[Squeak-e] Programming the VM

Stephen Pair spair at acm.org
Mon Feb 3 15:33:31 CET 2003

>> SqueakVM specialists: how hard is it to add an extra
> reference to all
>> objects? On Squeak level? On VM level?
> I would like to hear a little more about what this 'machine' actually
> is.  Is it a lexical closure?  Would all objects need to have this
> 'machine' reference?
> Squeak object headers are of variable size.  there are 3 different
> sizes so it uses 2 bits in the baseHeader to define the
> header size.  1
> word, 2 words, or 3 words are the possibilities, and the fourth value
> is for dead objects, I believe.  Stephen added an extra word for his
> LOOM impl, and basically all the code that reads or writes
> the headers
> would need to be changed to deal with it.

You are correct about header sizes and bit usages.  Coaxing squeak into
having extra headers is straightforward, but time consuming.  You'll
have an additional challenge by adding a pointer field (because you'll
need GC to trace it).  I'd also recommend that you bone up on the
InterpreterSimulator and use SystemTracer2 (on SqueakMap) for making
your new images and testing your new interpreter.  The
InterpreterSimulator is an invaluable debugging tool.

Since it is a pointer field, I'd probably explore adding the word
*below* the base header because that will be the least impact on
GC...however, it's also something that I've never attempted.  I have
added non-pointer headers to every object above the base header and I've
added the ability to insert a "state manager" between an object and its
class (path of least resistance).  If you added a pointer field above
the base header, you're going to have to do some very tricky things with
GC to get it to traverse that field (similar to the way GC currently
deals with the class header).

In an ideal world, I'd move the class header below the base header, but
this probably wasn't done because not every object has a class header
and testing that fact on every inst var access would probably be
expensive.  If you eliminated compact classes, then it would make sense
to move the class header below the base header (note, I have not done
that in my Chango VM even though I did eliminate compact classes).

If absolutely every object is going to have this pointer and it is a
pointer, I would start with a design that puts this pointer after the
base header.  If only some of the objects in the image will have this
pointer, you might try a similar hack that I did which inserts your
object between an object and its class.  Finally, you can put a pointer
above the header, but it's going to make for some ugly looking code in

- Stephen

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