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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/95349] Using std::launder(p) produces unexpected behavior where (p) produces expected behavior
Date: Tue, 16 Jun 2020 06:50:50 +0000 [thread overview]
Message-ID: <bug-95349-4-A1kL1AiC1V@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-95349-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95349
--- Comment #39 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 16 Jun 2020, andrew2085 at gmail dot com wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95349
>
> --- Comment #38 from Andrew Downing <andrew2085 at gmail dot com> ---
> > int *p;
> > int x;
> > if (<condition>)
> > p = &x;
> > else
> > p = malloc (4);
> > memcpy (p, q, 4);
> >
> > there is a single memcpy call and the standard says that both the dynamic
> > type transfers (from q) and that it does not (to x).
>
> I would say just that, that it both does and doesn't transfer the effective
> type. Meaning that you need to be conservative during optimization and consider
> p to alias both int and whatever type q is.
>
> > Note the C++ standard makes the placement new optional. Do you say that
> > your example is incorrect with the placement new elided?
>
> I'm not sure what you mean about the first part about it being optional. It
Somewhere the C++ standard says (or said in some "old" version) that
the lifetime of an object ends when "... or the storage is re-used".
Likewise lifetime of an object starts "when storage with the proper
alignment and size ... is obtained". Back in time when I designed
the way GCC currently works to satisfy placement new and friends I
concluded the safe thing to do is to treat every memory write
as changing the dynamic type of the memory location (because we have
to assume it is re-use of storage).
Thus for types without a non-trivial ctor/dtor you do not need to use
placement new. So take your example and remove the placement new.
Does that change its semantics?
next prev parent reply other threads:[~2020-06-16 6:50 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-26 20:45 [Bug c++/95349] New: " andrew2085 at gmail dot com
2020-05-27 8:04 ` [Bug c++/95349] " rguenth at gcc dot gnu.org
2020-05-27 9:14 ` redi at gcc dot gnu.org
2020-05-27 9:40 ` rguenther at suse dot de
2020-05-27 11:05 ` redi at gcc dot gnu.org
2020-05-27 14:45 ` andrew2085 at gmail dot com
2020-05-27 15:07 ` redi at gcc dot gnu.org
2020-05-27 15:19 ` andrew2085 at gmail dot com
2020-05-27 16:01 ` andrew2085 at gmail dot com
2020-05-29 10:59 ` ed at catmur dot uk
2020-05-29 11:23 ` rguenth at gcc dot gnu.org
2020-05-29 11:32 ` rguenth at gcc dot gnu.org
2020-05-29 13:53 ` ed at catmur dot uk
2020-05-29 14:15 ` redi at gcc dot gnu.org
2020-05-29 14:24 ` rguenther at suse dot de
2020-05-29 15:05 ` andrew2085 at gmail dot com
2020-05-29 18:07 ` richard-gccbugzilla at metafoo dot co.uk
2020-05-29 21:00 ` andrew2085 at gmail dot com
2020-05-29 21:50 ` richard-gccbugzilla at metafoo dot co.uk
2020-05-29 23:13 ` andrew2085 at gmail dot com
2020-05-29 23:25 ` richard-gccbugzilla at metafoo dot co.uk
2020-06-02 12:09 ` rguenth at gcc dot gnu.org
2020-06-02 12:20 ` rguenth at gcc dot gnu.org
2020-06-02 16:00 ` andrew2085 at gmail dot com
2020-06-02 16:23 ` rguenther at suse dot de
2020-06-02 16:34 ` andrew2085 at gmail dot com
2020-06-02 16:37 ` andrew2085 at gmail dot com
2020-06-02 17:54 ` rguenther at suse dot de
2020-06-02 18:43 ` andrew2085 at gmail dot com
2020-06-02 20:53 ` andrew2085 at gmail dot com
2020-06-03 6:52 ` rguenth at gcc dot gnu.org
2020-06-04 0:27 ` andrew2085 at gmail dot com
2020-06-04 6:14 ` rguenther at suse dot de
2020-06-04 16:05 ` andrew2085 at gmail dot com
2020-06-05 6:52 ` rguenth at gcc dot gnu.org
2020-06-05 14:30 ` andrew2085 at gmail dot com
2020-06-15 9:29 ` rguenth at gcc dot gnu.org
2020-06-15 21:45 ` richard-gccbugzilla at metafoo dot co.uk
2020-06-16 3:27 ` andrew2085 at gmail dot com
2020-06-16 6:50 ` rguenther at suse dot de [this message]
2020-06-16 6:57 ` rguenther at suse dot de
2020-06-16 13:56 ` andrew2085 at gmail dot com
2022-01-11 12:43 ` rguenth at gcc dot gnu.org
2022-01-11 12:48 ` rguenth at gcc dot gnu.org
2022-11-14 4:53 ` andrew2085 at gmail dot com
2024-06-03 8:02 ` Christopher.Nerz at de dot bosch.com
2024-06-03 8:51 ` rguenth at gcc dot gnu.org
2024-06-03 9:13 ` Christopher.Nerz at de dot bosch.com
2024-06-03 9:23 ` rguenth at gcc dot gnu.org
2024-06-03 10:26 ` Christopher.Nerz at de dot bosch.com
2024-06-03 11:19 ` rguenther at suse dot de
2024-06-03 15:53 ` redi at gcc dot gnu.org
2024-06-03 16:00 ` redi at gcc dot gnu.org
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