runtime/gc/collector/semi_space.cc

/*
 * Copyright (C) 2013 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "semi_space-inl.h"

#include <functional>
#include <numeric>
#include <climits>
#include <vector>

#include "base/logging.h"
#include "base/macros.h"
#include "base/mutex-inl.h"
#include "base/timing_logger.h"
#include "gc/accounting/heap_bitmap-inl.h"
#include "gc/accounting/mod_union_table.h"
#include "gc/accounting/remembered_set.h"
#include "gc/accounting/space_bitmap-inl.h"
#include "gc/heap.h"
#include "gc/space/bump_pointer_space.h"
#include "gc/space/bump_pointer_space-inl.h"
#include "gc/space/image_space.h"
#include "gc/space/large_object_space.h"
#include "gc/space/space-inl.h"
#include "indirect_reference_table.h"
#include "intern_table.h"
#include "jni_internal.h"
#include "mark_sweep-inl.h"
#include "monitor.h"
#include "mirror/art_field.h"
#include "mirror/art_field-inl.h"
#include "mirror/class-inl.h"
#include "mirror/class_loader.h"
#include "mirror/dex_cache.h"
#include "mirror/reference-inl.h"
#include "mirror/object-inl.h"
#include "mirror/object_array.h"
#include "mirror/object_array-inl.h"
#include "runtime.h"
#include "stack.h"
#include "thread-inl.h"
#include "thread_list.h"
#include "verifier/method_verifier.h"

using ::art::mirror::Class;
using ::art::mirror::Object;

namespace art {
namespace gc {
namespace collector {

static constexpr bool kProtectFromSpace = true;
static constexpr bool kStoreStackTraces = false;
static constexpr bool kUseBytesPromoted = true;
static constexpr size_t kBytesPromotedThreshold = 4 * MB;

void SemiSpace::BindBitmaps() {
  timings_.StartSplit("BindBitmaps");
  WriterMutexLock mu(self_, *Locks::heap_bitmap_lock_);
  // Mark all of the spaces we never collect as immune.
  for (const auto& space : GetHeap()->GetContinuousSpaces()) {
    if (space->GetLiveBitmap() != nullptr) {
      if (space == to_space_) {
        CHECK(to_space_->IsContinuousMemMapAllocSpace());
        to_space_->AsContinuousMemMapAllocSpace()->BindLiveToMarkBitmap();
      } else if (space->GetGcRetentionPolicy() == space::kGcRetentionPolicyNeverCollect
                 || space->GetGcRetentionPolicy() == space::kGcRetentionPolicyFullCollect
                 // Add the main free list space and the non-moving
                 // space to the immune space if a bump pointer space
                 // only collection.
                 || (generational_ && !whole_heap_collection_ &&
                     (space == GetHeap()->GetNonMovingSpace() ||
                      space == GetHeap()->GetPrimaryFreeListSpace()))) {
        CHECK(immune_region_.AddContinuousSpace(space)) << "Failed to add space " << *space;
      }
    }
  }
  if (generational_ && !whole_heap_collection_) {
    // We won't collect the large object space if a bump pointer space only collection.
    is_large_object_space_immune_ = true;
  }
  timings_.EndSplit();
}

SemiSpace::SemiSpace(Heap* heap, bool generational, const std::string& name_prefix)
    : GarbageCollector(heap,
                       name_prefix + (name_prefix.empty() ? "" : " ") + "marksweep + semispace"),
      to_space_(nullptr),
      from_space_(nullptr),
      generational_(generational),
      last_gc_to_space_end_(nullptr),
      bytes_promoted_(0),
      bytes_promoted_since_last_whole_heap_collection_(0),
      whole_heap_collection_(true),
      whole_heap_collection_interval_counter_(0),
      collector_name_(name_) {
}

void SemiSpace::InitializePhase() {
  timings_.Reset();
  TimingLogger::ScopedSplit split("InitializePhase", &timings_);
  mark_stack_ = heap_->mark_stack_.get();
  DCHECK(mark_stack_ != nullptr);
  immune_region_.Reset();
  is_large_object_space_immune_ = false;
  saved_bytes_ = 0;
  self_ = Thread::Current();
  // Do any pre GC verification.
  timings_.NewSplit("PreGcVerification");
  heap_->PreGcVerification(this);
  CHECK(from_space_->CanMoveObjects()) << "Attempting to move from " << *from_space_;
  // Set the initial bitmap.
  to_space_live_bitmap_ = to_space_->GetLiveBitmap();
}

void SemiSpace::ProcessReferences(Thread* self) {
  TimingLogger::ScopedSplit split("ProcessReferences", &timings_);
  WriterMutexLock mu(self, *Locks::heap_bitmap_lock_);
  GetHeap()->ProcessReferences(timings_, clear_soft_references_, &MarkedForwardingAddressCallback,
                               &MarkObjectCallback, &ProcessMarkStackCallback, this);
}

void SemiSpace::MarkingPhase() {
  if (kStoreStackTraces) {
    Locks::mutator_lock_->AssertExclusiveHeld(self_);
    // Store the stack traces into the runtime fault string in case we get a heap corruption
    // related crash later.
    ThreadState old_state = self_->SetStateUnsafe(kRunnable);
    std::ostringstream oss;
    Runtime* runtime = Runtime::Current();
    runtime->GetThreadList()->DumpForSigQuit(oss);
    runtime->GetThreadList()->DumpNativeStacks(oss);
    runtime->SetFaultMessage(oss.str());
    CHECK_EQ(self_->SetStateUnsafe(old_state), kRunnable);
  }

  if (generational_) {
    if (gc_cause_ == kGcCauseExplicit || gc_cause_ == kGcCauseForNativeAlloc ||
        clear_soft_references_) {
      // If an explicit, native allocation-triggered, or last attempt
      // collection, collect the whole heap (and reset the interval
      // counter to be consistent.)
      whole_heap_collection_ = true;
      if (!kUseBytesPromoted) {
        whole_heap_collection_interval_counter_ = 0;
      }
    }
    if (whole_heap_collection_) {
      VLOG(heap) << "Whole heap collection";
      name_ = collector_name_ + " whole";
    } else {
      VLOG(heap) << "Bump pointer space only collection";
      name_ = collector_name_ + " bps";
    }
  }

  if (!clear_soft_references_) {
    if (!generational_) {
      // If non-generational, always clear soft references.
      clear_soft_references_ = true;
    } else {
      // If generational, clear soft references if a whole heap collection.
      if (whole_heap_collection_) {
        clear_soft_references_ = true;
      }
    }
  }

  Locks::mutator_lock_->AssertExclusiveHeld(self_);

  TimingLogger::ScopedSplit split("MarkingPhase", &timings_);
  if (generational_) {
    // If last_gc_to_space_end_ is out of the bounds of the from-space
    // (the to-space from last GC), then point it to the beginning of
    // the from-space. For example, the very first GC or the
    // pre-zygote compaction.
    if (!from_space_->HasAddress(reinterpret_cast<mirror::Object*>(last_gc_to_space_end_))) {
      last_gc_to_space_end_ = from_space_->Begin();
    }
    // Reset this before the marking starts below.
    bytes_promoted_ = 0;
  }
  // Assume the cleared space is already empty.
  BindBitmaps();
  // Process dirty cards and add dirty cards to mod-union tables.
  heap_->ProcessCards(timings_, kUseRememberedSet && generational_);
  // Clear the whole card table since we can not get any additional dirty cards during the
  // paused GC. This saves memory but only works for pause the world collectors.
  timings_.NewSplit("ClearCardTable");
  heap_->GetCardTable()->ClearCardTable();
  // Need to do this before the checkpoint since we don't want any threads to add references to
  // the live stack during the recursive mark.
  timings_.NewSplit("SwapStacks");
  if (kUseThreadLocalAllocationStack) {
    heap_->RevokeAllThreadLocalAllocationStacks(self_);
  }
  heap_->SwapStacks(self_);
  WriterMutexLock mu(self_, *Locks::heap_bitmap_lock_);
  MarkRoots();
  // Mark roots of immune spaces.
  UpdateAndMarkModUnion();
  // Recursively mark remaining objects.
  MarkReachableObjects();
}

void SemiSpace::UpdateAndMarkModUnion() {
  for (auto& space : heap_->GetContinuousSpaces()) {
    // If the space is immune then we need to mark the references to other spaces.
    if (immune_region_.ContainsSpace(space)) {
      accounting::ModUnionTable* table = heap_->FindModUnionTableFromSpace(space);
      if (table != nullptr) {
        // TODO: Improve naming.
        TimingLogger::ScopedSplit split(
            space->IsZygoteSpace() ? "UpdateAndMarkZygoteModUnionTable" :
                                     "UpdateAndMarkImageModUnionTable",
                                     &timings_);
        table->UpdateAndMarkReferences(MarkHeapReferenceCallback, this);
      } else if (heap_->FindRememberedSetFromSpace(space) != nullptr) {
        DCHECK(kUseRememberedSet);
        // If a bump pointer space only collection, the non-moving
        // space is added to the immune space. The non-moving space
        // doesn't have a mod union table, but has a remembered
        // set. Its dirty cards will be scanned later in
        // MarkReachableObjects().
        DCHECK(generational_ && !whole_heap_collection_ &&
               (space == heap_->GetNonMovingSpace() || space == heap_->GetPrimaryFreeListSpace()))
            << "Space " << space->GetName() << " "
            << "generational_=" << generational_ << " "
            << "whole_heap_collection_=" << whole_heap_collection_ << " ";
      } else {
        DCHECK(!kUseRememberedSet);
        // If a bump pointer space only collection, the non-moving
        // space is added to the immune space. But the non-moving
        // space doesn't have a mod union table. Instead, its live
        // bitmap will be scanned later in MarkReachableObjects().
        DCHECK(generational_ && !whole_heap_collection_ &&
               (space == heap_->GetNonMovingSpace() || space == heap_->GetPrimaryFreeListSpace()))
            << "Space " << space->GetName() << " "
            << "generational_=" << generational_ << " "
            << "whole_heap_collection_=" << whole_heap_collection_ << " ";
      }
    }
  }
}

class SemiSpaceScanObjectVisitor {
 public:
  explicit SemiSpaceScanObjectVisitor(SemiSpace* ss) : semi_space_(ss) {}
  void operator()(Object* obj) const SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)
      EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_) {
    // TODO: fix NO_THREAD_SAFETY_ANALYSIS. ScanObject() requires an
    // exclusive lock on the mutator lock, but
    // SpaceBitmap::VisitMarkedRange() only requires the shared lock.
    DCHECK(obj != nullptr);
    semi_space_->ScanObject(obj);
  }
 private:
  SemiSpace* const semi_space_;
};

// Used to verify that there's no references to the from-space.
class SemiSpaceVerifyNoFromSpaceReferencesVisitor {
 public:
  explicit SemiSpaceVerifyNoFromSpaceReferencesVisitor(space::ContinuousMemMapAllocSpace* from_space) :
      from_space_(from_space) {}

  void operator()(Object* obj, MemberOffset offset, bool /* is_static */) const
      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) ALWAYS_INLINE {
    mirror::Object* ref = obj->GetFieldObject<mirror::Object>(offset, false);
    if (from_space_->HasAddress(ref)) {
      Runtime::Current()->GetHeap()->DumpObject(LOG(INFO), obj);
      LOG(FATAL) << ref << " found in from space";
    }
  }
 private:
  space::ContinuousMemMapAllocSpace* from_space_;
};

void SemiSpace::VerifyNoFromSpaceReferences(Object* obj) {
  DCHECK(!from_space_->HasAddress(obj)) << "Scanning object " << obj << " in from space";
  SemiSpaceVerifyNoFromSpaceReferencesVisitor visitor(from_space_);
  obj->VisitReferences<kMovingClasses>(visitor);
}

class SemiSpaceVerifyNoFromSpaceReferencesObjectVisitor {
 public:
  explicit SemiSpaceVerifyNoFromSpaceReferencesObjectVisitor(SemiSpace* ss) : semi_space_(ss) {}
  void operator()(Object* obj) const
      SHARED_LOCKS_REQUIRED(Locks::heap_bitmap_lock_, Locks::mutator_lock_) {
    DCHECK(obj != nullptr);
    semi_space_->VerifyNoFromSpaceReferences(obj);
  }
 private:
  SemiSpace* const semi_space_;
};

void SemiSpace::MarkReachableObjects() {
  timings_.StartSplit("MarkStackAsLive");
  accounting::ObjectStack* live_stack = heap_->GetLiveStack();
  heap_->MarkAllocStackAsLive(live_stack);
  live_stack->Reset();
  timings_.EndSplit();

  for (auto& space : heap_->GetContinuousSpaces()) {
    // If the space is immune and has no mod union table (the
    // non-moving space when the bump pointer space only collection is
    // enabled,) then we need to scan its live bitmap or dirty cards as roots
    // (including the objects on the live stack which have just marked
    // in the live bitmap above in MarkAllocStackAsLive().)
    if (immune_region_.ContainsSpace(space) &&
        heap_->FindModUnionTableFromSpace(space) == nullptr) {
      DCHECK(generational_ && !whole_heap_collection_ &&
             (space == GetHeap()->GetNonMovingSpace() || space == GetHeap()->GetPrimaryFreeListSpace()));
      accounting::RememberedSet* rem_set = heap_->FindRememberedSetFromSpace(space);
      if (kUseRememberedSet) {
        DCHECK(rem_set != nullptr);
        rem_set->UpdateAndMarkReferences(MarkHeapReferenceCallback, from_space_, this);
        if (kIsDebugBuild) {
          // Verify that there are no from-space references that
          // remain in the space, that is, the remembered set (and the
          // card table) didn't miss any from-space references in the
          // space.
          accounting::ContinuousSpaceBitmap* live_bitmap = space->GetLiveBitmap();
          SemiSpaceVerifyNoFromSpaceReferencesObjectVisitor visitor(this);
          live_bitmap->VisitMarkedRange(reinterpret_cast<uintptr_t>(space->Begin()),
                                        reinterpret_cast<uintptr_t>(space->End()),
                                        visitor);
        }
      } else {
        DCHECK(rem_set == nullptr);
        accounting::ContinuousSpaceBitmap* live_bitmap = space->GetLiveBitmap();
        SemiSpaceScanObjectVisitor visitor(this);
        live_bitmap->VisitMarkedRange(reinterpret_cast<uintptr_t>(space->Begin()),
                                      reinterpret_cast<uintptr_t>(space->End()),
                                      visitor);
      }
    }
  }

  if (is_large_object_space_immune_) {
    DCHECK(generational_ && !whole_heap_collection_);
    // Delay copying the live set to the marked set until here from
    // BindBitmaps() as the large objects on the allocation stack may
    // be newly added to the live set above in MarkAllocStackAsLive().
    GetHeap()->GetLargeObjectsSpace()->CopyLiveToMarked();

    // When the large object space is immune, we need to scan the
    // large object space as roots as they contain references to their
    // classes (primitive array classes) that could move though they
    // don't contain any other references.
    space::LargeObjectSpace* large_object_space = GetHeap()->GetLargeObjectsSpace();
    accounting::ObjectSet* large_live_objects = large_object_space->GetLiveObjects();
    SemiSpaceScanObjectVisitor visitor(this);
    for (const Object* obj : large_live_objects->GetObjects()) {
      visitor(const_cast<Object*>(obj));
    }
  }

  // Recursively process the mark stack.
  ProcessMarkStack();
}

void SemiSpace::ReclaimPhase() {
  TimingLogger::ScopedSplit split("ReclaimPhase", &timings_);
  ProcessReferences(self_);
  {
    ReaderMutexLock mu(self_, *Locks::heap_bitmap_lock_);
    SweepSystemWeaks();
  }
  // Record freed memory.
  uint64_t from_bytes = from_space_->GetBytesAllocated();
  uint64_t to_bytes = to_space_->GetBytesAllocated();
  uint64_t from_objects = from_space_->GetObjectsAllocated();
  uint64_t to_objects = to_space_->GetObjectsAllocated();
  CHECK_LE(to_objects, from_objects);
  int64_t freed_bytes = from_bytes - to_bytes;
  int64_t freed_objects = from_objects - to_objects;
  freed_bytes_.FetchAndAdd(freed_bytes);
  freed_objects_.FetchAndAdd(freed_objects);
  // Note: Freed bytes can be negative if we copy form a compacted space to a free-list backed
  // space.
  heap_->RecordFree(freed_objects, freed_bytes);

  timings_.StartSplit("PreSweepingGcVerification");
  heap_->PreSweepingGcVerification(this);
  timings_.EndSplit();
  {
    WriterMutexLock mu(self_, *Locks::heap_bitmap_lock_);
    // Reclaim unmarked objects.
    Sweep(false);
    // Swap the live and mark bitmaps for each space which we modified space. This is an
    // optimization that enables us to not clear live bits inside of the sweep. Only swaps unbound
    // bitmaps.
    timings_.StartSplit("SwapBitmaps");
    SwapBitmaps();
    timings_.EndSplit();
    // Unbind the live and mark bitmaps.
    TimingLogger::ScopedSplit split("UnBindBitmaps", &timings_);
    GetHeap()->UnBindBitmaps();
  }
  // TODO: Do this before doing verification since the from space may have objects which weren't
  // moved and point to dead objects.
  from_space_->Clear();
  // Protect the from space.
  VLOG(heap) << "Protecting space " << *from_space_;
  if (kProtectFromSpace) {
    from_space_->GetMemMap()->Protect(PROT_NONE);
  } else {
    from_space_->GetMemMap()->Protect(PROT_READ);
  }
  if (saved_bytes_ > 0) {
    VLOG(heap) << "Avoided dirtying " << PrettySize(saved_bytes_);
  }

  if (generational_) {
    // Record the end (top) of the to space so we can distinguish
    // between objects that were allocated since the last GC and the
    // older objects.
    last_gc_to_space_end_ = to_space_->End();
  }
}

void SemiSpace::ResizeMarkStack(size_t new_size) {
  std::vector<Object*> temp(mark_stack_->Begin(), mark_stack_->End());
  CHECK_LE(mark_stack_->Size(), new_size);
  mark_stack_->Resize(new_size);
  for (const auto& obj : temp) {
    mark_stack_->PushBack(obj);
  }
}

inline void SemiSpace::MarkStackPush(Object* obj) {
  if (UNLIKELY(mark_stack_->Size() >= mark_stack_->Capacity())) {
    ResizeMarkStack(mark_stack_->Capacity() * 2);
  }
  // The object must be pushed on to the mark stack.
  mark_stack_->PushBack(obj);
}

// Rare case, probably not worth inlining since it will increase instruction cache miss rate.
bool SemiSpace::MarkLargeObject(const Object* obj) {
  // TODO: support >1 discontinuous space.
  space::LargeObjectSpace* large_object_space = GetHeap()->GetLargeObjectsSpace();
  DCHECK(large_object_space->Contains(obj));
  accounting::ObjectSet* large_objects = large_object_space->GetMarkObjects();
  if (UNLIKELY(!large_objects->Test(obj))) {
    large_objects->Set(obj);
    return true;
  }
  return false;
}

static inline size_t CopyAvoidingDirtyingPages(void* dest, const void* src, size_t size) {
  if (LIKELY(size <= static_cast<size_t>(kPageSize))) {
    // We will dirty the current page and somewhere in the middle of the next page. This means
    // that the next object copied will also dirty that page.
    // TODO: Worth considering the last object copied? We may end up dirtying one page which is
    // not necessary per GC.
    memcpy(dest, src, size);
    return 0;
  }
  size_t saved_bytes = 0;
  byte* byte_dest = reinterpret_cast<byte*>(dest);
  if (kIsDebugBuild) {
    for (size_t i = 0; i < size; ++i) {
      CHECK_EQ(byte_dest[i], 0U);
    }
  }
  // Process the start of the page. The page must already be dirty, don't bother with checking.
  const byte* byte_src = reinterpret_cast<const byte*>(src);
  const byte* limit = byte_src + size;
  size_t page_remain = AlignUp(byte_dest, kPageSize) - byte_dest;
  // Copy the bytes until the start of the next page.
  memcpy(dest, src, page_remain);
  byte_src += page_remain;
  byte_dest += page_remain;
  DCHECK_ALIGNED(reinterpret_cast<uintptr_t>(byte_dest), kPageSize);
  DCHECK_ALIGNED(reinterpret_cast<uintptr_t>(byte_dest), sizeof(uintptr_t));
  DCHECK_ALIGNED(reinterpret_cast<uintptr_t>(byte_src), sizeof(uintptr_t));
  while (byte_src + kPageSize < limit) {
    bool all_zero = true;
    uintptr_t* word_dest = reinterpret_cast<uintptr_t*>(byte_dest);
    const uintptr_t* word_src = reinterpret_cast<const uintptr_t*>(byte_src);
    for (size_t i = 0; i < kPageSize / sizeof(*word_src); ++i) {
      // Assumes the destination of the copy is all zeros.
      if (word_src[i] != 0) {
        all_zero = false;
        word_dest[i] = word_src[i];
      }
    }
    if (all_zero) {
      // Avoided copying into the page since it was all zeros.
      saved_bytes += kPageSize;
    }
    byte_src += kPageSize;
    byte_dest += kPageSize;
  }
  // Handle the part of the page at the end.
  memcpy(byte_dest, byte_src, limit - byte_src);
  return saved_bytes;
}

mirror::Object* SemiSpace::MarkNonForwardedObject(mirror::Object* obj) {
  size_t object_size = obj->SizeOf();
  size_t bytes_allocated;
  mirror::Object* forward_address = nullptr;
  if (generational_ && reinterpret_cast<byte*>(obj) < last_gc_to_space_end_) {
    // If it's allocated before the last GC (older), move
    // (pseudo-promote) it to the main free list space (as sort
    // of an old generation.)
    size_t bytes_promoted;
    space::MallocSpace* promo_dest_space = GetHeap()->GetPrimaryFreeListSpace();
    forward_address = promo_dest_space->Alloc(self_, object_size, &bytes_promoted, nullptr);
    if (forward_address == nullptr) {
      // If out of space, fall back to the to-space.
      forward_address = to_space_->Alloc(self_, object_size, &bytes_allocated, nullptr);
    } else {
      GetHeap()->num_bytes_allocated_.FetchAndAdd(bytes_promoted);
      bytes_promoted_ += bytes_promoted;
      // Dirty the card at the destionation as it may contain
      // references (including the class pointer) to the bump pointer
      // space.
      GetHeap()->WriteBarrierEveryFieldOf(forward_address);
      // Handle the bitmaps marking.
      accounting::ContinuousSpaceBitmap* live_bitmap = promo_dest_space->GetLiveBitmap();
      DCHECK(live_bitmap != nullptr);
      accounting::ContinuousSpaceBitmap* mark_bitmap = promo_dest_space->GetMarkBitmap();
      DCHECK(mark_bitmap != nullptr);
      DCHECK(!live_bitmap->Test(forward_address));
      if (!whole_heap_collection_) {
        // If collecting the bump pointer spaces only, live_bitmap == mark_bitmap.
        DCHECK_EQ(live_bitmap, mark_bitmap);

        // If a bump pointer space only collection, delay the live
        // bitmap marking of the promoted object until it's popped off
        // the mark stack (ProcessMarkStack()). The rationale: we may
        // be in the middle of scanning the objects in the promo
        // destination space for
        // non-moving-space-to-bump-pointer-space references by
        // iterating over the marked bits of the live bitmap
        // (MarkReachableObjects()). If we don't delay it (and instead
        // mark the promoted object here), the above promo destination
        // space scan could encounter the just-promoted object and
        // forward the references in the promoted object's fields even
        // through it is pushed onto the mark stack. If this happens,
        // the promoted object would be in an inconsistent state, that
        // is, it's on the mark stack (gray) but its fields are
        // already forwarded (black), which would cause a
        // DCHECK(!to_space_->HasAddress(obj)) failure below.
      } else {
        // Mark forward_address on the live bit map.
        live_bitmap->Set(forward_address);
        // Mark forward_address on the mark bit map.
        DCHECK(!mark_bitmap->Test(forward_address));
        mark_bitmap->Set(forward_address);
      }
    }
    DCHECK(forward_address != nullptr);
  } else {
    // If it's allocated after the last GC (younger), copy it to the to-space.
    forward_address = to_space_->Alloc(self_, object_size, &bytes_allocated, nullptr);
  }
  // Copy over the object and add it to the mark stack since we still need to update its
  // references.
  saved_bytes_ +=
      CopyAvoidingDirtyingPages(reinterpret_cast<void*>(forward_address), obj, object_size);
  if (kUseBakerOrBrooksReadBarrier) {
    obj->AssertReadBarrierPointer();
    if (kUseBrooksReadBarrier) {
      DCHECK_EQ(forward_address->GetReadBarrierPointer(), obj);
      forward_address->SetReadBarrierPointer(forward_address);
    }
    forward_address->AssertReadBarrierPointer();
  }
  if (to_space_live_bitmap_ != nullptr) {
    to_space_live_bitmap_->Set(forward_address);
  }
  DCHECK(to_space_->HasAddress(forward_address) ||
         (generational_ && GetHeap()->GetPrimaryFreeListSpace()->HasAddress(forward_address)));
  return forward_address;
}

void SemiSpace::ProcessMarkStackCallback(void* arg) {
  reinterpret_cast<SemiSpace*>(arg)->ProcessMarkStack();
}

mirror::Object* SemiSpace::MarkObjectCallback(mirror::Object* root, void* arg) {
  auto ref = StackReference<mirror::Object>::FromMirrorPtr(root);
  reinterpret_cast<SemiSpace*>(arg)->MarkObject(&ref);
  return ref.AsMirrorPtr();
}

void SemiSpace::MarkHeapReferenceCallback(mirror::HeapReference<mirror::Object>* obj_ptr,
                                          void* arg) {
  reinterpret_cast<SemiSpace*>(arg)->MarkObject(obj_ptr);
}

void SemiSpace::MarkRootCallback(Object** root, void* arg, uint32_t /*thread_id*/,
                                 RootType /*root_type*/) {
  auto ref = StackReference<mirror::Object>::FromMirrorPtr(*root);
  reinterpret_cast<SemiSpace*>(arg)->MarkObject(&ref);
  if (*root != ref.AsMirrorPtr()) {
    *root = ref.AsMirrorPtr();
  }
}

// Marks all objects in the root set.
void SemiSpace::MarkRoots() {
  timings_.StartSplit("MarkRoots");
  // TODO: Visit up image roots as well?
  Runtime::Current()->VisitRoots(MarkRootCallback, this);
  timings_.EndSplit();
}

mirror::Object* SemiSpace::MarkedForwardingAddressCallback(mirror::Object* object, void* arg) {
  return reinterpret_cast<SemiSpace*>(arg)->GetMarkedForwardAddress(object);
}

void SemiSpace::SweepSystemWeaks() {
  timings_.StartSplit("SweepSystemWeaks");
  Runtime::Current()->SweepSystemWeaks(MarkedForwardingAddressCallback, this);
  timings_.EndSplit();
}

bool SemiSpace::ShouldSweepSpace(space::ContinuousSpace* space) const {
  return space != from_space_ && space != to_space_ && !immune_region_.ContainsSpace(space);
}

void SemiSpace::Sweep(bool swap_bitmaps) {
  DCHECK(mark_stack_->IsEmpty());
  TimingLogger::ScopedSplit("Sweep", &timings_);
  for (const auto& space : GetHeap()->GetContinuousSpaces()) {
    if (space->IsContinuousMemMapAllocSpace()) {
      space::ContinuousMemMapAllocSpace* alloc_space = space->AsContinuousMemMapAllocSpace();
      if (!ShouldSweepSpace(alloc_space)) {
        continue;
      }
      TimingLogger::ScopedSplit split(
          alloc_space->IsZygoteSpace() ? "SweepZygoteSpace" : "SweepAllocSpace", &timings_);
      size_t freed_objects = 0;
      size_t freed_bytes = 0;
      alloc_space->Sweep(swap_bitmaps, &freed_objects, &freed_bytes);
      heap_->RecordFree(freed_objects, freed_bytes);
      freed_objects_.FetchAndAdd(freed_objects);
      freed_bytes_.FetchAndAdd(freed_bytes);
    }
  }
  if (!is_large_object_space_immune_) {
    SweepLargeObjects(swap_bitmaps);
  }
}

void SemiSpace::SweepLargeObjects(bool swap_bitmaps) {
  DCHECK(!is_large_object_space_immune_);
  TimingLogger::ScopedSplit("SweepLargeObjects", &timings_);
  size_t freed_objects = 0;
  size_t freed_bytes = 0;
  GetHeap()->GetLargeObjectsSpace()->Sweep(swap_bitmaps, &freed_objects, &freed_bytes);
  freed_large_objects_.FetchAndAdd(freed_objects);
  freed_large_object_bytes_.FetchAndAdd(freed_bytes);
  GetHeap()->RecordFree(freed_objects, freed_bytes);
}

// Process the "referent" field in a java.lang.ref.Reference.  If the referent has not yet been
// marked, put it on the appropriate list in the heap for later processing.
void SemiSpace::DelayReferenceReferent(mirror::Class* klass, mirror::Reference* reference) {
  heap_->DelayReferenceReferent(klass, reference, MarkedForwardingAddressCallback, this);
}

class SemiSpaceMarkObjectVisitor {
 public:
  explicit SemiSpaceMarkObjectVisitor(SemiSpace* collector) : collector_(collector) {
  }

  void operator()(Object* obj, MemberOffset offset, bool /* is_static */) const ALWAYS_INLINE
      EXCLUSIVE_LOCKS_REQUIRED(Locks::mutator_lock_, Locks::heap_bitmap_lock_) {
    // Object was already verified when we scanned it.
    collector_->MarkObject(obj->GetFieldObjectReferenceAddr<kVerifyNone>(offset));
  }

  void operator()(mirror::Class* klass, mirror::Reference* ref) const
      SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)
      EXCLUSIVE_LOCKS_REQUIRED(Locks::heap_bitmap_lock_) {
    collector_->DelayReferenceReferent(klass, ref);
  }

 private:
  SemiSpace* const collector_;
};

// Visit all of the references of an object and update.
void SemiSpace::ScanObject(Object* obj) {
  DCHECK(!from_space_->HasAddress(obj)) << "Scanning object " << obj << " in from space";
  SemiSpaceMarkObjectVisitor visitor(this);
  obj->VisitReferences<kMovingClasses>(visitor, visitor);
}

// Scan anything that's on the mark stack.
void SemiSpace::ProcessMarkStack() {
  space::MallocSpace* promo_dest_space = nullptr;
  accounting::ContinuousSpaceBitmap* live_bitmap = nullptr;
  if (generational_ && !whole_heap_collection_) {
    // If a bump pointer space only collection (and the promotion is
    // enabled,) we delay the live-bitmap marking of promoted objects
    // from MarkObject() until this function.
    promo_dest_space = GetHeap()->GetPrimaryFreeListSpace();
    live_bitmap = promo_dest_space->GetLiveBitmap();
    DCHECK(live_bitmap != nullptr);
    accounting::ContinuousSpaceBitmap* mark_bitmap = promo_dest_space->GetMarkBitmap();
    DCHECK(mark_bitmap != nullptr);
    DCHECK_EQ(live_bitmap, mark_bitmap);
  }
  timings_.StartSplit("ProcessMarkStack");
  while (!mark_stack_->IsEmpty()) {
    Object* obj = mark_stack_->PopBack();
    if (generational_ && !whole_heap_collection_ && promo_dest_space->HasAddress(obj)) {
      // obj has just been promoted. Mark the live bitmap for it,
      // which is delayed from MarkObject().
      DCHECK(!live_bitmap->Test(obj));
      live_bitmap->Set(obj);
    }
    ScanObject(obj);
  }
  timings_.EndSplit();
}

inline Object* SemiSpace::GetMarkedForwardAddress(mirror::Object* obj) const
    SHARED_LOCKS_REQUIRED(Locks::heap_bitmap_lock_) {
  // All immune objects are assumed marked.
  if (immune_region_.ContainsObject(obj)) {
    return obj;
  }
  if (from_space_->HasAddress(obj)) {
    // Returns either the forwarding address or nullptr.
    return GetForwardingAddressInFromSpace(obj);
  } else if (to_space_->HasAddress(obj)) {
    // Should be unlikely.
    // Already forwarded, must be marked.
    return obj;
  }
  return heap_->GetMarkBitmap()->Test(obj) ? obj : nullptr;
}

void SemiSpace::SetToSpace(space::ContinuousMemMapAllocSpace* to_space) {
  DCHECK(to_space != nullptr);
  to_space_ = to_space;
}

void SemiSpace::SetFromSpace(space::ContinuousMemMapAllocSpace* from_space) {
  DCHECK(from_space != nullptr);
  from_space_ = from_space;
}

void SemiSpace::FinishPhase() {
  TimingLogger::ScopedSplit split("FinishPhase", &timings_);
  Heap* heap = GetHeap();
  timings_.NewSplit("PostGcVerification");
  heap->PostGcVerification(this);
  // Null the "to" and "from" spaces since compacting from one to the other isn't valid until
  // further action is done by the heap.
  to_space_ = nullptr;
  from_space_ = nullptr;
  CHECK(mark_stack_->IsEmpty());
  mark_stack_->Reset();
  if (generational_) {
    // Decide whether to do a whole heap collection or a bump pointer
    // only space collection at the next collection by updating
    // whole_heap_collection.
    if (!whole_heap_collection_) {
      if (!kUseBytesPromoted) {
        // Enable whole_heap_collection once every
        // kDefaultWholeHeapCollectionInterval collections.
        --whole_heap_collection_interval_counter_;
        DCHECK_GE(whole_heap_collection_interval_counter_, 0);
        if (whole_heap_collection_interval_counter_ == 0) {
          whole_heap_collection_ = true;
        }
      } else {
        // Enable whole_heap_collection if the bytes promoted since
        // the last whole heap collection exceeds a threshold.
        bytes_promoted_since_last_whole_heap_collection_ += bytes_promoted_;
        if (bytes_promoted_since_last_whole_heap_collection_ >= kBytesPromotedThreshold) {
          whole_heap_collection_ = true;
        }
      }
    } else {
      if (!kUseBytesPromoted) {
        DCHECK_EQ(whole_heap_collection_interval_counter_, 0);
        whole_heap_collection_interval_counter_ = kDefaultWholeHeapCollectionInterval;
        whole_heap_collection_ = false;
      } else {
        // Reset it.
        bytes_promoted_since_last_whole_heap_collection_ = bytes_promoted_;
        whole_heap_collection_ = false;
      }
    }
  }
  // Clear all of the spaces' mark bitmaps.
  WriterMutexLock mu(Thread::Current(), *Locks::heap_bitmap_lock_);
  heap_->ClearMarkedObjects();
}

void SemiSpace::RevokeAllThreadLocalBuffers() {
  timings_.StartSplit("(Paused)RevokeAllThreadLocalBuffers");
  GetHeap()->RevokeAllThreadLocalBuffers();
  timings_.EndSplit();
}

}  // namespace collector
}  // namespace gc
}  // namespace art