From: "Linus Björnstam" <linus.bjornstam@veryfast.biz>
To: "Ludovic Courtès" <ludo@gnu.org>
Cc: "Aleix Conchillo Flaqué" <aconchillo@gmail.com>,
guile-user <guile-user@gnu.org>,
guile-devel <guile-devel@gnu.org>
Subject: Re: Guile's time execution issues
Date: Mon, 04 May 2020 22:50:25 +0200 [thread overview]
Message-ID: <d7951a62-a1df-44e2-bbc0-2fb9e0bc4079@www.fastmail.com> (raw)
In-Reply-To: <874ksvqx7k.fsf@gnu.org>
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 <linus.bjornstam@veryfast.biz> 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’.
>
next prev parent reply other threads:[~2020-05-04 20:50 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-21 22:03 Guile's time execution issues Aleix Conchillo Flaqué
2020-04-22 13:47 ` Linus Björnstam
2020-04-26 17:16 ` Ludovic Courtès
2020-04-26 23:14 ` Aleix Conchillo Flaqué
2020-05-02 14:11 ` Ludovic Courtès
2020-05-04 0:32 ` Aleix Conchillo Flaqué
2020-05-04 9:36 ` Ludovic Courtès
2020-05-04 11:19 ` Linus Björnstam
2020-05-04 20:09 ` Ludovic Courtès
2020-05-04 20:50 ` Linus Björnstam [this message]
2020-05-08 11:31 ` Linus Björnstam
2020-05-04 18:47 ` Linus Björnstam
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