src/v8/src/snapshot/deserializer.cc

// Copyright 2016 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "src/snapshot/deserializer.h"

#include "src/api.h"
#include "src/assembler-inl.h"
#include "src/bootstrapper.h"
#include "src/deoptimizer.h"
#include "src/external-reference-table.h"
#include "src/heap/heap-inl.h"
#include "src/isolate.h"
#include "src/macro-assembler.h"
#include "src/objects-inl.h"
#include "src/snapshot/builtin-deserializer.h"
#include "src/snapshot/natives.h"
#include "src/snapshot/startup-deserializer.h"
#include "src/v8.h"
#include "src/v8threads.h"

namespace v8 {
namespace internal {

void Deserializer::DecodeReservation(
    Vector<const SerializedData::Reservation> res) {
  DCHECK_EQ(0, reservations_[NEW_SPACE].size());
  STATIC_ASSERT(NEW_SPACE == 0);
  int current_space = NEW_SPACE;
  for (auto& r : res) {
    reservations_[current_space].push_back({r.chunk_size(), NULL, NULL});
    if (r.is_last()) current_space++;
  }
  DCHECK_EQ(kNumberOfSpaces, current_space);
  for (int i = 0; i < kNumberOfPreallocatedSpaces; i++) current_chunk_[i] = 0;
}

void Deserializer::RegisterDeserializedObjectsForBlackAllocation() {
  isolate_->heap()->RegisterDeserializedObjectsForBlackAllocation(
      reservations_, deserialized_large_objects_, allocated_maps_);
}

bool Deserializer::ReserveSpace() {
#ifdef DEBUG
  for (int i = NEW_SPACE; i < kNumberOfSpaces; ++i) {
    DCHECK(reservations_[i].size() > 0);
  }
#endif  // DEBUG
  DCHECK(allocated_maps_.empty());
  if (!isolate_->heap()->ReserveSpace(reservations_, &allocated_maps_))
    return false;
  for (int i = 0; i < kNumberOfPreallocatedSpaces; i++) {
    high_water_[i] = reservations_[i][0].start;
  }
  return true;
}

// static
bool Deserializer::ReserveSpace(StartupDeserializer* startup_deserializer,
                                BuiltinDeserializer* builtin_deserializer) {
  const int first_space = NEW_SPACE;
  const int last_space = SerializerDeserializer::kNumberOfSpaces;
  Isolate* isolate = startup_deserializer->isolate();

  // Create a set of merged reservations to reserve space in one go.
  // The BuiltinDeserializer's reservations are ignored, since our actual
  // requirements vary based on whether lazy deserialization is enabled.
  // Instead, we manually determine the required code-space.

  DCHECK(builtin_deserializer->ReservesOnlyCodeSpace());
  Heap::Reservation merged_reservations[kNumberOfSpaces];
  for (int i = first_space; i < last_space; i++) {
    merged_reservations[i] = startup_deserializer->reservations_[i];
  }

  Heap::Reservation builtin_reservations =
      builtin_deserializer->CreateReservationsForEagerBuiltins();
  DCHECK(!builtin_reservations.empty());

  for (const auto& c : builtin_reservations) {
    merged_reservations[CODE_SPACE].push_back(c);
  }

  if (!isolate->heap()->ReserveSpace(merged_reservations,
                                     &startup_deserializer->allocated_maps_)) {
    return false;
  }

  DisallowHeapAllocation no_allocation;

  // Distribute the successful allocations between both deserializers.
  // There's nothing to be done here except for code space.

  {
    const int num_builtin_reservations =
        static_cast<int>(builtin_reservations.size());
    for (int i = num_builtin_reservations - 1; i >= 0; i--) {
      const auto& c = merged_reservations[CODE_SPACE].back();
      DCHECK_EQ(c.size, builtin_reservations[i].size);
      DCHECK_EQ(c.size, c.end - c.start);
      builtin_reservations[i].start = c.start;
      builtin_reservations[i].end = c.end;
      merged_reservations[CODE_SPACE].pop_back();
    }

    builtin_deserializer->InitializeBuiltinsTable(builtin_reservations);
  }

  // Write back startup reservations.

  for (int i = first_space; i < last_space; i++) {
    startup_deserializer->reservations_[i].swap(merged_reservations[i]);
  }

  for (int i = first_space; i < kNumberOfPreallocatedSpaces; i++) {
    startup_deserializer->high_water_[i] =
        startup_deserializer->reservations_[i][0].start;
    builtin_deserializer->high_water_[i] = nullptr;
  }

  return true;
}

bool Deserializer::ReservesOnlyCodeSpace() const {
  for (int space = NEW_SPACE; space < kNumberOfSpaces; space++) {
    if (space == CODE_SPACE) continue;
    const auto& r = reservations_[space];
    for (const Heap::Chunk& c : r)
      if (c.size != 0) return false;
  }
  return true;
}

void Deserializer::Initialize(Isolate* isolate) {
  DCHECK_NULL(isolate_);
  DCHECK_NOT_NULL(isolate);
  isolate_ = isolate;
  DCHECK_NULL(external_reference_table_);
  external_reference_table_ = ExternalReferenceTable::instance(isolate);
#ifdef DEBUG
  // Count the number of external references registered through the API.
  num_api_references_ = 0;
  if (isolate_->api_external_references() != nullptr) {
    while (isolate_->api_external_references()[num_api_references_] != 0) {
      num_api_references_++;
    }
  }
#endif  // DEBUG
  CHECK_EQ(magic_number_,
           SerializedData::ComputeMagicNumber(external_reference_table_));
}

void Deserializer::SortMapDescriptors() {
  for (const auto& address : allocated_maps_) {
    Map* map = Map::cast(HeapObject::FromAddress(address));
    if (map->instance_descriptors()->number_of_descriptors() > 1) {
      map->instance_descriptors()->Sort();
    }
  }
}

bool Deserializer::IsLazyDeserializationEnabled() const {
  return FLAG_lazy_deserialization && !isolate()->serializer_enabled();
}

Deserializer::~Deserializer() {
#ifdef DEBUG
  // Do not perform checks if we aborted deserialization.
  if (source_.position() == 0) return;
  // Check that we only have padding bytes remaining.
  while (source_.HasMore()) DCHECK_EQ(kNop, source_.Get());
  for (int space = 0; space < kNumberOfPreallocatedSpaces; space++) {
    int chunk_index = current_chunk_[space];
    DCHECK_EQ(reservations_[space].size(), chunk_index + 1);
    DCHECK_EQ(reservations_[space][chunk_index].end, high_water_[space]);
  }
  DCHECK_EQ(allocated_maps_.size(), next_map_index_);
#endif  // DEBUG
}

// This is called on the roots.  It is the driver of the deserialization
// process.  It is also called on the body of each function.
void Deserializer::VisitRootPointers(Root root, Object** start, Object** end) {
  // The space must be new space.  Any other space would cause ReadChunk to try
  // to update the remembered using NULL as the address.
  ReadData(start, end, NEW_SPACE, NULL);
}

void Deserializer::Synchronize(VisitorSynchronization::SyncTag tag) {
  static const byte expected = kSynchronize;
  CHECK_EQ(expected, source_.Get());
  deserializing_builtins_ = (tag == VisitorSynchronization::kHandleScope);
}

void Deserializer::DeserializeDeferredObjects() {
  for (int code = source_.Get(); code != kSynchronize; code = source_.Get()) {
    switch (code) {
      case kAlignmentPrefix:
      case kAlignmentPrefix + 1:
      case kAlignmentPrefix + 2:
        SetAlignment(code);
        break;
      default: {
        int space = code & kSpaceMask;
        DCHECK(space <= kNumberOfSpaces);
        DCHECK(code - space == kNewObject);
        HeapObject* object = GetBackReferencedObject(space);
        int size = source_.GetInt() << kPointerSizeLog2;
        Address obj_address = object->address();
        Object** start = reinterpret_cast<Object**>(obj_address + kPointerSize);
        Object** end = reinterpret_cast<Object**>(obj_address + size);
        bool filled = ReadData(start, end, space, obj_address);
        CHECK(filled);
        DCHECK(CanBeDeferred(object));
        PostProcessNewObject(object, space);
      }
    }
  }
}

StringTableInsertionKey::StringTableInsertionKey(String* string)
    : StringTableKey(ComputeHashField(string)), string_(string) {
  DCHECK(string->IsInternalizedString());
}

bool StringTableInsertionKey::IsMatch(Object* string) {
  // We know that all entries in a hash table had their hash keys created.
  // Use that knowledge to have fast failure.
  if (Hash() != String::cast(string)->Hash()) return false;
  // We want to compare the content of two internalized strings here.
  return string_->SlowEquals(String::cast(string));
}

Handle<String> StringTableInsertionKey::AsHandle(Isolate* isolate) {
  return handle(string_, isolate);
}

uint32_t StringTableInsertionKey::ComputeHashField(String* string) {
  // Make sure hash_field() is computed.
  string->Hash();
  return string->hash_field();
}

HeapObject* Deserializer::PostProcessNewObject(HeapObject* obj, int space) {
  if (deserializing_user_code()) {
    if (obj->IsString()) {
      String* string = String::cast(obj);
      // Uninitialize hash field as the hash seed may have changed.
      string->set_hash_field(String::kEmptyHashField);
      if (string->IsInternalizedString()) {
        // Canonicalize the internalized string. If it already exists in the
        // string table, set it to forward to the existing one.
        StringTableInsertionKey key(string);
        String* canonical = StringTable::LookupKeyIfExists(isolate_, &key);
        if (canonical == NULL) {
          new_internalized_strings_.push_back(handle(string));
          return string;
        } else {
          string->SetForwardedInternalizedString(canonical);
          return canonical;
        }
      }
    } else if (obj->IsScript()) {
      new_scripts_.push_back(handle(Script::cast(obj)));
    } else {
      DCHECK(CanBeDeferred(obj));
    }
  }
  if (obj->IsAllocationSite()) {
    // Allocation sites are present in the snapshot, and must be linked into
    // a list at deserialization time.
    AllocationSite* site = AllocationSite::cast(obj);
    // TODO(mvstanton): consider treating the heap()->allocation_sites_list()
    // as a (weak) root. If this root is relocated correctly, this becomes
    // unnecessary.
    if (isolate_->heap()->allocation_sites_list() == Smi::kZero) {
      site->set_weak_next(isolate_->heap()->undefined_value());
    } else {
      site->set_weak_next(isolate_->heap()->allocation_sites_list());
    }
    isolate_->heap()->set_allocation_sites_list(site);
  } else if (obj->IsCode()) {
    // We flush all code pages after deserializing the startup snapshot. In that
    // case, we only need to remember code objects in the large object space.
    // When deserializing user code, remember each individual code object.
    if (deserializing_user_code() || space == LO_SPACE) {
      new_code_objects_.push_back(Code::cast(obj));
    }
  } else if (obj->IsAccessorInfo()) {
    if (isolate_->external_reference_redirector()) {
      accessor_infos_.push_back(AccessorInfo::cast(obj));
    }
  } else if (obj->IsExternalOneByteString()) {
    DCHECK(obj->map() == isolate_->heap()->native_source_string_map());
    ExternalOneByteString* string = ExternalOneByteString::cast(obj);
    DCHECK(string->is_short());
    string->set_resource(
        NativesExternalStringResource::DecodeForDeserialization(
            string->resource()));
    isolate_->heap()->RegisterExternalString(string);
  } else if (obj->IsJSArrayBuffer()) {
    JSArrayBuffer* buffer = JSArrayBuffer::cast(obj);
    // Only fixup for the off-heap case.
    if (buffer->backing_store() != nullptr) {
      Smi* store_index = reinterpret_cast<Smi*>(buffer->backing_store());
      void* backing_store = off_heap_backing_stores_[store_index->value()];

      buffer->set_backing_store(backing_store);
      buffer->set_allocation_base(backing_store);
      isolate_->heap()->RegisterNewArrayBuffer(buffer);
    }
  } else if (obj->IsFixedTypedArrayBase()) {
    FixedTypedArrayBase* fta = FixedTypedArrayBase::cast(obj);
    // Only fixup for the off-heap case.
    if (fta->base_pointer() == nullptr) {
      Smi* store_index = reinterpret_cast<Smi*>(fta->external_pointer());
      void* backing_store = off_heap_backing_stores_[store_index->value()];

      fta->set_external_pointer(backing_store);
    }
  }
  if (FLAG_rehash_snapshot && can_rehash_ && !deserializing_user_code()) {
    if (obj->IsString()) {
      // Uninitialize hash field as we are going to reinitialize the hash seed.
      String* string = String::cast(obj);
      string->set_hash_field(String::kEmptyHashField);
    } else if (obj->IsTransitionArray() &&
               TransitionArray::cast(obj)->number_of_entries() > 1) {
      transition_arrays_.push_back(TransitionArray::cast(obj));
    }
  }
  // Check alignment.
  DCHECK_EQ(0, Heap::GetFillToAlign(obj->address(), obj->RequiredAlignment()));
  return obj;
}

int Deserializer::MaybeReplaceWithDeserializeLazy(int builtin_id) {
  DCHECK(Builtins::IsBuiltinId(builtin_id));
  return (IsLazyDeserializationEnabled() && Builtins::IsLazy(builtin_id) &&
          !deserializing_builtins_)
             ? Builtins::kDeserializeLazy
             : builtin_id;
}

HeapObject* Deserializer::GetBackReferencedObject(int space) {
  HeapObject* obj;
  SerializerReference back_reference =
      SerializerReference::FromBitfield(source_.GetInt());
  if (space == LO_SPACE) {
    uint32_t index = back_reference.large_object_index();
    obj = deserialized_large_objects_[index];
  } else if (space == MAP_SPACE) {
    int index = back_reference.map_index();
    DCHECK(index < next_map_index_);
    obj = HeapObject::FromAddress(allocated_maps_[index]);
  } else {
    DCHECK(space < kNumberOfPreallocatedSpaces);
    uint32_t chunk_index = back_reference.chunk_index();
    DCHECK_LE(chunk_index, current_chunk_[space]);
    uint32_t chunk_offset = back_reference.chunk_offset();
    Address address = reservations_[space][chunk_index].start + chunk_offset;
    if (next_alignment_ != kWordAligned) {
      int padding = Heap::GetFillToAlign(address, next_alignment_);
      next_alignment_ = kWordAligned;
      DCHECK(padding == 0 || HeapObject::FromAddress(address)->IsFiller());
      address += padding;
    }
    obj = HeapObject::FromAddress(address);
  }
  if (deserializing_user_code() && obj->IsInternalizedString()) {
    obj = String::cast(obj)->GetForwardedInternalizedString();
  }
  hot_objects_.Add(obj);
  return obj;
}

// This routine writes the new object into the pointer provided and then
// returns true if the new object was in young space and false otherwise.
// The reason for this strange interface is that otherwise the object is
// written very late, which means the FreeSpace map is not set up by the
// time we need to use it to mark the space at the end of a page free.
void Deserializer::ReadObject(int space_number, Object** write_back) {
  Address address;
  HeapObject* obj;
  int size = source_.GetInt() << kObjectAlignmentBits;

  if (next_alignment_ != kWordAligned) {
    int reserved = size + Heap::GetMaximumFillToAlign(next_alignment_);
    address = Allocate(space_number, reserved);
    obj = HeapObject::FromAddress(address);
    // If one of the following assertions fails, then we are deserializing an
    // aligned object when the filler maps have not been deserialized yet.
    // We require filler maps as padding to align the object.
    Heap* heap = isolate_->heap();
    DCHECK(heap->free_space_map()->IsMap());
    DCHECK(heap->one_pointer_filler_map()->IsMap());
    DCHECK(heap->two_pointer_filler_map()->IsMap());
    obj = heap->AlignWithFiller(obj, size, reserved, next_alignment_);
    address = obj->address();
    next_alignment_ = kWordAligned;
  } else {
    address = Allocate(space_number, size);
    obj = HeapObject::FromAddress(address);
  }

  isolate_->heap()->OnAllocationEvent(obj, size);
  Object** current = reinterpret_cast<Object**>(address);
  Object** limit = current + (size >> kPointerSizeLog2);

  if (ReadData(current, limit, space_number, address)) {
    // Only post process if object content has not been deferred.
    obj = PostProcessNewObject(obj, space_number);
  }

  Object* write_back_obj = obj;
  UnalignedCopy(write_back, &write_back_obj);
#ifdef DEBUG
  if (obj->IsCode()) {
    DCHECK(space_number == CODE_SPACE || space_number == LO_SPACE);
  } else {
    DCHECK(space_number != CODE_SPACE);
  }
#endif  // DEBUG
}

// We know the space requirements before deserialization and can
// pre-allocate that reserved space. During deserialization, all we need
// to do is to bump up the pointer for each space in the reserved
// space. This is also used for fixing back references.
// We may have to split up the pre-allocation into several chunks
// because it would not fit onto a single page. We do not have to keep
// track of when to move to the next chunk. An opcode will signal this.
// Since multiple large objects cannot be folded into one large object
// space allocation, we have to do an actual allocation when deserializing
// each large object. Instead of tracking offset for back references, we
// reference large objects by index.
Address Deserializer::Allocate(int space_index, int size) {
  if (space_index == LO_SPACE) {
    AlwaysAllocateScope scope(isolate_);
    LargeObjectSpace* lo_space = isolate_->heap()->lo_space();
    Executability exec = static_cast<Executability>(source_.Get());
    AllocationResult result = lo_space->AllocateRaw(size, exec);
    HeapObject* obj = result.ToObjectChecked();
    deserialized_large_objects_.push_back(obj);
    return obj->address();
  } else if (space_index == MAP_SPACE) {
    DCHECK_EQ(Map::kSize, size);
    return allocated_maps_[next_map_index_++];
  } else {
    DCHECK(space_index < kNumberOfPreallocatedSpaces);
    Address address = high_water_[space_index];
    DCHECK_NOT_NULL(address);
    high_water_[space_index] += size;
#ifdef DEBUG
    // Assert that the current reserved chunk is still big enough.
    const Heap::Reservation& reservation = reservations_[space_index];
    int chunk_index = current_chunk_[space_index];
    DCHECK_LE(high_water_[space_index], reservation[chunk_index].end);
#endif
    if (space_index == CODE_SPACE) SkipList::Update(address, size);
    return address;
  }
}

Object* Deserializer::ReadDataSingle() {
  Object* o;
  Object** start = &o;
  Object** end = start + 1;
  int source_space = NEW_SPACE;
  Address current_object = nullptr;

  CHECK(ReadData(start, end, source_space, current_object));

  return o;
}

bool Deserializer::ReadData(Object** current, Object** limit, int source_space,
                            Address current_object_address) {
  Isolate* const isolate = isolate_;
  // Write barrier support costs around 1% in startup time.  In fact there
  // are no new space objects in current boot snapshots, so it's not needed,
  // but that may change.
  bool write_barrier_needed =
      (current_object_address != NULL && source_space != NEW_SPACE &&
       source_space != CODE_SPACE);
  while (current < limit) {
    byte data = source_.Get();
    switch (data) {
#define CASE_STATEMENT(where, how, within, space_number) \
  case where + how + within + space_number:              \
    STATIC_ASSERT((where & ~kWhereMask) == 0);           \
    STATIC_ASSERT((how & ~kHowToCodeMask) == 0);         \
    STATIC_ASSERT((within & ~kWhereToPointMask) == 0);   \
    STATIC_ASSERT((space_number & ~kSpaceMask) == 0);

#define CASE_BODY(where, how, within, space_number_if_any)                   \
  current = ReadDataCase<where, how, within, space_number_if_any>(           \
      isolate, current, current_object_address, data, write_barrier_needed); \
  break;

// This generates a case and a body for the new space (which has to do extra
// write barrier handling) and handles the other spaces with fall-through cases
// and one body.
#define ALL_SPACES(where, how, within)           \
  CASE_STATEMENT(where, how, within, NEW_SPACE)  \
  CASE_BODY(where, how, within, NEW_SPACE)       \
  CASE_STATEMENT(where, how, within, OLD_SPACE)  \
  CASE_STATEMENT(where, how, within, CODE_SPACE) \
  CASE_STATEMENT(where, how, within, MAP_SPACE)  \
  CASE_STATEMENT(where, how, within, LO_SPACE)   \
  CASE_BODY(where, how, within, kAnyOldSpace)

#define FOUR_CASES(byte_code) \
  case byte_code:             \
  case byte_code + 1:         \
  case byte_code + 2:         \
  case byte_code + 3:

#define SIXTEEN_CASES(byte_code) \
  FOUR_CASES(byte_code)          \
  FOUR_CASES(byte_code + 4)      \
  FOUR_CASES(byte_code + 8)      \
  FOUR_CASES(byte_code + 12)

#define SINGLE_CASE(where, how, within, space) \
  CASE_STATEMENT(where, how, within, space)    \
  CASE_BODY(where, how, within, space)

      // Deserialize a new object and write a pointer to it to the current
      // object.
      ALL_SPACES(kNewObject, kPlain, kStartOfObject)
      // Deserialize a new code object and write a pointer to its first
      // instruction to the current code object.
      ALL_SPACES(kNewObject, kFromCode, kInnerPointer)
      // Find a recently deserialized object using its offset from the current
      // allocation point and write a pointer to it to the current object.
      ALL_SPACES(kBackref, kPlain, kStartOfObject)
      ALL_SPACES(kBackrefWithSkip, kPlain, kStartOfObject)
#if V8_CODE_EMBEDS_OBJECT_POINTER
      // Deserialize a new object from pointer found in code and write
      // a pointer to it to the current object. Required only for MIPS, PPC, ARM
      // or S390 with embedded constant pool, and omitted on the other
      // architectures because it is fully unrolled and would cause bloat.
      ALL_SPACES(kNewObject, kFromCode, kStartOfObject)
      // Find a recently deserialized code object using its offset from the
      // current allocation point and write a pointer to it to the current
      // object. Required only for MIPS, PPC, ARM or S390 with embedded
      // constant pool.
      ALL_SPACES(kBackref, kFromCode, kStartOfObject)
      ALL_SPACES(kBackrefWithSkip, kFromCode, kStartOfObject)
#endif
      // Find a recently deserialized code object using its offset from the
      // current allocation point and write a pointer to its first instruction
      // to the current code object or the instruction pointer in a function
      // object.
      ALL_SPACES(kBackref, kFromCode, kInnerPointer)
      ALL_SPACES(kBackrefWithSkip, kFromCode, kInnerPointer)
      // Find an object in the roots array and write a pointer to it to the
      // current object.
      SINGLE_CASE(kRootArray, kPlain, kStartOfObject, 0)
#if V8_CODE_EMBEDS_OBJECT_POINTER
      // Find an object in the roots array and write a pointer to it to in code.
      SINGLE_CASE(kRootArray, kFromCode, kStartOfObject, 0)
#endif
      // Find an object in the partial snapshots cache and write a pointer to it
      // to the current object.
      SINGLE_CASE(kPartialSnapshotCache, kPlain, kStartOfObject, 0)
      SINGLE_CASE(kPartialSnapshotCache, kFromCode, kStartOfObject, 0)
      SINGLE_CASE(kPartialSnapshotCache, kFromCode, kInnerPointer, 0)
      // Find an external reference and write a pointer to it to the current
      // object.
      SINGLE_CASE(kExternalReference, kPlain, kStartOfObject, 0)
      // Find an external reference and write a pointer to it in the current
      // code object.
      SINGLE_CASE(kExternalReference, kFromCode, kStartOfObject, 0)
      // Find an object in the attached references and write a pointer to it to
      // the current object.
      SINGLE_CASE(kAttachedReference, kPlain, kStartOfObject, 0)
      SINGLE_CASE(kAttachedReference, kFromCode, kStartOfObject, 0)
      SINGLE_CASE(kAttachedReference, kFromCode, kInnerPointer, 0)
      // Find a builtin and write a pointer to it to the current object.
      SINGLE_CASE(kBuiltin, kPlain, kStartOfObject, 0)
      SINGLE_CASE(kBuiltin, kFromCode, kStartOfObject, 0)
      SINGLE_CASE(kBuiltin, kFromCode, kInnerPointer, 0)

#undef CASE_STATEMENT
#undef CASE_BODY
#undef ALL_SPACES

      case kSkip: {
        int size = source_.GetInt();
        current = reinterpret_cast<Object**>(
            reinterpret_cast<intptr_t>(current) + size);
        break;
      }

      case kInternalReferenceEncoded:
      case kInternalReference: {
        // Internal reference address is not encoded via skip, but by offset
        // from code entry.
        int pc_offset = source_.GetInt();
        int target_offset = source_.GetInt();
        Code* code =
            Code::cast(HeapObject::FromAddress(current_object_address));
        DCHECK(0 <= pc_offset && pc_offset <= code->instruction_size());
        DCHECK(0 <= target_offset && target_offset <= code->instruction_size());
        Address pc = code->entry() + pc_offset;
        Address target = code->entry() + target_offset;
        Assembler::deserialization_set_target_internal_reference_at(
            isolate, pc, target, data == kInternalReference
                                     ? RelocInfo::INTERNAL_REFERENCE
                                     : RelocInfo::INTERNAL_REFERENCE_ENCODED);
        break;
      }

      case kNop:
        break;

      case kNextChunk: {
        int space = source_.Get();
        DCHECK(space < kNumberOfPreallocatedSpaces);
        int chunk_index = current_chunk_[space];
        const Heap::Reservation& reservation = reservations_[space];
        // Make sure the current chunk is indeed exhausted.
        CHECK_EQ(reservation[chunk_index].end, high_water_[space]);
        // Move to next reserved chunk.
        chunk_index = ++current_chunk_[space];
        CHECK_LT(chunk_index, reservation.size());
        high_water_[space] = reservation[chunk_index].start;
        break;
      }

      case kDeferred: {
        // Deferred can only occur right after the heap object header.
        DCHECK(current == reinterpret_cast<Object**>(current_object_address +
                                                     kPointerSize));
        HeapObject* obj = HeapObject::FromAddress(current_object_address);
        // If the deferred object is a map, its instance type may be used
        // during deserialization. Initialize it with a temporary value.
        if (obj->IsMap()) Map::cast(obj)->set_instance_type(FILLER_TYPE);
        current = limit;
        return false;
      }

      case kSynchronize:
        // If we get here then that indicates that you have a mismatch between
        // the number of GC roots when serializing and deserializing.
        CHECK(false);
        break;

      // Deserialize raw data of variable length.
      case kVariableRawData: {
        int size_in_bytes = source_.GetInt();
        byte* raw_data_out = reinterpret_cast<byte*>(current);
        source_.CopyRaw(raw_data_out, size_in_bytes);
        current = reinterpret_cast<Object**>(
            reinterpret_cast<intptr_t>(current) + size_in_bytes);
        break;
      }

      // Deserialize raw code directly into the body of the code object.
      // Do not move current.
      case kVariableRawCode: {
        int size_in_bytes = source_.GetInt();
        source_.CopyRaw(current_object_address + Code::kDataStart,
                        size_in_bytes);
        break;
      }

      case kVariableRepeat: {
        int repeats = source_.GetInt();
        Object* object = current[-1];
        DCHECK(!isolate->heap()->InNewSpace(object));
        for (int i = 0; i < repeats; i++) UnalignedCopy(current++, &object);
        break;
      }

      case kOffHeapBackingStore: {
        int byte_length = source_.GetInt();
        byte* backing_store = static_cast<byte*>(
            isolate->array_buffer_allocator()->AllocateUninitialized(
                byte_length));
        CHECK_NOT_NULL(backing_store);
        source_.CopyRaw(backing_store, byte_length);
        off_heap_backing_stores_.push_back(backing_store);
        break;
      }

      case kApiReference: {
        int skip = source_.GetInt();
        current = reinterpret_cast<Object**>(
            reinterpret_cast<Address>(current) + skip);
        uint32_t reference_id = static_cast<uint32_t>(source_.GetInt());
        DCHECK_WITH_MSG(reference_id < num_api_references_,
                        "too few external references provided through the API");
        Address address = reinterpret_cast<Address>(
            isolate->api_external_references()[reference_id]);
        memcpy(current, &address, kPointerSize);
        current++;
        break;
      }

      case kAlignmentPrefix:
      case kAlignmentPrefix + 1:
      case kAlignmentPrefix + 2:
        SetAlignment(data);
        break;

      STATIC_ASSERT(kNumberOfRootArrayConstants == Heap::kOldSpaceRoots);
      STATIC_ASSERT(kNumberOfRootArrayConstants == 32);
      SIXTEEN_CASES(kRootArrayConstantsWithSkip)
      SIXTEEN_CASES(kRootArrayConstantsWithSkip + 16) {
        int skip = source_.GetInt();
        current = reinterpret_cast<Object**>(
            reinterpret_cast<intptr_t>(current) + skip);
        // Fall through.
      }

      SIXTEEN_CASES(kRootArrayConstants)
      SIXTEEN_CASES(kRootArrayConstants + 16) {
        int id = data & kRootArrayConstantsMask;
        Heap::RootListIndex root_index = static_cast<Heap::RootListIndex>(id);
        Object* object = isolate->heap()->root(root_index);
        DCHECK(!isolate->heap()->InNewSpace(object));
        UnalignedCopy(current++, &object);
        break;
      }

      STATIC_ASSERT(kNumberOfHotObjects == 8);
      FOUR_CASES(kHotObjectWithSkip)
      FOUR_CASES(kHotObjectWithSkip + 4) {
        int skip = source_.GetInt();
        current = reinterpret_cast<Object**>(
            reinterpret_cast<Address>(current) + skip);
        // Fall through.
      }

      FOUR_CASES(kHotObject)
      FOUR_CASES(kHotObject + 4) {
        int index = data & kHotObjectMask;
        Object* hot_object = hot_objects_.Get(index);
        UnalignedCopy(current, &hot_object);
        if (write_barrier_needed && isolate->heap()->InNewSpace(hot_object)) {
          Address current_address = reinterpret_cast<Address>(current);
          isolate->heap()->RecordWrite(
              HeapObject::FromAddress(current_object_address),
              reinterpret_cast<Object**>(current_address), hot_object);
        }
        current++;
        break;
      }

      // Deserialize raw data of fixed length from 1 to 32 words.
      STATIC_ASSERT(kNumberOfFixedRawData == 32);
      SIXTEEN_CASES(kFixedRawData)
      SIXTEEN_CASES(kFixedRawData + 16) {
        byte* raw_data_out = reinterpret_cast<byte*>(current);
        int size_in_bytes = (data - kFixedRawDataStart) << kPointerSizeLog2;
        source_.CopyRaw(raw_data_out, size_in_bytes);
        current = reinterpret_cast<Object**>(raw_data_out + size_in_bytes);
        break;
      }

      STATIC_ASSERT(kNumberOfFixedRepeat == 16);
      SIXTEEN_CASES(kFixedRepeat) {
        int repeats = data - kFixedRepeatStart;
        Object* object;
        UnalignedCopy(&object, current - 1);
        DCHECK(!isolate->heap()->InNewSpace(object));
        for (int i = 0; i < repeats; i++) UnalignedCopy(current++, &object);
        break;
      }

#undef SIXTEEN_CASES
#undef FOUR_CASES
#undef SINGLE_CASE

      default:
        CHECK(false);
    }
  }
  CHECK_EQ(limit, current);
  return true;
}

template <int where, int how, int within, int space_number_if_any>
Object** Deserializer::ReadDataCase(Isolate* isolate, Object** current,
                                    Address current_object_address, byte data,
                                    bool write_barrier_needed) {
  bool emit_write_barrier = false;
  bool current_was_incremented = false;
  int space_number = space_number_if_any == kAnyOldSpace ? (data & kSpaceMask)
                                                         : space_number_if_any;
  if (where == kNewObject && how == kPlain && within == kStartOfObject) {
    ReadObject(space_number, current);
    emit_write_barrier = (space_number == NEW_SPACE);
  } else {
    Object* new_object = NULL; /* May not be a real Object pointer. */
    if (where == kNewObject) {
      ReadObject(space_number, &new_object);
    } else if (where == kBackref) {
      emit_write_barrier = (space_number == NEW_SPACE);
      new_object = GetBackReferencedObject(data & kSpaceMask);
    } else if (where == kBackrefWithSkip) {
      int skip = source_.GetInt();
      current =
          reinterpret_cast<Object**>(reinterpret_cast<Address>(current) + skip);
      emit_write_barrier = (space_number == NEW_SPACE);
      new_object = GetBackReferencedObject(data & kSpaceMask);
    } else if (where == kRootArray) {
      int id = source_.GetInt();
      Heap::RootListIndex root_index = static_cast<Heap::RootListIndex>(id);
      new_object = isolate->heap()->root(root_index);
      emit_write_barrier = isolate->heap()->InNewSpace(new_object);
      hot_objects_.Add(HeapObject::cast(new_object));
    } else if (where == kPartialSnapshotCache) {
      int cache_index = source_.GetInt();
      new_object = isolate->partial_snapshot_cache()->at(cache_index);
      emit_write_barrier = isolate->heap()->InNewSpace(new_object);
    } else if (where == kExternalReference) {
      int skip = source_.GetInt();
      current =
          reinterpret_cast<Object**>(reinterpret_cast<Address>(current) + skip);
      uint32_t reference_id = static_cast<uint32_t>(source_.GetInt());
      Address address = external_reference_table_->address(reference_id);
      new_object = reinterpret_cast<Object*>(address);
    } else if (where == kAttachedReference) {
      int index = source_.GetInt();
      new_object = *attached_objects_[index];
      emit_write_barrier = isolate->heap()->InNewSpace(new_object);
    } else {
      DCHECK(where == kBuiltin);
      int builtin_id = MaybeReplaceWithDeserializeLazy(source_.GetInt());
      new_object = isolate->builtins()->builtin(builtin_id);
      emit_write_barrier = false;
    }
    if (within == kInnerPointer) {
      DCHECK(how == kFromCode);
      if (where == kBuiltin) {
        // At this point, new_object may still be uninitialized, thus the
        // unchecked Code cast.
        new_object = reinterpret_cast<Object*>(
            reinterpret_cast<Code*>(new_object)->instruction_start());
      } else if (new_object->IsCode()) {
        new_object = reinterpret_cast<Object*>(
            Code::cast(new_object)->instruction_start());
      } else {
        Cell* cell = Cell::cast(new_object);
        new_object = reinterpret_cast<Object*>(cell->ValueAddress());
      }
    }
    if (how == kFromCode) {
      Address location_of_branch_data = reinterpret_cast<Address>(current);
      Assembler::deserialization_set_special_target_at(
          isolate, location_of_branch_data,
          Code::cast(HeapObject::FromAddress(current_object_address)),
          reinterpret_cast<Address>(new_object));
      location_of_branch_data += Assembler::kSpecialTargetSize;
      current = reinterpret_cast<Object**>(location_of_branch_data);
      current_was_incremented = true;
    } else {
      UnalignedCopy(current, &new_object);
    }
  }
  if (emit_write_barrier && write_barrier_needed) {
    Address current_address = reinterpret_cast<Address>(current);
    SLOW_DCHECK(isolate->heap()->ContainsSlow(current_object_address));
    isolate->heap()->RecordWrite(
        HeapObject::FromAddress(current_object_address),
        reinterpret_cast<Object**>(current_address),
        *reinterpret_cast<Object**>(current_address));
  }
  if (!current_was_incremented) {
    current++;
  }

  return current;
}

}  // namespace internal
}  // namespace v8