShenandoahHeap::notify_gc_no_progress() {
  Atomic::inc(&_gc_no_progress_count);
}

inline size_t ShenandoahHeap::get_gc_no_progress_count() const {
  return Atomic::load(&_gc_no_progress_count);
}

inline size_t ShenandoahHeap::heap_region_index_containing(const void* addr) const {
  uintptr_t region_start = ((uintptr_t) addr);
  uintptr_t index = (region_start - (uintptr_t) base()) >> ShenandoahHeapRegion::region_size_bytes_shift();
  assert(index < num_regions(), "Region index is in bounds: " PTR_FORMAT, p2i(addr));
  return index;
}

inline ShenandoahHeapRegion* ShenandoahHeap::heap_region_containing(const void* addr) const {
  size_t index = heap_region_index_containing(addr);
  ShenandoahHeapRegion* const result = get_region(index);
  assert(addr >= result->bottom() && addr < result->end(), "Heap region contains the address: " PTR_FORMAT, p2i(addr));
  return result;
}

inline void ShenandoahHeap::enter_evacuation(Thread* t) {
  _oom_evac_handler.enter_evacuation(t);
}

inline void ShenandoahHeap::leave_evacuation(Thread* t) {
  _oom_evac_handler.leave_evacuation(t);
}

template <class T>
inline void ShenandoahHeap::non_conc_update_with_forwarded(T* p) {
  T o = RawAccess<>::oop_load(p);
  if (!CompressedOops::is_null(o)) {
    oop obj = CompressedOops::decode_not_null(o);
    if (in_collection_set(obj)) {
      // Corner case: when evacuation fails, there are objects in collection
      // set that are not really forwarded. We can still go and try and update them
      // (uselessly) to simplify the common path.
      shenandoah_assert_forwarded_except(p, obj, cancelled_gc());
      oop fwd = ShenandoahBarrierSet::resolve_forwarded_not_null(obj);
      shenandoah_assert_not_in_cset_except(p, fwd, cancelled_gc());

      // Unconditionally store the update: no concurrent updates expected.
      RawAccess<IS_NOT_NULL>::oop_store(p, fwd);
    }
  }
}

template <class T>
inline void ShenandoahHeap::conc_update_with_forwarded(T* p) {
  T o = RawAccess<>::oop_load(p);
  if (!CompressedOops::is_null(o)) {
    oop obj = CompressedOops::decode_not_null(o);
    if (in_collection_set(obj)) {
      // Corner case: when evacuation fails, there are objects in collection
      // set that are not really forwarded. We can still go and try CAS-update them
      // (uselessly) to simplify the common path.
      shenandoah_assert_forwarded_except(p, obj, cancelled_gc());
      oop fwd = ShenandoahBarrierSet::resolve_forwarded_not_null(obj);
      shenandoah_assert_not_in_cset_except(p, fwd, cancelled_gc());

      // Sanity check: we should not be updating the cset regions themselves,
      // unless we are recovering from the evacuation failure.
      shenandoah_assert_not_in_cset_loc_except(p, !is_in(p) || cancelled_gc());

      // Either we succeed in updating the reference, or something else gets in our way.
      // We don't care if that is another concurrent GC update, or another mutator update.
      atomic_update_oop(fwd, p, obj);
    }
  }
}

// Atomic updates of heap location. This is only expected to work with updating the same
// logical object with its forwardee. The reason why we need stronger-than-relaxed memory
// ordering has to do with coordination with GC barriers and mutator accesses.
//
// In essence, stronger CAS access is required to maintain the transitive chains that mutator
// accesses build by themselves. To illustrate this point, consider the following example.
//
// Suppose "o" is the object that has a field "x" and the reference to "o" is stored
// to field at "addr", which happens to be Java volatile field. Normally, the accesses to volatile
// field at "addr" would be matched with release/acquire barriers. This changes when GC moves
// the object under mutator feet.
//
// Thread 1 (Java)
//         // --- previous access starts here
//         ...
//   T1.1: store(&o.x, 1, mo_relaxed)
//   T1.2: store(&addr, o, mo_release) // volatile store
//
//         // --- new access starts here
//         // LRB: copy and install the new copy to fwdptr
//   T1.3: var copy = copy(o)
//   T1.4: cas(&fwd, t, copy, mo_release) // pointer-mediated publication
//         <access continues>
//
// Thread 2 (GC updater)
//   T2.1: var f = load(&fwd, mo_{consume|acquire}) // pointer-mediated acquisition
//   T2.2: cas(&addr, o, f, mo_release) // this method
//
// Thread 3 (Java)
//   T3.1: var o = load(&addr, mo_acquire) // volatile read
//   T3.2: if (o != null)
//   T3.3:   var r = load(&o.x, mo_relaxed)
//
// r is guaranteed to contain "1".
//
// Without GC involvement, there is synchronizes-with edge from T1.2 to T3.1,
// which guarantees this. With GC involvement, when LRB copies the object and
// another thread updates the reference to it, we need to have the transitive edge
// from T1.4 to T2.1 (that one is guaranteed by forwarding accesses), plus the edge
// from T2.2 to T3.1 (which is brought by this CAS).
//
// Note that we do not need to "acquire" in these methods, because we do not read the
// failure witnesses contents on any path, and "release" is enough.
//

inline void ShenandoahHeap::atomic_update_oop(oop update, oop* addr, oop compare) {
  assert(is_aligned(addr, HeapWordSize), "Address should be aligned: " PTR_FORMAT, p2i(addr));
  Atomic::cmpxchg(addr, compare, update, memory_order_release);
}

inline void ShenandoahHeap::atomic_update_oop(oop update, narrowOop* addr, narrowOop compare) {
  assert(is_aligned(addr, sizeof(narrowOop)), "Address should be aligned: " PTR_FORMAT, p2i(addr));
  narrowOop u = CompressedOops::encode(update);
  Atomic::cmpxchg(addr, compare, u, memory_order_release);
}

inline void ShenandoahHeap::atomic_update_oop(oop update, narrowOop* addr, oop compare) {
  assert(is_aligned(addr, sizeof(narrowOop)), "Address should be aligned: " PTR_FORMAT, p2i(addr));
  narrowOop c = CompressedOops::encode(compare);
  narrowOop u = CompressedOops::encode(update);
  Atomic::cmpxchg(addr, c, u, memory_order_release);
}

inline bool ShenandoahHeap::atomic_update_oop_check(oop update, oop* addr, oop compare) {
  assert(is_aligned(addr, HeapWordSize), "Address should be aligned: " PTR_FORMAT, p2i(addr));
  return (oop) Atomic::cmpxchg(addr, compare, update, memory_order_release) == compare;
}

inline bool ShenandoahHeap::atomic_update_oop_check(oop update, narrowOop* addr, narrowOop compare) {
  assert(is_aligned(addr, sizeof(narrowOop)), "Address should be aligned: " PTR_FORMAT, p2i(addr));
  narrowOop u = CompressedOops::encode(update);
  return (narrowOop) Atomic::cmpxchg(addr, compare, u, memory_order_release) == compare;
}

inline bool ShenandoahHeap::atomic_update_oop_check(oop update, narrowOop* addr, oop compare) {
  assert(is_aligned(addr, sizeof(narrowOop)), "Address should be aligned: " PTR_FORMAT, p2i(addr));
  narrowOop c = CompressedOops::encode(compare);
  narrowOop u = CompressedOops::encode(update);
  return CompressedOops::decode(Atomic::cmpxchg(addr, c, u, memory_order_release)) == compare;
}

// The memory ordering discussion above does not apply for methods that store nulls:
// then, there is no transitive reads in mutator (as we see nulls), and we can do
// relaxed memory ordering there.

inline void ShenandoahHeap::atomic_clear_oop(oop* addr, oop compare) {
  assert(is_aligned(addr, HeapWordSize), "Address should be aligned: " PTR_FORMAT, p2i(addr));
  Atomic::cmpxchg(addr, compare, oop(), memory_order_relaxed);
}

inline void ShenandoahHeap::atomic_clear_oop(narrowOop* addr, oop compare) {
  assert(is_aligned(addr, sizeof(narrowOop)), "Address should be aligned: " PTR_FORMAT, p2i(addr));
  narrowOop cmp = CompressedOops::encode(compare);
  Atomic::cmpxchg(addr, cmp, narrowOop(), memory_order_relaxed);
}

inline void ShenandoahHeap::atomic_clear_oop(narrowOop* addr, narrowOop compare) {
  assert(is_aligned(addr, sizeof(narrowOop)), "Address should be aligned: " PTR_FORMAT, p2i(addr));
  Atomic::cmpxchg(addr, compare, narrowOop(), memory_order_relaxed);
}

inline bool ShenandoahHeap::cancelled_gc() const {
  return _cancelled_gc.get() != GCCause::_no_gc;
}

inline bool ShenandoahHeap::check_cancelled_gc_and_yield(bool sts_active) {
  if (sts_active && !cancelled_gc()) {
    if (SuspendibleThreadSet::should_yield()) {
      SuspendibleThreadSet::yield();
    }
  }
  return cancelled_gc();
}

inline GCCause::Cause ShenandoahHeap::cancelled_cause() const {
  return _cancelled_gc.get();
}

inline void ShenandoahHeap::clear_cancelled_gc(bool clear_oom_handler) {
  _cancelled_gc.set(GCCause::_no_gc);
  if (_cancel_requested_time > 0) {
    log_debug(gc)("GC cancellation took %.3fs", (os::elapsedTime() - _cancel_requested_time));
    _cancel_requested_time = 0;
  }

  if (clear_oom_handler) {
    _oom_evac_handler.clear();
  }
}

inline HeapWord* ShenandoahHeap::allocate_from_gclab(Thread* thread, size_t size) {
  assert(UseTLAB, "TLABs should be enabled");

  PLAB* gclab = ShenandoahThreadLocalData::gclab(thread);
  if (gclab == nullptr) {
    assert(!thread->is_Java_thread() && !thread->is_Worker_thread(),
           "Performance: thread should have GCLAB: %s", thread->name());
    // No GCLABs in this thread, fallback to shared allocation
    return nullptr;
  }
  HeapWord* obj = gclab->allocate(size);
  if (obj != nullptr) {
    return obj;
  }
  return allocate_from_gclab_slow(thread, size);
}

void ShenandoahHeap::increase_object_age(oop obj, uint additional_age) {
  // This operates on new copy of an object. This means that the object's mark-word
  // is thread-local and therefore safe to access. However, when the mark is
  // displaced (i.e. stack-locked or monitor-locked), then it must be considered
  // a shared memory location. It can be accessed by other threads.
  // In particular, a competing evacuating thread can succeed to install its copy
  // as the forwardee and continue to unlock the object, at which point 'our'
  // write to the foreign stack-location would potentially over-write random
  // information on that stack. Writing to a monitor is less problematic,
  // but still not safe: while the ObjectMonitor would not randomly disappear,
  // the other thread would also write to the same displaced header location,
  // possibly leading to increase the age twice.
  // For all these reasons, we take the conservative approach and not attempt
  // to increase the age when the header is displaced.
  markWord w = obj->mark();
  // The mark-word has been copied from the original object. It can not be
  // inflating, because inflation can not be interrupted by a safepoint,
  // and after a safepoint, a Java thread would first have to successfully
  // evacuate the object before it could inflate the monitor.
  assert(!w.is_being_inflated() || LockingMode == LM_LIGHTWEIGHT, "must not inflate monitor before evacuation of object succeeds");
  // It is possible that we have copied the object after another thread has
  // already successfully completed evacuation. While harmless (we would never
  // publish our copy), don't even attempt to modify the age when that
  // happens.
  if (!w.has_displaced_mark_helper() && !w.is_marked()) {
    w = w.set_age(MIN2(markWord::max_age, w.age() + additional_age));
    obj->set_mark(w);
  }
}

// Return the object's age, or a sentinel value when the age can't
// necessarily be determined because of concurrent locking by the
// mutator
uint ShenandoahHeap::get_object_age(oop obj) {
  markWord w = obj->mark();
  assert(!w.is_marked(), "must not be forwarded");
  if (UseObjectMonitorTable) {
    assert(LockingMode == LM_LIGHTWEIGHT, "Must use LW locking, too");
    assert(w.age() <= markWord::max_age, "Impossible!");
    return w.age();
  }
  if (w.has_monitor()) {
    w = w.monitor()->header();
  } else if (w.is_being_inflated() || w.has_displaced_mark_helper()) {
    // Informs caller that we aren't able to determine the age
    return markWord::max_age + 1; // sentinel
  }
  assert(w.age() <= markWord::max_age, "Impossible!");
  return w.age();
}

inline bool ShenandoahHeap::is_in_active_generation(oop obj) const {
  if (!mode()->is_generational()) {
    // everything is the same single generation
    assert(is_in_reserved(obj), "Otherwise shouldn't return true below");
    return true;
  }

  ShenandoahGeneration* const gen = active_generation();

  if (gen == nullptr) {
    // no collection is happening: only expect this to be called
    // when concurrent processing is active, but that could change
    return false;
  }

  assert(is_in_reserved(obj), "only check if is in active generation for objects (" PTR_FORMAT ") in heap", p2i(obj));
  assert(gen->is_old() || gen->is_young() || gen->is_global(),
         "Active generation must be old, young, or global");

  size_t index = heap_region_containing(obj)->index();

  // No flickering!
  assert(gen == active_generation(), "Race?");

  switch (region_affiliation(index)) {
  case ShenandoahAffiliation::FREE:
    // Free regions are in old, young, and global collections
    return true;
  case ShenandoahAffiliation::YOUNG_GENERATION:
    // Young regions are in young and global collections, not in old collections
    return !gen->is_old();
  case ShenandoahAffiliation::OLD_GENERATION:
    // Old regions are in old and global collections, not in young collections
    return !gen->is_young();
  default:
    assert(false, "Bad affiliation (%d) for region %zu", region_affiliation(index), index);
    return false;
  }
}

inline bool ShenandoahHeap::is_in_young(const void* p) const {
  return is_in_reserved(p) && (region_affiliation(heap_region_index_containing(p)) == ShenandoahAffiliation::YOUNG_GENERATION);
}

inline bool ShenandoahHeap::is_in_old(const void* p) const {
  return is_in_reserved(p) && (region_affiliation(heap_region_index_containing(p)) == ShenandoahAffiliation::OLD_GENERATION);
}

inline bool ShenandoahHeap::is_in_old_during_young_collection(oop obj) const {
  return active_generation()->is_young() && is_in_old(obj);
}

inline ShenandoahAffiliation ShenandoahHeap::region_affiliation(const ShenandoahHeapRegion *r) const {
  return region_affiliation(r->index());
}

inline void ShenandoahHeap::assert_lock_for_affiliation(ShenandoahAffiliation orig_affiliation,
                                                        ShenandoahAffiliation new_affiliation) {
  // A lock is required when changing from FREE to NON-FREE.  Though it may be possible to elide the lock when
  // transitioning from in-use to FREE, the current implementation uses a lock for this transition.  A lock is
  // not required to change from YOUNG to OLD (i.e. when promoting humongous region).
  //
  //         new_affiliation is:     FREE   YOUNG   OLD
  //  orig_affiliation is:  FREE      X       L      L
  //                       YOUNG      L       X
  //                         OLD      L       X      X
  //  X means state transition won't happen (so don't care)
  //  L means lock should be held
  //  Blank means no lock required because affiliation visibility will not be required until subsequent safepoint
  //
  // Note: during full GC, all transitions between states are possible.  During Full GC, we should be in a safepoint.

  if (orig_affiliation == ShenandoahAffiliation::FREE) {
    shenandoah_assert_heaplocked_or_safepoint();
  }
}

inline void ShenandoahHeap::set_affiliation(ShenandoahHeapRegion* r, ShenandoahAffiliation new_affiliation) {
#ifdef ASSERT
  assert_lock_for_affiliation(region_affiliation(r), new_affiliation);
#endif
  Atomic::store(_affiliations + r->index(), (uint8_t) new_affiliation);
}

inline ShenandoahAffiliation ShenandoahHeap::region_affiliation(size_t index) const {
  return (ShenandoahAffiliation) Atomic::load(_affiliations + index);
}

inline bool ShenandoahHeap::requires_marking(const void* entry) const {
  oop obj = cast_to_oop(entry);
  return !_marking_context->is_marked_strong(obj);
}

inline bool ShenandoahHeap::in_collection_set(oop p) const {
  assert(collection_set() != nullptr, "Sanity");
  return collection_set()->is_in(p);
}

inline bool ShenandoahHeap::in_collection_set_loc(void* p) const {
  assert(collection_set() != nullptr, "Sanity");
  return collection_set()->is_in_loc(p);
}

inline bool ShenandoahHeap::is_idle() const {
  return _gc_state_changed ? _gc_state.is_clear() : ShenandoahThreadLocalData::gc_state(Thread::current()) == 0;
}

inline bool ShenandoahHeap::has_forwarded_objects() const {
  return is_gc_state(HAS_FORWARDED);
}

inline bool ShenandoahHeap::is_concurrent_mark_in_progress() const {
  return is_gc_state(MARKING);
}

inline bool ShenandoahHeap::is_concurrent_young_mark_in_progress() const {
  return is_gc_state(YOUNG_MARKING);
}

inline bool ShenandoahHeap::is_concurrent_old_mark_in_progress() const {
  return is_gc_state(OLD_MARKING);
}

inline bool ShenandoahHeap::is_evacuation_in_progress() const {
  return is_gc_state(EVACUATION);
}

inline bool ShenandoahHeap::is_update_refs_in_progress() const {
  return is_gc_state(UPDATE_REFS);
}

inline bool ShenandoahHeap::is_concurrent_weak_root_in_progress() const {
  return is_gc_state(WEAK_ROOTS);
}

inline bool ShenandoahHeap::is_degenerated_gc_in_progress() const {
  return _degenerated_gc_in_progress.is_set();
}

inline bool ShenandoahHeap::is_full_gc_in_progress() const {
  return _full_gc_in_progress.is_set();
}

inline bool ShenandoahHeap::is_full_gc_move_in_progress() const {
  return _full_gc_move_in_progress.is_set();
}

inline bool ShenandoahHeap::is_stw_gc_in_progress() const {
  return is_full_gc_in_progress() || is_degenerated_gc_in_progress();
}

inline bool ShenandoahHeap::is_concurrent_strong_root_in_progress() const {
  return _concurrent_strong_root_in_progress.is_set();
}

template<class T>
inline void ShenandoahHeap::marked_object_iterate(ShenandoahHeapRegion* region, T* cl) {
  marked_object_iterate(region, cl, region->top());
}

template<class T>
inline void ShenandoahHeap::marked_object_iterate(ShenandoahHeapRegion* region, T* cl, HeapWord* limit) {
  assert(! region->is_humongous_continuation(), "no humongous continuation regions here");

  ShenandoahMarkingContext* const ctx = marking_context();

  HeapWord* tams = ctx->top_at_mark_start(region);

  size_t skip_bitmap_delta = 1;
  HeapWord* start = region->bottom();
  HeapWord* end = MIN2(tams, region->end());

  // Step 1. Scan below the TAMS based on bitmap data.
  HeapWord* limit_bitmap = MIN2(limit, tams);

  // Try to scan the initial candidate. If the candidate is above the TAMS, it would
  // fail the subsequent "< limit_bitmap" checks, and fall through to Step 2.
  HeapWord* cb = ctx->get_next_marked_addr(start, end);

  intx dist = ShenandoahMarkScanPrefetch;
  if (dist > 0) {
    // Batched scan that prefetches the oop data, anticipating the access to
    // either header, oop field, or forwarding pointer. Not that we cannot
    // touch anything in oop, while it still being prefetched to get enough
    // time for prefetch to work. This is why we try to scan the bitmap linearly,
    // disregarding the object size. However, since we know forwarding pointer
    // precedes the object, we can skip over it. Once we cannot trust the bitmap,
    // there is no point for prefetching the oop contents, as oop->size() will
    // touch it prematurely.

    // No variable-length arrays in standard C++, have enough slots to fit
    // the prefetch distance.
    static const int SLOT_COUNT = 256;
    guarantee(dist <= SLOT_COUNT, "adjust slot count");
    HeapWord* slots[SLOT_COUNT];

    int avail;
    do {
      avail = 0;
      for (int c = 0; (c < dist) && (cb < limit_bitmap); c++) {
        Prefetch::read(cb, oopDesc::mark_offset_in_bytes());
        slots[avail++] = cb;
        cb += skip_bitmap_delta;
        if (cb < limit_bitmap) {
          cb = ctx->get_next_marked_addr(cb, limit_bitmap);
        }
      }

      for (int c = 0; c < avail; c++) {
        assert (slots[c] < tams,  "only objects below TAMS here: "  PTR_FORMAT " (" PTR_FORMAT ")", p2i(slots[c]), p2i(tams));
        assert (slots[c] < limit, "only objects below limit here: " PTR_FORMAT " (" PTR_FORMAT ")", p2i(slots[c]), p2i(limit));
        oop obj = cast_to_oop(slots[c]);
        assert(oopDesc::is_oop(obj), "sanity");
        assert(ctx->is_marked(obj), "object expected to be marked");
        cl->do_object(obj);
      }
    } while (avail > 0);
  } else {
    while (cb < limit_bitmap) {
      assert (cb < tams,  "only objects below TAMS here: "  PTR_FORMAT " (" PTR_FORMAT ")", p2i(cb), p2i(tams));
      assert (cb < limit, "only objects below limit here: " PTR_FORMAT " (" PTR_FORMAT ")", p2i(cb), p2i(limit));
      oop obj = cast_to_oop(cb);
      assert(oopDesc::is_oop(obj), "sanity");
      assert(ctx->is_marked(obj), "object expected to be marked");
      cl->do_object(obj);
      cb += skip_bitmap_delta;
      if (cb < limit_bitmap) {
        cb = ctx->get_next_marked_addr(cb, limit_bitmap);
      }
    }
  }

  // Step 2. Accurate size-based traversal, happens past the TAMS.
  // This restarts the scan at TAMS, which makes sure we traverse all objects,
  // regardless of what happened at Step 1.
  HeapWord* cs = tams;
  while (cs < limit) {
    assert (cs >= tams, "only objects past TAMS here: "   PTR_FORMAT " (" PTR_FORMAT ")", p2i(cs), p2i(tams));
    assert (cs < limit, "only objects below limit here: " PTR_FORMAT " (" PTR_FORMAT ")", p2i(cs), p2i(limit));
    oop obj = cast_to_oop(cs);
    assert(oopDesc::is_oop(obj), "sanity");
    assert(ctx->is_marked(obj), "object expected to be marked");
    size_t size = ShenandoahForwarding::size(obj);
    cl->do_object(obj);
    cs += size;
  }
}

template <class T>
class ShenandoahObjectToOopClosure : public ObjectClosure {
  T* _cl;
public:
  ShenandoahObjectToOopClosure(T* cl) : _cl(cl) {}

  void do_object(oop obj) {
    obj->oop_iterate(_cl);
  }
};

template <class T>
class ShenandoahObjectToOopBoundedClosure : public ObjectClosure {
  T* _cl;
  MemRegion _bounds;
public:
  ShenandoahObjectToOopBoundedClosure(T* cl, HeapWord* bottom, HeapWord* top) :
    _cl(cl), _bounds(bottom, top) {}

  void do_object(oop obj) {
    obj->oop_iterate(_cl, _bounds);
  }
};

template<class T>
inline void ShenandoahHeap::marked_object_oop_iterate(ShenandoahHeapRegion* region, T* cl, HeapWord* top) {
  if (region->is_humongous()) {
    HeapWord* bottom = region->bottom();
    if (top > bottom) {
      region = region->humongous_start_region();
      ShenandoahObjectToOopBoundedClosure<T> objs(cl, bottom, top);
      marked_object_iterate(region, &objs);
    }
  } else {
    ShenandoahObjectToOopClosure<T> objs(cl);
    marked_object_iterate(region, &objs, top);
  }
}

inline ShenandoahHeapRegion* ShenandoahHeap::get_region(size_t region_idx) const {
  if (region_idx < _num_regions) {
    return _regions[region_idx];
  } else {
    return nullptr;
  }
}

inline ShenandoahMarkingContext* ShenandoahHeap::marking_context() const {
  return _marking_context;
}

#endif // SHARE_GC_SHENANDOAH_SHENANDOAHHEAP_INLINE_HPP