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 5AA6538515F4; Wed, 21 Apr 2021 08:35:46 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 5AA6538515F4
From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/100162] missed optimization for dead code
 elimination at -O3 (vs. -O2)
Date: Wed, 21 Apr 2021 08:35:46 +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: rguenth 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: version cc everconfirmed cf_reconfirmed_on
 bug_status keywords
Message-ID: <bug-100162-4-sekVkXMcvv@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-100162-4@http.gcc.gnu.org/bugzilla/>
References: <bug-100162-4@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: Wed, 21 Apr 2021 08:35:46 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100162

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unknown                     |12.0
                 CC|                            |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-04-21
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |missed-optimization

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Optimized by DOM3 which sees the following difference:

-  <bb 2> [local count: 118111601]:
+  <bb 2> [local count: 955630225]:
   b.1_1 =3D b;
-  c[0][b.1_1] =3D 2;
-  c[1][b.1_1] =3D 2;
-  c[2][b.1_1] =3D 2;
-  c[3][b.1_1] =3D 2;
+  _27 =3D (sizetype) b.1_1;
+  _28 =3D _27 * 4;
+  vectp_c.13_26 =3D &c + _28;
+  MEM <vector(4) int> [(int *)vectp_c.13_26] =3D { 2, 2, 2, 2 };
+  vectp_c.12_30 =3D vectp_c.13_26 + 16;
   c[4][b.1_1] =3D 2;
   a =3D 5;
   _5 =3D b.1_1 !=3D 0;
   _6 =3D (int) _5;
-  _8 =3D _6 / 2;
+  _7 =3D c[0][0];
+  _8 =3D _6 / _7;
   if (_8 !=3D 0)

here c[0][b.1_1] takes advantage of get_ref_base_and_extent honoring the kn=
own
array size of [1] while the pointer based access is not constrained this way
which makes matching c[0][0] to *(&c + _28) =3D { 2, 2, 2, 2 } difficult.

The realistic chance is to catch this by improving value-numbering done
on the not unrolled loop earlier:

  <bb 3> [local count: 955630225]:
  # a.3_19 =3D PHI <_2(3), 0(2)>
  c[a.3_19][b.1_1] =3D 2;
  _2 =3D a.3_19 + 1;
  if (_2 <=3D 4)
    goto <bb 3>; [89.00%]
  else
    goto <bb 4>; [11.00%]

  <bb 4> [local count: 118111600]:
  a =3D _2;
  _5 =3D b.1_1 !=3D 0;
  _6 =3D (int) _5;
  _7 =3D c[0][0];

where we could use SCEV & friends to lookup c[0][0] at the c[a.3_19][b.1_1]
definition in vn_reference_lookup_3.

That might also help to look through loop abstraction earlier.=