SnapshotCreator: start from existing snapshot if we have one

src/snapshot/code-serializer.h

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_SNAPSHOT_CODE_SERIALIZER_H_
 #define V8_SNAPSHOT_CODE_SERIALIZER_H_
 
 #include "src/parsing/preparse-data.h"
 #include "src/snapshot/serializer.h"
 
@@ skipping 67 matching lines @@
 class SerializedCodeData : public SerializedData {
  public:
   enum SanityCheckResult {
     CHECK_SUCCESS = 0,
     MAGIC_NUMBER_MISMATCH = 1,
     VERSION_MISMATCH = 2,
     SOURCE_MISMATCH = 3,
     CPU_FEATURES_MISMATCH = 4,
     FLAGS_MISMATCH = 5,
     CHECKSUM_MISMATCH = 6,
-    INVALID_HEADER = 7
+    INVALID_HEADER = 7,
+    LENGTH_MISMATCH = 8
   };
 
   // The data header consists of uint32_t-sized entries:
-  // [0] magic number and external reference count
-  // [1] version hash
-  // [2] source hash
-  // [3] cpu features
-  // [4] flag hash
-  // [5] number of code stub keys
-  // [6] number of reservation size entries
-  // [7] payload length
-  // [8] payload checksum part 1
-  // [9] payload checksum part 2
+  // [0] magic number and (internally provided) external reference count
+  // [1] extra (API-provided) external reference count
+  // [2] version hash
+  // [3] source hash
+  // [4] cpu features
+  // [5] flag hash
+  // [6] number of code stub keys
+  // [7] number of reservation size entries
+  // [8] payload length
+  // [9] payload checksum part 1
+  // [10] payload checksum part 2
   // ...  reservations
   // ...  code stub keys
   // ...  serialized payload
-  static const int kVersionHashOffset = kMagicNumberOffset + kInt32Size;
   static const int kSourceHashOffset = kVersionHashOffset + kInt32Size;
   static const int kCpuFeaturesOffset = kSourceHashOffset + kInt32Size;
   static const int kFlagHashOffset = kCpuFeaturesOffset + kInt32Size;
   static const int kNumReservationsOffset = kFlagHashOffset + kInt32Size;
   static const int kNumCodeStubKeysOffset = kNumReservationsOffset + kInt32Size;
   static const int kPayloadLengthOffset = kNumCodeStubKeysOffset + kInt32Size;
   static const int kChecksum1Offset = kPayloadLengthOffset + kInt32Size;
   static const int kChecksum2Offset = kChecksum1Offset + kInt32Size;
-  static const int kHeaderSize = kChecksum2Offset + kInt32Size;
+  static const int kUnalignedHeaderSize = kChecksum2Offset + kInt32Size;
+  static const int kHeaderSize = POINTER_SIZE_ALIGN(kUnalignedHeaderSize);
 
   // Used when consuming.
   static const SerializedCodeData FromCachedData(
       Isolate* isolate, ScriptData* cached_data, uint32_t expected_source_hash,
       SanityCheckResult* rejection_result);
 
   // Used when producing.
   SerializedCodeData(const List<byte>* payload, const CodeSerializer* cs);
 
   // Return ScriptData object and relinquish ownership over it to the caller.
@@ skipping 16 matching lines @@
   }
 
   SanityCheckResult SanityCheck(Isolate* isolate,
                                 uint32_t expected_source_hash) const;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_CODE_SERIALIZER_H_