[Vm-dev] how many free bits we have in Object Header?

Adrian Lienhard adi at netstyle.ch
Tue May 4 12:12:42 UTC 2010

Hi Mariano,

I see two solutions
- make the identityHash (even) smaller
- remove compact classes

The former would probably not be hard to implement (just change some bit masks and then rehash all sets). I've brought up the idea about the latter some time ago on this mailing list. The idea is to remove compact classes to get a larger identityHash (trading memory against speed). After the removal, only two header types are left and hence you gain a spare bit. I think this shouldn't be too much work either, but I haven't come around to implementing it (yet).

Good luck ;)

On May 4, 2010, at 13:13 , Mariano Martinez Peck wrote:

>>> Now...the question is, is there another free bit in the object header ? I
>>> need another one :(
>> Based on the comment at the bottom of Interpreter>>internalIsImmutable:
>> I suspect that somebody else has their eye on that last available bit
>> also ;-)
> Yes, I know :(   But for the moment I don't care to "integrate" my stuff.  I
> just want to play and experiment. And even for that, I need 2 bits :(
> Of course, if I once make something good to integrate, I will have double
> problem :(
> So...no solution ?

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