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 41C01388C031; Wed, 10 Jun 2020 09:08:12 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 41C01388C031
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1591780092;
 bh=c6LwRgNOCuIMjJEp+JKEyWc0+OSdWrsfBD5yntMliRg=;
 h=From:To:Subject:Date:From;
 b=ZcXxUSmFwA6BNESRHs+LQFWQr4YMhmEurmR8iu7BWqvxf7Yo7dmWqFUGGY84jCYph
 qBs+NbDG04Q5dUwTdb02ptKEI1auJCq07l8SZwHvOxVJRSbL6ZLR0MyhtqaMadfDi1
 8Rav/qoG44gEnqJnEDwPLz9Zmkh2O9p0eJpoXQFc=
From: "burnus at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/95622] New: force_output flag on a variable prevents
 optimization / regresses c-c++-common/goacc/kernels-alias-ipa-pta-2.c
Date: Wed, 10 Jun 2020 09:08:12 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: middle-end
X-Bugzilla-Version: 10.0
X-Bugzilla-Keywords: missed-optimization, openacc, openmp
X-Bugzilla-Severity: normal
X-Bugzilla-Who: burnus 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
 keywords bug_severity priority component assigned_to reporter cc
 target_milestone
Message-ID: <bug-95622-4@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: <http://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: <http://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Wed, 10 Jun 2020 09:08:12 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95622

            Bug ID: 95622
           Summary: force_output flag on a variable prevents optimization
                    / regresses
                    c-c++-common/goacc/kernels-alias-ipa-pta-2.c
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization, openacc, openmp
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

The following only shows up as FAIL in the testsuite if  ENABLE_OFFLOAD  is
true, even though it is only a compile-time test.   [Hence, it can make sen=
se
to not only "make check-target-libgomp" with an offloading compiler but also
gcc/.]

Somewhat similar to PR 68773.

The testcase: c-c++-common/goacc/kernels-alias-ipa-pta-2.c
=E2=80=A6
  {
    a[0] =3D 0;
    b[0] =3D 1;
    c[0] =3D a[0];
  }
=E2=80=A6

Without ENABLE_OFFLOAD or before
r11-1075-g1c0fdaf79e3618fd7512608a2e5c62b6b306e9e8 (for PR94848 + PR95551):

  MEM[(unsigned int *)_5] =3D 0;  // a[0] =3D 0
  MEM[(unsigned int *)_4] =3D 1;  // b[0] =3D 1
  MEM[(unsigned int *)_3] =3D 0;  // c[0] =3D a[0]

With r11-1075 + offloading compiler, force_output is on offloading variables
(to ensure they are not optimized away), the last line becomes

  _8 =3D MEM[(unsigned int *)_5];  // _8 =3D a[0]
  MEM[(unsigned int *)_3] =3D _8;  // c[0] =3D _8

Expected: force_output does not affect the optimization. (Especially as "a"
cannot be optimized away as it is also used for "copyout".)

[A very well optimizing compiler could see that nothing uses a/b/c after the
target section and could replace the whole function body by "{ }"=E2=80=A6]=