From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 6CB743851C1C; Thu, 15 Apr 2021 21:39:10 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 6CB743851C1C
From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/85563] [8/9/10/11 regression] -Wmaybe-uninitialized
 false alarm regression with __builtin_unreachable and GCC 8
Date: Thu, 15 Apr 2021 21:39:10 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: middle-end
X-Bugzilla-Version: 8.0.1
X-Bugzilla-Keywords: deferred, diagnostic, missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: msebor at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P2
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: 11.0
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: keywords cf_reconfirmed_on cc cf_known_to_fail
Message-ID: <bug-85563-4-ltsg1KVtWU@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-85563-4@http.gcc.gnu.org/bugzilla/>
References: <bug-85563-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Thu, 15 Apr 2021 21:39:10 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D85563

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
   Last reconfirmed|2018-04-30 00:00:00         |2021-4-15
                 CC|                            |msebor at gcc dot gnu.org
      Known to fail|8.0.1                       |10.2.0, 11.0, 8.3.0, 9.3.0

--- Comment #18 from Martin Sebor <msebor at gcc dot gnu.org> ---
Reconfirming with GCC 11.

Inverting the reachability expression:

  ((CONSP (Vframe_list)) ? (void) 0 : __builtin_unreachable ());

to

  (!(CONSP (Vframe_list)) ? __builtin_unreachable () : (void) 0);

or changing it to a corresponding if statement avoids the warning and impro=
ves
the emitted code, implying there is a missed optimization opportunity there
somewhere.

I also reduced the test from comment #6 a bit further to make it easier to
read:

struct A
{
  struct A *p, *q;
};

extern void *p;

inline _Bool
f (void* a)
{
  return ((__INTPTR_TYPE__) (a) & ~(- (1 << 3))) =3D=3D 3;
}

inline struct A *
g (void* a)
{
  return (struct A *)((char *)a - 3);
}

extern void* foo (void*);

void bar (void)
{
#if NOWARN
  if (!f (p))
    __builtin_unreachable ();
#else
  f (p) ? (void)0 : __builtin_unreachable ();
#endif

  void* q;
  for (void *r =3D p; f (r) && ((q =3D g (r)->p), 1); r =3D g (r)->q)
    ;

  foo (q);
}=