From: "Mattias Engdegård" <mattias.engdegard@gmail.com>
To: Eli Zaretskii <eliz@gnu.org>
Cc: 65491@debbugs.gnu.org, monnier@iro.umontreal.ca
Subject: bug#65491: [PATCH] Improve performance allocating vectors
Date: Sat, 16 Sep 2023 19:03:33 +0200 [thread overview]
Message-ID: <DB1F1173-0C57-45A6-AE7E-F5D156F9103C@gmail.com> (raw)
In-Reply-To: <83sf7egitx.fsf@gnu.org>
16 sep. 2023 kl. 18.54 skrev Eli Zaretskii <eliz@gnu.org>:
> It does, but LISP_WORD_TAG(type) is a 64=bit type with the only bits
> set above 32 bit, so how casting it to uintptr_t is TRT?
Because XUNTAG is used to get the pointer part; we don't want the tag bits.
> Why did you need to change the original cast in the first place?
The commit message tried to explain that, but in essence, the old code untagged a Lisp_Object value by casting it to char *, then do pointer arithmetic on that, and then cast the result to whatever pointer we want.
The C standard severely restricts pointer arithmetic: in particular, for P+X where X is an integer and P is a pointer, P cannot be null (nor will P+X, since both P and P+X must be pointers to objects in the same array).
This means that XUNTAG could never reliably untag a null pointer and this did cause mayhem in some places. We have just been lucky not to trigger it so far but I noticed when attempting to make some innocent-looking changes.
next prev parent reply other threads:[~2023-09-16 17:03 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-24 9:59 bug#65491: [PATCH] Improve performance allocating vectors Ihor Radchenko
2023-08-26 7:14 ` Eli Zaretskii
2023-08-26 7:27 ` Ihor Radchenko
2023-08-26 7:31 ` Eli Zaretskii
2023-08-26 7:51 ` Ihor Radchenko
2023-08-26 8:07 ` Ihor Radchenko
2023-08-26 9:01 ` Eli Zaretskii
2023-08-26 7:47 ` Ihor Radchenko
2023-08-26 12:01 ` Mattias Engdegård
2023-08-26 14:54 ` Ihor Radchenko
2023-08-26 14:55 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-08-27 9:54 ` Mattias Engdegård
2023-09-16 14:58 ` Mattias Engdegård
2023-09-16 16:12 ` Eli Zaretskii
2023-09-16 16:17 ` Eli Zaretskii
2023-09-16 16:32 ` Mattias Engdegård
2023-09-16 16:54 ` Eli Zaretskii
2023-09-16 17:03 ` Mattias Engdegård [this message]
2023-09-16 17:11 ` Eli Zaretskii
2023-09-17 3:02 ` Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-09-17 17:02 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-09-18 2:19 ` Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-09-18 2:27 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-09-18 3:08 ` Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-09-18 4:10 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-09-16 16:54 ` Mattias Engdegård
2023-09-16 17:09 ` Eli Zaretskii
2023-09-16 17:22 ` Mattias Engdegård
2023-09-16 18:19 ` Eli Zaretskii
2023-09-16 19:04 ` Mattias Engdegård
2023-09-16 19:46 ` Paul Eggert
2023-09-17 5:18 ` Eli Zaretskii
2023-09-17 15:22 ` Paul Eggert
2023-09-17 16:15 ` Eli Zaretskii
2023-09-17 16:37 ` Paul Eggert
2023-09-17 16:44 ` Eli Zaretskii
2023-09-18 16:10 ` Mattias Engdegård
2023-09-18 17:13 ` Eli Zaretskii
2023-09-19 13:28 ` Mattias Engdegård
2023-09-19 14:04 ` Eli Zaretskii
2023-09-19 14:05 ` Mattias Engdegård
2023-09-25 16:06 ` Mattias Engdegård
2023-08-27 16:21 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-08-28 10:14 ` Ihor Radchenko
2023-08-28 16:32 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-08-28 12:47 ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
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