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 497F13858001; Tue, 13 Oct 2020 11:10:31 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 497F13858001
From: "acoplan at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/97400] New: [10/11 Regression] SVE: wrong code since
 r10-3906-g96eb7d7a64
Date: Tue, 13 Oct 2020 11:10:31 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: acoplan 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
 bug_severity priority component assigned_to reporter target_milestone
 attachments.created
Message-ID: <bug-97400-4@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: Tue, 13 Oct 2020 11:10:31 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97400

            Bug ID: 97400
           Summary: [10/11 Regression] SVE: wrong code since
                    r10-3906-g96eb7d7a64
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: acoplan at gcc dot gnu.org
  Target Milestone: ---

Created attachment 49364
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D49364&action=3Dedit
assembly generated at r10-3906

AArch64 GCC miscompiles the following testcase:

int a[256];
int c, f;
int *d[10];
int main(void)
{
  for (; f < 256; f++) {
    a[f] =3D c =3D 9;
    for (; c >=3D 0; c--) {
      d[c] =3D 0;
    }
  }
  return a[255];
}

with -O3 -march=3Darmv8.2-a+sve since
r10-3906-g96eb7d7a642085f651e9940f0ee75568d7c4441d7. The program should exit
with status code 9 but instead exits with status code 0.

The program produces the correct result at
r10-3681-g3faf75d458529592007436a0972f44e14ebf46f6, but between these two
revisions, GCC ICEs on this input, so the bad commit lies somewhere in betw=
een
these.

To reproduce the issue back to r10-3906, you need to add -fno-common to the
command line (this became the default in GCC 10).

Examining the broken assembly code, it appears that the scalar epilogue for=
 the
inner loop tramples backwards through d into the end of a:

.L8:
        add     x1, x4, 1032          // x1 =3D &d[0]
        sub     w5, w0, #1            // w5 =3D -1
        sub     w13, w0, #2
        sub     w12, w0, #3
        sub     w11, w0, #4
        sub     w9, w0, #5
        str     xzr, [x1, w0, sxtw 3]
        sub     w8, w0, #6
        str     xzr, [x1, w5, sxtw 3] // incorrectly sets a[255] =3D 0
[...]

The layout of .bss here is:

f (4 bytes) | padding (4 bytes) | a (1024 bytes) | d (80 bytes) | c (4 byte=
s)

I've attached the broken assembly code generated by GCC at
r10-3906-g96eb7d7a642085f651e9940f0ee75568d7c4441d7.=