Closures (addendum)

[Scott E. Fahlman <Fahlman at Cmu-20c>]

By the way, on the Vax, at least, it is not necessarily the case that
dynamic closures cost you an extra cycle on every special variable
reference.  When you pick up a special value, you have to check for the
unbound marker anyway, and if things are arranged properly this same
check can pick off the case of an EVC-pointer or whatever.  So the only
extra cost is on function entry or when you actually do have a ref to a
closed-over or unbound special.  If there were no checking at all of the
special value before returning it, then the EVC-forward check would
indeed cost you a cycle.  Perhaps Hedrick is picking off unbound
specials some other way (or not at all?) and the closure cost is
therefore real to him.  Evan if it cost a whole cycle per special ref,
I'm not sure that that would slow the language down noticeably, since
specials are somewhere down below 10% of all variable refs in the
assorted statistics I've seen.  We can't let them become aribitrarily
bad, but maybe an extra cycle or two is tolerable.

As I said, if we can get the same effect with lexical vars, we should do
it that way.

-- Scott