public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "ed at catmur dot uk" <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: Fri, 29 May 2020 13:53:55 +0000	[thread overview]
Message-ID: <bug-95349-4-vnbecwAJ1i@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 #12 from Ed Catmur <ed at catmur dot uk> ---
(In reply to Richard Biener from comment #11)
> Note that for C++ types you can apply memcpy to the placement new is not
> needed since object re-use terminates lifetime of the previous object and
> starts lifetime of a new one.

Under P0593R6 it has the effect of implicitly creating objects on demand.
Effectively it is supposed to "curse" the double and "bless" the subsequent
uint64_t. Invoking P0593 may be jumping the gun since it's still in LWG, but
Richard (Smith) wants it retroactively applied to C++20 IS as a DR, and that
could still happen.

> Note that while your example performs memcpy dances you are probably
> after a solution that elides all generated code?

Sure, I assume that memcpy of anything smaller than a page will be elided :)

> Note that I do not belive making your examples work as you intend is
> possible in an actual implementation without sacrifying all
> type-based alias analysis.

Ouch. You might be asked to if and when P0593 goes in (again, assuming I've
understood it correctly). Would it be appropriate to find out what Ville
thinks?

  parent reply	other threads:[~2020-05-29 13:53 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 [this message]
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
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

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=bug-95349-4-vnbecwAJ1i@http.gcc.gnu.org/bugzilla/ \
    --to=gcc-bugzilla@gcc.gnu.org \
    --cc=gcc-bugs@gcc.gnu.org \
    /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. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).