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 E93313857C69; Mon,  5 Apr 2021 20:15:29 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org E93313857C69
From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/99918] New: suboptimal code for bool bitfield
 tests
Date: Mon, 05 Apr 2021 20:15:29 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: msebor 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: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-99918-4@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: Mon, 05 Apr 2021 20:15:30 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99918

            Bug ID: 99918
           Summary: suboptimal code for bool bitfield tests
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC does a better job folding operations involving plain Booleans than it d=
oes
with bool bit-fields.  The example below shows that in f() the return state=
ment
is folded to zero while in g() it's not.  This is behind a class of
-Wmaybe-uninitialized warnings.

$ cat z.c && gcc -O2 -S -Wall -fdump-tree-optimized=3D/dev/stdout z.c
struct A { _Bool i, j; };

_Bool f (struct A a)
{
  if (a.i)
    a.j =3D 0;
  else
    a.j =3D a.i;

  return a.j;    // folded to 0
}

struct B { _Bool i: 1, j: 1; };

_Bool g (struct B b)
{
  if (b.i)
    b.j =3D 0;
  else
    b.j =3D b.i;

  return b.j;    // not folded
}


;; Function f (f, funcdef_no=3D0, decl_uid=3D1946, cgraph_uid=3D1, symbol_o=
rder=3D0)

_Bool f (struct A a)
{
  <bb 2> [local count: 1073741824]:
  return 0;

}



;; Function g (g, funcdef_no=3D1, decl_uid=3D1953, cgraph_uid=3D2, symbol_o=
rder=3D1)

Removing basic block 5
_Bool g (struct B b)
{
  _Bool b$j;
  unsigned char _1;
  unsigned char _2;
  _Bool _3;

  <bb 2> [local count: 1073741824]:
  _1 =3D VIEW_CONVERT_EXPR<unsigned char>(b);
  _2 =3D _1 & 1;
  if (_2 !=3D 0)
    goto <bb 4>; [50.00%]
  else
    goto <bb 3>; [50.00%]

  <bb 3> [local count: 536870913]:
  _3 =3D b.i;

  <bb 4> [local count: 1073741824]:
  # b$j_5 =3D PHI <0(2), _3(3)>
  return b$j_5;

}=