Re: Guile's time execution issues

[Ludovic Courtès]

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’.