[Vm-dev] Re: allObjectsDo: (was Re: [squeak-dev] The Inbox:
David T. Lewis
lewis at mail.msen.com
Mon Jan 13 23:59:15 UTC 2014
On Mon, Jan 13, 2014 at 09:26:31AM -0800, Eliot Miranda wrote:
> Hi David,
> On Sun, Jan 12, 2014 at 5:13 PM, David T. Lewis <lewis at mail.msen.com> wrote:
> > On Sun, Jan 12, 2014 at 09:53:27PM +0100, Bert Freudenberg wrote:
> > >
> > > On 12.01.2014, at 20:42, David T. Lewis <lewis at mail.msen.com> wrote:
> > >
> > > > It is worth noting that allObjectsDo: relies on assumptions about how
> > > > the objects memory works internally. It requires that #someObject will
> > > > always answer the object at the lowest address in the object memory,
> > and
> > > > also that a newly allocated object will always be placed at a higher
> > > > address location than all other objects. Either of these assumptions is
> > > > likely to be a problem as new and better object memories and garbage
> > > > collectors are implemented.
> > > >
> > > > Dave
> > >
> > > Right (as Eliot's vm-dev post shows).
> > >
> > > So IMHO the only sensible semantics of allObjectsDo: is as in
> > "allObjects do:" -
> > > which might be implemented as a primitive in some VMs soonish. It
> > *should not*
> > > enumerate objects created after calling the method.
> > >
> > On Sun, Jan 12, 2014 at 12:01:00PM -0800, Eliot Miranda wrote:
> > >
> > > The bug is in implementing allObjects in terms of someObject and
> > nextObject
> > > in the first place. It's cheap and cheerful but horribly error-prone and
> > > restrictive. It's cheap because the collection of objects doesn't have
> > to
> > > be created, and on the original 16-bit Smalltalk machines that was really
> > > important. It's horribly restrictive because it assumes much about the
> > > implementation.
> > >
> > > Before closures a sentinel wasn't even needed because enumerating the
> > block
> > > didn't create a new object (the block context was reused). So the code
> > had
> > > to be rewritten just to support closures.
> > >
> > > Spur has a generation scavenger operating in a distinct new space and
> > that
> > > doesn't jive well with a consistent ordering at all. So far the system
> > is
> > > limping along by tenuring all objects on someObject and someInstance (so
> > > that newSpace is either empty, or doesn't contain any instances of a
> > > specific class) and having nextObject enumerate only objects in oldSpace.
> > >
> > > But I think now we can afford a primitive that answers all the objects
> > > (remember that average object size means that such a collection will be ~
> > > 10% of the heap, average object size in Squeak V3 is about 10.6 words).
> > At
> > > least that's what Spur will do, along with an allInstancesOf: primitive.
> > > And then the become example won't cause any problems at all. Far more
> > > reliable. I suppose there are circumstances when enumerating without a
> > > container is the only feasible approach, but VisualWorks has got along
> > with
> > > only an allObjects primitive for a long time now. I suspect we can too.
> > >
> > Implementation attached. Works on interpreter VM, not yet tested on Cog but
> > it should be ok there also. If no objections or better suggestions I will
> > commit it to VMMaker tomorrow.
> > InterpreterPrimitives>>primitiveAllObjects
> > "Answer an array of all objects that exist when the primitive is
> > called, excluding those
> > that may be garbage collected as a side effect of allocating the
> > result array. Multiple
> > references to nil in the last slots of the array are an indication
> > that garbage collection
> > occured, such that some of the unreferenced objects that existed
> > at the time of calling
> > the primitive no longer exist. Sender is responsible for handling
> > multiple references to
> > nil in the result array."
> Instead of filling the unused slots with nil or 0, I think you should
> shorten the object so that it contains each object only once, and contains
> only the objects. Cog contains some code for shortening. See
That would be a better solution. However, I cannot offer an implementation
in the near term because of:
shorten: obj toIndexableSize: nSlots
"Currently this works for pointer objects only, and is almost certainly wrong for 64 bits."
Given that this is currently intended for pointer objects, it is probably
fairly straightforward to get it working on the 64-bit object memory. In fact,
it might already work as written. But I think that it will take some time to
test so it's not going to happen tonight.
We could consider a variation on Bert's suggestion, in which the result array
might have trailing zeros if garbage collection has occurred. Later the primitive
can be improved with shorten:toIndexableSize: after which the trailing zeros
will never occur in practice. That would still put the burden on the image
to ignore the trailing junk, so I don't know if it would be worth doing.
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