Fix crash in V8 during serializing objects requiring alignment.

src/serialize.cc

Index: src/serialize.cc
diff --git a/src/serialize.cc b/src/serialize.cc
index 8222ff66d85cfb11cc542ff6c0b6e850303ccdff..bcc168ffa7ce1e5a606d493e748be6f483ff63f7 100644
--- a/src/serialize.cc
+++ b/src/serialize.cc
@@ -1657,14 +1657,17 @@ void Serializer::ObjectSerializer::SerializePrologue(AllocationSpace space,
     }
     back_reference = serializer_->AllocateLargeObject(size);
   } else {
+    bool needs_double_align = false;
     if (object_->NeedsToEnsureDoubleAlignment()) {
       // Add wriggle room for double alignment padding.
       back_reference = serializer_->Allocate(space, size + kPointerSize);
-      sink_->PutInt(kDoubleAlignmentSentinel, "DoubleAlignSentinel");
+      needs_double_align = true;
     } else {
       back_reference = serializer_->Allocate(space, size);
     }
     sink_->Put(kNewObject + reference_representation_ + space, "NewObject");

[Yang, 2014/12/10 20:47:58]

How about we simply put this line (outputting the new object) before the if-else
clause?

[Slava Chigrin, 2014/12/11 09:16:40]

I am afraid this will not work - Serializer::Allocate may output kNextChunk to
the sink, so resulting binary stream will differ. It will be

kNewObject, [kNewChunk, chunk_space], [kDoubleAlignmentSentinel], object_size, 

whereas correct order is 
[kNewChunk, chunk_space], kNewObject, [kDoubleAlignmentSentinel], object_size

On 2014/12/10 20:47:58, Yang wrote:

+    if (needs_double_align)

[Slava Chigrin, 2014/12/10 18:13:58]

Deserializer::ReadObject expects DoubleAlignmentSentinel be just before size
field, not before "NewObject" tag.

[Yang, 2014/12/10 20:45:48]

Thanks for catching this! I guess I got confused when refactoring a couple of
things when implementing this.

+      sink_->PutInt(kDoubleAlignmentSentinel, "DoubleAlignSentinel");
     int encoded_size = size >> kObjectAlignmentBits;
     DCHECK_NE(kDoubleAlignmentSentinel, encoded_size);
     sink_->PutInt(encoded_size, "ObjectSizeInWords");