From: Alan Mackenzie <acm@muc.de>
To: "Mattias Engdegård" <mattias.engdegard@gmail.com>
Cc: 65051@debbugs.gnu.org, Eli Zaretskii <eliz@gnu.org>,
Stefan Monnier <monnier@iro.umontreal.ca>
Subject: bug#65051: internal_equal manipulates symbols with position without checking symbols-with-pos-enabled.
Date: Mon, 7 Aug 2023 09:44:55 +0000 [thread overview]
Message-ID: <ZNC9F_4-xUUkyMlf@ACM> (raw)
In-Reply-To: <65A42652-DB4A-4FA6-8ADA-0D5BEB00F54C@gmail.com>
Hello, Mattias.
On Mon, Aug 07, 2023 at 10:58:41 +0200, Mattias Engdegård wrote:
> 6 aug. 2023 kl. 17.02 skrev Alan Mackenzie <acm@muc.de>:
> >>> together with an unsigned integer called the @dfn{position}. These
> >>> -objects are intended for use by the byte compiler, which records in
> >>> -them the position of each symbol occurrence and uses those positions
> >>> -in warning and error messages.
> >>> +objects are stored internally much like vectors
> >> Not sure why we want to say how they are stored here. They can be
> >> stored in bubble memory for all the user cares.
> > The point is, they are _not_ stored in the obarray. Eli specifically
> > asked me to clarify this point, yesterday.
> Oh that part is perfectly fine (thank you), we just don't need to say
> that the sympos objects are stored "like vectors" -- that just confuses
> the reader.
Why not? It's true, and I doubt it will cause confusion. I think we
need to say something positive in that place (since we're following it
with a negative). Perhaps you could suggest an alternative.
> >>> +When @code{symbols-with-pos-enabled} is @code{nil}, any symbols with
> >>> +position continue to exist, but do not behave as symbols, or have the
> >>> +other useful properties outlined in the previous paragraph. @code{eq}
> >>> +returns @code{t} when given identical arguments, and @code{equal}
> >>> +returns @code{t} when given arguments with @code{equal} components.
> >> Since the components are bare symbols and fixnums, equality and
> >> identity for them are equivalent, right?
> > No. If there are two distinct SWPs with the same bare symbol and the
> > same position, they should be equal, but not eq. But the real point is
> > to contrast how equal and eq work when symbols-with-pos-enabled is nil
> > with when it is non-nil.
> I meant that the components of equal sympos objects aren't merely equal
> but identical. (This is a very minor quibble; you can keep the text if
> you like.)
The current proposed text has a more subtle intention. It says that eq
and equal behave just like they always have done for everything when
symbols-with-pos-enabled is nil.
> >> OK. This reduces the number of branches in the hot path for ordinary
> >> (non-sympos) code by one while adding one to sym-pos code, and that
> >> should be a fair trade-off. The new branch should be well-predicted but
> >> is still consuming resources.
> > I did some simple timings on the old and new code, and the new code is
> > not slower.
> This is not easy to measure and details matter, but as I said -- there
> is no reason to believe that your changes should be a regression in the
> important measure, rather the opposite.
Agreed.
> >>> + if (SYMBOL_WITH_POS_P(o1)) /* symbols_with_pos_enabled is false. */
> >>> + return (internal_equal (XSYMBOL_WITH_POS (o1)->sym,
> >>> + XSYMBOL_WITH_POS (o2)->sym,
> >>> + equal_kind, depth + 1, ht)
> >>> + && internal_equal (XSYMBOL_WITH_POS (o1)->pos,
> >>> + XSYMBOL_WITH_POS (o2)->pos,
> >>> + equal_kind, depth + 1, ht));
> >> Why recurse here if the components are a bare symbol and a fixnum,
> >> respectively?
> > Maybe in case they might somehow be something else?
> No, we must be able to assume that internal invariants hold when we
> offer no way for them to be violated. Let's just change the calls to
> BASE_EQ and be done with it.
OK, I think you're right, here, I'll change that.
> >> However we should make an effort to prevent the compiler from
> >> optimising (eq X X) -> t etc, which it is completely entitled to doing,
> >> ....
> > Why? (eq X X) is t in all circumstances, whether X is a symbol, a cons
> > structure, or anything else. What am I missing, here?
> If the compiler transforms (eq foo1 foo1) into t then the test won't
> actually exercise the implementation of `eq`.
Ah! You're talking about the tests. OK. In my tests, I timed (equal a
b) where a and b were variables which were either equal or not.
> >> .... and also test both the interpreted and compiled version of `eq`
> >> and `equal`.
> > They're the same code in both cases. I'm missing something here, too, I
> > think.
> Byte-code doesn't call Feq, it uses its own implementation. They should
> work identically but as we are checking edge cases here we'd better be
> sure about that.
> >> The test bytecomp--eq-symbols-with-pos-enabled already does most of
> >> this for a different reason. Perhaps it can be extended to cover
> >> `equal` as well?
> > I don't have such a test in my repository anywhere. Are you sure you
> > wrote it right?
> It was added in 44d7fd3805.
OK. That commit is recent, then? If so, I'll see it soon.
--
Alan Mackenzie (Nuremberg, Germany).
next prev parent reply other threads:[~2023-08-07 9:44 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-04 14:00 bug#65051: internal_equal manipulates symbols with position without checking symbols-with-pos-enabled Alan Mackenzie
2023-08-04 14:32 ` Eli Zaretskii
2023-08-04 14:59 ` Alan Mackenzie
2023-08-04 15:27 ` Eli Zaretskii
2023-08-04 17:06 ` Alan Mackenzie
2023-08-04 18:01 ` Eli Zaretskii
2023-08-05 10:45 ` Alan Mackenzie
2023-08-05 10:57 ` Eli Zaretskii
2023-08-05 11:52 ` Alan Mackenzie
2023-08-05 12:13 ` Eli Zaretskii
2023-08-05 13:04 ` Alan Mackenzie
2023-08-05 13:13 ` Eli Zaretskii
2023-08-13 16:14 ` Alan Mackenzie
2023-08-05 14:40 ` Mattias Engdegård
2023-08-05 16:59 ` Alan Mackenzie
2023-08-05 17:02 ` Mattias Engdegård
2023-08-05 21:07 ` Alan Mackenzie
2023-08-06 13:37 ` Mattias Engdegård
2023-08-06 15:02 ` Alan Mackenzie
2023-08-07 8:58 ` Mattias Engdegård
2023-08-07 9:44 ` Alan Mackenzie [this message]
2023-08-09 18:45 ` Mattias Engdegård
2023-08-07 3:30 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-08-07 9:20 ` Alan Mackenzie
2023-08-08 2:56 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-08-08 15:33 ` Alan Mackenzie
2023-08-10 3:28 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-08-10 9:14 ` Alan Mackenzie
2023-08-10 14:28 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-08-10 18:35 ` Alan Mackenzie
2023-08-12 5:36 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-08-12 6:10 ` Eli Zaretskii
2023-08-12 18:46 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-08-12 19:10 ` Eli Zaretskii
2023-08-13 15:27 ` Alan Mackenzie
2023-08-12 10:41 ` Alan Mackenzie
2023-08-12 18:07 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-08-13 13:52 ` Alan Mackenzie
2023-08-12 21:59 ` Alan Mackenzie
2023-08-11 0:51 ` Dmitry Gutov
2023-08-11 10:42 ` Alan Mackenzie
2023-08-11 11:18 ` Dmitry Gutov
2023-08-11 12:05 ` Alan Mackenzie
2023-08-11 13:19 ` Dmitry Gutov
2023-08-11 14:04 ` Alan Mackenzie
2023-08-11 18:15 ` Dmitry Gutov
[not found] ` <handler.65051.B.169115764532326.ack@debbugs.gnu.org>
2023-09-04 12:57 ` bug#65051: Acknowledgement (internal_equal manipulates symbols with position without checking symbols-with-pos-enabled.) Alan Mackenzie
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ZNC9F_4-xUUkyMlf@ACM \
--to=acm@muc.de \
--cc=65051@debbugs.gnu.org \
--cc=eliz@gnu.org \
--cc=mattias.engdegard@gmail.com \
--cc=monnier@iro.umontreal.ca \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this external index
https://git.savannah.gnu.org/cgit/emacs.git
https://git.savannah.gnu.org/cgit/emacs/org-mode.git
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.