From: Stefan Monnier <monnier@iro.umontreal.ca>
To: emacs-devel@gnu.org
Subject: Re: `aset` on strings, changing the size in bytes
Date: Sun, 09 Sep 2018 20:18:04 -0400 [thread overview]
Message-ID: <jwvzhwqky6g.fsf-monnier+gmane.emacs.devel@gnu.org> (raw)
In-Reply-To: 83y3cbqm81.fsf@gnu.org
>> >> The need for an indirection (a String_Object has to hold a pointer to an
>> >> sdata object rather than being able to keep its payload directly in the
>> >> Lisp_String object (using FLEXIBLE_ARRAY_MEMBER))
>> > Why is that a problem?
>> It slows down every string access, and increases the heap size of every
>> string (currently they're something like N bytes of payload plus
>> 5 words where 2 of those 5 words are due to the extra indirection).
>> For a feature that's almost never used, I think it's pretty costly.
> It can be (and was) used by memory-allocation infrastructure,
> especially with very large strings. Are we sure we want to lose it
> for slowdown that should be hardly perceptible?
As mentioned in my original message, this indirection is used currently
for 2 purposes:
1- to implement `aset`.
2- to implement string compaction in the GC.
For this reason, either one of the two may be considered to "come for
free" if you presume the other one as a given.
Point nb 2 is a purely internal implementation detail, which we
could change whenever we feel like this decision gets in the way of
something preferable.
Point nb 1 OTOH cannot be changed so easily because it would be
a backward-incompatible change.
Stefan
next prev parent reply other threads:[~2018-09-10 0:18 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-07 19:52 `aset` on strings, changing the size in bytes Stefan Monnier
2018-09-07 21:33 ` Johan Bockgård
2018-09-07 23:12 ` Paul Eggert
2018-09-07 23:41 ` John Wiegley
2018-09-08 5:17 ` Richard Stallman
2018-09-08 6:34 ` Eli Zaretskii
2018-09-08 2:04 ` Stefan Monnier
2018-09-08 2:17 ` Paul Eggert
2018-09-08 6:41 ` Eli Zaretskii
2018-09-08 18:03 ` Stefan Monnier
2018-09-08 18:20 ` Eli Zaretskii
2018-09-08 18:36 ` Stefan Monnier
2018-09-08 20:59 ` Eli Zaretskii
2018-09-08 22:09 ` Stefan Monnier
2018-09-09 5:22 ` Eli Zaretskii
2018-09-10 0:18 ` Stefan Monnier [this message]
2018-09-09 6:07 ` Richard Stallman
2018-09-09 6:26 ` Eli Zaretskii
2018-09-09 14:44 ` Noam Postavsky
2018-09-09 15:17 ` Eli Zaretskii
2018-09-09 16:27 ` Noam Postavsky
2018-09-10 5:48 ` Richard Stallman
2018-09-10 3:03 ` Stefan Monnier
2018-10-16 21:05 ` Stefan Monnier
2018-09-10 5:47 ` Richard Stallman
2018-09-10 5:48 ` Richard Stallman
2018-09-10 3:02 ` Stefan Monnier
2018-09-08 6:03 ` Helmut Eller
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