Lucid 3.0.2 Optimization botch

[Jon L White <jonl@lucid.com>]

re: If you rewrite the Lucid part of FIN to do this mattress-pad stuff then
    I can install it in PCL and fix this longstanding problem.

	Just doing the tail-recursive exit will probably make the matter 
	essentially moot for all practical purposes.

    The problem crops up all the time here for people using PCL in Lucid
    Lisp.
    -------

The problem with the "mattress pad" is that it has to be presented as 
a "binary patch" -- that is, a small segment of machine code presented 
as hex digits -- for each and every hardware platform.  I can easily do 
this for the 68K and SPARC if you so wish, but note that numerous Lucid 
users are on other platforms.  Do you wanna try to do the tail-recursive 
cache-miss code yourself?  I can show you my version, although it may have 
Lucid-specific parts in it; it will take a little thought to see how to 
retro-fit it (but not much).

The reason why not doing a tail-recursive call is such a big lose is
in the relative period of time in which dcode (procedure) consistency
is required:

   (1) non-tail:  over the interval of open-coded cache lookup, *** as
       well as over the call to 'dcode-cache-miss' *** which can take
       not only an arbitrarily long period of time, but will ocasionally
       directly overwrite the dcode itself, due to cache growth.

   (2) tail:  only over the interval of the open-coded cache lookup [which
       we all know to be very small, right, or else PCL wouldn't be viable
       at all];  the liklihood of a multiprocessing collision here is about 
       the same as the liklihood of a multiprocessing collision revealing 
       one of the few unprotected "critical sections" in the Lucid 3.0 
       system -- not zero, but epsilonically small.  Also, there is no  
       problem with cache growth or other updates than can occur in the
       "miss" case.


Of course, all other implementations would benefit from the fix to dcodes 
to be tail recursive when "miss"'ing.  I'm pretty sure that VaxLisp has 
compiler OPTIMIZE settings that can produce morphologically different code; 
and there ought to be other such compilers around.


-- JonL --


P.S. I've been advised privately that the credits for the "mattress pad"
     idea (and name) really belong to another Franz person -- that jkf
     simply wrote the first version of Franz's "fin".  My apologies.