[Vm-dev] Root flag & root objects

Andreas Raab andreas.raab at gmx.de
Sat Feb 21 19:01:06 UTC 2009

Eliot Miranda wrote:
> So yes, when creating a new image, all objects are in oldSpace and so no 
> objects need to be in the rot table or have their root bit set.


> This raises another point of ignorance for me.  I would have thought 
> that it is OK for the root bit to be set or the root table to contain 
> objects that are not in old space or do not/no longer refer to objects 
> in youngSpace because the GC will remove these from the root table on 
> GC.  But I don't see where objects get cleared from the rootTable.  I 
> only see the RootTable being voided when it gets full (rather drastic). 
>  Can anyone say if this happens or not, and if so, where it happens?

I think you're right. Short of a full GC the root table may never get 

   - Andreas

> Looks to me like either markAndTrace: or a specialized version, say 
> markAndTraceRootObject:, should maintain a flag, refersToYoungObject, 
> and clear the object from the rootTable (and clear the root bit) 
> if refersToYoungObject is false at the end of the scan.   But with the 
> pointer reversal algorithm it is hard to see where to maintain this flag.
> Can those who know enlighten this ignorant soul?
> On Sat, Feb 21, 2009 at 4:42 AM, Igor Stasenko <siguctua at gmail.com 
> <mailto:siguctua at gmail.com>> wrote:
>     Hi,
>     i'd like to ask, is it safe to clear the root flag for every object
>     in system.
>     I'm artificially creating a new image, and i'd like to know, what
>     objects should be marked as roots initially, or VM can care about it
>     itself?
>     --
>     Best regards,
>     Igor Stasenko AKA sig.

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