[snapshot] allow custom chunk sizes.

Bug: v8:7887
Change-Id: I3904981f06efcb1fb83e863d0be6a16ebaaf17f2
Reviewed-on: https://chromium-review.googlesource.com/1113930Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54189}

BUILD.gn

@@ -2345,6 +2345,7 @@ v8_source_set("v8_base") {
     "src/snapshot/partial-deserializer.h",
     "src/snapshot/partial-serializer.cc",
     "src/snapshot/partial-serializer.h",
+    "src/snapshot/references.h",
     "src/snapshot/serializer-common.cc",
     "src/snapshot/serializer-common.h",
     "src/snapshot/serializer.cc",

src/address-map.h

@@ -69,181 +69,6 @@ class RootIndexMap {
   DISALLOW_COPY_AND_ASSIGN(RootIndexMap);
 };
 
-class SerializerReference {
- public:
-  SerializerReference() : bitfield_(Special(kInvalidValue)) {}
-
-  static SerializerReference FromBitfield(uint32_t bitfield) {
-    return SerializerReference(bitfield);
-  }
-
-  static SerializerReference BackReference(AllocationSpace space,
-                                           uint32_t chunk_index,
-                                           uint32_t chunk_offset) {
-    DCHECK(IsAligned(chunk_offset, kObjectAlignment));
-    DCHECK_NE(LO_SPACE, space);
-    return SerializerReference(
-        SpaceBits::encode(space) | ChunkIndexBits::encode(chunk_index) |
-        ChunkOffsetBits::encode(chunk_offset >> kObjectAlignmentBits));
-  }
-
-  static SerializerReference MapReference(uint32_t index) {
-    return SerializerReference(SpaceBits::encode(MAP_SPACE) |
-                               ValueIndexBits::encode(index));
-  }
-
-  static SerializerReference OffHeapBackingStoreReference(uint32_t index) {
-    return SerializerReference(SpaceBits::encode(kExternalSpace) |
-                               ValueIndexBits::encode(index));
-  }
-
-  static SerializerReference LargeObjectReference(uint32_t index) {
-    return SerializerReference(SpaceBits::encode(LO_SPACE) |
-                               ValueIndexBits::encode(index));
-  }
-
-  static SerializerReference AttachedReference(uint32_t index) {
-    return SerializerReference(SpaceBits::encode(kAttachedReferenceSpace) |
-                               ValueIndexBits::encode(index));
-  }
-
-  static SerializerReference DummyReference() {
-    return SerializerReference(Special(kDummyValue));
-  }
-
-  bool is_valid() const { return bitfield_ != Special(kInvalidValue); }
-
-  bool is_back_reference() const {
-    return SpaceBits::decode(bitfield_) <= LAST_SPACE;
-  }
-
-  AllocationSpace space() const {
-    DCHECK(is_back_reference());
-    return static_cast<AllocationSpace>(SpaceBits::decode(bitfield_));
-  }
-
-  uint32_t chunk_offset() const {
-    DCHECK(is_back_reference());
-    return ChunkOffsetBits::decode(bitfield_) << kObjectAlignmentBits;
-  }
-
-  uint32_t map_index() const {
-    DCHECK(is_back_reference());
-    return ValueIndexBits::decode(bitfield_);
-  }
-
-  bool is_off_heap_backing_store_reference() const {
-    return SpaceBits::decode(bitfield_) == kExternalSpace;
-  }
-
-  uint32_t off_heap_backing_store_index() const {
-    DCHECK(is_off_heap_backing_store_reference());
-    return ValueIndexBits::decode(bitfield_);
-  }
-
-  uint32_t large_object_index() const {
-    DCHECK(is_back_reference());
-    return ValueIndexBits::decode(bitfield_);
-  }
-
-  uint32_t chunk_index() const {
-    DCHECK(is_back_reference());
-    return ChunkIndexBits::decode(bitfield_);
-  }
-
-  uint32_t back_reference() const {
-    DCHECK(is_back_reference());
-    return bitfield_ & (ChunkOffsetBits::kMask | ChunkIndexBits::kMask);
-  }
-
-  bool is_attached_reference() const {
-    return SpaceBits::decode(bitfield_) == kAttachedReferenceSpace;
-  }
-
-  int attached_reference_index() const {
-    DCHECK(is_attached_reference());
-    return ValueIndexBits::decode(bitfield_);
-  }
-
- private:
-  explicit SerializerReference(uint32_t bitfield) : bitfield_(bitfield) {}
-
-  inline static uint32_t Special(int value) {
-    return SpaceBits::encode(kSpecialValueSpace) |
-           ValueIndexBits::encode(value);
-  }
-
-  // We use the 32-bit bitfield to encode either a back reference, a special
-  // value, or an attached reference index.
-  // Back reference:
-  //   [ Space index             ] [ Chunk index ] [ Chunk offset ]
-  //   [ LO_SPACE                ] [ large object index           ]
-  // Special value
-  //   [ kSpecialValueSpace      ] [ Special value index          ]
-  // Attached reference
-  //   [ kAttachedReferenceSpace ] [ Attached reference index     ]
-  // External
-  //   [ kExternalSpace          ] [ External reference index     ]
-
-  static const int kChunkOffsetSize = kPageSizeBits - kObjectAlignmentBits;
-  static const int kChunkIndexSize = 32 - kChunkOffsetSize - kSpaceTagSize;
-  static const int kValueIndexSize = kChunkOffsetSize + kChunkIndexSize;
-
-  static const int kSpecialValueSpace = LAST_SPACE + 1;
-  static const int kAttachedReferenceSpace = kSpecialValueSpace + 1;
-  static const int kExternalSpace = kAttachedReferenceSpace + 1;
-  STATIC_ASSERT(kExternalSpace < (1 << kSpaceTagSize));
-
-  static const int kInvalidValue = 0;
-  static const int kDummyValue = 1;
-
-  // The chunk offset can also be used to encode the index of special values.
-  class ChunkOffsetBits : public BitField<uint32_t, 0, kChunkOffsetSize> {};
-  class ChunkIndexBits
-      : public BitField<uint32_t, ChunkOffsetBits::kNext, kChunkIndexSize> {};
-  class ValueIndexBits : public BitField<uint32_t, 0, kValueIndexSize> {};
-  STATIC_ASSERT(ChunkIndexBits::kNext == ValueIndexBits::kNext);
-  class SpaceBits : public BitField<int, kValueIndexSize, kSpaceTagSize> {};
-  STATIC_ASSERT(SpaceBits::kNext == 32);
-
-  uint32_t bitfield_;
-
-  friend class SerializerReferenceMap;
-};
-
-// Mapping objects to their location after deserialization.
-// This is used during building, but not at runtime by V8.
-class SerializerReferenceMap {
- public:
-  SerializerReferenceMap()
-      : no_allocation_(), map_(), attached_reference_index_(0) {}
-
-  SerializerReference Lookup(void* obj) {
-    Maybe<uint32_t> maybe_index = map_.Get(obj);
-    return maybe_index.IsJust() ? SerializerReference(maybe_index.FromJust())
-                                : SerializerReference();
-  }
-
-  void Add(void* obj, SerializerReference b) {
-    DCHECK(b.is_valid());
-    DCHECK(map_.Get(obj).IsNothing());
-    map_.Set(obj, b.bitfield_);
-  }
-
-  SerializerReference AddAttachedReference(HeapObject* attached_reference) {
-    SerializerReference reference =
-        SerializerReference::AttachedReference(attached_reference_index_++);
-    Add(attached_reference, reference);
-    return reference;
-  }
-
- private:
-  DisallowHeapAllocation no_allocation_;
-  PointerToIndexHashMap<void*> map_;
-  int attached_reference_index_;
-  DISALLOW_COPY_AND_ASSIGN(SerializerReferenceMap);
-};
-
 }  // namespace internal
 }  // namespace v8
 

src/external-reference-table.h

@@ -8,7 +8,6 @@
 #include <vector>
 
 #include "src/accessors.h"
-#include "src/address-map.h"
 #include "src/builtins/builtins.h"
 #include "src/external-reference.h"
 

src/flag-definitions.h

@@ -1080,6 +1080,7 @@ DEFINE_BOOL(profile_deserialization, false,
             "Print the time it takes to deserialize the snapshot.")
 DEFINE_BOOL(serialization_statistics, false,
             "Collect statistics on serialized objects.")
+DEFINE_UINT(serialization_chunk_size, 0, "Custom size for serialization chunks")
 
 // Regexp
 DEFINE_BOOL(regexp_optimization, true, "generate optimized regexp code")

src/globals.h

@@ -584,7 +584,7 @@ enum AllocationSpace {
   FIRST_GROWABLE_PAGED_SPACE = OLD_SPACE,
   LAST_GROWABLE_PAGED_SPACE = MAP_SPACE
 };
-constexpr int kSpaceTagSize = 4;
+constexpr int kSpaceTagSize = 3;
 STATIC_ASSERT(FIRST_SPACE == 0);
 
 enum AllocationAlignment { kWordAligned, kDoubleAligned, kDoubleUnaligned };

src/snapshot/builtin-serializer-allocator.cc

@@ -18,9 +18,7 @@ SerializerReference BuiltinSerializerAllocator::Allocate(AllocationSpace space,
   // system.  Instead of worrying about chunk indices and offsets, we simply
   // need to generate unique offsets here.
 
-  const uint32_t virtual_chunk_index = 0;
-  const auto ref = SerializerReference::BackReference(
-      CODE_SPACE, virtual_chunk_index, virtual_chunk_offset_);
+  const auto ref = SerializerReference::BuiltinReference(virtual_chunk_offset_);
 
   virtual_chunk_size_ += size;
   virtual_chunk_offset_ += kObjectAlignment;  // Needs to be aligned.
@@ -31,11 +29,8 @@ SerializerReference BuiltinSerializerAllocator::Allocate(AllocationSpace space,
 #ifdef DEBUG
 bool BuiltinSerializerAllocator::BackReferenceIsAlreadyAllocated(
     SerializerReference reference) const {
-  DCHECK(reference.is_back_reference());
-  AllocationSpace space = reference.space();
-  DCHECK_EQ(space, CODE_SPACE);
-  DCHECK_EQ(reference.chunk_index(), 0);
-  return reference.chunk_offset() < virtual_chunk_offset_;
+  DCHECK(reference.is_builtin_reference());
+  return reference.builtin_offset() < virtual_chunk_offset_;
 }
 #endif  // DEBUG
 

src/snapshot/code-serializer.cc

@@ -31,6 +31,11 @@ ScriptData::ScriptData(const byte* data, int length)
   }
 }
 
+CodeSerializer::CodeSerializer(Isolate* isolate, uint32_t source_hash)
+    : Serializer(isolate), source_hash_(source_hash) {
+  allocator()->UseCustomChunkSize(FLAG_serialization_chunk_size);
+}
+
 // static
 ScriptCompiler::CachedData* CodeSerializer::Serialize(
     Handle<SharedFunctionInfo> info) {
@@ -253,7 +258,7 @@ void CodeSerializer::SerializeGeneric(HeapObject* heap_object,
 void CodeSerializer::SerializeCodeStub(Code* code_stub, HowToCode how_to_code,
                                        WhereToPoint where_to_point) {
   // We only arrive here if we have not encountered this code stub before.
-  DCHECK(!reference_map()->Lookup(code_stub).is_valid());
+  DCHECK(!reference_map()->LookupReference(code_stub).is_valid());
   uint32_t stub_key = code_stub->stub_key();
   DCHECK(CodeStub::MajorKeyFromKey(stub_key) != CodeStub::NoCache);
   DCHECK(!CodeStub::GetCode(isolate(), stub_key).is_null());

src/snapshot/code-serializer.h

@@ -58,8 +58,7 @@ class CodeSerializer : public Serializer<> {
   uint32_t source_hash() const { return source_hash_; }
 
  protected:
-  explicit CodeSerializer(Isolate* isolate, uint32_t source_hash)
-      : Serializer(isolate), source_hash_(source_hash) {}
+  CodeSerializer(Isolate* isolate, uint32_t source_hash);
   ~CodeSerializer() override { OutputStatistics("CodeSerializer"); }
 
   virtual void SerializeCodeObject(Code* code_object, HowToCode how_to_code,

src/snapshot/default-serializer-allocator.cc

@@ -5,6 +5,7 @@
 #include "src/snapshot/default-serializer-allocator.h"
 
 #include "src/heap/heap-inl.h"
+#include "src/snapshot/references.h"
 #include "src/snapshot/serializer.h"
 #include "src/snapshot/snapshot-source-sink.h"
 
@@ -19,18 +20,34 @@ DefaultSerializerAllocator::DefaultSerializerAllocator(
   }
 }
 
+void DefaultSerializerAllocator::UseCustomChunkSize(uint32_t chunk_size) {
+  custom_chunk_size_ = chunk_size;
+}
+
+static uint32_t PageSizeOfSpace(int space) {
+  return static_cast<uint32_t>(
+      MemoryAllocator::PageAreaSize(static_cast<AllocationSpace>(space)));
+}
+
+uint32_t DefaultSerializerAllocator::TargetChunkSize(int space) {
+  if (custom_chunk_size_ == 0) return PageSizeOfSpace(space);
+  DCHECK_LE(custom_chunk_size_, PageSizeOfSpace(space));
+  return custom_chunk_size_;
+}
+
 SerializerReference DefaultSerializerAllocator::Allocate(AllocationSpace space,
                                                          uint32_t size) {
   DCHECK(space >= 0 && space < kNumberOfPreallocatedSpaces);
-  DCHECK(size > 0 && size <= MaxChunkSizeInSpace(space));
+  DCHECK(size > 0 && size <= PageSizeOfSpace(space));
 
   // Maps are allocated through AllocateMap.
   DCHECK_NE(MAP_SPACE, space);
 
-  uint32_t new_chunk_size = pending_chunk_[space] + size;
-  if (new_chunk_size > MaxChunkSizeInSpace(space)) {
-    // The new chunk size would not fit onto a single page. Complete the
-    // current chunk and start a new one.
+  uint32_t old_chunk_size = pending_chunk_[space];
+  uint32_t new_chunk_size = old_chunk_size + size;
+  // Start a new chunk if the new size exceeds the target chunk size.
+  // We may exceed the target chunk size if the single object size does.
+  if (new_chunk_size > TargetChunkSize(space)) {
     serializer_->PutNextChunk(space);
     completed_chunks_[space].push_back(pending_chunk_[space]);
     pending_chunk_[space] = 0;
@@ -136,13 +153,5 @@ void DefaultSerializerAllocator::OutputStatistics() {
   PrintF("%16d\n", large_objects_total_size_);
 }
 
-// static
-uint32_t DefaultSerializerAllocator::MaxChunkSizeInSpace(int space) {
-  DCHECK(0 <= space && space < kNumberOfPreallocatedSpaces);
-
-  return static_cast<uint32_t>(
-      MemoryAllocator::PageAreaSize(static_cast<AllocationSpace>(space)));
-}
-
 }  // namespace internal
 }  // namespace v8

src/snapshot/default-serializer-allocator.h

@@ -23,6 +23,8 @@ class DefaultSerializerAllocator final {
   SerializerReference AllocateLargeObject(uint32_t size);
   SerializerReference AllocateOffHeapBackingStore();
 
+  void UseCustomChunkSize(uint32_t chunk_size);
+
 #ifdef DEBUG
   bool BackReferenceIsAlreadyAllocated(
       SerializerReference back_reference) const;
@@ -33,13 +35,13 @@ class DefaultSerializerAllocator final {
   void OutputStatistics();
 
  private:
+  uint32_t TargetChunkSize(int space);
+
   static constexpr int kNumberOfPreallocatedSpaces =
       SerializerDeserializer::kNumberOfPreallocatedSpaces;
   static constexpr int kNumberOfSpaces =
       SerializerDeserializer::kNumberOfSpaces;
 
-  static uint32_t MaxChunkSizeInSpace(int space);
-
   // Objects from the same space are put into chunks for bulk-allocation
   // when deserializing. We have to make sure that each chunk fits into a
   // page. So we track the chunk size in pending_chunk_ of a space, but
@@ -61,6 +63,8 @@ class DefaultSerializerAllocator final {
   // from index 0.
   uint32_t seen_backing_stores_index_ = 1;
 
+  uint32_t custom_chunk_size_ = 0;
+
   // The current serializer.
   Serializer<DefaultSerializerAllocator>* const serializer_;
 

src/snapshot/deserializer.cc

@@ -276,33 +276,37 @@ int Deserializer<AllocatorT>::MaybeReplaceWithDeserializeLazy(int builtin_id) {
 template <class AllocatorT>
 HeapObject* Deserializer<AllocatorT>::GetBackReferencedObject(int space) {
   HeapObject* obj;
-  SerializerReference back_reference =
-      SerializerReference::FromBitfield(source_.GetInt());
-
   switch (space) {
     case LO_SPACE:
-      obj = allocator()->GetLargeObject(back_reference.large_object_index());
+      obj = allocator()->GetLargeObject(source_.GetInt());
       break;
     case MAP_SPACE:
-      obj = allocator()->GetMap(back_reference.map_index());
+      obj = allocator()->GetMap(source_.GetInt());
       break;
-    case RO_SPACE:
+    case RO_SPACE: {
+      uint32_t chunk_index = source_.GetInt();
+      uint32_t chunk_offset = source_.GetInt();
       if (isolate()->heap()->deserialization_complete()) {
         PagedSpace* read_only_space = isolate()->heap()->read_only_space();
         Page* page = read_only_space->first_page();
-        for (uint32_t i = 0; i < back_reference.chunk_index(); ++i) {
+        for (uint32_t i = 0; i < chunk_index; ++i) {
           page = page->next_page();
         }
-        Address address = page->OffsetToAddress(back_reference.chunk_offset());
+        Address address = page->OffsetToAddress(chunk_offset);
         obj = HeapObject::FromAddress(address);
-        break;
+      } else {
+        obj = allocator()->GetObject(static_cast<AllocationSpace>(space),
+                                     chunk_index, chunk_offset);
       }
-      V8_FALLTHROUGH;
-    default:
+      break;
+    }
+    default: {
+      uint32_t chunk_index = source_.GetInt();
+      uint32_t chunk_offset = source_.GetInt();
       obj = allocator()->GetObject(static_cast<AllocationSpace>(space),
-                                   back_reference.chunk_index(),
-                                   back_reference.chunk_offset());
+                                   chunk_index, chunk_offset);
       break;
+    }
   }
 
   if (deserializing_user_code() && obj->IsThinString()) {

src/snapshot/partial-serializer.cc

@@ -20,6 +20,7 @@ PartialSerializer::PartialSerializer(
       can_be_rehashed_(true),
       context_(nullptr) {
   InitializeCodeAddressMap();
+  allocator()->UseCustomChunkSize(FLAG_serialization_chunk_size);
 }
 
 PartialSerializer::~PartialSerializer() {
@@ -139,7 +140,7 @@ void PartialSerializer::SerializeEmbedderFields() {
     HandleScope scope(isolate());
     Handle<JSObject> obj(embedder_field_holders_.back(), isolate());
     embedder_field_holders_.pop_back();
-    SerializerReference reference = reference_map()->Lookup(*obj);
+    SerializerReference reference = reference_map()->LookupReference(*obj);
     DCHECK(reference.is_back_reference());
     int embedder_fields_count = obj->GetEmbedderFieldCount();
     for (int i = 0; i < embedder_fields_count; i++) {

src/snapshot/references.h

+// Copyright 2018 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_SNAPSHOT_REFERENCES_H_
+#define V8_SNAPSHOT_REFERENCES_H_
+
+#include "src/assert-scope.h"
+#include "src/base/hashmap.h"
+#include "src/utils.h"
+
+namespace v8 {
+namespace internal {
+
+class SerializerReference {
+ private:
+  enum SpecialValueType {
+    kSpecialValue,
+    kAttachedReference,
+    kOffHeapBackingStore,
+    kBuiltinReference,
+  };
+
+  static const int kSpecialValueSpace = LAST_SPACE + 1;
+  static const int kInvalidValue = 0;
+  static const int kDummyValue = 1;
+
+  SerializerReference(SpecialValueType type, uint32_t value)
+      : bitfield_(SpaceBits::encode(kSpecialValueSpace) |
+                  SpecialValueTypeBits::encode(type)),
+        value_(value) {}
+
+ public:
+  SerializerReference() : SerializerReference(kSpecialValue, kInvalidValue) {}
+
+  SerializerReference(uint32_t space, uint32_t chunk_index,
+                      uint32_t chunk_offset)
+      : bitfield_(SpaceBits::encode(space) |
+                  ChunkIndexBits::encode(chunk_index)),
+        value_(chunk_offset) {}
+
+  static SerializerReference BackReference(AllocationSpace space,
+                                           uint32_t chunk_index,
+                                           uint32_t chunk_offset) {
+    DCHECK(IsAligned(chunk_offset, kObjectAlignment));
+    DCHECK_LT(space, LO_SPACE);
+    return SerializerReference(space, chunk_index, chunk_offset);
+  }
+
+  static SerializerReference MapReference(uint32_t index) {
+    return SerializerReference(MAP_SPACE, 0, index);
+  }
+
+  static SerializerReference OffHeapBackingStoreReference(uint32_t index) {
+    return SerializerReference(kOffHeapBackingStore, index);
+  }
+
+  static SerializerReference LargeObjectReference(uint32_t index) {
+    return SerializerReference(LO_SPACE, 0, index);
+  }
+
+  static SerializerReference AttachedReference(uint32_t index) {
+    return SerializerReference(kAttachedReference, index);
+  }
+
+  static SerializerReference BuiltinReference(uint32_t index) {
+    return SerializerReference(kBuiltinReference, index);
+  }
+
+  static SerializerReference DummyReference() {
+    return SerializerReference(kSpecialValue, kDummyValue);
+  }
+
+  bool is_valid() const {
+    return SpaceBits::decode(bitfield_) != kSpecialValueSpace ||
+           SpecialValueTypeBits::decode(bitfield_) != kSpecialValue ||
+           value_ != kInvalidValue;
+  }
+
+  bool is_back_reference() const {
+    return SpaceBits::decode(bitfield_) <= LAST_SPACE;
+  }
+
+  AllocationSpace space() const {
+    DCHECK(is_back_reference());
+    return static_cast<AllocationSpace>(SpaceBits::decode(bitfield_));
+  }
+
+  uint32_t chunk_offset() const {
+    DCHECK(is_back_reference());
+    return value_;
+  }
+
+  uint32_t chunk_index() const {
+    DCHECK(space() != MAP_SPACE && space() != LO_SPACE);
+    return ChunkIndexBits::decode(bitfield_);
+  }
+
+  uint32_t map_index() const {
+    DCHECK_EQ(MAP_SPACE, SpaceBits::decode(bitfield_));
+    return value_;
+  }
+
+  bool is_off_heap_backing_store_reference() const {
+    return SpaceBits::decode(bitfield_) == kSpecialValueSpace &&
+           SpecialValueTypeBits::decode(bitfield_) == kOffHeapBackingStore;
+  }
+
+  uint32_t off_heap_backing_store_index() const {
+    DCHECK(is_off_heap_backing_store_reference());
+    return value_;
+  }
+
+  uint32_t large_object_index() const {
+    DCHECK_EQ(LO_SPACE, SpaceBits::decode(bitfield_));
+    return value_;
+  }
+
+  bool is_attached_reference() const {
+    return SpaceBits::decode(bitfield_) == kSpecialValueSpace &&
+           SpecialValueTypeBits::decode(bitfield_) == kAttachedReference;
+  }
+
+  uint32_t attached_reference_index() const {
+    DCHECK(is_attached_reference());
+    return value_;
+  }
+
+  bool is_builtin_reference() const {
+    return SpaceBits::decode(bitfield_) == kSpecialValueSpace &&
+           SpecialValueTypeBits::decode(bitfield_) == kBuiltinReference;
+  }
+
+  uint32_t builtin_offset() const {
+    DCHECK(is_builtin_reference());
+    return value_;
+  }
+
+ private:
+  class SpaceBits : public BitField<int, 0, kSpaceTagSize> {};
+  class ChunkIndexBits
+      : public BitField<uint32_t, SpaceBits::kNext, 32 - kSpaceTagSize> {};
+  class SpecialValueTypeBits
+      : public BitField<SpecialValueType, SpaceBits::kNext,
+                        32 - kSpaceTagSize> {};
+
+  // We use two fields to store a reference.
+  // In case of a normal back reference, the bitfield_ stores the space and
+  // the chunk index. In case of special references, it uses a special value
+  // for space and stores the special value type.
+  uint32_t bitfield_;
+  // value_ stores either chunk offset or special value.
+  uint32_t value_;
+
+  friend class SerializerReferenceMap;
+};
+
+class SerializerReferenceMap
+    : public base::TemplateHashMapImpl<uintptr_t, SerializerReference,
+                                       base::KeyEqualityMatcher<intptr_t>,
+                                       base::DefaultAllocationPolicy> {
+ public:
+  typedef base::TemplateHashMapEntry<uintptr_t, SerializerReference> Entry;
+
+  SerializerReferenceMap() : no_allocation_(), attached_reference_index_(0) {}
+
+  SerializerReference LookupReference(void* value) const {
+    uintptr_t key = Key(value);
+    Entry* entry = Lookup(key, Hash(key));
+    if (entry == nullptr) return SerializerReference();
+    return entry->value;
+  }
+
+  void Add(void* obj, SerializerReference reference) {
+    DCHECK(reference.is_valid());
+    DCHECK(!LookupReference(obj).is_valid());
+    uintptr_t key = Key(obj);
+    LookupOrInsert(key, Hash(key))->value = reference;
+  }
+
+  SerializerReference AddAttachedReference(void* attached_reference) {
+    SerializerReference reference =
+        SerializerReference::AttachedReference(attached_reference_index_++);
+    Add(attached_reference, reference);
+    return reference;
+  }
+
+ private:
+  static inline uintptr_t Key(void* value) {
+    return reinterpret_cast<uintptr_t>(value);
+  }
+
+  static uint32_t Hash(uintptr_t key) { return static_cast<uint32_t>(key); }
+
+  DisallowHeapAllocation no_allocation_;
+  int attached_reference_index_;
+  DISALLOW_COPY_AND_ASSIGN(SerializerReferenceMap);
+};
+
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_SNAPSHOT_REFERENCES_H_

src/snapshot/serializer-common.h

@@ -9,6 +9,7 @@
 #include "src/base/bits.h"
 #include "src/external-reference-table.h"
 #include "src/globals.h"
+#include "src/snapshot/references.h"
 #include "src/utils.h"
 #include "src/visitors.h"
 

src/snapshot/serializer.cc

@@ -177,7 +177,7 @@ bool Serializer<AllocatorT>::SerializeBackReference(HeapObject* obj,
                                                     HowToCode how_to_code,
                                                     WhereToPoint where_to_point,
                                                     int skip) {
-  SerializerReference reference = reference_map_.Lookup(obj);
+  SerializerReference reference = reference_map_.LookupReference(obj);
   if (!reference.is_valid()) return false;
   // Encode the location of an already deserialized object in order to write
   // its location into a later object.  We can encode the location as an
@@ -291,7 +291,21 @@ template <class AllocatorT>
 void Serializer<AllocatorT>::PutBackReference(HeapObject* object,
                                               SerializerReference reference) {
   DCHECK(allocator()->BackReferenceIsAlreadyAllocated(reference));
-  sink_.PutInt(reference.back_reference(), "BackRefValue");
+  switch (reference.space()) {
+    case MAP_SPACE:
+      sink_.PutInt(reference.map_index(), "BackRefChunkOffset");
+      break;
+
+    case LO_SPACE:
+      sink_.PutInt(reference.large_object_index(), "BackRefChunkOffset");
+      break;
+
+    default:
+      sink_.PutInt(reference.chunk_index(), "BackRefChunkIndex");
+      sink_.PutInt(reference.chunk_offset(), "BackRefChunkOffset");
+      break;
+  }
+
   hot_objects_.Add(object);
 }
 
@@ -406,7 +420,7 @@ template <class AllocatorT>
 int32_t Serializer<AllocatorT>::ObjectSerializer::SerializeBackingStore(
     void* backing_store, int32_t byte_length) {
   SerializerReference reference =
-      serializer_->reference_map()->Lookup(backing_store);
+      serializer_->reference_map()->LookupReference(backing_store);
 
   // Serialize the off-heap backing store.
   if (!reference.is_valid()) {
@@ -678,7 +692,7 @@ void Serializer<AllocatorT>::ObjectSerializer::SerializeDeferred() {
   int size = object_->Size();
   Map* map = object_->map();
   SerializerReference back_reference =
-      serializer_->reference_map()->Lookup(object_);
+      serializer_->reference_map()->LookupReference(object_);
   DCHECK(back_reference.is_back_reference());
 
   // Serialize the rest of the object.

src/snapshot/serializer.h

@@ -141,7 +141,7 @@ class Serializer : public SerializerDeserializer {
   const std::vector<byte>* Payload() const { return sink_.data(); }
 
   bool ReferenceMapContains(HeapObject* o) {
-    return reference_map()->Lookup(o).is_valid();
+    return reference_map()->LookupReference(o).is_valid();
   }
 
   Isolate* isolate() const { return isolate_; }
@@ -219,7 +219,7 @@ class Serializer : public SerializerDeserializer {
   Code* CopyCode(Code* code);
 
   void QueueDeferredObject(HeapObject* obj) {
-    DCHECK(reference_map_.Lookup(obj).is_back_reference());
+    DCHECK(reference_map_.LookupReference(obj).is_back_reference());
     deferred_objects_.push_back(obj);
   }
 

src/snapshot/snapshot-common.cc

@@ -260,8 +260,9 @@ v8::StartupData Snapshot::CreateSnapshotBlob(
             reinterpret_cast<const char*>(startup_snapshot->RawData().start()),
             payload_length);
   if (FLAG_profile_deserialization) {
-    PrintF("Snapshot blob consists of:\n%10d bytes for startup\n",
-           payload_length);
+    PrintF("Snapshot blob consists of:\n%10d bytes in %d chunks for startup\n",
+           payload_length,
+           static_cast<uint32_t>(startup_snapshot->Reservations().size()));
   }
   payload_offset += payload_length;
 
@@ -286,7 +287,8 @@ v8::StartupData Snapshot::CreateSnapshotBlob(
         reinterpret_cast<const char*>(context_snapshot->RawData().start()),
         payload_length);
     if (FLAG_profile_deserialization) {
-      PrintF("%10d bytes for context #%d\n", payload_length, i);
+      PrintF("%10d bytes in %d chunks for context #%d\n", payload_length,
+             static_cast<uint32_t>(context_snapshot->Reservations().size()), i);
     }
     payload_offset += payload_length;
   }

test/cctest/test-serialize.cc

@@ -299,9 +299,7 @@ static void SanityCheck(v8::Isolate* v8_isolate) {
   isolate->factory()->InternalizeOneByteString(STATIC_CHAR_VECTOR("Empty"));
 }
 
-UNINITIALIZED_TEST(StartupSerializerOnce) {
-  DisableLazyDeserialization();
-  DisableAlwaysOpt();
+void TestStartupSerializerOnceImpl() {
   v8::Isolate* isolate = TestIsolate::NewInitialized();
   StartupBlobs blobs = Serialize(isolate);
   isolate->Dispose();
@@ -319,6 +317,40 @@ UNINITIALIZED_TEST(StartupSerializerOnce) {
   blobs.Dispose();
 }
 
+UNINITIALIZED_TEST(StartupSerializerOnce) {
+  DisableLazyDeserialization();
+  DisableAlwaysOpt();
+  TestStartupSerializerOnceImpl();
+}
+
+UNINITIALIZED_TEST(StartupSerializerOnce32) {
+  DisableLazyDeserialization();
+  DisableAlwaysOpt();
+  FLAG_serialization_chunk_size = 32;
+  TestStartupSerializerOnceImpl();
+}
+
+UNINITIALIZED_TEST(StartupSerializerOnce1K) {
+  DisableLazyDeserialization();
+  DisableAlwaysOpt();
+  FLAG_serialization_chunk_size = 1 * KB;
+  TestStartupSerializerOnceImpl();
+}
+
+UNINITIALIZED_TEST(StartupSerializerOnce4K) {
+  DisableLazyDeserialization();
+  DisableAlwaysOpt();
+  FLAG_serialization_chunk_size = 4 * KB;
+  TestStartupSerializerOnceImpl();
+}
+
+UNINITIALIZED_TEST(StartupSerializerOnce32K) {
+  DisableLazyDeserialization();
+  DisableAlwaysOpt();
+  FLAG_serialization_chunk_size = 32 * KB;
+  TestStartupSerializerOnceImpl();
+}
+
 UNINITIALIZED_TEST(StartupSerializerRootMapDependencies) {
   DisableAlwaysOpt();
   v8::SnapshotCreator snapshot_creator;