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.bjorns...@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’. >
