Make snapshots reproducible.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index e8c985048455e406c77c6b4c6814261f2cb7ab41..6c2b84c9ab99d9ed1a1adf3753313d00339e7c21 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -5196,6 +5196,9 @@ class Code: public HeapObject {
 
   void ReplaceNthCell(int n, Cell* replace_with);
 
+  // Slightly ugly, used in ObjectSerializer only.

[Erik Corry, 2013/11/04 12:41:14]

Rather than insulting your own code it would be more helpful to spend a few
words explaining why it's needed.

[Sven Panne, 2013/11/04 13:03:19]

Well, that would just be yet another general v8 comment: What we are lacking
here again is a clear distinction between raw data structure manipulation and
some semantics layered upon that. I just want the "raw stuff" here.

Furthermore, from a more conceptual POV this doesn't really belong into the Code
class at all, it just lives here because we suffer from the common OOP problem
of the "tyranny of the hierarchy". Duplicating READ_FIELD/WRITE_FIELD in the
serializer would have been worse.

I am not sure how to put that into a non-insulting but enlightening comment...
:-)

[Erik Corry, 2013/11/04 13:20:02]

Let me suggest:

"The entire code object including its header is copied verbatim to the snapshot
so that it can be written in one, fast, memcpy during deserialization.  The
deserializer will overwrite some pointers, rather like a runtime linker, but the
random allocation addresses used in the mksnapshot process would still be
present in the unlinked snapshot data, which would make snapshot production
non-reproducible.  This method wipes out the to-be-overwritten header data for
reproducible snapshots."

+  inline void WipeOutHeader();
+
   class ExtraICStateStrictMode: public BitField<StrictModeFlag, 0, 1> {};
   class ExtraICStateKeyedAccessStoreMode:
       public BitField<KeyedAccessStoreMode, 1, 4> {};  // NOLINT