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From Robin Garner <robin.gar...@anu.edu.au>
Subject Re: [DRLVM][JET] write barrier for Java (mmtk)
Date Wed, 11 Oct 2006 08:39:30 GMT
I think you must be looking at a fairly old version of MMTk. 
writeBarrier is an instance method of a MutatorContext (in org.mmtk.plan).

MutatorContext exists to hold unsynchronized thread-local data 
structures.  Particularly relevant to the write barrier, each mutator 
context has its own thread-local remset.  All of the mutator context 
methods of MMTk need fast access to the MMTk thread local data 
structures, which is why they are instance methods.  The other critical 
instance method of a MutatorContext is 'alloc', which also has it's 
thread-local chunk of the space(s) it allocates into.

As far as the VM is concerned, it will be calling instance methods of a 
final class.  The various classes in org.mmtk.plan.* aren't final, but 
the VM interface code is expected to wrap the currently selected plan in 
  some final class.  JikesRVM wraps the currently selected plan classes 
in a 'SelectedPlan', 'SelectedMutatorContext' etc.

As far as the VM.barriers.performWriteInBarrier() call is concerned,
the optimization required to devirtualize a call to a final method of a 
static final field shouldn't be too hard to implement.  MMTk recently 
moved away from using static methods for this part of the interface, to 
the current abstract factory, and improved the structure of the software 
significantly.  We don't want to go back!

 >                                  I erroneously thought we could call
 > VM.barriers.performWriteInBarrier() directly.  This sort of, kind of 
breaks
 > MMTk architecture.

well, it less 'breaks the architecture' than performs a no-op :)

-- robin

Weldon Washburn wrote:
> Ooops.  I really tangled things up.  You are right about how we are 
> supposed
> to find the Java write barrier method.  It is located in 
> Plan.writeBarrier().
> Each GC algorithm has a Plan class that overrides the writeBarrier()
> method.  I erroneously thought we could call
> VM.barriers.performWriteInBarrier() directly.  This sort of, kind of breaks
> MMTk architecture.  By design, each GC algorithm in MMTk is supposed to 
> call
> Plan.writeBarrier() which, in turn, will call
> VM.barriers.performWriteInBarrier.
> 
> Sorry for the confusion.
> 
> 
> 
> 
> On 10/10/06, Mikhail Fursov <mike.fursov@gmail.com> wrote:
>>
>> Yes, we can run the usual inliner after helpers are inlined.
>> The only problem I want to notice is that once we have different helpers
>> for
>> different GCs it's a bad idea to use virtual method calls in performance
>> sensitive helpers. You are allowed to do it, but the better solution 
>> is to
>> teach the helper to use a final implementation of the Barrier and replace
>> the helper once the implementation of the Barrier class is changed.
>>
>> On 10/11/06, Rana Dasgupta <rdasgupt@gmail.com> wrote:
>> >
>> > Makes sense, using a standard barrier invocation fastpath. But I assume
>> > that
>> > the MMTk WB helper that it will call needs to be inlined too.
>> >
>> > Thanks
>> >
>> >
>> > On 10/10/06, Mikhail Fursov <mike.fursov@gmail.com> wrote:
>> > >
>> > > Weldon,
>> > > > I thought about slightly different approach.
>> > > > Why not to write fast-path VM helper like was proposed in the 
>> thread
>> > > > "[drlvm]Extending..."
>> > > > This helper (a static method) can be inlined by JIT without any
>> > > > devirtualization and call any method needed from MMTk or native
>> > > > implementation. So JIT won't know if it works with MMTk or with a
>> > native
>> > > > GC:
>> > > > all you need is just to replace the Java version of the helper.
>> > > > ?
>> > > >
>> > >
>> > >
>> >
>> >
>>
>>
>> -- 
>> Mikhail Fursov
>>
>>
> 
> 


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