Pure (side-effect-free) calls into c/c++?

[Linas Vepstas <linasvepstas@gmail.com>]

So, I've got lots of C code wrapped up in guile, and I'd like to declare
many of these functions to be pure functions, side-effect-free, thus
hopefully garnering some optimizations.  Is this possible? How would I do
it? A cursory google-search reveals no clues.

To recap, I've got functions f and g that call into c++, but are pure (i.e.
always return the same value for the same arguments).   I've got
user-written code that looks like this:

(define (foo x)
    (g  (f 42) (f x) (f 43))

and from what I can tell, `f` is getting called three times whenever the
user calls `foo`. I could tell the user to re-write their code to cache,
manually: viz:

(define c42 (f 42))
(define c43 (f 43))
(define (foo x) (g c42 (f x) c43))

but asking the users to do this is .. cumbersome.  And barely worth it: `f`
takes under maybe 10 microseconds to run; so most simple-minded caching
stunts don't pay off. But since `foo` is called millions/billions of times,
I'm motivated to find something spiffy.

Ideas? suggestions?

-- Linas
-- 
cassette tapes - analog TV - film cameras - you