Design question: why not reuse the CMS slots in mark word?

[Zhengyu Gu]

Hi Aleksey,

Mark word can also be used by BasicLock (none-inflated lock) and displace mark word on stack, so could make
finding brooks pointer complicated.

Thanks,

-Zhengyu


On 09/19/2016 04:26 PM, Aleksey Shipilev wrote:
> Hi,
>
> I tried to search for the answer to this on my own, to no avail. Our
> current forwarding pointer is laid out before the object, which makes
> the allocation logic to reserve more space than "visible" to the object
> instance. Obviously, you would want to have the forwarding pointer
> within the object.
>
> My question is: is there a reason we cannot reuse the mark word
> mechanics already laid out for CMS? See the bit format here:
>   http://hg.openjdk.java.net/jdk9/jdk9/hotspot/file/6bddcf692e1d/src/share/vm/oops/markOop.hpp#l30
>
> It would seem that both 32 bit and 64 bit modes have the place for the
> forwarding pointer (cut alignment bits out):
>   //             PromotedObject*:29 ---------->| promo_bits:3 ----->|
> (CMS promoted object)
>   //  PromotedObject*:61 --------------------->| promo_bits:3 ----->|
> (CMS promoted object)
>   //  narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->|
> (COOPs && CMS promoted object)
>
> Of course, we trade in monitor biasing, complicate hashcode storage, a
> bit slower forwarding, plus we need to support compressed forwarding
> pointers, but other than that? Maybe I miss some blocker issue, but
> somebody had explored and discovered it already?
>
> Thanks,
> -Aleksey
>