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

From: Zelphir Kaltstahl
Subject: Re: Pure (side-effect-free) calls into c/c++?
Date: Sat, 11 Jan 2020 15:13:10 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2

Hello Linas,

On 1/10/20 11:36 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

I don't know exactly how to do it, but in theory, you could provide the
user a macro, which looks for calls of `f` and makes it so, that these
calls are only done once. My macro skills are not so great yet, so I
don't know how to do that. I am just thinking, that in theory this
should be possible, perhaps with a simplified case, that assumes, that
the user does not redefine `f` inside the expression given to the macro.

Just outputting the idea, not sure it is a good idea.



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