[Vm-dev] flat objects in Self (was: What are classes defined with varibleSubclass?)

[Eliot Miranda]

On Thu, Jun 9, 2011 at 12:14 PM, Jecel Assumpcao Jr. <jecel at merlintec.com>wrote:

>
> Eliot Miranda wrote on Wed, 8 Jun 2011 10:01:17 -0700
> > When Squeak was implemented it was decided to use flat objects in the VM,
> > following the lead of David Ungar's Berkeley Smalltalk implementation,
> and of
> > the subsequent Self implementations, all of which also use flat objects.
>
> Given the background you mention and that all Vector objects in Self
> (Array in Smalltalk-80) can have named slots as well as indexed ones,
> you would think that the use of flat objects would be popular. But they
> are actually very rare. Methods objects, for example, have a named
> variable called "literals" that holds a Vector and another variable
> named "bytecodes" that holds a ByteVector. The same is true for the
> various kinds of Collections.
>

Are you sure that Self allows named and indexable slots?  What's an example
in Self?


> > Flat objects makes for faster allocation and faster inst var access
> (since
> > accessing an inst var doesn't require the double indirection of following
> the
> > pointer  to the header and then the pointer to the body).  But it makes
> become
> > very much more expensive, since in the worst case the VM must scan the
> entire
> > heap looking for references to the objects in the become operation and
> > replacing them by references to their corresponding objects.
>
> In theory there is no become in Self. In practice, the various object
> building primitives (such as _AddSlots: or _RemoveSlots:) do their work
> in place and so have the same cost as become. They are normally invoked
> by the user from the GUI and not used programatically, so their
> performance isn't too critical (and scanning the entire heap is the most
> optimized operation of all, obtained by a special tag for header words
> and by allocating bytes and pointers from different ends of the heap).
>
> > The solution David Ungar developed, which was adopted by Squeak, was to
> > unflatten objects that used become to grow, such as OrderedCollection,
> Set
> > and DIctionary, and use an array to hold their variable part.
>
> Do you mean at runtime or at design time (converting the image once)? If
> the latter, I had the impression that this was first proposed at
> Tektronix (after the versions described in the green book). But I don't
> trust my memory on this.
>

Im sure it was incremental.  There is a full become: in BS but its slow and
hence to speed the system up changes were made to certain core classes to
reduce becomming.  IIRC these changes were available in a change set called
unbecomming.st :)

cheers
Eliot

>
> -- Jecel
>
>
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