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

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

This is step towards removal of class_ field.

Committed: https://code.google.com/p/dart/source/detail?r=8098

[Vyacheslav Egorov (Google), 2012-05-29 15:26:28]

Hi Siva,

Here is snapshoting related CL extracted from big CL:
http://codereview.chromium.org/10450014/.

This CL switches snapshot generator to use RawObject::tags_ field to store
object ids for serialized objects (method MarkObject) instead of
RawObject::class_ field.

Please take a look.

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

lgtm

https://chromiumcodereview.appspot.com/10444059/diff/1/runtime/vm/snapshot.cc
File runtime/vm/snapshot.cc (right):

https://chromiumcodereview.appspot.com/10444059/diff/1/runtime/vm/snapshot.cc...
runtime/vm/snapshot.cc:550: raw->ptr()->tags_ = forward_list_[i]->tags();  //
Restore original class.
Comment should be restore original tags_ (will become header finally).

https://chromiumcodereview.appspot.com/10444059/diff/1/runtime/vm/snapshot.cc...
runtime/vm/snapshot.cc:589: value = SerializedHeaderData::update(object_id,
value);
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 07:48:34]

Thanks for the review, Siva!

https://chromiumcodereview.appspot.com/10444059/diff/1/runtime/vm/snapshot.cc
File runtime/vm/snapshot.cc (right):

https://chromiumcodereview.appspot.com/10444059/diff/1/runtime/vm/snapshot.cc...
runtime/vm/snapshot.cc:589: value = SerializedHeaderData::update(object_id,
value);
On 2012/05/30 00:35:29, asiva wrote:
> 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).

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?