runtime/vm/app_snapshot.cc

// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

#include <memory>
#include <utility>

#include "vm/app_snapshot.h"

#include "platform/assert.h"
#include "vm/bootstrap.h"
#include "vm/bss_relocs.h"
#include "vm/canonical_tables.h"
#include "vm/class_id.h"
#include "vm/code_observers.h"
#include "vm/compiler/api/print_filter.h"
#include "vm/compiler/assembler/disassembler.h"
#include "vm/dart.h"
#include "vm/dart_entry.h"
#include "vm/dispatch_table.h"
#include "vm/flag_list.h"
#include "vm/growable_array.h"
#include "vm/heap/heap.h"
#include "vm/image_snapshot.h"
#include "vm/native_entry.h"
#include "vm/object.h"
#include "vm/object_store.h"
#include "vm/program_visitor.h"
#include "vm/raw_object_fields.h"
#include "vm/stub_code.h"
#include "vm/symbols.h"
#include "vm/timeline.h"
#include "vm/v8_snapshot_writer.h"
#include "vm/version.h"
#include "vm/zone_text_buffer.h"

#if !defined(DART_PRECOMPILED_RUNTIME)
#include "vm/compiler/backend/code_statistics.h"
#include "vm/compiler/backend/il_printer.h"
#include "vm/compiler/relocation.h"
#endif  // !defined(DART_PRECOMPILED_RUNTIME)

namespace dart {

#if !defined(DART_PRECOMPILED_RUNTIME)
DEFINE_FLAG(bool,
            print_cluster_information,
            false,
            "Print information about clusters written to snapshot");
#endif

#if defined(DART_PRECOMPILER)
DEFINE_FLAG(charp,
            write_v8_snapshot_profile_to,
            nullptr,
            "Write a snapshot profile in V8 format to a file.");
DEFINE_FLAG(bool,
            print_array_optimization_candidates,
            false,
            "Print information about how many array are candidates for Smi and "
            "ROData optimizations.");
#endif  // defined(DART_PRECOMPILER)

// Forward declarations.
class Serializer;
class Deserializer;

namespace {

// Serialized clusters are identified by their CID. So to insert custom clusters
// we need to assign them a CID that is otherwise never serialized.
static constexpr intptr_t kDeltaEncodedTypedDataCid = kNativePointer;

// StorageTrait for HashTable which allows to create hash tables backed by
// zone memory. Used to compute cluster order for canonical clusters.
struct GrowableArrayStorageTraits {
  class Array : public ZoneAllocated {
   public:
    explicit Array(Zone* zone, intptr_t length)
        : length_(length), array_(zone->Alloc<ObjectPtr>(length)) {}

    intptr_t Length() const { return length_; }
    void SetAt(intptr_t index, const Object& value) const {
      array_[index] = value.ptr();
    }
    ObjectPtr At(intptr_t index) const { return array_[index]; }

   private:
    intptr_t length_ = 0;
    ObjectPtr* array_ = nullptr;
    DISALLOW_COPY_AND_ASSIGN(Array);
  };

  using ArrayPtr = Array*;
  class ArrayHandle : public ZoneAllocated {
   public:
    explicit ArrayHandle(ArrayPtr ptr) : ptr_(ptr) {}
    ArrayHandle() {}

    void SetFrom(const ArrayHandle& other) { ptr_ = other.ptr_; }
    void Clear() { ptr_ = nullptr; }
    bool IsNull() const { return ptr_ == nullptr; }
    ArrayPtr ptr() { return ptr_; }

    intptr_t Length() const { return ptr_->Length(); }
    void SetAt(intptr_t index, const Object& value) const {
      ptr_->SetAt(index, value);
    }
    ObjectPtr At(intptr_t index) const { return ptr_->At(index); }

   private:
    ArrayPtr ptr_ = nullptr;
    DISALLOW_COPY_AND_ASSIGN(ArrayHandle);
  };

  static ArrayHandle& PtrToHandle(ArrayPtr ptr) {
    return *new ArrayHandle(ptr);
  }

  static void SetHandle(ArrayHandle& dst, const ArrayHandle& src) {  // NOLINT
    dst.SetFrom(src);
  }

  static void ClearHandle(ArrayHandle& dst) {  // NOLINT
    dst.Clear();
  }

  static ArrayPtr New(Zone* zone, intptr_t length, Heap::Space space) {
    return new (zone) Array(zone, length);
  }

  static bool IsImmutable(const ArrayHandle& handle) { return false; }

  static ObjectPtr At(ArrayHandle* array, intptr_t index) {
    return array->At(index);
  }

  static void SetAt(ArrayHandle* array, intptr_t index, const Object& value) {
    array->SetAt(index, value);
  }
};
}  // namespace

#if defined(DART_PRECOMPILER) && !defined(TARGET_ARCH_IA32)

static void RelocateCodeObjects(
    bool is_vm,
    GrowableArray<CodePtr>* code_objects,
    GrowableArray<ImageWriterCommand>* image_writer_commands) {
  auto thread = Thread::Current();
  auto isolate_group =
      is_vm ? Dart::vm_isolate()->group() : thread->isolate_group();

  WritableCodePages writable_code_pages(thread, isolate_group);
  CodeRelocator::Relocate(thread, code_objects, image_writer_commands, is_vm);
}

#endif  // defined(DART_PRECOMPILER) && !defined(TARGET_ARCH_IA32)

class SerializationCluster : public ZoneAllocated {
 public:
  static constexpr intptr_t kSizeVaries = -1;
  explicit SerializationCluster(const char* name,
                                intptr_t cid,
                                intptr_t target_instance_size = kSizeVaries,
                                bool is_canonical = false)
      : name_(name),
        cid_(cid),
        target_instance_size_(target_instance_size),
        is_canonical_(is_canonical),
        is_immutable_(Object::ShouldHaveImmutabilityBitSet(cid)) {
    ASSERT(target_instance_size == kSizeVaries || target_instance_size >= 0);
  }
  virtual ~SerializationCluster() {}

  // Add [object] to the cluster and push its outgoing references.
  virtual void Trace(Serializer* serializer, ObjectPtr object) = 0;

  // Write the cluster type and information needed to allocate the cluster's
  // objects. For fixed sized objects, this is just the object count. For
  // variable sized objects, this is the object count and length of each object.
  virtual void WriteAlloc(Serializer* serializer) = 0;

  // Write the byte and reference data of the cluster's objects.
  virtual void WriteFill(Serializer* serializer) = 0;

  void WriteAndMeasureAlloc(Serializer* serializer);
  void WriteAndMeasureFill(Serializer* serializer);

  const char* name() const { return name_; }
  intptr_t cid() const { return cid_; }
  bool is_canonical() const { return is_canonical_; }
  bool is_immutable() const { return is_immutable_; }
  intptr_t size() const { return size_; }
  intptr_t num_objects() const { return num_objects_; }

  // Returns number of bytes needed for deserialized objects in
  // this cluster. Printed in --print_snapshot_sizes_verbose statistics.
  //
  // In order to calculate this size, clusters of fixed-size objects
  // can pass instance size as [target_instance_size] constructor parameter.
  // Otherwise clusters should count [target_memory_size] in
  // their [WriteAlloc] methods.
  intptr_t target_memory_size() const { return target_memory_size_; }

 protected:
  const char* const name_;
  const intptr_t cid_;
  const intptr_t target_instance_size_;
  const bool is_canonical_;
  const bool is_immutable_;
  intptr_t size_ = 0;
  intptr_t num_objects_ = 0;
  intptr_t target_memory_size_ = 0;
};

class DeserializationCluster : public ZoneAllocated {
 public:
  explicit DeserializationCluster(const char* name,
                                  bool is_canonical = false,
                                  bool is_immutable = false)
      : name_(name),
        is_canonical_(is_canonical),
        is_immutable_(is_immutable),
        start_index_(-1),
        stop_index_(-1) {}
  virtual ~DeserializationCluster() {}

  // Allocate memory for all objects in the cluster and write their addresses
  // into the ref array. Do not touch this memory.
  virtual void ReadAlloc(Deserializer* deserializer) = 0;

  // Initialize the cluster's objects. Do not touch the memory of other objects.
  virtual void ReadFill(Deserializer* deserializer) = 0;

  // Complete any action that requires the full graph to be deserialized, such
  // as rehashing.
  virtual void PostLoad(Deserializer* deserializer, const Array& refs) {
    // We only need to worry about how canonical values are handled during
    // deserialization if there may be multiple loading units, which only
    // happens in the precompiled runtime.
#if defined(DART_PRECOMPILED_RUNTIME)
    if (is_canonical()) {
      FATAL("%s needs canonicalization but doesn't define PostLoad", name());
    }
#endif
  }

  const char* name() const { return name_; }
  bool is_canonical() const { return is_canonical_; }

 protected:
  void ReadAllocFixedSize(Deserializer* deserializer, intptr_t instance_size);

  const char* const name_;
  const bool is_canonical_;
  const bool is_immutable_;
  // The range of the ref array that belongs to this cluster.
  intptr_t start_index_;
  intptr_t stop_index_;
};

class SerializationRoots {
 public:
  virtual ~SerializationRoots() {}
  virtual void AddBaseObjects(Serializer* serializer) = 0;
  virtual void PushRoots(Serializer* serializer) = 0;
  virtual void WriteRoots(Serializer* serializer) = 0;

  virtual const CompressedStackMaps& canonicalized_stack_map_entries() const;
};

class DeserializationRoots {
 public:
  virtual ~DeserializationRoots() {}
  virtual void AddBaseObjects(Deserializer* deserializer) = 0;
  virtual void ReadRoots(Deserializer* deserializer) = 0;
  virtual void PostLoad(Deserializer* deserializer, const Array& refs) = 0;
};

// Reference value for objects that either are not reachable from the roots or
// should never have a reference in the snapshot (because they are dropped,
// for example). Should be the default value for Heap::GetObjectId.
static constexpr intptr_t kUnreachableReference = 0;
COMPILE_ASSERT(kUnreachableReference == WeakTable::kNoValue);
static constexpr intptr_t kFirstReference = 1;

// Reference value for traced objects that have not been allocated their final
// reference ID.
static constexpr intptr_t kUnallocatedReference = -1;

static constexpr bool IsAllocatedReference(intptr_t ref) {
  return ref > kUnreachableReference;
}

static constexpr bool IsArtificialReference(intptr_t ref) {
  return ref < kUnallocatedReference;
}

static constexpr bool IsReachableReference(intptr_t ref) {
  return ref == kUnallocatedReference || IsAllocatedReference(ref);
}

class CodeSerializationCluster;

class Serializer : public ThreadStackResource {
 public:
  Serializer(Thread* thread,
             Snapshot::Kind kind,
             NonStreamingWriteStream* stream,
             ImageWriter* image_writer_,
             bool vm_,
             V8SnapshotProfileWriter* profile_writer = nullptr);
  ~Serializer();

  void AddBaseObject(ObjectPtr base_object,
                     const char* type = nullptr,
                     const char* name = nullptr);

  intptr_t AssignRef(ObjectPtr object);
  intptr_t AssignArtificialRef(ObjectPtr object = nullptr);

  intptr_t GetCodeIndex(CodePtr code);

  void Push(ObjectPtr object, intptr_t cid_override = kIllegalCid);
  void PushWeak(ObjectPtr object);

  void AddUntracedRef() { num_written_objects_++; }

  void Trace(ObjectPtr object, intptr_t cid_override);

  void UnexpectedObject(ObjectPtr object, const char* message);
#if defined(SNAPSHOT_BACKTRACE)
  ObjectPtr ParentOf(ObjectPtr object) const;
  ObjectPtr ParentOf(const Object& object) const;
#endif

  SerializationCluster* NewClusterForClass(intptr_t cid, bool is_canonical);

  void ReserveHeader() {
    // Make room for recording snapshot buffer size.
    stream_->SetPosition(Snapshot::kHeaderSize);
  }

  void FillHeader(Snapshot::Kind kind) {
    Snapshot* header = reinterpret_cast<Snapshot*>(stream_->buffer());
    header->set_magic();
    header->set_length(stream_->bytes_written());
    header->set_kind(kind);
  }

  void WriteVersionAndFeatures(bool is_vm_snapshot);

  ZoneGrowableArray<Object*>* Serialize(SerializationRoots* roots);
  void PrintSnapshotSizes();

  NonStreamingWriteStream* stream() { return stream_; }
  intptr_t bytes_written() { return stream_->bytes_written(); }
  intptr_t bytes_heap_allocated() { return bytes_heap_allocated_; }

  class WritingObjectScope : ValueObject {
   public:
    WritingObjectScope(Serializer* serializer,
                       const char* type,
                       ObjectPtr object,
                       StringPtr name)
        : WritingObjectScope(
              serializer,
              ReserveId(serializer,
                        type,
                        object,
                        String::ToCString(serializer->thread(), name)),
              object) {}

    WritingObjectScope(Serializer* serializer,
                       const char* type,
                       ObjectPtr object,
                       const char* name)
        : WritingObjectScope(serializer,
                             ReserveId(serializer, type, object, name),
                             object) {}

    WritingObjectScope(Serializer* serializer,
                       const V8SnapshotProfileWriter::ObjectId& id,
                       ObjectPtr object = nullptr);

    WritingObjectScope(Serializer* serializer, ObjectPtr object)
        : WritingObjectScope(serializer,
                             serializer->GetProfileId(object),
                             object) {}

    ~WritingObjectScope();

   private:
    static V8SnapshotProfileWriter::ObjectId ReserveId(Serializer* serializer,
                                                       const char* type,
                                                       ObjectPtr object,
                                                       const char* name);

   private:
    Serializer* const serializer_;
    const ObjectPtr old_object_;
    const V8SnapshotProfileWriter::ObjectId old_id_;
    const classid_t old_cid_;
  };

  // Writes raw data to the stream (basic type).
  // sizeof(T) must be in {1,2,4,8}.
  template <typename T>
  void Write(T value) {
    BaseWriteStream::Raw<sizeof(T), T>::Write(stream_, value);
  }
  void WriteRefId(intptr_t value) { stream_->WriteRefId(value); }
  void WriteUnsigned(intptr_t value) { stream_->WriteUnsigned(value); }
  void WriteUnsigned64(uint64_t value) { stream_->WriteUnsigned(value); }

  void WriteWordWith32BitWrites(uword value) {
    stream_->WriteWordWith32BitWrites(value);
  }

  void WriteBytes(const void* addr, intptr_t len) {
    stream_->WriteBytes(addr, len);
  }
  void Align(intptr_t alignment, intptr_t offset = 0) {
    stream_->Align(alignment, offset);
  }

  V8SnapshotProfileWriter::ObjectId GetProfileId(ObjectPtr object) const;
  V8SnapshotProfileWriter::ObjectId GetProfileId(intptr_t ref) const;

  void WriteRootRef(ObjectPtr object, const char* name = nullptr) {
    intptr_t id = RefId(object);
    WriteRefId(id);
    if (profile_writer_ != nullptr) {
      profile_writer_->AddRoot(GetProfileId(object), name);
    }
  }

  // Record a reference from the currently written object to the given object
  // and return reference id for the given object.
  void AttributeReference(ObjectPtr object,
                          const V8SnapshotProfileWriter::Reference& reference);

  void AttributeElementRef(ObjectPtr object, intptr_t index) {
    AttributeReference(object,
                       V8SnapshotProfileWriter::Reference::Element(index));
  }

  void WriteElementRef(ObjectPtr object, intptr_t index) {
    AttributeElementRef(object, index);
    WriteRefId(RefId(object));
  }

  void AttributePropertyRef(ObjectPtr object, const char* property) {
    AttributeReference(object,
                       V8SnapshotProfileWriter::Reference::Property(property));
  }

  void WritePropertyRef(ObjectPtr object, const char* property) {
    AttributePropertyRef(object, property);
    WriteRefId(RefId(object));
  }

  void WriteOffsetRef(ObjectPtr object, intptr_t offset) {
    intptr_t id = RefId(object);
    WriteRefId(id);
    if (profile_writer_ != nullptr) {
      if (auto const property = offsets_table_->FieldNameForOffset(
              object_currently_writing_.cid_, offset)) {
        AttributePropertyRef(object, property);
      } else {
        AttributeElementRef(object, offset);
      }
    }
  }

  template <typename T, typename... P>
  void WriteFromTo(T obj, P&&... args) {
    auto* from = obj->untag()->from();
    auto* to = obj->untag()->to_snapshot(kind(), args...);
    WriteRange(obj, from, to);
  }

  template <typename T>
  DART_NOINLINE void WriteRange(ObjectPtr obj, T from, T to) {
    for (auto* p = from; p <= to; p++) {
      WriteOffsetRef(
          p->Decompress(obj->heap_base()),
          reinterpret_cast<uword>(p) - reinterpret_cast<uword>(obj->untag()));
    }
  }

  template <typename T, typename... P>
  void PushFromTo(T obj, P&&... args) {
    auto* from = obj->untag()->from();
    auto* to = obj->untag()->to_snapshot(kind(), args...);
    PushRange(obj, from, to);
  }

  template <typename T>
  DART_NOINLINE void PushRange(ObjectPtr obj, T from, T to) {
    for (auto* p = from; p <= to; p++) {
      Push(p->Decompress(obj->heap_base()));
    }
  }

  void WriteTokenPosition(TokenPosition pos) { Write(pos.Serialize()); }

  void WriteCid(intptr_t cid) {
    COMPILE_ASSERT(UntaggedObject::kClassIdTagSize <= 32);
    Write<int32_t>(cid);
  }

  // Sorts Code objects and reorders instructions before writing snapshot.
  // Builds binary search table for stack maps.
  void PrepareInstructions(const CompressedStackMaps& canonical_smap);

  void WriteInstructions(InstructionsPtr instr,
                         uint32_t unchecked_offset,
                         CodePtr code,
                         bool deferred);
  uint32_t GetDataOffset(ObjectPtr object) const;
  void TraceDataOffset(uint32_t offset);
  intptr_t GetDataSize() const;

  void WriteDispatchTable(const Array& entries);

  Heap* heap() const { return heap_; }
  Zone* zone() const { return zone_; }
  Snapshot::Kind kind() const { return kind_; }
  intptr_t next_ref_index() const { return next_ref_index_; }

  void DumpCombinedCodeStatistics();

  V8SnapshotProfileWriter* profile_writer() const { return profile_writer_; }

  // If the given [obj] was not included into the snapshot and have not
  // yet gotten an artificial node created for it create an artificial node
  // in the profile representing this object.
  // Returns true if [obj] has an artificial profile node associated with it.
  bool CreateArtificialNodeIfNeeded(ObjectPtr obj);

  bool InCurrentLoadingUnitOrRoot(ObjectPtr obj);
  void RecordDeferredCode(CodePtr ptr);
  GrowableArray<LoadingUnitSerializationData*>* loading_units() const {
    return loading_units_;
  }
  void set_loading_units(GrowableArray<LoadingUnitSerializationData*>* units) {
    loading_units_ = units;
  }
  intptr_t current_loading_unit_id() const { return current_loading_unit_id_; }
  void set_current_loading_unit_id(intptr_t id) {
    current_loading_unit_id_ = id;
  }

  // Returns the reference ID for the object. Fails for objects that have not
  // been allocated a reference ID yet, so should be used only after all
  // WriteAlloc calls.
  intptr_t RefId(ObjectPtr object) const;

  // Same as RefId, but allows artificial and unreachable references. Still
  // fails for unallocated references.
  intptr_t UnsafeRefId(ObjectPtr object) const;

  // Whether the object is reachable.
  bool IsReachable(ObjectPtr object) const {
    return IsReachableReference(heap_->GetObjectId(object));
  }
  // Whether the object has an allocated reference.
  bool HasRef(ObjectPtr object) const {
    return IsAllocatedReference(heap_->GetObjectId(object));
  }
  // Whether the object only appears in the V8 snapshot profile.
  bool HasArtificialRef(ObjectPtr object) const {
    return IsArtificialReference(heap_->GetObjectId(object));
  }
  // Whether a node for the object already has been added to the V8 snapshot
  // profile.
  bool HasProfileNode(ObjectPtr object) const {
    ASSERT(profile_writer_ != nullptr);
    return profile_writer_->HasId(GetProfileId(object));
  }
  bool IsWritten(ObjectPtr object) const {
    return heap_->GetObjectId(object) > num_base_objects_;
  }

 private:
  const char* ReadOnlyObjectType(intptr_t cid);
  void FlushProfile();

  Heap* heap_;
  Zone* zone_;
  Snapshot::Kind kind_;
  NonStreamingWriteStream* stream_;
  ImageWriter* image_writer_;
  SerializationCluster** canonical_clusters_by_cid_;
  SerializationCluster** clusters_by_cid_;
  CodeSerializationCluster* code_cluster_ = nullptr;

  struct StackEntry {
    ObjectPtr obj;
    intptr_t cid_override;
  };
  GrowableArray<StackEntry> stack_;

  intptr_t num_cids_;
  intptr_t num_tlc_cids_;
  intptr_t num_base_objects_;
  intptr_t num_written_objects_;
  intptr_t next_ref_index_;

  intptr_t dispatch_table_size_ = 0;
  intptr_t bytes_heap_allocated_ = 0;
  intptr_t instructions_table_len_ = 0;
  intptr_t instructions_table_rodata_offset_ = 0;

  // True if writing VM snapshot, false for Isolate snapshot.
  bool vm_;

  V8SnapshotProfileWriter* profile_writer_ = nullptr;
  struct ProfilingObject {
    ObjectPtr object_ = nullptr;
    // Unless within a WritingObjectScope, any bytes written are attributed to
    // the artificial root.
    V8SnapshotProfileWriter::ObjectId id_ =
        V8SnapshotProfileWriter::kArtificialRootId;
    intptr_t last_stream_position_ = 0;
    intptr_t cid_ = -1;
  } object_currently_writing_;
  OffsetsTable* offsets_table_ = nullptr;

#if defined(SNAPSHOT_BACKTRACE)
  ObjectPtr current_parent_;
  GrowableArray<Object*> parent_pairs_;
#endif

#if defined(DART_PRECOMPILER)
  IntMap<intptr_t> deduped_instructions_sources_;
  IntMap<intptr_t> code_index_;
#endif

  intptr_t current_loading_unit_id_ = 0;
  GrowableArray<LoadingUnitSerializationData*>* loading_units_ = nullptr;
  ZoneGrowableArray<Object*>* objects_ = new ZoneGrowableArray<Object*>();

  DISALLOW_IMPLICIT_CONSTRUCTORS(Serializer);
};

#define AutoTraceObject(obj)                                                   \
  Serializer::WritingObjectScope scope_##__COUNTER__(s, name(), obj, nullptr)

#define AutoTraceObjectName(obj, str)                                          \
  Serializer::WritingObjectScope scope_##__COUNTER__(s, name(), obj, str)

#define WriteFieldValue(field, value) s->WritePropertyRef(value, #field);

#define WriteFromTo(obj, ...) s->WriteFromTo(obj, ##__VA_ARGS__);

#define PushFromTo(obj, ...) s->PushFromTo(obj, ##__VA_ARGS__);

#define WriteField(obj, field) s->WritePropertyRef(obj->untag()->field, #field)
#define WriteCompressedField(obj, name)                                        \
  s->WritePropertyRef(obj->untag()->name(), #name "_")

class Deserializer : public ThreadStackResource {
 public:
  Deserializer(Thread* thread,
               Snapshot::Kind kind,
               const uint8_t* buffer,
               intptr_t size,
               const uint8_t* data_buffer,
               const uint8_t* instructions_buffer,
               bool is_non_root_unit,
               intptr_t offset = 0);
  ~Deserializer();

  // Verifies the image alignment.
  //
  // Returns ApiError::null() on success and an ApiError with an an appropriate
  // message otherwise.
  ApiErrorPtr VerifyImageAlignment();

  ObjectPtr Allocate(intptr_t size);
  static void InitializeHeader(ObjectPtr raw,
                               intptr_t cid,
                               intptr_t size,
                               bool is_canonical = false) {
    InitializeHeader(raw, cid, size, is_canonical,
                     ShouldHaveImmutabilityBitSetCid(cid));
  }
  static void InitializeHeader(ObjectPtr raw,
                               intptr_t cid,
                               intptr_t size,
                               bool is_canonical,
                               bool is_immutable);

  // Reads raw data (for basic types).
  // sizeof(T) must be in {1,2,4,8}.
  template <typename T>
  T Read() {
    return ReadStream::Raw<sizeof(T), T>::Read(&stream_);
  }
  intptr_t ReadRefId() { return stream_.ReadRefId(); }
  intptr_t ReadUnsigned() { return stream_.ReadUnsigned(); }
  uint64_t ReadUnsigned64() { return stream_.ReadUnsigned<uint64_t>(); }
  void ReadBytes(uint8_t* addr, intptr_t len) { stream_.ReadBytes(addr, len); }

  uword ReadWordWith32BitReads() { return stream_.ReadWordWith32BitReads(); }

  intptr_t position() const { return stream_.Position(); }
  void set_position(intptr_t p) { stream_.SetPosition(p); }
  const uint8_t* AddressOfCurrentPosition() const {
    return stream_.AddressOfCurrentPosition();
  }

  void Advance(intptr_t value) { stream_.Advance(value); }
  void Align(intptr_t alignment, intptr_t offset = 0) {
    stream_.Align(alignment, offset);
  }

  void AddBaseObject(ObjectPtr base_object) { AssignRef(base_object); }

  void AssignRef(ObjectPtr object) {
    ASSERT(next_ref_index_ <= num_objects_);
    refs_->untag()->data()[next_ref_index_] = object;
    next_ref_index_++;
  }

  ObjectPtr Ref(intptr_t index) const {
    ASSERT(index > 0);
    ASSERT(index <= num_objects_);
    return refs_->untag()->element(index);
  }

  CodePtr GetCodeByIndex(intptr_t code_index, uword* entry_point) const;
  uword GetEntryPointByCodeIndex(intptr_t code_index) const;

  // If |code_index| corresponds to a non-discarded Code object returns
  // index within the code cluster that corresponds to this Code object.
  // Otherwise, if |code_index| corresponds to the discarded Code then
  // returns -1.
  static intptr_t CodeIndexToClusterIndex(const InstructionsTable& table,
                                          intptr_t code_index);

  ObjectPtr ReadRef() { return Ref(ReadRefId()); }

  TokenPosition ReadTokenPosition() {
    return TokenPosition::Deserialize(Read<int32_t>());
  }

  intptr_t ReadCid() {
    COMPILE_ASSERT(UntaggedObject::kClassIdTagSize <= 32);
    return Read<int32_t>();
  }

  void ReadInstructions(CodePtr code, bool deferred);
  void EndInstructions();
  ObjectPtr GetObjectAt(uint32_t offset) const;

  void Deserialize(DeserializationRoots* roots);

  DeserializationCluster* ReadCluster();

  void ReadDispatchTable() {
    ReadDispatchTable(&stream_, /*deferred=*/false, InstructionsTable::Handle(),
                      -1, -1);
  }
  void ReadDispatchTable(ReadStream* stream,
                         bool deferred,
                         const InstructionsTable& root_instruction_table,
                         intptr_t deferred_code_start_index,
                         intptr_t deferred_code_end_index);

  intptr_t next_index() const { return next_ref_index_; }
  Heap* heap() const { return heap_; }
  Zone* zone() const { return zone_; }
  Snapshot::Kind kind() const {
#if defined(DART_PRECOMPILED_RUNTIME)
    return Snapshot::kFullAOT;
#else
    return kind_;
#endif
  }
  bool is_non_root_unit() const { return is_non_root_unit_; }
  void set_code_start_index(intptr_t value) { code_start_index_ = value; }
  intptr_t code_start_index() const { return code_start_index_; }
  void set_code_stop_index(intptr_t value) { code_stop_index_ = value; }
  intptr_t code_stop_index() const { return code_stop_index_; }
  const InstructionsTable& instructions_table() const {
    return instructions_table_;
  }
  intptr_t num_base_objects() const { return num_base_objects_; }

  // This serves to make the snapshot cursor, ref table and null be locals
  // during ReadFill, which allows the C compiler to see they are not aliased
  // and can be kept in registers.
  class Local : public ReadStream {
   public:
    explicit Local(Deserializer* d)
        : ReadStream(d->stream_.buffer_, d->stream_.current_, d->stream_.end_),
          d_(d),
          refs_(d->refs_),
          null_(Object::null()) {
#if defined(DEBUG)
      // Can't mix use of Deserializer::Read*.
      d->stream_.current_ = nullptr;
#endif
    }
    ~Local() { d_->stream_.current_ = current_; }

    ObjectPtr Ref(intptr_t index) const {
      ASSERT(index > 0);
      ASSERT(index <= d_->num_objects_);
      return refs_->untag()->element(index);
    }

    template <typename T>
    T Read() {
      return ReadStream::Raw<sizeof(T), T>::Read(this);
    }
    uint64_t ReadUnsigned64() { return ReadUnsigned<uint64_t>(); }

    ObjectPtr ReadRef() { return Ref(ReadRefId()); }
    TokenPosition ReadTokenPosition() {
      return TokenPosition::Deserialize(Read<int32_t>());
    }

    intptr_t ReadCid() {
      COMPILE_ASSERT(UntaggedObject::kClassIdTagSize <= 32);
      return Read<int32_t>();
    }

    template <typename T, typename... P>
    void ReadFromTo(T obj, P&&... params) {
      auto* from = obj->untag()->from();
      auto* to_snapshot = obj->untag()->to_snapshot(d_->kind(), params...);
      auto* to = obj->untag()->to(params...);
      for (auto* p = from; p <= to_snapshot; p++) {
        *p = ReadRef();
      }
      // This is necessary because, unlike Object::Allocate, the clustered
      // deserializer allocates object without null-initializing them. Instead,
      // each deserialization cluster is responsible for initializing every
      // field, ensuring that every field is written to exactly once.
      for (auto* p = to_snapshot + 1; p <= to; p++) {
        *p = null_;
      }
    }

   private:
    Deserializer* const d_;
    const ArrayPtr refs_;
    const ObjectPtr null_;
  };

 private:
  Heap* heap_;
  PageSpace* old_space_;
  FreeList* freelist_;
  Zone* zone_;
  Snapshot::Kind kind_;
  ReadStream stream_;
  ImageReader* image_reader_;
  intptr_t num_base_objects_;
  intptr_t num_objects_;
  intptr_t num_clusters_;
  ArrayPtr refs_;
  intptr_t next_ref_index_;
  intptr_t code_start_index_ = 0;
  intptr_t code_stop_index_ = 0;
  intptr_t instructions_index_ = 0;
  DeserializationCluster** clusters_;
  const bool is_non_root_unit_;
  InstructionsTable& instructions_table_;
};

DART_FORCE_INLINE
ObjectPtr Deserializer::Allocate(intptr_t size) {
  return UntaggedObject::FromAddr(
      old_space_->AllocateSnapshotLocked(freelist_, size));
}

void Deserializer::InitializeHeader(ObjectPtr raw,
                                    intptr_t class_id,
                                    intptr_t size,
                                    bool is_canonical,
                                    bool is_immutable) {
  ASSERT(Utils::IsAligned(size, kObjectAlignment));
  uword tags = 0;
  tags = UntaggedObject::ClassIdTag::update(class_id, tags);
  tags = UntaggedObject::SizeTag::update(size, tags);
  tags = UntaggedObject::CanonicalBit::update(is_canonical, tags);
  tags = UntaggedObject::AlwaysSetBit::update(true, tags);
  tags = UntaggedObject::NotMarkedBit::update(true, tags);
  tags = UntaggedObject::OldAndNotRememberedBit::update(true, tags);
  tags = UntaggedObject::NewOrEvacuationCandidateBit::update(false, tags);
  tags = UntaggedObject::ImmutableBit::update(is_immutable, tags);
  raw->untag()->tags_ = tags;
}

#if !defined(DART_PRECOMPILED_RUNTIME)
void SerializationCluster::WriteAndMeasureAlloc(Serializer* serializer) {
  intptr_t start_size = serializer->bytes_written();
  intptr_t start_data = serializer->GetDataSize();
  intptr_t start_objects = serializer->next_ref_index();
  uint32_t tags = UntaggedObject::ClassIdTag::encode(cid_) |
                  UntaggedObject::CanonicalBit::encode(is_canonical()) |
                  UntaggedObject::ImmutableBit::encode(is_immutable());
  serializer->Write<uint32_t>(tags);
  WriteAlloc(serializer);
  intptr_t stop_size = serializer->bytes_written();
  intptr_t stop_data = serializer->GetDataSize();
  intptr_t stop_objects = serializer->next_ref_index();
  if (FLAG_print_cluster_information) {
    OS::PrintErr("Snapshot 0x%" Pp " (%" Pd "), ", start_size,
                 stop_size - start_size);
    OS::PrintErr("Data 0x%" Pp " (%" Pd "): ", start_data,
                 stop_data - start_data);
    OS::PrintErr("Alloc %s (%" Pd ")\n", name(), stop_objects - start_objects);
  }
  size_ += (stop_size - start_size) + (stop_data - start_data);
  num_objects_ += (stop_objects - start_objects);
  if (target_instance_size_ != kSizeVaries) {
    target_memory_size_ += num_objects_ * target_instance_size_;
  }
}

void SerializationCluster::WriteAndMeasureFill(Serializer* serializer) {
  intptr_t start = serializer->bytes_written();
  WriteFill(serializer);
  intptr_t stop = serializer->bytes_written();
  if (FLAG_print_cluster_information) {
    OS::PrintErr("Snapshot 0x%" Pp " (%" Pd "): Fill %s\n", start, stop - start,
                 name());
  }
  size_ += (stop - start);
}
#endif  // !DART_PRECOMPILED_RUNTIME

DART_NOINLINE
void DeserializationCluster::ReadAllocFixedSize(Deserializer* d,
                                                intptr_t instance_size) {
  start_index_ = d->next_index();
  intptr_t count = d->ReadUnsigned();
  for (intptr_t i = 0; i < count; i++) {
    d->AssignRef(d->Allocate(instance_size));
  }
  stop_index_ = d->next_index();
}

#if !defined(DART_PRECOMPILED_RUNTIME)
static UnboxedFieldBitmap CalculateTargetUnboxedFieldsBitmap(
    Serializer* s,
    intptr_t class_id) {
  const auto unboxed_fields_bitmap_host =
      s->isolate_group()->class_table()->GetUnboxedFieldsMapAt(class_id);

  UnboxedFieldBitmap unboxed_fields_bitmap;
  if (unboxed_fields_bitmap_host.IsEmpty() ||
      kWordSize == compiler::target::kWordSize) {
    unboxed_fields_bitmap = unboxed_fields_bitmap_host;
  } else {
    ASSERT(kWordSize == 8 && compiler::target::kWordSize == 4);
    // A new bitmap is built if the word sizes in the target and
    // host are different
    unboxed_fields_bitmap.Reset();
    intptr_t target_i = 0, host_i = 0;

    while (host_i < UnboxedFieldBitmap::Length()) {
      // Each unboxed field has constant length, therefore the number of
      // words used by it should double when compiling from 64-bit to 32-bit.
      if (unboxed_fields_bitmap_host.Get(host_i++)) {
        unboxed_fields_bitmap.Set(target_i++);
        unboxed_fields_bitmap.Set(target_i++);
      } else {
        // For object pointers, the field is always one word length
        target_i++;
      }
    }
  }

  return unboxed_fields_bitmap;
}

class ClassSerializationCluster : public SerializationCluster {
 public:
  explicit ClassSerializationCluster(intptr_t num_cids)
      : SerializationCluster("Class",
                             kClassCid,
                             compiler::target::Class::InstanceSize()),
        predefined_(kNumPredefinedCids),
        objects_(num_cids) {}
  ~ClassSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    ClassPtr cls = Class::RawCast(object);
    intptr_t class_id = cls->untag()->id_;

    if (class_id == kIllegalCid) {
      // Classes expected to be dropped by the precompiler should not be traced.
      s->UnexpectedObject(cls, "Class with illegal cid");
    }
    if (class_id < kNumPredefinedCids) {
      // These classes are allocated by Object::Init or Object::InitOnce, so the
      // deserializer must find them in the class table instead of allocating
      // them.
      predefined_.Add(cls);
    } else {
      objects_.Add(cls);
    }

    PushFromTo(cls);
  }

  void WriteAlloc(Serializer* s) {
    intptr_t count = predefined_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      ClassPtr cls = predefined_[i];
      s->AssignRef(cls);
      AutoTraceObject(cls);
      intptr_t class_id = cls->untag()->id_;
      s->WriteCid(class_id);
    }
    count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      ClassPtr cls = objects_[i];
      s->AssignRef(cls);
    }
  }

  void WriteFill(Serializer* s) {
    intptr_t count = predefined_.length();
    for (intptr_t i = 0; i < count; i++) {
      WriteClass(s, predefined_[i]);
    }
    count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      WriteClass(s, objects_[i]);
    }
  }

 private:
  void WriteClass(Serializer* s, ClassPtr cls) {
    AutoTraceObjectName(cls, cls->untag()->name());
    WriteFromTo(cls);
    intptr_t class_id = cls->untag()->id_;
    if (class_id == kIllegalCid) {
      s->UnexpectedObject(cls, "Class with illegal cid");
    }
    s->WriteCid(class_id);
    if (s->kind() != Snapshot::kFullAOT) {
      s->Write<uint32_t>(cls->untag()->kernel_offset_);
    }
    s->Write<int32_t>(Class::target_instance_size_in_words(cls));
    s->Write<int32_t>(Class::target_next_field_offset_in_words(cls));
    s->Write<int32_t>(Class::target_type_arguments_field_offset_in_words(cls));
    s->Write<int16_t>(cls->untag()->num_type_arguments_);
    s->Write<uint16_t>(cls->untag()->num_native_fields_);
    if (s->kind() != Snapshot::kFullAOT) {
      s->WriteTokenPosition(cls->untag()->token_pos_);
      s->WriteTokenPosition(cls->untag()->end_token_pos_);
      s->WriteCid(cls->untag()->implementor_cid_);
    }
    s->Write<uint32_t>(cls->untag()->state_bits_);

    if (!ClassTable::IsTopLevelCid(class_id)) {
      const auto unboxed_fields_map =
          CalculateTargetUnboxedFieldsBitmap(s, class_id);
      s->WriteUnsigned64(unboxed_fields_map.Value());
    }
  }

  GrowableArray<ClassPtr> predefined_;
  GrowableArray<ClassPtr> objects_;
};
#endif  // !DART_PRECOMPILED_RUNTIME

class ClassDeserializationCluster : public DeserializationCluster {
 public:
  ClassDeserializationCluster() : DeserializationCluster("Class") {}
  ~ClassDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    predefined_start_index_ = d->next_index();
    intptr_t count = d->ReadUnsigned();
    ClassTable* table = d->isolate_group()->class_table();
    for (intptr_t i = 0; i < count; i++) {
      intptr_t class_id = d->ReadCid();
      ASSERT(table->HasValidClassAt(class_id));
      ClassPtr cls = table->At(class_id);
      ASSERT(cls != nullptr);
      d->AssignRef(cls);
    }
    predefined_stop_index_ = d->next_index();

    start_index_ = d->next_index();
    count = d->ReadUnsigned();
    for (intptr_t i = 0; i < count; i++) {
      d->AssignRef(d->Allocate(Class::InstanceSize()));
    }
    stop_index_ = d->next_index();
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    for (intptr_t id = predefined_start_index_; id < predefined_stop_index_;
         id++) {
      ClassPtr cls = static_cast<ClassPtr>(d.Ref(id));
      d.ReadFromTo(cls);
      intptr_t class_id = d.ReadCid();
      cls->untag()->id_ = class_id;
#if !defined(DART_PRECOMPILED_RUNTIME)
      ASSERT(d_->kind() != Snapshot::kFullAOT);
      cls->untag()->kernel_offset_ = d.Read<uint32_t>();
#endif
      if (!IsInternalVMdefinedClassId(class_id)) {
        cls->untag()->host_instance_size_in_words_ = d.Read<int32_t>();
        cls->untag()->host_next_field_offset_in_words_ = d.Read<int32_t>();
#if defined(DART_PRECOMPILER)
        // Only one pair is serialized. The target field only exists when
        // DART_PRECOMPILER is defined
        cls->untag()->target_instance_size_in_words_ =
            cls->untag()->host_instance_size_in_words_;
        cls->untag()->target_next_field_offset_in_words_ =
            cls->untag()->host_next_field_offset_in_words_;
#endif  // defined(DART_PRECOMPILER)
      } else {
        d.Read<int32_t>();  // Skip.
        d.Read<int32_t>();  // Skip.
      }
      cls->untag()->host_type_arguments_field_offset_in_words_ =
          d.Read<int32_t>();
#if defined(DART_PRECOMPILER)
      cls->untag()->target_type_arguments_field_offset_in_words_ =
          cls->untag()->host_type_arguments_field_offset_in_words_;
#endif  // defined(DART_PRECOMPILER)
      cls->untag()->num_type_arguments_ = d.Read<int16_t>();
      cls->untag()->num_native_fields_ = d.Read<uint16_t>();
#if !defined(DART_PRECOMPILED_RUNTIME)
      ASSERT(d_->kind() != Snapshot::kFullAOT);
      cls->untag()->token_pos_ = d.ReadTokenPosition();
      cls->untag()->end_token_pos_ = d.ReadTokenPosition();
      cls->untag()->implementor_cid_ = d.ReadCid();
#endif  // !defined(DART_PRECOMPILED_RUNTIME)
      cls->untag()->state_bits_ = d.Read<uint32_t>();
      d.ReadUnsigned64();  // Skip unboxed fields bitmap.
    }

    ClassTable* table = d_->isolate_group()->class_table();
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      ClassPtr cls = static_cast<ClassPtr>(d.Ref(id));
      Deserializer::InitializeHeader(cls, kClassCid, Class::InstanceSize());
      d.ReadFromTo(cls);

      intptr_t class_id = d.ReadCid();
      ASSERT(class_id >= kNumPredefinedCids);
      cls->untag()->id_ = class_id;

#if !defined(DART_PRECOMPILED_RUNTIME)
      ASSERT(d_->kind() != Snapshot::kFullAOT);
      cls->untag()->kernel_offset_ = d.Read<uint32_t>();
#endif
      cls->untag()->host_instance_size_in_words_ = d.Read<int32_t>();
      cls->untag()->host_next_field_offset_in_words_ = d.Read<int32_t>();
      cls->untag()->host_type_arguments_field_offset_in_words_ =
          d.Read<int32_t>();
#if defined(DART_PRECOMPILER)
      cls->untag()->target_instance_size_in_words_ =
          cls->untag()->host_instance_size_in_words_;
      cls->untag()->target_next_field_offset_in_words_ =
          cls->untag()->host_next_field_offset_in_words_;
      cls->untag()->target_type_arguments_field_offset_in_words_ =
          cls->untag()->host_type_arguments_field_offset_in_words_;
#endif  // defined(DART_PRECOMPILER)
      cls->untag()->num_type_arguments_ = d.Read<int16_t>();
      cls->untag()->num_native_fields_ = d.Read<uint16_t>();
#if !defined(DART_PRECOMPILED_RUNTIME)
      ASSERT(d_->kind() != Snapshot::kFullAOT);
      cls->untag()->token_pos_ = d.ReadTokenPosition();
      cls->untag()->end_token_pos_ = d.ReadTokenPosition();
      cls->untag()->implementor_cid_ = d.ReadCid();
#endif  // !defined(DART_PRECOMPILED_RUNTIME)
      cls->untag()->state_bits_ = d.Read<uint32_t>();

      table->AllocateIndex(class_id);
      table->SetAt(class_id, cls);

      if (!ClassTable::IsTopLevelCid(class_id)) {
        const UnboxedFieldBitmap unboxed_fields_map(d.ReadUnsigned64());
        table->SetUnboxedFieldsMapAt(class_id, unboxed_fields_map);
      }
    }
  }

 private:
  intptr_t predefined_start_index_;
  intptr_t predefined_stop_index_;
};

// Super classes for writing out clusters which contain objects grouped into
// a canonical set (e.g. String, Type, TypeArguments, etc).
// To save space in the snapshot we avoid writing such canonical sets
// explicitly as Array objects into the snapshot and instead utilize a different
// encoding: objects in a cluster representing a canonical set are sorted
// to appear in the same order they appear in the Array representing the set,
// and we additionally write out array of values describing gaps between
// objects.
//
// In some situations not all canonical objects of the some type need to
// be added to the resulting canonical set because they are cached in some
// special way (see Type::Canonicalize as an example, which caches declaration
// types in a special way). In this case subclass can set
// kAllCanonicalObjectsAreIncludedIntoSet to |false| and override
// IsInCanonicalSet filter.
#if !defined(DART_PRECOMPILED_RUNTIME)
template <typename SetType,
          typename HandleType,
          typename PointerType,
          bool kAllCanonicalObjectsAreIncludedIntoSet = true>
class CanonicalSetSerializationCluster : public SerializationCluster {
 protected:
  CanonicalSetSerializationCluster(intptr_t cid,
                                   bool is_canonical,
                                   bool represents_canonical_set,
                                   const char* name,
                                   intptr_t target_instance_size = 0)
      : SerializationCluster(name, cid, target_instance_size, is_canonical),
        represents_canonical_set_(represents_canonical_set) {}

  virtual bool IsInCanonicalSet(Serializer* s, PointerType ptr) {
    // Must override this function if kAllCanonicalObjectsAreIncludedIntoSet
    // is set to |false|.
    ASSERT(kAllCanonicalObjectsAreIncludedIntoSet);
    return true;
  }

  void ReorderObjects(Serializer* s) {
    if (!represents_canonical_set_) {
      return;
    }

    // Sort objects before writing them out so that they appear in the same
    // order as they would appear in a CanonicalStringSet.
    using ZoneCanonicalSet =
        HashTable<typename SetType::Traits, 0, 0, GrowableArrayStorageTraits>;

    // Compute required capacity for the hashtable (to avoid overallocating).
    intptr_t required_capacity = 0;
    for (auto ptr : objects_) {
      if (kAllCanonicalObjectsAreIncludedIntoSet || IsInCanonicalSet(s, ptr)) {
        required_capacity++;
      }
    }
    // Over-allocate capacity so a few inserts can happen at startup without
    // causing a rehash.
    const intptr_t kSpareCapacity = 32;
    required_capacity = static_cast<intptr_t>(
        static_cast<double>(required_capacity + kSpareCapacity) /
        HashTables::kMaxLoadFactor);

    intptr_t num_occupied = 0;

    // Build canonical set out of objects that should belong to it.
    // Objects that don't belong to it are copied to the prefix of objects_.
    ZoneCanonicalSet table(
        s->zone(), HashTables::New<ZoneCanonicalSet>(required_capacity));
    HandleType& element = HandleType::Handle(s->zone());
    for (auto ptr : objects_) {
      if (kAllCanonicalObjectsAreIncludedIntoSet || IsInCanonicalSet(s, ptr)) {
        element ^= ptr;
        intptr_t entry = -1;
        const bool present = table.FindKeyOrDeletedOrUnused(element, &entry);
        ASSERT(!present);
        table.InsertKey(entry, element);
      } else {
        objects_[num_occupied++] = ptr;
      }
    }

    const auto prefix_length = num_occupied;

    // Compute objects_ order and gaps based on canonical set layout.
    auto& arr = table.Release();
    intptr_t last_occupied = ZoneCanonicalSet::kFirstKeyIndex - 1;
    for (intptr_t i = ZoneCanonicalSet::kFirstKeyIndex, length = arr.Length();
         i < length; i++) {
      ObjectPtr v = arr.At(i);
      ASSERT(v != ZoneCanonicalSet::DeletedMarker().ptr());
      if (v != ZoneCanonicalSet::UnusedMarker().ptr()) {
        const intptr_t unused_run_length = (i - 1) - last_occupied;
        gaps_.Add(unused_run_length);
        objects_[num_occupied++] = static_cast<PointerType>(v);
        last_occupied = i;
      }
    }
    ASSERT(num_occupied == objects_.length());
    ASSERT(prefix_length == (objects_.length() - gaps_.length()));
    table_length_ = arr.Length();
  }

  void WriteCanonicalSetLayout(Serializer* s) {
    if (represents_canonical_set_) {
      s->WriteUnsigned(table_length_);
      s->WriteUnsigned(objects_.length() - gaps_.length());
      for (auto gap : gaps_) {
        s->WriteUnsigned(gap);
      }
      target_memory_size_ +=
          compiler::target::Array::InstanceSize(table_length_);
    }
  }

  GrowableArray<PointerType> objects_;

 private:
  const bool represents_canonical_set_;
  GrowableArray<intptr_t> gaps_;
  intptr_t table_length_ = 0;
};
#endif

template <typename SetType, bool kAllCanonicalObjectsAreIncludedIntoSet = true>
class CanonicalSetDeserializationCluster : public DeserializationCluster {
 public:
  CanonicalSetDeserializationCluster(bool is_canonical,
                                     bool is_root_unit,
                                     const char* name)
      : DeserializationCluster(name, is_canonical),
        is_root_unit_(is_root_unit),
        table_(SetType::ArrayHandle::Handle()) {}

  void BuildCanonicalSetFromLayout(Deserializer* d) {
    if (!is_root_unit_ || !is_canonical()) {
      return;
    }

    const auto table_length = d->ReadUnsigned();
    first_element_ = d->ReadUnsigned();
    const intptr_t count = stop_index_ - (start_index_ + first_element_);
    auto table = StartDeserialization(d, table_length, count);
    for (intptr_t i = start_index_ + first_element_; i < stop_index_; i++) {
      table.FillGap(d->ReadUnsigned());
      table.WriteElement(d, d->Ref(i));
    }
    table_ = table.Finish();
  }

 protected:
  const bool is_root_unit_;
  intptr_t first_element_;
  typename SetType::ArrayHandle& table_;

  void VerifyCanonicalSet(Deserializer* d,
                          const Array& refs,
                          const typename SetType::ArrayHandle& current_table) {
#if defined(DEBUG)
    // First check that we are not overwriting a table and loosing information.
    if (!current_table.IsNull()) {
      SetType current_set(d->zone(), current_table.ptr());
      ASSERT(current_set.NumOccupied() == 0);
      current_set.Release();
    }

    // Now check that manually created table behaves correctly as a canonical
    // set.
    SetType canonical_set(d->zone(), table_.ptr());
    Object& key = Object::Handle();
    for (intptr_t i = start_index_ + first_element_; i < stop_index_; i++) {
      key = refs.At(i);
      ASSERT(canonical_set.GetOrNull(key) != Object::null());
    }
    canonical_set.Release();
#endif  // defined(DEBUG)
  }

 private:
  struct DeserializationFinger {
    typename SetType::ArrayPtr table;
    intptr_t current_index;
    ObjectPtr gap_element;

    void FillGap(int length) {
      for (intptr_t j = 0; j < length; j++) {
        table->untag()->data()[current_index + j] = gap_element;
      }
      current_index += length;
    }

    void WriteElement(Deserializer* d, ObjectPtr object) {
      table->untag()->data()[current_index++] = object;
    }

    typename SetType::ArrayPtr Finish() {
      if (table != SetType::ArrayHandle::null()) {
        FillGap(Smi::Value(table->untag()->length()) - current_index);
      }
      auto result = table;
      table = SetType::ArrayHandle::null();
      return result;
    }
  };

  static DeserializationFinger StartDeserialization(Deserializer* d,
                                                    intptr_t length,
                                                    intptr_t count) {
    const intptr_t instance_size = SetType::ArrayHandle::InstanceSize(length);
    typename SetType::ArrayPtr table =
        static_cast<typename SetType::ArrayPtr>(d->Allocate(instance_size));
    Deserializer::InitializeHeader(table, SetType::Storage::ArrayCid,
                                   instance_size);
    if ((SetType::Storage::ArrayCid == kArrayCid) &&
        Array::UseCardMarkingForAllocation(length)) {
      table->untag()->SetCardRememberedBitUnsynchronized();
      Page::Of(table)->AllocateCardTable();
    }
    InitTypeArgsOrNext(table);
    table->untag()->length_ = Smi::New(length);
    for (intptr_t i = 0; i < SetType::kFirstKeyIndex; i++) {
      table->untag()->data()[i] = Smi::New(0);
    }
    table->untag()->data()[SetType::kOccupiedEntriesIndex] = Smi::New(count);
    return {table, SetType::kFirstKeyIndex, SetType::UnusedMarker().ptr()};
  }

  static void InitTypeArgsOrNext(ArrayPtr table) {
    table->untag()->type_arguments_ = TypeArguments::null();
  }
  static void InitTypeArgsOrNext(WeakArrayPtr table) {
    table->untag()->next_seen_by_gc_ = WeakArray::null();
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
class TypeParametersSerializationCluster : public SerializationCluster {
 public:
  TypeParametersSerializationCluster()
      : SerializationCluster("TypeParameters",
                             kTypeParametersCid,
                             compiler::target::TypeParameters::InstanceSize()) {
  }
  ~TypeParametersSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    TypeParametersPtr type_params = TypeParameters::RawCast(object);
    objects_.Add(type_params);
    PushFromTo(type_params);
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      TypeParametersPtr type_params = objects_[i];
      s->AssignRef(type_params);
    }
  }

  void WriteFill(Serializer* s) {
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      TypeParametersPtr type_params = objects_[i];
      AutoTraceObject(type_params);
      WriteFromTo(type_params);
    }
  }

 private:
  GrowableArray<TypeParametersPtr> objects_;
};
#endif  // !DART_PRECOMPILED_RUNTIME

class TypeParametersDeserializationCluster : public DeserializationCluster {
 public:
  TypeParametersDeserializationCluster()
      : DeserializationCluster("TypeParameters") {}
  ~TypeParametersDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    ReadAllocFixedSize(d, TypeParameters::InstanceSize());
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    ASSERT(!is_canonical());  // Never canonical.
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      TypeParametersPtr type_params = static_cast<TypeParametersPtr>(d.Ref(id));
      Deserializer::InitializeHeader(type_params, kTypeParametersCid,
                                     TypeParameters::InstanceSize());
      d.ReadFromTo(type_params);
    }
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
class TypeArgumentsSerializationCluster
    : public CanonicalSetSerializationCluster<CanonicalTypeArgumentsSet,
                                              TypeArguments,
                                              TypeArgumentsPtr> {
 public:
  TypeArgumentsSerializationCluster(bool is_canonical,
                                    bool represents_canonical_set)
      : CanonicalSetSerializationCluster(kTypeArgumentsCid,
                                         is_canonical,
                                         represents_canonical_set,
                                         "TypeArguments") {}
  ~TypeArgumentsSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    TypeArgumentsPtr type_args = TypeArguments::RawCast(object);
    objects_.Add(type_args);

    s->Push(type_args->untag()->instantiations());
    const intptr_t length = Smi::Value(type_args->untag()->length());
    for (intptr_t i = 0; i < length; i++) {
      s->Push(type_args->untag()->element(i));
    }
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    ReorderObjects(s);
    for (intptr_t i = 0; i < count; i++) {
      TypeArgumentsPtr type_args = objects_[i];
      s->AssignRef(type_args);
      AutoTraceObject(type_args);
      const intptr_t length = Smi::Value(type_args->untag()->length());
      s->WriteUnsigned(length);
      target_memory_size_ +=
          compiler::target::TypeArguments::InstanceSize(length);
    }
    WriteCanonicalSetLayout(s);
  }

  void WriteFill(Serializer* s) {
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      TypeArgumentsPtr type_args = objects_[i];
      AutoTraceObject(type_args);
      const intptr_t length = Smi::Value(type_args->untag()->length());
      s->WriteUnsigned(length);
      intptr_t hash = Smi::Value(type_args->untag()->hash());
      s->Write<int32_t>(hash);
      const intptr_t nullability =
          Smi::Value(type_args->untag()->nullability());
      s->WriteUnsigned(nullability);
      WriteField(type_args, instantiations());
      for (intptr_t j = 0; j < length; j++) {
        s->WriteElementRef(type_args->untag()->element(j), j);
      }
    }
  }
};
#endif  // !DART_PRECOMPILED_RUNTIME

class TypeArgumentsDeserializationCluster
    : public CanonicalSetDeserializationCluster<CanonicalTypeArgumentsSet> {
 public:
  explicit TypeArgumentsDeserializationCluster(bool is_canonical,
                                               bool is_root_unit)
      : CanonicalSetDeserializationCluster(is_canonical,
                                           is_root_unit,
                                           "TypeArguments") {}
  ~TypeArgumentsDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    start_index_ = d->next_index();
    const intptr_t count = d->ReadUnsigned();
    for (intptr_t i = 0; i < count; i++) {
      const intptr_t length = d->ReadUnsigned();
      d->AssignRef(d->Allocate(TypeArguments::InstanceSize(length)));
    }
    stop_index_ = d->next_index();
    BuildCanonicalSetFromLayout(d);
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    const bool mark_canonical = is_root_unit_ && is_canonical();
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      TypeArgumentsPtr type_args = static_cast<TypeArgumentsPtr>(d.Ref(id));
      const intptr_t length = d.ReadUnsigned();
      Deserializer::InitializeHeader(type_args, kTypeArgumentsCid,
                                     TypeArguments::InstanceSize(length),
                                     mark_canonical);
      type_args->untag()->length_ = Smi::New(length);
      type_args->untag()->hash_ = Smi::New(d.Read<int32_t>());
      type_args->untag()->nullability_ = Smi::New(d.ReadUnsigned());
      type_args->untag()->instantiations_ = static_cast<ArrayPtr>(d.ReadRef());
      for (intptr_t j = 0; j < length; j++) {
        type_args->untag()->types()[j] =
            static_cast<AbstractTypePtr>(d.ReadRef());
      }
    }
  }

  void PostLoad(Deserializer* d, const Array& refs) override {
    if (!table_.IsNull()) {
      auto object_store = d->isolate_group()->object_store();
      VerifyCanonicalSet(
          d, refs, Array::Handle(object_store->canonical_type_arguments()));
      object_store->set_canonical_type_arguments(table_);
    } else if (!is_root_unit_ && is_canonical()) {
      TypeArguments& type_arg = TypeArguments::Handle(d->zone());
      for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
        type_arg ^= refs.At(i);
        type_arg = type_arg.Canonicalize(d->thread());
        refs.SetAt(i, type_arg);
      }
    }
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
class PatchClassSerializationCluster : public SerializationCluster {
 public:
  PatchClassSerializationCluster()
      : SerializationCluster("PatchClass",
                             kPatchClassCid,
                             compiler::target::PatchClass::InstanceSize()) {}
  ~PatchClassSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    PatchClassPtr cls = PatchClass::RawCast(object);
    objects_.Add(cls);
    PushFromTo(cls);
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      PatchClassPtr cls = objects_[i];
      s->AssignRef(cls);
    }
  }

  void WriteFill(Serializer* s) {
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      PatchClassPtr cls = objects_[i];
      AutoTraceObject(cls);
      WriteFromTo(cls);
      if (s->kind() != Snapshot::kFullAOT) {
        s->Write<int32_t>(cls->untag()->kernel_library_index_);
      }
    }
  }

 private:
  GrowableArray<PatchClassPtr> objects_;
};
#endif  // !DART_PRECOMPILED_RUNTIME

class PatchClassDeserializationCluster : public DeserializationCluster {
 public:
  PatchClassDeserializationCluster() : DeserializationCluster("PatchClass") {}
  ~PatchClassDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    ReadAllocFixedSize(d, PatchClass::InstanceSize());
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    ASSERT(!is_canonical());  // Never canonical.
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      PatchClassPtr cls = static_cast<PatchClassPtr>(d.Ref(id));
      Deserializer::InitializeHeader(cls, kPatchClassCid,
                                     PatchClass::InstanceSize());
      d.ReadFromTo(cls);
#if !defined(DART_PRECOMPILED_RUNTIME)
      ASSERT(d_->kind() != Snapshot::kFullAOT);
      cls->untag()->kernel_library_index_ = d.Read<int32_t>();
#endif
    }
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
class FunctionSerializationCluster : public SerializationCluster {
 public:
  FunctionSerializationCluster()
      : SerializationCluster("Function",
                             kFunctionCid,
                             compiler::target::Function::InstanceSize()) {}
  ~FunctionSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    Snapshot::Kind kind = s->kind();
    FunctionPtr func = Function::RawCast(object);
    objects_.Add(func);

    PushFromTo(func);
    if (kind == Snapshot::kFullAOT) {
      s->Push(func->untag()->code());
    } else if (kind == Snapshot::kFullJIT) {
      NOT_IN_PRECOMPILED(s->Push(func->untag()->unoptimized_code()));
      s->Push(func->untag()->code());
      s->Push(func->untag()->ic_data_array_or_bytecode());
    }
    if (kind != Snapshot::kFullAOT) {
      NOT_IN_PRECOMPILED(s->Push(func->untag()->positional_parameter_names()));
    }
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      FunctionPtr func = objects_[i];
      s->AssignRef(func);
    }
  }

  void WriteFill(Serializer* s) {
    Snapshot::Kind kind = s->kind();
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      FunctionPtr func = objects_[i];
      AutoTraceObjectName(func, MakeDisambiguatedFunctionName(s, func));
      WriteFromTo(func);
      if (kind == Snapshot::kFullAOT) {
#if defined(DART_PRECOMPILER)
        CodePtr code = func->untag()->code();
        const auto code_index = s->GetCodeIndex(code);
        s->WriteUnsigned(code_index);
        s->AttributePropertyRef(code, "code_");
#else
        UNREACHABLE();
#endif
      } else if (s->kind() == Snapshot::kFullJIT) {
        NOT_IN_PRECOMPILED(WriteCompressedField(func, unoptimized_code));
        WriteCompressedField(func, code);
        WriteCompressedField(func, ic_data_array_or_bytecode);
      }

      if (kind != Snapshot::kFullAOT) {
        NOT_IN_PRECOMPILED(
            WriteCompressedField(func, positional_parameter_names));
      }

#if defined(DART_PRECOMPILER) && !defined(PRODUCT)
      TokenPosition token_pos = func->untag()->token_pos_;
      if (kind == Snapshot::kFullAOT) {
        // We use then token_pos property to store the line number
        // in AOT snapshots.
        intptr_t line = -1;
        const Function& function = Function::Handle(func);
        const Script& script = Script::Handle(function.script());
        if (!script.IsNull()) {
          script.GetTokenLocation(token_pos, &line, nullptr);
        }
        token_pos = line == -1 ? TokenPosition::kNoSource
                               : TokenPosition::Deserialize(line);
      }
      s->WriteTokenPosition(token_pos);
#else
      if (kind != Snapshot::kFullAOT) {
        s->WriteTokenPosition(func->untag()->token_pos_);
      }
#endif
      if (kind != Snapshot::kFullAOT) {
        s->WriteTokenPosition(func->untag()->end_token_pos_);
        s->Write<uint32_t>(func->untag()->kernel_offset_);
        s->Write<bool>(func->untag()->is_optimizable_);
      }
      s->Write<uint32_t>(func->untag()->kind_tag_);
    }
  }

  static const char* MakeDisambiguatedFunctionName(Serializer* s,
                                                   FunctionPtr f) {
    if (s->profile_writer() == nullptr) {
      return nullptr;
    }

    REUSABLE_FUNCTION_HANDLESCOPE(s->thread());
    Function& fun = reused_function_handle.Handle();
    fun = f;
    ZoneTextBuffer printer(s->thread()->zone());
    fun.PrintName(NameFormattingParams::DisambiguatedUnqualified(
                      Object::NameVisibility::kInternalName),
                  &printer);
    return printer.buffer();
  }

 private:
  GrowableArray<FunctionPtr> objects_;
};
#endif  // !DART_PRECOMPILED_RUNTIME

template <bool need_entry_point_for_non_discarded>
DART_FORCE_INLINE static CodePtr GetCodeAndEntryPointByIndex(
    const Deserializer* d,
    intptr_t code_index,
    uword* entry_point) {
  code_index -= 1;  // 0 is reserved for LazyCompile stub.

  // In root unit and VM isolate snapshot code_indices are self-contained
  // they point into instruction table and/or into the code cluster.
  // In non-root units we might also refer to code objects from the
  // parent unit which means code_index is biased by num_base_objects_
  const intptr_t base = d->is_non_root_unit() ? d->num_base_objects() : 0;
  if (code_index < base) {
    CodePtr code = static_cast<CodePtr>(d->Ref(code_index));
    if (need_entry_point_for_non_discarded) {
      *entry_point = Code::EntryPointOf(code);
    }
    return code;
  }
  code_index -= base;

  // At this point code_index is referring to a code object which is either
  // discarded or exists in the Code cluster. Non-discarded Code objects
  // are associated with the tail of the instruction table and have the
  // same order there and in the Code cluster. This means that
  // subtracting first_entry_with_code yields index into the Code cluster.
  // This also works for deferred code objects in root unit's snapshot
  // due to the choice of encoding (see Serializer::GetCodeIndex).
  const intptr_t first_entry_with_code =
      d->instructions_table().rodata()->first_entry_with_code;
  if (code_index < first_entry_with_code) {
    *entry_point = d->instructions_table().EntryPointAt(code_index);
    return StubCode::UnknownDartCode().ptr();
  } else {
    const intptr_t cluster_index = code_index - first_entry_with_code;
    CodePtr code =
        static_cast<CodePtr>(d->Ref(d->code_start_index() + cluster_index));
    if (need_entry_point_for_non_discarded) {
      *entry_point = Code::EntryPointOf(code);
    }
    return code;
  }
}

CodePtr Deserializer::GetCodeByIndex(intptr_t code_index,
                                     uword* entry_point) const {
  // See Serializer::GetCodeIndex for how code_index is encoded.
  if (code_index == 0) {
    return StubCode::LazyCompile().ptr();
  } else if (FLAG_precompiled_mode) {
    return GetCodeAndEntryPointByIndex<
        /*need_entry_point_for_non_discarded=*/false>(this, code_index,
                                                      entry_point);
  } else {
    // -1 below because 0 is reserved for LazyCompile stub.
    const intptr_t ref = code_start_index_ + code_index - 1;
    ASSERT(code_start_index_ <= ref && ref < code_stop_index_);
    return static_cast<CodePtr>(Ref(ref));
  }
}

intptr_t Deserializer::CodeIndexToClusterIndex(const InstructionsTable& table,
                                               intptr_t code_index) {
  // Note: code indices we are interpreting here originate from the root
  // loading unit which means base is equal to 0.
  // See comments which clarify the connection between code_index and
  // index into the Code cluster.
  ASSERT(FLAG_precompiled_mode);
  const intptr_t first_entry_with_code = table.rodata()->first_entry_with_code;
  return code_index - 1 - first_entry_with_code;
}

uword Deserializer::GetEntryPointByCodeIndex(intptr_t code_index) const {
  // See Deserializer::GetCodeByIndex which this code repeats.
  ASSERT(FLAG_precompiled_mode);
  uword entry_point = 0;
  GetCodeAndEntryPointByIndex</*need_entry_point_for_non_discarded=*/true>(
      this, code_index, &entry_point);
  return entry_point;
}

class FunctionDeserializationCluster : public DeserializationCluster {
 public:
  FunctionDeserializationCluster() : DeserializationCluster("Function") {}
  ~FunctionDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    ReadAllocFixedSize(d, Function::InstanceSize());
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    ASSERT(!is_canonical());  // Never canonical.
    Snapshot::Kind kind = d_->kind();

    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      FunctionPtr func = static_cast<FunctionPtr>(d.Ref(id));
      Deserializer::InitializeHeader(func, kFunctionCid,
                                     Function::InstanceSize());
      d.ReadFromTo(func);

#if defined(DEBUG)
      func->untag()->entry_point_ = 0;
      func->untag()->unchecked_entry_point_ = 0;
#endif

#if defined(DART_PRECOMPILED_RUNTIME)
      ASSERT(kind == Snapshot::kFullAOT);
      const intptr_t code_index = d.ReadUnsigned();
      uword entry_point = 0;
      CodePtr code = d_->GetCodeByIndex(code_index, &entry_point);
      func->untag()->code_ = code;
      if (entry_point != 0) {
        func->untag()->entry_point_ = entry_point;
        func->untag()->unchecked_entry_point_ = entry_point;
      }
#else
      ASSERT(kind != Snapshot::kFullAOT);
      if (kind == Snapshot::kFullJIT) {
        func->untag()->unoptimized_code_ = static_cast<CodePtr>(d.ReadRef());
        func->untag()->code_ = static_cast<CodePtr>(d.ReadRef());
        func->untag()->ic_data_array_or_bytecode_ = d.ReadRef();
      }
#endif

#if !defined(DART_PRECOMPILED_RUNTIME)
      ASSERT(kind != Snapshot::kFullAOT);
      func->untag()->positional_parameter_names_ =
          static_cast<ArrayPtr>(d.ReadRef());
#endif
#if !defined(DART_PRECOMPILED_RUNTIME) ||                                      \
    (defined(DART_PRECOMPILED_RUNTIME) && !defined(PRODUCT))
      func->untag()->token_pos_ = d.ReadTokenPosition();
#endif
#if !defined(DART_PRECOMPILED_RUNTIME)
      func->untag()->end_token_pos_ = d.ReadTokenPosition();
      func->untag()->kernel_offset_ = d.Read<uint32_t>();
      func->untag()->unboxed_parameters_info_.Reset();
      func->untag()->is_optimizable_ = d.Read<bool>();
#endif

      func->untag()->kind_tag_ = d.Read<uint32_t>();
#if !defined(DART_PRECOMPILED_RUNTIME)
      func->untag()->usage_counter_ = 0;
      func->untag()->optimized_instruction_count_ = 0;
      func->untag()->optimized_call_site_count_ = 0;
      func->untag()->deoptimization_counter_ = 0;
      func->untag()->state_bits_ = 0;
      func->untag()->inlining_depth_ = 0;
#endif
    }
  }

  void PostLoad(Deserializer* d, const Array& refs) override {
    if (d->kind() == Snapshot::kFullAOT) {
      Function& func = Function::Handle(d->zone());
      for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
        func ^= refs.At(i);
        auto const code = func.ptr()->untag()->code();
        ASSERT(code->IsCode());
        if (!Code::IsUnknownDartCode(code)) {
          uword entry_point = code->untag()->entry_point_;
          ASSERT(entry_point != 0);
          func.ptr()->untag()->entry_point_ = entry_point;
          uword unchecked_entry_point = code->untag()->unchecked_entry_point_;
          ASSERT(unchecked_entry_point != 0);
          func.ptr()->untag()->unchecked_entry_point_ = unchecked_entry_point;
        }
      }
    } else if (d->kind() == Snapshot::kFullJIT) {
      Function& func = Function::Handle(d->zone());
      Code& code = Code::Handle(d->zone());
      for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
        func ^= refs.At(i);
        code = func.CurrentCode();
        if (func.HasCode() && !code.IsDisabled()) {
          func.SetInstructionsSafe(code);  // Set entrypoint.
          func.SetWasCompiled(true);
        } else {
          func.ClearCodeSafe();  // Set code and entrypoint to lazy compile stub
        }
      }
    } else {
      Function& func = Function::Handle(d->zone());
      for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
        func ^= refs.At(i);
        func.ClearCodeSafe();  // Set code and entrypoint to lazy compile stub.
      }
    }
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
class ClosureDataSerializationCluster : public SerializationCluster {
 public:
  ClosureDataSerializationCluster()
      : SerializationCluster("ClosureData",
                             kClosureDataCid,
                             compiler::target::ClosureData::InstanceSize()) {}
  ~ClosureDataSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    ClosureDataPtr data = ClosureData::RawCast(object);
    objects_.Add(data);

    if (s->kind() != Snapshot::kFullAOT) {
      s->Push(data->untag()->context_scope());
    }
    s->Push(data->untag()->parent_function());
    s->Push(data->untag()->closure());
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      ClosureDataPtr data = objects_[i];
      s->AssignRef(data);
    }
  }

  void WriteFill(Serializer* s) {
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      ClosureDataPtr data = objects_[i];
      AutoTraceObject(data);
      if (s->kind() != Snapshot::kFullAOT) {
        WriteCompressedField(data, context_scope);
      }
      WriteCompressedField(data, parent_function);
      WriteCompressedField(data, closure);
      s->WriteUnsigned(static_cast<uint32_t>(data->untag()->packed_fields_));
    }
  }

 private:
  GrowableArray<ClosureDataPtr> objects_;
};
#endif  // !DART_PRECOMPILED_RUNTIME

class ClosureDataDeserializationCluster : public DeserializationCluster {
 public:
  ClosureDataDeserializationCluster() : DeserializationCluster("ClosureData") {}
  ~ClosureDataDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    ReadAllocFixedSize(d, ClosureData::InstanceSize());
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    ASSERT(!is_canonical());  // Never canonical.
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      ClosureDataPtr data = static_cast<ClosureDataPtr>(d.Ref(id));
      Deserializer::InitializeHeader(data, kClosureDataCid,
                                     ClosureData::InstanceSize());
      if (d_->kind() == Snapshot::kFullAOT) {
        data->untag()->context_scope_ = ContextScope::null();
      } else {
        data->untag()->context_scope_ =
            static_cast<ContextScopePtr>(d.ReadRef());
      }
      data->untag()->parent_function_ = static_cast<FunctionPtr>(d.ReadRef());
      data->untag()->closure_ = static_cast<ClosurePtr>(d.ReadRef());
      data->untag()->packed_fields_ = d.ReadUnsigned<uint32_t>();
    }
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
class FfiTrampolineDataSerializationCluster : public SerializationCluster {
 public:
  FfiTrampolineDataSerializationCluster()
      : SerializationCluster(
            "FfiTrampolineData",
            kFfiTrampolineDataCid,
            compiler::target::FfiTrampolineData::InstanceSize()) {}
  ~FfiTrampolineDataSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    FfiTrampolineDataPtr data = FfiTrampolineData::RawCast(object);
    objects_.Add(data);
    PushFromTo(data);
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      s->AssignRef(objects_[i]);
    }
  }

  void WriteFill(Serializer* s) {
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      FfiTrampolineDataPtr const data = objects_[i];
      AutoTraceObject(data);
      WriteFromTo(data);
      s->Write<int32_t>(data->untag()->callback_id_);
      s->Write<uint8_t>(data->untag()->ffi_function_kind_);
    }
  }

 private:
  GrowableArray<FfiTrampolineDataPtr> objects_;
};
#endif  // !DART_PRECOMPILED_RUNTIME

class FfiTrampolineDataDeserializationCluster : public DeserializationCluster {
 public:
  FfiTrampolineDataDeserializationCluster()
      : DeserializationCluster("FfiTrampolineData") {}
  ~FfiTrampolineDataDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    ReadAllocFixedSize(d, FfiTrampolineData::InstanceSize());
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    ASSERT(!is_canonical());  // Never canonical.
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      FfiTrampolineDataPtr data = static_cast<FfiTrampolineDataPtr>(d.Ref(id));
      Deserializer::InitializeHeader(data, kFfiTrampolineDataCid,
                                     FfiTrampolineData::InstanceSize());
      d.ReadFromTo(data);
      data->untag()->callback_id_ = d.Read<int32_t>();
      data->untag()->ffi_function_kind_ = d.Read<uint8_t>();
    }
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
class FieldSerializationCluster : public SerializationCluster {
 public:
  FieldSerializationCluster()
      : SerializationCluster("Field",
                             kFieldCid,
                             compiler::target::Field::InstanceSize()) {}
  ~FieldSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    FieldPtr field = Field::RawCast(object);
    objects_.Add(field);

    Snapshot::Kind kind = s->kind();

    s->Push(field->untag()->name());
    s->Push(field->untag()->owner());
    s->Push(field->untag()->type());
    // Write out the initializer function
    s->Push(field->untag()->initializer_function());

    if (kind != Snapshot::kFullAOT) {
      s->Push(field->untag()->guarded_list_length());
    }
    if (kind == Snapshot::kFullJIT) {
      s->Push(field->untag()->dependent_code());
    }
    // Write out either the initial static value or field offset.
    if (Field::StaticBit::decode(field->untag()->kind_bits_)) {
      s->Push(field->untag()->host_offset_or_field_id());
    } else {
      s->Push(Smi::New(Field::TargetOffsetOf(field)));
    }
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      FieldPtr field = objects_[i];
      s->AssignRef(field);
    }
  }

  void WriteFill(Serializer* s) {
    Snapshot::Kind kind = s->kind();
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      FieldPtr field = objects_[i];
      AutoTraceObjectName(field, field->untag()->name());

      WriteCompressedField(field, name);
      WriteCompressedField(field, owner);
      WriteCompressedField(field, type);
      // Write out the initializer function and initial value if not in AOT.
      WriteCompressedField(field, initializer_function);
      if (kind != Snapshot::kFullAOT) {
        WriteCompressedField(field, guarded_list_length);
      }
      if (kind == Snapshot::kFullJIT) {
        WriteCompressedField(field, dependent_code);
      }

      if (kind != Snapshot::kFullAOT) {
        s->WriteTokenPosition(field->untag()->token_pos_);
        s->WriteTokenPosition(field->untag()->end_token_pos_);
        s->WriteCid(field->untag()->guarded_cid_);
        s->WriteCid(field->untag()->is_nullable_);
        s->Write<int8_t>(field->untag()->static_type_exactness_state_);
        s->Write<uint32_t>(field->untag()->kernel_offset_);
      }
      s->Write<uint16_t>(field->untag()->kind_bits_);

      // Write out either the initial static value or field offset.
      if (Field::StaticBit::decode(field->untag()->kind_bits_)) {
        WriteFieldValue("id", field->untag()->host_offset_or_field_id());
      } else {
        WriteFieldValue("offset", Smi::New(Field::TargetOffsetOf(field)));
      }
    }
  }

 private:
  GrowableArray<FieldPtr> objects_;
};
#endif  // !DART_PRECOMPILED_RUNTIME

class FieldDeserializationCluster : public DeserializationCluster {
 public:
  FieldDeserializationCluster() : DeserializationCluster("Field") {}
  ~FieldDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    ReadAllocFixedSize(d, Field::InstanceSize());
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    ASSERT(!is_canonical());  // Never canonical.
#if !defined(DART_PRECOMPILED_RUNTIME)
    Snapshot::Kind kind = d_->kind();
#endif
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      FieldPtr field = static_cast<FieldPtr>(d.Ref(id));
      Deserializer::InitializeHeader(field, kFieldCid, Field::InstanceSize());
      d.ReadFromTo(field);
#if !defined(DART_PRECOMPILED_RUNTIME)
      ASSERT(d_->kind() != Snapshot::kFullAOT);
      field->untag()->guarded_list_length_ = static_cast<SmiPtr>(d.ReadRef());
      if (kind == Snapshot::kFullJIT) {
        field->untag()->dependent_code_ =
            static_cast<WeakArrayPtr>(d.ReadRef());
      }
      field->untag()->token_pos_ = d.ReadTokenPosition();
      field->untag()->end_token_pos_ = d.ReadTokenPosition();
      field->untag()->guarded_cid_ = d.ReadCid();
      field->untag()->is_nullable_ = d.ReadCid();
      const int8_t static_type_exactness_state = d.Read<int8_t>();
#if defined(TARGET_ARCH_X64)
      field->untag()->static_type_exactness_state_ =
          static_type_exactness_state;
#else
      // We might produce core snapshots using X64 VM and then consume
      // them in IA32 or ARM VM. In which case we need to simply ignore
      // static type exactness state written into snapshot because non-X64
      // builds don't have this feature enabled.
      // TODO(dartbug.com/34170) Support other architectures.
      USE(static_type_exactness_state);
      field->untag()->static_type_exactness_state_ =
          StaticTypeExactnessState::NotTracking().Encode();
#endif  // defined(TARGET_ARCH_X64)
      field->untag()->kernel_offset_ = d.Read<uint32_t>();
#endif
      field->untag()->kind_bits_ = d.Read<uint16_t>();

      field->untag()->host_offset_or_field_id_ =
          static_cast<SmiPtr>(d.ReadRef());
#if !defined(DART_PRECOMPILED_RUNTIME)
      field->untag()->target_offset_ =
          Smi::Value(field->untag()->host_offset_or_field_id());
#endif  //  !defined(DART_PRECOMPILED_RUNTIME)
    }
  }

  void PostLoad(Deserializer* d, const Array& refs) override {
    Field& field = Field::Handle(d->zone());
    if (!IsolateGroup::Current()->use_field_guards()) {
      for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
        field ^= refs.At(i);
        field.set_guarded_cid_unsafe(kDynamicCid);
        field.set_is_nullable_unsafe(true);
        field.set_guarded_list_length_unsafe(Field::kNoFixedLength);
        field.set_guarded_list_length_in_object_offset_unsafe(
            Field::kUnknownLengthOffset);
        field.set_static_type_exactness_state_unsafe(
            StaticTypeExactnessState::NotTracking());
      }
    } else {
      for (intptr_t i = start_index_, n = stop_index_; i < n; i++) {
        field ^= refs.At(i);
        field.InitializeGuardedListLengthInObjectOffset(/*unsafe=*/true);
      }
    }
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
class ScriptSerializationCluster : public SerializationCluster {
 public:
  ScriptSerializationCluster()
      : SerializationCluster("Script",
                             kScriptCid,
                             compiler::target::Script::InstanceSize()) {}
  ~ScriptSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    ScriptPtr script = Script::RawCast(object);
    objects_.Add(script);
    auto* from = script->untag()->from();
    auto* to = script->untag()->to_snapshot(s->kind());
    for (auto* p = from; p <= to; p++) {
      const intptr_t offset =
          reinterpret_cast<uword>(p) - reinterpret_cast<uword>(script->untag());
      const ObjectPtr obj = p->Decompress(script->heap_base());
      if (offset == Script::line_starts_offset()) {
        // Line starts are delta encoded.
        s->Push(obj, kDeltaEncodedTypedDataCid);
      } else {
        s->Push(obj);
      }
    }
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      ScriptPtr script = objects_[i];
      s->AssignRef(script);
    }
  }

  void WriteFill(Serializer* s) {
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      ScriptPtr script = objects_[i];
      AutoTraceObjectName(script, script->untag()->url());
      WriteFromTo(script);
      if (s->kind() != Snapshot::kFullAOT) {
        // Clear out the max position cache in snapshots to ensure no
        // differences in the snapshot due to triggering caching vs. not.
        int32_t written_flags =
            UntaggedScript::CachedMaxPositionBitField::update(
                0, script->untag()->flags_and_max_position_);
        written_flags = UntaggedScript::HasCachedMaxPositionBit::update(
            false, written_flags);
        s->Write<int32_t>(written_flags);
      }
      s->Write<int32_t>(script->untag()->kernel_script_index_);
    }
  }

 private:
  GrowableArray<ScriptPtr> objects_;
};
#endif  // !DART_PRECOMPILED_RUNTIME

class ScriptDeserializationCluster : public DeserializationCluster {
 public:
  ScriptDeserializationCluster() : DeserializationCluster("Script") {}
  ~ScriptDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    ReadAllocFixedSize(d, Script::InstanceSize());
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    ASSERT(!is_canonical());  // Never canonical.
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      ScriptPtr script = static_cast<ScriptPtr>(d.Ref(id));
      Deserializer::InitializeHeader(script, kScriptCid,
                                     Script::InstanceSize());
      d.ReadFromTo(script);
#if !defined(DART_PRECOMPILED_RUNTIME)
      script->untag()->flags_and_max_position_ = d.Read<int32_t>();
#endif
      script->untag()->kernel_script_index_ = d.Read<int32_t>();
      script->untag()->load_timestamp_ = 0;
    }
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
class LibrarySerializationCluster : public SerializationCluster {
 public:
  LibrarySerializationCluster()
      : SerializationCluster("Library",
                             kLibraryCid,
                             compiler::target::Library::InstanceSize()) {}
  ~LibrarySerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    LibraryPtr lib = Library::RawCast(object);
    objects_.Add(lib);
    PushFromTo(lib);
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      LibraryPtr lib = objects_[i];
      s->AssignRef(lib);
    }
  }

  void WriteFill(Serializer* s) {
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      LibraryPtr lib = objects_[i];
      AutoTraceObjectName(lib, lib->untag()->url());
      WriteFromTo(lib);
      s->Write<int32_t>(lib->untag()->index_);
      s->Write<uint16_t>(lib->untag()->num_imports_);
      s->Write<int8_t>(lib->untag()->load_state_);
      s->Write<uint8_t>(lib->untag()->flags_);
      if (s->kind() != Snapshot::kFullAOT) {
        s->Write<uint32_t>(lib->untag()->kernel_library_index_);
      }
    }
  }

 private:
  GrowableArray<LibraryPtr> objects_;
};
#endif  // !DART_PRECOMPILED_RUNTIME

class LibraryDeserializationCluster : public DeserializationCluster {
 public:
  LibraryDeserializationCluster() : DeserializationCluster("Library") {}
  ~LibraryDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    ReadAllocFixedSize(d, Library::InstanceSize());
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    ASSERT(!is_canonical());  // Never canonical.
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      LibraryPtr lib = static_cast<LibraryPtr>(d.Ref(id));
      Deserializer::InitializeHeader(lib, kLibraryCid, Library::InstanceSize());
      d.ReadFromTo(lib);
      lib->untag()->native_entry_resolver_ = nullptr;
      lib->untag()->native_entry_symbol_resolver_ = nullptr;
      lib->untag()->ffi_native_resolver_ = nullptr;
      lib->untag()->index_ = d.Read<int32_t>();
      lib->untag()->num_imports_ = d.Read<uint16_t>();
      lib->untag()->load_state_ = d.Read<int8_t>();
      lib->untag()->flags_ =
          UntaggedLibrary::InFullSnapshotBit::update(true, d.Read<uint8_t>());
#if !defined(DART_PRECOMPILED_RUNTIME)
      ASSERT(d_->kind() != Snapshot::kFullAOT);
      lib->untag()->kernel_library_index_ = d.Read<uint32_t>();
#endif
    }
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
class NamespaceSerializationCluster : public SerializationCluster {
 public:
  NamespaceSerializationCluster()
      : SerializationCluster("Namespace",
                             kNamespaceCid,
                             compiler::target::Namespace::InstanceSize()) {}
  ~NamespaceSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    NamespacePtr ns = Namespace::RawCast(object);
    objects_.Add(ns);
    PushFromTo(ns);
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      NamespacePtr ns = objects_[i];
      s->AssignRef(ns);
    }
  }

  void WriteFill(Serializer* s) {
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      NamespacePtr ns = objects_[i];
      AutoTraceObject(ns);
      WriteFromTo(ns);
    }
  }

 private:
  GrowableArray<NamespacePtr> objects_;
};
#endif  // !DART_PRECOMPILED_RUNTIME

class NamespaceDeserializationCluster : public DeserializationCluster {
 public:
  NamespaceDeserializationCluster() : DeserializationCluster("Namespace") {}
  ~NamespaceDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    ReadAllocFixedSize(d, Namespace::InstanceSize());
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    ASSERT(!is_canonical());  // Never canonical.
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      NamespacePtr ns = static_cast<NamespacePtr>(d.Ref(id));
      Deserializer::InitializeHeader(ns, kNamespaceCid,
                                     Namespace::InstanceSize());
      d.ReadFromTo(ns);
    }
  }
};

#if !defined(DART_PRECOMPILED_RUNTIME)
// KernelProgramInfo objects are not written into a full AOT snapshot.
class KernelProgramInfoSerializationCluster : public SerializationCluster {
 public:
  KernelProgramInfoSerializationCluster()
      : SerializationCluster(
            "KernelProgramInfo",
            kKernelProgramInfoCid,
            compiler::target::KernelProgramInfo::InstanceSize()) {}
  ~KernelProgramInfoSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    KernelProgramInfoPtr info = KernelProgramInfo::RawCast(object);
    objects_.Add(info);
    PushFromTo(info);
  }

  void WriteAlloc(Serializer* s) {
    const intptr_t count = objects_.length();
    s->WriteUnsigned(count);
    for (intptr_t i = 0; i < count; i++) {
      KernelProgramInfoPtr info = objects_[i];
      s->AssignRef(info);
    }
  }

  void WriteFill(Serializer* s) {
    const intptr_t count = objects_.length();
    for (intptr_t i = 0; i < count; i++) {
      KernelProgramInfoPtr info = objects_[i];
      AutoTraceObject(info);
      WriteFromTo(info);
    }
  }

 private:
  GrowableArray<KernelProgramInfoPtr> objects_;
};

// Since KernelProgramInfo objects are not written into full AOT snapshots,
// one will never need to read them from a full AOT snapshot.
class KernelProgramInfoDeserializationCluster : public DeserializationCluster {
 public:
  KernelProgramInfoDeserializationCluster()
      : DeserializationCluster("KernelProgramInfo") {}
  ~KernelProgramInfoDeserializationCluster() {}

  void ReadAlloc(Deserializer* d) override {
    ReadAllocFixedSize(d, KernelProgramInfo::InstanceSize());
  }

  void ReadFill(Deserializer* d_) override {
    Deserializer::Local d(d_);

    ASSERT(!is_canonical());  // Never canonical.
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      KernelProgramInfoPtr info = static_cast<KernelProgramInfoPtr>(d.Ref(id));
      Deserializer::InitializeHeader(info, kKernelProgramInfoCid,
                                     KernelProgramInfo::InstanceSize());
      d.ReadFromTo(info);
    }
  }

  void PostLoad(Deserializer* d, const Array& refs) override {
    Array& array = Array::Handle(d->zone());
    KernelProgramInfo& info = KernelProgramInfo::Handle(d->zone());
    for (intptr_t id = start_index_, n = stop_index_; id < n; id++) {
      info ^= refs.At(id);
      array = HashTables::New<UnorderedHashMap<SmiTraits>>(16, Heap::kOld);
      info.set_libraries_cache(array);
      array = HashTables::New<UnorderedHashMap<SmiTraits>>(16, Heap::kOld);
      info.set_classes_cache(array);
    }
  }
};

class CodeSerializationCluster : public SerializationCluster {
 public:
  explicit CodeSerializationCluster(Heap* heap)
      : SerializationCluster("Code", kCodeCid), array_(Array::Handle()) {}
  ~CodeSerializationCluster() {}

  void Trace(Serializer* s, ObjectPtr object) {
    CodePtr code = Code::RawCast(object);

    const bool is_deferred = !s->InCurrentLoadingUnitOrRoot(code);
    if (is_deferred) {
      s->RecordDeferredCode(code);
    } else {
      objects_.Add(code);
    }

    // Even if this code object is itself deferred we still need to scan
    // the pool for references to other code objects (which might reside
    // in the current loading unit).
    ObjectPoolPtr pool = code->untag()->object_pool_;
    if (s->kind() == Snapshot::kFullAOT) {
      TracePool(s, pool, /*only_call_targets=*/is_deferred);
    } else {
      if (s->InCurrentLoadingUnitOrRoot(pool)) {
        s->Push(pool);
      } else {
        TracePool(s, pool, /*only_call_targets=*/true);
      }
    }

    if (s->kind() == Snapshot::kFullJIT) {
      s->Push(code->untag()->deopt_info_array_);
      s->Push(code->untag()->static_calls_target_table_);
      s->Push(code->untag()->compressed_stackmaps_);
    } else if (s->kind() == Snapshot::kFullAOT) {
      // Note: we don't trace compressed_stackmaps_ because we are going to emit
      // a separate mapping table into RO data which is not going to be a real
      // heap object.
#if defined(DART_PRECOMPILER)
      auto const calls_array = code->untag()->static_calls_target_table_;
      if (calls_array != Array::null()) {
        // Some Code entries in the static calls target table may only be
        // accessible via here, so push the Code objects.
        array_ = calls_array;
        for (auto entry : StaticCallsTable(array_)) {
          auto kind = Code::KindField::decode(
              Smi::Value(entry.Get<Code::kSCallTableKindAndOffset>()));
          switch (kind) {
            case Code::kCallViaCode:
              // Code object in the pool.
              continue;
            case Code::kPcRelativeTTSCall:
              // TTS will be reachable through type object which itself is
              // in the pool.
              continue;
            case Code::kPcRelativeCall:
            case Code::kPcRelativeTailCall:
              auto destination = entry.Get<Code::kSCallTableCodeOrTypeTarget>();
              ASSERT(destination->IsHeapObject() && destination->IsCode());
              s->Push(destination);
          }
        }
      }
#else
      UNREACHABLE();
#endif
    }

    if (Code::IsDiscarded(code)) {
      ASSERT(s->kind() == Snapshot::kFullAOT && FLAG_dwarf_stack_traces_mode &&
             !FLAG_retain_code_objects);
      // Only object pool and static call table entries and the compressed
      // stack maps should be pushed.
      return;
    }

    s->Push(code->untag()->owner_);
    s->Push(code->untag()->exception_handlers_);
    s->Push(code->untag()->pc_descriptors_);
    s->Push(code->untag()->catch_entry_);
    if (!FLAG_precompiled_mode || !FLAG_dwarf_stack_traces_mode) {
      s->Push(code->untag()->inlined_id_to_function_);
      if (s->InCurrentLoadingUnitOrRoot(code->untag()->code_source_map_)) {
        s->Push(code->untag()->code_source_map_);
      }
    }
#if !defined(PRODUCT)
    s->Push(code->untag()->return_address_metadata_);
    if (FLAG_code_comments) {
      s->Push(code->untag()->comments_);
    }
#endif
  }

  void TracePool(Serializer* s, ObjectPoolPtr pool, bool only_call_targets) {
    if (pool == ObjectPool::null()) {
      return;
    }

    const intptr_t length = pool->untag()->length_;
    uint8_t* entry_bits = pool->untag()->entry_bits();
    for (intptr_t i = 0; i < length; i++) {
      auto entry_type = ObjectPool::TypeBits::decode(entry_bits[i]);
      if (entry_type == ObjectPool::EntryType::kTaggedObject) {
        const ObjectPtr target = pool->untag()->data()[i].raw_obj_;
        // A field is a call target because its initializer may be called
        // indirectly by passing the field to the runtime. A const closure
        // is a call target because its function may be called indirectly
        // via a closure call.
        intptr_t cid = target->GetClassId();
        if (!only_call_targets || (cid == kCodeCid) || (cid == kFunctionCid) ||
            (cid == kFieldCid) || (cid == kClosureCid)) {
          s->Push(target);
        } else if (cid >= kNumPredefinedCids) {
          s->Push(s->isolate_group()->class_table()->At(cid));
        }
      }
    }
  }

  struct CodeOrderInfo {
    CodePtr code;
    intptr_t not_discarded;  // 1 if this code was not discarded and