[LLVMdev] More Garbage Collection Questions

[Talin]

Gordon Henriksen wrote:
> Can you be more specific the algorithm for which you need type  
> metadata in a write barrier? No algorithms I am aware of perform any  
> tracing from a write barrier.
>   
This one does:

http://citeseer.ist.psu.edu/cache/papers/cs2/442/http:zSzzSzwww.cs.technion.ac.ilzSz~erezzSzPaperszSzms-sliding-views.pdf/an-on-the-fly.pdf
> Write barriers are commonly used to record references from old- 
> generation objects to new-generation ones, either by recording the  
> referencing object, the referencing field, or using a card table. For  
> these purposes, the addresses are sufficient.
>   
In the paper cited above, the write barrier checks to see if the object 
being mutated has been traced; If it hasn't, then it records the values 
of all pointers contained in the object into a buffer - for which you 
need the location of the pointers.
> In concurrent collectors—by which I mean those that run  
> asynchronously of the mutator—write barriers may be used to mark the  
> written object pointer. Even byte arrays as you describe require an  
> object header in which to store mark bits, else the collector cannot  
> know whether the object has been marked (mark-sweep collectors) or  
> copied (copying collectors).
>   
In my design, the mark bits are maintained by the allocator as part of 
the allocation header of each memory block.

In other words, I'm trying to maintain a strict separation between the 
allocator/collector and the actual objects stored in the heap. The 
allocator/collector "owns" all of the bits that are before the start 
address of the object, while the language-specific type system owns all 
of the bits after the start address. As a result the collector isn't 
tied to a specific language.

-- Talin