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RE: ObjC configured --with-objc-gc needs external Boehm gc


> From: Helge Hess [mailto:helge.hess@skyrix.com]
> "Boehm, Hans" wrote:
> > I can't think of any fundamental problems.  I suspect 
> Objective C normally
> > wants the collector built to recognize all interior 
> pointers, where gcj
> > builds it without that option, so that interior pointers 
> are recognized only
> > if they're on a stack.  This is currently a build time 
> option, but that
> > probably doesn't save very much anymore over making it a 
> runtime option.
> 
> Hm, I don't remember - all-interior-ptrs are required for 
> untyped memory
> ? The ObjC runtime uses typed allocation, so it might not require
> all-interior-ptrs.
The question is really whether the last pointer to an object can point to
the inside of the object.  If you can get and save a pointer to a field in
object A, save it inside object B, and then drop the pointer to A, you need
ALL_INTERIOR_POINTERS.  Thus C/C++ more or less need it, while Java doesn't.
If you can embed C structs inside Objective C objects, I suspect Objective C
also needs it?
  
> 
> > Does Objective C normally use the collector with thread support?
> 
> I think that this is 50/50. The problem is that threading currently
> slows down the runtime to some degree and therefore is often 
> turned off
> if threading is not required.
Aside from a tuning problem in current collector versions, it should
currently cost basically one test-and-set (probably 5-30 cycles depending on
the processor) on entry, and a store on exit of each malloc call.  The
uniprocessor cost with thread-local allocation (as in GC6.0) is probably
about the same, though it scales much better on multiprocessors.

Hans

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