When generate snapshot use tags_ instead of class_ to store object ids.

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
diff --git a/runtime/vm/snapshot.cc b/runtime/vm/snapshot.cc
index f9f37749579c7e56d9b2bd3c2dfa4b6a0879a2c9..244e2aed25a015c30ef4d853fedc81a4d12b0b2f 100644
--- a/runtime/vm/snapshot.cc
+++ b/runtime/vm/snapshot.cc
@@ -523,7 +523,7 @@ void SnapshotWriter::WriteObject(RawObject* rawobj) {
 
   // Check if it is a code object in that case just write a Null object
   // as we do not want code objects in the snapshot.
-  if (rawobj->ptr()->class_ == Object::code_class()) {
+  if (RawObject::ClassTag::decode(GetObjectTags(rawobj)) == kCode) {
     WriteIndexedObject(Object::kNullObject);
     return;
   }
@@ -547,7 +547,7 @@ void SnapshotWriter::UnmarkAll() {
   NoGCScope no_gc;
   for (intptr_t i = 0; i < forward_list_.length(); i++) {
     RawObject* raw = forward_list_[i]->raw();
-    raw->ptr()->class_ = forward_list_[i]->cls();  // Restore original class.
+    raw->ptr()->tags_ = forward_list_[i]->tags();  // Restore original class.

[siva, 2012/05/30 00:35:29]

Comment should be restore original tags_ (will become header finally).

   }
 }
 
@@ -569,6 +569,17 @@ void SnapshotWriter::WriteFullSnapshot() {
 }
 
 
+uword SnapshotWriter::GetObjectTags(RawObject* raw) {
+  uword tags = raw->ptr()->tags_;
+  if (SerializedHeaderTag::decode(tags) == kObjectId) {
+    intptr_t id = SerializedHeaderData::decode(tags);
+    return forward_list_[id - kMaxPredefinedObjectIds]->tags();
+  } else {
+    return tags;
+  }
+}
+
+
 intptr_t SnapshotWriter::MarkObject(RawObject* raw, RawClass* cls) {
   NoGCScope no_gc;
   intptr_t object_id = forward_list_.length() + kMaxPredefinedObjectIds;
@@ -576,8 +587,9 @@ intptr_t SnapshotWriter::MarkObject(RawObject* raw, RawClass* cls) {
   uword value = 0;
   value = SerializedHeaderTag::update(kObjectId, value);
   value = SerializedHeaderData::update(object_id, value);

[siva, 2012/05/30 00:35:29]

The SerializedHeaderTag bits collide with the reserved GC bits in tags
(kFreeBit, kMarkBit).

Currently this is not a problem as the GC bits are only used during GC and
should not collide during snapshot writing but there is a proposal to pre mark
certain objects in order to speed up the marking phase. When that is implemented
we will have issues with the bit collision.

Could you add a TODO above enum SerializedHeaderType in snapshot.h to resolve
this by using different bits (maybe the canonical/snapshot bits can be reused).

[Vyacheslav Egorov (Google), 2012/05/30 07:48:35]

Actually there is one change I did not merge from "big CL": there I just made
kObjectId == kFreeBit because to my understanding no free list element should
ever be serialized into snapshot, so there will never be a clash.

Here a decided to go a different way: no object in the heap will ever have
kFreeBit and kMarkBit both set up. Because when kFreeBit is set the rest of
tags_ is a pointer to the next free element which is 2 word aligned and thus has
0 at lower 3 (or 4 on x64) bits. Thus if kFreeBit is set kMarkBit (2nd bit) is
zero.

I think I will just merge the change that makes kObjectId to be equal to
kFreeBit.

What do you think?

-  raw->ptr()->class_ = reinterpret_cast<RawClass*>(value);
-  ForwardObjectNode* node = new ForwardObjectNode(raw, cls);
+  uword tags = raw->ptr()->tags_;
+  raw->ptr()->tags_ = value;
+  ForwardObjectNode* node = new ForwardObjectNode(raw, tags);
   ASSERT(node != NULL);
   forward_list_.Add(node);
   return object_id;
@@ -586,16 +598,18 @@ intptr_t SnapshotWriter::MarkObject(RawObject* raw, RawClass* cls) {
 
 void SnapshotWriter::WriteInlinedObject(RawObject* raw) {
   NoGCScope no_gc;
-  RawClass* cls = raw->ptr()->class_;
+  uword tags = raw->ptr()->tags_;
 
   // Check if object has already been serialized, in that
   // case just write the object id out.
-  if (SerializedHeaderTag::decode(reinterpret_cast<uword>(cls)) == kObjectId) {
-    intptr_t id = SerializedHeaderData::decode(reinterpret_cast<intptr_t>(cls));
+  if (SerializedHeaderTag::decode(tags) == kObjectId) {
+    intptr_t id = SerializedHeaderData::decode(tags);
     WriteIndexedObject(id);
     return;
   }
 
+  RawClass* cls = class_table_->At(RawObject::ClassTag::decode(tags));
+
   // Object is being serialized, add it to the forward ref list and mark
   // it so that future references to this object in the snapshot will use
   // an object id, instead of trying to serialize it again.
@@ -617,7 +631,7 @@ void SnapshotWriter::WriteInlinedObject(RawObject* raw) {
     WriteIntptrValue(SerializedHeaderData::encode(kInstanceId));
 
     // Write out the tags.
-    WriteIntptrValue(raw->ptr()->tags_);
+    WriteIntptrValue(tags);
 
     // Write out the class information for this object.
     WriteObject(cls);