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Re: Guile's time execution issues

From: Ludovic Courtès
Subject: Re: Guile's time execution issues
Date: Mon, 04 May 2020 22:09:03 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux)


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: 

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)
         ((@@ (ice-9 match) error)
          "no matching pattern"
--8<---------------cut here---------------end--------------->8---

but even that sounds unlikely.

You’re compiling with -O2, right?


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