[serializer] reserve maps one by one to avoid fragmentation.

src/address-map.h

 // Copyright 2015 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_ADDRESS_MAP_H_
 #define V8_ADDRESS_MAP_H_
 
 #include "src/assert-scope.h"
 #include "src/base/hashmap.h"
 #include "src/objects.h"
@@ skipping 59 matching lines @@
   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 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));
   }
 
@@ skipping 10 matching lines @@
   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_);
+  }
+
   uint32_t large_object_index() const {
     DCHECK(is_back_reference());
-    DCHECK(chunk_index() == 0);
-    return ChunkOffsetBits::decode(bitfield_);
+    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);
@@ skipping 83 matching lines @@
   DisallowHeapAllocation no_allocation_;
   base::HashMap map_;
   int attached_reference_index_;
   DISALLOW_COPY_AND_ASSIGN(SerializerReferenceMap);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_ADDRESS_MAP_H_