SnapshotCreator: start from existing snapshot if we have one

src/snapshot/code-serializer.h

Index: src/snapshot/code-serializer.h
diff --git a/src/snapshot/code-serializer.h b/src/snapshot/code-serializer.h
index 4d87a731f7587a1462fc506fbe440251762a948a..d290cf9c4b1cf117c30128190c951242983d87b9 100644
--- a/src/snapshot/code-serializer.h
+++ b/src/snapshot/code-serializer.h
@@ -85,24 +85,27 @@ class SerializedCodeData : public SerializedData {
     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 kVersionHashOffset =
+      kExtraExternalReferencesOffset + kInt32Size;
   static const int kSourceHashOffset = kVersionHashOffset + kInt32Size;
   static const int kCpuFeaturesOffset = kSourceHashOffset + kInt32Size;
   static const int kFlagHashOffset = kCpuFeaturesOffset + kInt32Size;
@@ -137,7 +140,8 @@ class SerializedCodeData : public SerializedData {
       : SerializedData(const_cast<byte*>(data), size) {}
 
   Vector<const byte> DataWithoutHeader() const {
-    return Vector<const byte>(data_ + kHeaderSize, size_ - kHeaderSize);
+    const int kPaddedStart = POINTER_SIZE_ALIGN(kHeaderSize);

[Yang, 2017/03/07 11:28:11]

Let's define kHeaderSize to be padded. Also see reason below.

[Jakob Kummerow, 2017/03/07 12:39:15]

Done.

+    return Vector<const byte>(data_ + kPaddedStart, size_ - kPaddedStart);
   }
 
   SanityCheckResult SanityCheck(Isolate* isolate,