From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-496756-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 71314 invoked by alias); 9 Sep 2015 14:42:46 -0000
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
Received: (qmail 70756 invoked by uid 48); 9 Sep 2015 14:42:42 -0000
From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/65892] gcc fails to implement N685 aliasing of union members
Date: Wed, 09 Sep 2015 14:42:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 5.0
X-Bugzilla-Keywords: alias
X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.org
X-Bugzilla-Status: SUSPENDED
X-Bugzilla-Resolution:
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags:
X-Bugzilla-Changed-Fields:
Message-ID: <bug-65892-4-AoT6fycsuv@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-65892-4@http.gcc.gnu.org/bugzilla/>
References: <bug-65892-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-SW-Source: 2015-09/txt/msg00734.txt.bz2

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892

--- Comment #16 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Message [c++std-core-20893] on the C++ core reflector on 2011-12-14 supports
the GCC view that a C++ compiler can apply strict aliasing rules to p1->m and
p2->m unless the fact they come from the same object is visible to the
compiler.

<quote>
| Either I'm mistaken, or it is a 'magical property of unions'. If the
| compiler can figure out two objects didn't come form the same union,
| it can assume they do not alias,

Yes.  I believe I filled a core issue regarding this -- coming from
controversy originating from optimizations performed by compilers
today (especially without full program analysis) -- and the resolution
is that if intend to play type punning games with X and Y you better let
the type checker and optimizer see your cards upfront.  Otherwise, you
are on your own.  I can't remember the issue number right now.  In the
end, I think that was the right decision.
</quote>

But I can't find the core issue referred to.