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 3E3853850432; Thu, 18 Mar 2021 11:29:52 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 3E3853850432
From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/99588] variable set but not used warning on static _Atomic
 assignment
Date: Thu, 18 Mar 2021 11:29:52 +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: 11.0
X-Bugzilla-Keywords: diagnostic
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jakub at gcc dot gnu.org
X-Bugzilla-Status: NEW
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: cc
Message-ID: <bug-99588-4-5dpbt3Y0cI@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-99588-4@http.gcc.gnu.org/bugzilla/>
References: <bug-99588-4@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, 18 Mar 2021 11:29:52 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99588

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
More complete testcase:
void bar (int, ...);
void f1 (void) { static _Atomic int x =3D 0; bar (0, x); }
void f2 (void) { static _Atomic int x =3D 0; bar (0, x +=3D 1); }
void f3 (void) { static _Atomic int x =3D 0; bar (x); }
void f4 (void) { static _Atomic int x =3D 0; bar (x +=3D 1); }
void f5 (void) { static _Atomic int x =3D 0; bar (x =3D 1); }
void f6 (void) { static _Atomic int x =3D 0; x =3D 1; }
void f7 (void) { static _Atomic int x =3D 0; x +=3D 3; }
void f8 (void) { _Atomic int x =3D 0; bar (0, x); }
void f9 (void) { _Atomic int x =3D 0; bar (0, x +=3D 1); }
void f10 (void) { _Atomic int x =3D 0; bar (x); }
void f11 (void) { _Atomic int x =3D 0; bar (x +=3D 1); }
void f12 (void) { _Atomic int x =3D 0; bar (x =3D 1); }
void f13 (void) { _Atomic int x =3D 0; x =3D 1; }
void f14 (void) { _Atomic int x =3D 0; x +=3D 3; }

With -D_Atomic=3D we warn on f6 and f13 and nothing else,
otherwise on everything but f1, f3, f8, f10.
The reason for not warning on the f{1,3,8,10} is
convert_lvalue_to_rvalue:
      /* EXPR is always read.  */
      mark_exp_read (exp.value);
So for consistency with the non-_Atomic x op=3D expr at least should
mark_exp_read too in build_atomic_assign (i.e. something like:
  if (modifycode !=3D NOP_EXPR)
    mark_exp_read (lhs);
somewhere early.  That results in warning on f{5,6,12,13}, f5 and f12 still
being false positives.=