public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109039] [12/13 Regression] Miscompilation with no_unique_address and bitfields
Date: Thu, 09 Mar 2023 14:56:59 +0000	[thread overview]
Message-ID: <bug-109039-4-fKezCr8roh@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-109039-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109039

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
And there doesn't even have to be any NSDMI, even
struct X {
  short x0 : 7;
  short x1 : 8;
  X () : x0 (1), x1 (2) {}
  int get () { return x0 + x1; }
};

struct S {
  [[no_unique_address]] X x;
  char c;
  S () : c (0) {}
};

S s;

int
main ()
{
  if (s.x.x0 != 1 || s.x.x1 != 2 || s.c != 0)
    __builtin_abort ();
}
is miscompiled since that revision.  With sizeof (s) == 2 there aren't enough
bytes to
fit the 15 bits of bitfields and another byte.  S::x.x0 is on x86_64 at offset
0, low 7
bits, then S::x.x1 next 8 bits, and S::c is at offset 1, so overlapping the
high 7 bits of S::x.x1.

  parent reply	other threads:[~2023-03-09 14:56 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-03-06 10:46 [Bug c++/109039] New: " coillol at yandex dot ru
2023-03-06 14:44 ` [Bug c++/109039] [12/13 Regression] " ppalka at gcc dot gnu.org
2023-03-06 14:59 ` jakub at gcc dot gnu.org
2023-03-09 14:56 ` jakub at gcc dot gnu.org [this message]
2023-03-09 16:57 ` jakub at gcc dot gnu.org
2023-03-09 17:12 ` jakub at gcc dot gnu.org
2023-03-10 19:37 ` cvs-commit at gcc dot gnu.org
2023-03-10 19:45 ` [Bug c++/109039] [12 " jakub at gcc dot gnu.org
2023-03-19  5:30 ` cvs-commit at gcc dot gnu.org
2023-03-20 10:29 ` jakub 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-109039-4-fKezCr8roh@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).