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 657AC385E009; Fri, 27 Mar 2020 18:27:58 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 657AC385E009
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1585333678;
 bh=pE2Rk9OgnN4JThQf4TxnSPp2jWA2p4zIfAOmekwnS+M=;
 h=From:To:Subject:Date:From;
 b=UaNNtZ00BX/FcIb3R6DfTZeK2yF2IGR/mFDF8SBn2ja210p1hw7uNA5yZTpvltRbZ
 uQPf/mVC7g2vLZSGFaOcmjFA0EVSw51TdCIk6IDof1LZYmDoKnaWochHw7lAamPNRI
 i5C5ZI7FuhJXpbVEBzdS98lYzVvpG2fuTW9RtFzI=
From: "satzeni at nvidia dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/94366] New: OpenMP Parallel Reduction with "&&" operator does
 not compute correct result
Date: Fri, 27 Mar 2020 18:27:58 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 9.3.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: satzeni at nvidia dot com
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
 attachments.created
Message-ID: <bug-94366-4@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: <http://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: Fri, 27 Mar 2020 18:27:58 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94366

            Bug ID: 94366
           Summary: OpenMP Parallel Reduction with "&&" operator does not
                    compute correct result
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: satzeni at nvidia dot com
  Target Milestone: ---

Created attachment 48137
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D48137&action=3Dedit
reproducer

Consider this program:

#include <assert.h>
#include <omp.h>
#include <stdio.h>

static int expect =3D 1;
static int result;

int main() {
    int a =3D 2;  /* or any another even number */

#pragma omp parallel reduction(&& : a)
    {
        a =3D a && 1;
    }
    result =3D a ? 1 : 0;
    assert(result =3D=3D 1);
}

Compiler and run with:

$ gcc -fopenmp test.c
$ OMP_NUM_THREADS=3D2 ./a.out

GCC is not correctly computing the logical-and reduction on variable "a".

Inside the parallel region "a" value is 1, but right after the parallel reg=
ion
is 0 failing the assertion. The correct result should be 1 as the initial v=
alue
is 2 and the logical-and between 2 && 1 should be non zero.

Looking at the assembly code, GCC is using a bitwise =E2=80=9Cand=E2=80=9D =
instruction to
perform the =E2=80=9C&&=E2=80=9D operation:

<main._omp_fn.0+26>    mov    (%rbx),%edx
<main._omp_fn.0+28>    jmp    0x400782 <main._omp_fn.0+34>
<main._omp_fn.0+30>    xchg   %ax,%ax
<main._omp_fn.0+32>    mov    %eax,%edx
<main._omp_fn.0+34>    mov    %edx,%ecx
<main._omp_fn.0+36>    mov    %edx,%eax
<main._omp_fn.0+38>    and    $0x1,%ecx
<main._omp_fn.0+41>    lock cmpxchg %ecx,(%rbx)

However the the value of =E2=80=9Ca=E2=80=9D is not reduced to =E2=80=9C1=
=E2=80=9D before it is passed to the
parallel region.

<main+15>              movl   $0x2,(%rsp)
<main+22>              callq  0x4005c0 <GOMP_parallel_start@plt>=