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 DDDC63840C10; Sat, 27 Jun 2020 15:57:24 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org DDDC63840C10
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1593273444;
 bh=iTlz07goXVzgsu/73wvqcqog+bNyfkEa5mV+ShewHlk=;
 h=From:To:Subject:Date:In-Reply-To:References:From;
 b=hD0l8rvQ8K3qyuB/VvbYwlM2vVPMh0IH0XKQTBLyPENpNY/8lNFo1idZOk30PxwLL
 2ToodQHwxLADFLtxHMlhGj0WSlYCxYb0H6ra8NpEoEpzB4fgQrYtlJvkaK/dHgQ5tI
 pApB+xNY4/EOaufojCDOWqrLs15wk4oeSt1gOV04=
From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/95663] static_cast checks for null even when
 the pointer is dereferenced
Date: Sat, 27 Jun 2020 15:57:24 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 10.1.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jakub at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
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-95663-4-ZVirs3T7dJ@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-95663-4@http.gcc.gnu.org/bugzilla/>
References: <bug-95663-4@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: <http://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: <http://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Sat, 27 Jun 2020 15:57:25 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95663

--- Comment #15 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
In the particular case we are talking about, security/non-security, it does=
n't
make sense to do anything but optimize it into straight line code, any
__builtin_trap or similar will just hurt.  If you feel e.g. by default addi=
ng
__builtin_unreachable is too dangerous in some cases, it can just handle
similar cases manually and optimize away the conditional if there are no
side-effect statements in between.
We are talking about:
  <bb 2> [local count: 1073741824]:
  if (base_2(D) !=3D 0B)
    goto <bb 3>; [70.00%]
  else
    goto <bb 4>; [30.00%]

  <bb 3> [local count: 751619281]:
  iftmp.1_3 =3D base_2(D) + (sizetype)-4;

  <bb 4> [local count: 1073741824]:
  # iftmp.1_1 =3D PHI <iftmp.1_3(3), 0B(2)>
  _5 =3D MEM[(const struct Derived *)iftmp.1_1].D.2340.y;
where without -fno-delete-null-pointer-checks and without -fwrapv-pointer, =
we
can assume: 1) pointers in valid programs don't wrap 2) the first page is n=
ot
mapped
As offsetof (Derived, D.2340.y) is >=3D 4 and < 4096 here, we don't need to=
 even
care about pointer wrapping, just rely on accesses to 0 .. 4095 offsets to
trap.
If the offsetof would be 0, it would be about pointer wrapping, whether we =
are
ok if instead of dereferencing *(int *)0 we dereference *(int *)-4 instead.=