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From Egor Pasko <egor.pa...@gmail.com>
Subject Re: [DRLVM][JIT] write barrier broken by new jit opts?
Date Tue, 09 Jan 2007 11:24:28 GMT
On the 0x257 day of Apache Harmony Robin Garner wrote:
> I disagree.  Partly from the point of view of eventual support for MMTk,
> but also on more general grounds, in terms of support for future GC
> algorithms.
> 
> If the memory manager requires that the compiler invoke the write barrier
> for each pointer store, then it should honour the contract and do so.
> 
> If you want to provide an object remembering barrier that the compiler can
> call as an optimization, then by all means do so, but the GC should not be
> forced to work around compiler 'optimizations' that break fundamental
> interface contracts.

That's what I think we should do. I see it like this:

* JIT asks GC if it has_object_update_wb_feature() 

* JIT checks CompilationInterface.insertWriteBarriers()

* if both true, JIT inserts code that update whole objects with a
  single "object/array update barrier". If only second is true,
  generate an ordinary code.

* "object update barrier" should NOT be a GC safe point

The scheme is not complicated and easy to implement in JIT. Just need
to put the right checks in right places. It should not be difficult to
implement the feature in GCv5, isn't it?

In spite of introducng more state-dependant code, I think, the feature
is worth it from performance point of view.

> While object remembering barriers may be adequate for GCV5 there are a
> large family of GC algorithms that they are inadequate for, including
> reference counting and concurrent/incremental GCs.
> 
> regards,
> Robin
> 
> > Hi, I found write barrier in DRLVM can't catch all reference fields
> > updates, and the problem is identified to be caused by new jit opts
> > that do not observe the write barrier invariant for fields updates.
> > For example, JIT may generate code to copy consecutive fields of an
> > object without invoking write barrier. In order to revive the write
> > barrier functionality while not sacrificing the opt performance, I'd
> > suggest to introduce an object remember write barrier which will be
> > invoked after the object is copied. So the JIT doesn't need to insert
> > barrier for each field store. GC has an interface
> > gc_heap_wrote_object(p_obj) for this case.  I think it's ok to insert
> > only the runtime native call at first. Then later we can consider to
> > inline the object remembering barrier as well as the slot remembering
> > barrier.
> >
> > Thanks,
> > xiaofeng
> >
> 
> 
> 

-- 
Egor Pasko


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