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

From: Linas Vepstas
Subject: Re: Pure (side-effect-free) calls into c/c++?
Date: Sat, 11 Jan 2020 12:11:04 -0600

Hmm Thanks.  Perhaps I should have been more clear.  I'm talking about a
handful of values that behave like constants, and NOT about memoization.

So here's a bit more detail. The only thing the C++ code is doing is
stuffing the values into a giant hash table... in C++. So its already very
fast. The return value is a pointer into that hash table.  Replicating that
hash table a second time, in guile is ... well ... it has tens/hundreds of
millions of entries, it would blow out RAM.  (And the other functions,
those that are actually CPU-intensive, already got the Grand Wazoo
treatment for memization; some in guile, some in C++)

For this particular issue, I'm thinking I need to understand macros,
somehow.  The point being that there is just a tiny handful of values --
some dozens, maybe hundreds, that some human has written into the code, and
are being treated as if they were literal constants (because, in the C++
code, they *are* constants -- they're really just fixed entries in a symbol
table) and so I want to automatically replace these by the actual constant
value (the location in the c++ table).  To recap:  a macro that would

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


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

so that guild can treat c42 and c43 as constants (boxes, I guess).

-- Linas

On Sat, Jan 11, 2020 at 8:39 AM Matt Wette <address@hidden> wrote:

> On 1/10/20 2:36 PM, Linas Vepstas wrote:
> > 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
> read this:
> and look at

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