From: "Mattias Engdegård" <mattias.engdegard@gmail.com>
To: Eli Zaretskii <eliz@gnu.org>
Cc: 58168@debbugs.gnu.org
Subject: bug#58168: string-lessp glitches and inconsistencies
Date: Mon, 3 Oct 2022 21:48:14 +0200 [thread overview]
Message-ID: <BC625893-642A-4B8B-9309-1DCC5E4594B3@gmail.com> (raw)
In-Reply-To: <83tu4mais1.fsf@gnu.org>
2 okt. 2022 kl. 07.36 skrev Eli Zaretskii <eliz@gnu.org>:
>> Comparison between objects is not only useful when someone cares about their order, as in presenting a sorted list to the user. Often what is important is an ability to impose an order, preferably total, for use in building and searching data structures. I came across this bug when implementing a string set.
>
> Always converting to multibyte handles this case, doesn't it?
I don't think it does -- string= treats raw bytes in unibyte and multibyte strings as distinct; converting to multibyte does not preserve (in)equality.
>> Actually I was talking about multibyte-multibyte comparisons.
>
> Then why did you mention raw bytes? their multibyte representation
> presents no performance problems
In a way they do -- the way raw bytes are represented (they start with C0 or C1) causes memcmp to sort them between U+007F and U+0080. If we accept that then comparisons are fast since memcmp will compare many character per data-dependent branch. The current code requires several data-dependent branches for each character.
While we could probably bring down the comparison cost slightly by clever hand-coding, it's unlikely to be even nearly as fast as a memcmp and much messier. Since users are unlikely to care much about the ordering between raw bytes and something else (as long as there is an order), it would be a cheap way to improve performance while at the same time fixing the string< / string= mismatch.
> You can compare under the assumption that a unibyte string is
> pure-ASCII until you bump into the first non-ASCII one. If that
> happens, abandon the comparison, convert the unibyte string to its
> multibyte representation, and compare again.
I don't quite see how that would improve performance but may be missing something.
next prev parent reply other threads:[~2022-10-03 19:48 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-29 16:24 bug#58168: string-lessp glitches and inconsistencies Mattias Engdegård
2022-09-29 17:00 ` Mattias Engdegård
2022-09-29 17:11 ` Eli Zaretskii
2022-09-30 20:04 ` Mattias Engdegård
2022-10-01 5:22 ` Eli Zaretskii
2022-10-01 19:57 ` Mattias Engdegård
2022-10-02 5:36 ` Eli Zaretskii
2022-10-03 19:48 ` Mattias Engdegård [this message]
2022-10-04 5:55 ` Eli Zaretskii
2022-10-04 17:40 ` Richard Stallman
2022-10-04 18:07 ` Eli Zaretskii
2022-10-06 9:05 ` Mattias Engdegård
2022-10-06 11:06 ` Eli Zaretskii
2022-10-07 14:23 ` Mattias Engdegård
2022-10-08 7:35 ` Eli Zaretskii
2022-10-14 14:39 ` Mattias Engdegård
2022-10-14 15:31 ` Eli Zaretskii
2022-10-17 12:44 ` Mattias Engdegård
2022-09-30 13:52 ` Lars Ingebrigtsen
2022-09-30 20:12 ` Mattias Engdegård
2022-10-01 5:34 ` Eli Zaretskii
2022-10-01 11:51 ` Mattias Engdegård
2022-10-01 10:02 ` Lars Ingebrigtsen
2022-10-01 10:12 ` Eli Zaretskii
2022-10-01 13:37 ` Mattias Engdegård
2022-10-01 13:43 ` Lars Ingebrigtsen
2022-10-03 19:48 ` Mattias Engdegård
2022-10-04 10:44 ` Lars Ingebrigtsen
2022-10-04 11:37 ` Eli Zaretskii
2022-10-04 14:44 ` Mattias Engdegård
2022-10-04 16:24 ` Eli Zaretskii
2022-10-06 9:05 ` Mattias Engdegård
2022-10-06 11:13 ` Eli Zaretskii
2022-10-06 12:43 ` Mattias Engdegård
2022-10-06 14:34 ` Eli Zaretskii
2022-10-07 14:45 ` Mattias Engdegård
2022-10-07 15:33 ` Eli Zaretskii
2022-10-08 17:13 ` Mattias Engdegård
2022-10-01 13:51 ` Eli Zaretskii
2022-10-01 5:30 ` Eli Zaretskii
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