_AddSlots: in programs

Jecel Mattos Assumpcao Jr jecel at lsi.usp.br
Tue Sep 17 16:29:50 UTC 1991

> The system keeps dependency information to keep track of the fact that
> the machine code for bar depends on b being a data slot.  During the
> _Define:, bar's code is marked as "outdated", and it's stack frame is
> marked.  When control returns to bar (after the change), a trap
> handler is invoked which recompiles bar before continuing execution.

It seems to me that depending on what the compiler has inlined, and how
it moved code around and optimized some of it away, that it isn't easy to
continue execution after recompiling a method. I haven't had time to
think this through, but the new compilation of the method might organize
its stack/registers differently. Just remapping the instruction pointer 
from the old method to the new seems quite complicated.

I realize that you must have all this worked out already on the old Self
just to be able to dump the stack correctly considering all the tricks
the compiler might play. The OOPSLA'89 paper mentions scope description
and bytecode mappings as the solution. How much overhead do you get for
keeping all that information. Would it be too costly to throw it away
and, when you need it, to recompile the old version of the method to 
regenerate all this?

-- Jecel

P.S.: I hope my last message didn't cause any confusion - I copied the
      date to try to make it clear I was referring to an old message.

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