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 94CC33835820; Sun,  2 May 2021 11:27:22 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 94CC33835820
From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/100366] spurious warning - std::vector::clear
 followed by std::vector::insert(vec.end(), ...) with -O2
Date: Sun, 02 May 2021 11:27:22 +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: 11.1.0
X-Bugzilla-Keywords: diagnostic, missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: glisse 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: cf_reconfirmed_on everconfirmed component keywords
 bug_status
Message-ID: <bug-100366-4-eFGc3Zh3bn@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-100366-4@http.gcc.gnu.org/bugzilla/>
References: <bug-100366-4@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: Sun, 02 May 2021 11:27:22 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100366

Marc Glisse <glisse at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-05-02
     Ever confirmed|0                           |1
          Component|c++                         |tree-optimization
           Keywords|                            |diagnostic,
                   |                            |missed-optimization
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
Assuming the warning happens during the strlen pass, we are still missing a=
 lot
of optimizations at that point

  if (_6 !=3D _7)
    goto <bb 4>; [70.00%]
  else
    goto <bb 3>; [30.00%]

  <bb 3> [local count: 322122544]:
  _158 =3D _7 - _6;

once VRP2 (2 passes after strlen) replaces _158 with 0 and propagates it, m=
aybe
the code becomes nice enough to avoid confusing this fragile warning (I did=
n't
check).

Before FRE3, we have

  _6 =3D vec_2(D)->D.33506._M_impl.D.32819._M_start;
  _7 =3D vec_2(D)->D.33506._M_impl.D.32819._M_finish;
  if (_6 !=3D _7)
    goto <bb 3>; [70.00%]
  else
    goto <bb 4>; [30.00%]

  <bb 4> [local count: 1073741824]:
  _5 =3D MEM[(char * const &)vec_2(D) + 8];
  MEM[(struct __normal_iterator *)&D.33862] =3D{v} {CLOBBER};
  MEM[(struct __normal_iterator *)&D.33862]._M_current =3D _5;
  __position =3D D.33862;
  _12 =3D MEM[(const char * const &)vec_2(D)];
  _13 =3D MEM[(const char * const &)&__position];
  _14 =3D _13 - _12;

and after FRE3

  <bb 4> [local count: 1073741824]:
  _5 =3D MEM[(char * const &)vec_2(D) + 8];
  MEM[(struct __normal_iterator *)&D.33862] =3D{v} {CLOBBER};
  MEM[(struct __normal_iterator *)&D.33862]._M_current =3D _5;
  __position =3D D.33862;
  _14 =3D _5 - _6;

Only PRE manages to notice that _5 is the same as _7, which is already late.
And it then takes until VRP2 to realize that _7 - _6 must be 0 in the else
branch of _6 !=3D _7.

* I am not sure why FRE manages to optimize _12 and not _5, that seems like=
 the
first thing to check (maybe the +8 means it is obviously "partial")
* I don't know if some other pass than VRP could learn that b-a is 0 if not
a!=3Db.=