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 807373858036; Fri,  5 Nov 2021 20:20:18 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 807373858036
From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/102650] [12 Regression] Dead Code Elimination
 Regression at -O3 (trunk vs 11.2.0)
Date: Fri, 05 Nov 2021 20:20:18 +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: 12.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: amacleod at redhat dot com
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: 12.0
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: 
Message-ID: <bug-102650-4-faZkskGvwc@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-102650-4@http.gcc.gnu.org/bugzilla/>
References: <bug-102650-4@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: Fri, 05 Nov 2021 20:20:18 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102650

--- Comment #3 from Andrew Macleod <amacleod at redhat dot com> ---
I figured running ranger as VRP2 would fix this... but alas, there is some
interference :-)

After fre5:
  <bb 4> [local count: 118111600]:
  # prephitmp_24 =3D PHI <1(3), 4(2)>
  d.5_16 =3D d;
  if (d.5_16 <=3D 0)
    goto <bb 5>; [89.00%]
  else
    goto <bb 6>; [11.00%]

  <bb 5> [local count: 955630225]:
  d =3D 1;

  <bb 6> [local count: 118111600]:
  # e_4 =3D PHI <prephitmp_24(5), e_17(D)(4)>
  if (e_4 =3D=3D 0)
    goto <bb 7>; [33.00%]
  else
    goto <bb 8>; [67.00%]

  <bb 7> [local count: 38976828]:
  foo ();

We know prephitmp_24 is [1,1] [4,4], and e_17 is undefined on 4->6, so rang=
er
will evaluate that PHI as [1,1], [4,4] and fold the condition as never true.

Unfortuntely, the next opass is threasd2 and its decides to thread this,
producing:

 <bb 4> [local count: 118111600]:
  # prephitmp_24 =3D PHI <1(3), 4(2)>
  d.5_16 =3D d;
  if (d.5_16 <=3D 0)
    goto <bb 5>; [89.00%]
  else
    goto <bb 6>; [11.00%]

  <bb 5> [local count: 105119324]:
  d =3D 1;
  goto <bb 8>; [100.00%]

  <bb 6> [local count: 12992276]:
  # e_4 =3D PHI <e_17(D)(4)>
  if (e_4 =3D=3D 0)
    goto <bb 7>; [66.33%]
  else
    goto <bb 8>; [33.67%]

  <bb 7> [local count: 38976828]:
  foo ();

  <bb 8> [local count: 118111600]:
  return 0;

now we have a condition if (e_4 =3D=3D 0) and e_4 is UNDEFINED, and so rang=
er
leaves it alone.

Is there any pass that examines branches using undefined values and decides
which way to fold it is most profitable?  probably not.=20=20

Running this with --param=3Dvrp1-mode=3Dranger resolves the problem because=
 we
don't introduce this situation before it's taken care of,a nd we fold the
condition based on the PHI.=