Recoding Lisp programs in C

michael b maxwell michaelm at bcsaic.UUCP
Thu Oct 3 03:14:03 AEST 1985

The common wisdom is that AI-type applications should be developed in an AI 
language such as Lisp or Prolog, for obvious reasons.  At the same time, 
it is frequently asserted that a mature Lisp program can be recoded in C 
for increased speed and decreased size.

My question is, how much faster, and how much smaller?

Obviously this depends on many things.  Let's assume for the purposes of
discussion reasonably good Lisp and C compilers.  (e.g. Frantz Liszt, etc.;
I don't "speak" C, so insert your favorite C compiler here!)  Let's also
assume you've made what modifications in your Lisp program you can to speed
it up, e.g. doing (sstatus translink on), using localf (assuming that
works, which it doesn't on my Sun...), setting up the allocation of space
for different forms of data so as to optimize garbage collection, etc.
Likewise whatever optimizations you can make to the C version.

The improvements gained by recoding in C also obviously depend on what kind 
of a program you're recoding; so what programs are helped most?  
Pointer-intensive ones??  Arithmetic-intensive ones?

On a large program, recoding the entire thing in C is probably not
worthwile (I would guess).  How can you tell what parts would best benefit
from translation?  (I assume here a Lisp like Frantz, which allows
calling of C functions from Lisp code.)  Or does this really gain you

Have many people really done the translation?  Or is it one of those things 
that everyone talks about, but no one does anything about? :-)  What gains 
did you realize?  Are there any published studies?
Mike Maxwell
Boeing Artificial Intelligence Center

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