2014/1/13 Eliot Miranda <eliot.miranda at gmail.com>

> Hi Nicolas,
>
>
> On Mon, Jan 13, 2014 at 10:20 AM, Nicolas Cellier <
> nicolas.cellier.aka.nice at gmail.com> wrote:
>
>>
>> 2014/1/13 Eliot Miranda <eliot.miranda at gmail.com>
>>
>>> 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
>>> [New]ObjectMemory>>shorten:toIndexableSize:.
>>>
>>> HTH
>>> --
>>> best,
>>> Eliot
>>>
>>>
>>> Oh, I missed this one. I can't check now, could it cleanly address my
>> hugly trick of modifying the header like explained at
>>
>> http://smallissimo.blogspot.fr/2013/04/still-hacking-largeintegerplugins-in.html?
>>
>
> It would on SqueakV3.  On Spur it could be more difficult as there's no
> support for one-double-word free objects, so one can't free a singe 64-bit
> pair (objects are rounded up to 64-bits in length).  But perhaps you should
> inflict this on me to force my hand ;-).  The problem is that a single
> 64-bit word is big enough for an object header but not for an object header
> plus an indirection in the first slot, which takes 129 bits, when rounded
> up to a 64-bit boundary.  So such fragments are only reclaimable if an
> object next to them is freed.  They can't be compacted because there's no
> forwarding pointer.  So I expect in Spur I'd just have to disable the code
> and always create the copy.  However, what *can* be shortened, always, is
> the last object in eden.  So if the object is newly created it can easily
> be shortened; all that needs ot happen is the allocation pointer,
> freeStart, is cut-back.
>
> HTH,
> Eliot
>
>
> It was more for the fun of hacking than for solving a bottleneck: indeed,
macro benchmark show that benefit is really tiny.
I think that case of LargeInteger to be shorten will most often be the last
created but if it's too complex for such small return, just let a hacker do
it ;)
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