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 EF0803858404; Fri,  9 Sep 2022 08:15:14 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org EF0803858404
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1662711314;
	bh=pKU4t2UnXNHWOyje0yUelWa2WdkQ3RgUrPXi6cStq54=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=xR9ZpNFApK61yr+dz0Ixev3hSyLrWBOJn9r4XEp87o3ok8fyAFjYMxvjFdUtvH0wC
	 gNkbiL4hXbSmC2qExfaRLNcHLgmZ4yMZ0sUWZKwZmVT/K9j2IAZ4hoPRXmDH9aoSuu
	 gQCGaKr/H35AhlmtNAmxaqaU5RMJBeogOoWmG+ao=
From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/106892] Wrong code at -O3 on x86_64-linux-gnu
Date: Fri, 09 Sep 2022 08:15:14 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: wrong-code
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: bug_status cc keywords
Message-ID: <bug-106892-4-vGCKb3EpkB@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-106892-4@http.gcc.gnu.org/bugzilla/>
References: <bug-106892-4@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
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106892

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW
                 CC|                            |rguenth at gcc dot gnu.org
           Keywords|                            |wrong-code

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Li Shaohua from comment #3)
> Yes, I reduced it too much. Here is the new one with return value in g()
> function.
>=20
> a, b, c, d, e;
> f[8];
> g() {
>   while (a)
>     a >>=3D 4;
>   return 0;
> }
> h(i) {
>   if (i >=3D '0')
>     return i - '0';
> }
> j(i) {
>   b =3D 2;
>   for (; g() <=3D 7; b++)
>     if (i) {
>       for (; e <=3D 7; e++) {
>         c =3D 1;
>         for (; c <=3D 7; c++) {
>           d =3D h(b + 48);
>           f[-d + 4] ^=3D 3;
>         }
>       }
>       return;
>     }
> }
> main() {
>   j(1);
>   printf("%d\n", f[2]);
> }

When I apply the same "fix" to h() (add a return 0 or __builtin_unreachable=
 ())
the code works again.

Note I think the missing return stmt isn't reached at runtime.

Disabling either unswitching or loop splitting makes the issue go away,
enabling both ontop of -O2 doesn't trigger it.

We early inline h into j where h looks like

int h (int i)
{
  int _3;

  <bb 2> :
  if (i_1(D) > 47)
    goto <bb 3>; [INV]
  else
    goto <bb 4>; [INV]

  <bb 3> :
  _3 =3D i_1(D) + -48;
  // predicted unlikely by early return (on trees) predictor.
  return _3;

  <bb 4> :
  return;

}

which results in

  b.2_1 =3D b;
  _2 =3D b.2_1 + 48;
  _3 =3D h (_2);
  d =3D _3;

becoming

  b.2_1 =3D b;
  _2 =3D b.2_1 + 48;
  if (_2 > 47)
    goto <bb 6>; [34.00%]
  else
    goto <bb 7>; [66.00%]

  <bb 6> :
  _30 =3D _2 + -48;
  _43 =3D _30;

  <bb 7> :
  # _39 =3D PHI <_43(6), _37(5)>
  _3 =3D _39;
  d =3D _3;

note that the return; case results in _37 to be used which is completely
random.

That said, our handling of an unreachable missing return in the IL is
likely counter productive.

Cleaned up testcase:

int a, b, c, d, e;
int f[8];
int g() {
  while (a)
    a >>=3D 4;
  return 0;
}
int h(int i) {
  if (i >=3D '0')
    return i - '0';
  //__builtin_unreachable ();
}
void j(int i) {
  b =3D 2;
  for (; g() <=3D 7; b++)
    if (i) {
      for (; e <=3D 7; e++) {
        c =3D 1;
        for (; c <=3D 7; c++) {
          d =3D h(b + 48);
          f[-d + 4] ^=3D 3;
        }
      }
      return;
    }
}
int main() {
  j(1);
  if (f[2] !=3D 0)
    __builtin_abort ();
}=