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Re: Guile's time execution issues
From: |
Linus Björnstam |
Subject: |
Re: Guile's time execution issues |
Date: |
Mon, 04 May 2020 22:50:25 +0200 |
User-agent: |
Cyrus-JMAP/3.3.0-dev0-351-g9981f4f-fmstable-20200421v1 |
You didn't see my other reply. The matching code isn't suboptimal. The equality
predicate is The problem is that match compares using equal? even for literal
chars (where eqv? is a lot faster). It would be a rather trivial optimization
to do, either to match.scm (meaning: breaking with upstream and use
syntax-case) or to the guile compiler in general (changing equal? to eqv, when
there are character literals), which seems ok-ish for this use-case but at very
little benefit in general.
A long-term goal of mine is to write a pattern matcher with the optimisations
that the racket matcher does (among other things: some serious list matching
reordering!). That is a daunting task though.
--
Linus Björnstam
On Mon, 4 May 2020, at 22:09, Ludovic Courtès wrote:
> Hi,
>
> Linus Björnstam <address@hidden> skribis:
>
> > On Mon, 4 May 2020, at 11:36, Ludovic Courtès wrote:
> >
> >> > One thing I found is that `match` is slow. The code looked nicer but had
> >> > to change it back to lets and conds as the performance
> >> > increase was ~2 seconds.
> >>
> >> Oh, in which case exactly? And are you sure your hand-written code is
> >> equivalent to the ‘match’ code (it’s common for hand-written code to be
> >> more lax than ‘match’)?
> >>
> >> One thing to pay attention to is the use of ‘list?’, which is O(N), and
> >> is implied by ellipses in ‘match’. If you want to use ‘match’ in a way
> >> that avoids ‘list?’, write patterns such as (a . b) instead of (a b ...).
> >> It doesn’t have the same meaning, but often the end result is the same,
> >> for instance because you’ll later match on ‘b’ anyway.
> >>
> >> (I wish we can one day have a proper list type disjoint from pairs…)
> >
> > The change is here: he is only matching against chars and predicates:
> > https://github.com/aconchillo/guile-json/commit/ad4b06d86e4822466983d00f55474c8f664b538d
>
> It would be nice if you could pinpoint which one of these changes causes
> a difference, because:
>
> --8<---------------cut here---------------start------------->8---
> scheme@(guile-user)> ,optimize (match (peek-char port) ((? eof-object?)
> x) ((? whitespace?) w) (_ e))
> $84 = (let ((v (peek-char port)))
> (cond ((eof-object? v) x)
> ((whitespace? v) w)
> (else e)))
> --8<---------------cut here---------------end--------------->8---
>
> What might make a difference is the code bloat when using ‘or’:
>
> --8<---------------cut here---------------start------------->8---
> scheme@(guile-user)> ,optimize (match (peek-char port) ((or #\a #\b #\c #\d)
> x))
> $86 = (let ((v (peek-char port)))
> (cond ((equal? v #\a) x)
> ((equal? v #\b) x)
> ((equal? v #\c) x)
> ((equal? v #\d) x)
> (else
> ((@@ (ice-9 match) error)
> 'match
> "no matching pattern"
> v)
> #f)))
> --8<---------------cut here---------------end--------------->8---
>
> but even that sounds unlikely.
>
> You’re compiling with -O2, right?
>
> Thanks,
> Ludo’.
>