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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103300] [12 Regression] wrong code at -O3 on
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Date: Thu, 18 Nov 2021 10:37:19 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103300

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |matz at gcc dot gnu.org

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> (In reply to hubicka from comment #2)
> > Needs -O2  -floop-unroll-and-jam   --param early-inlining-insns=3D14
> > to fail, so I guess it may be issue with unrol-and-jam.
>=20
> The major difference I see between GCC 11 and GCC 12 is how tree-loop-im
> handles the load/store of a and c. In GCC 11, it was an unconditional move
> of the store of a and c while in GCC 12 we get some interesting branches:
>   <bb 9> [local count: 35059055]:
>   # a_lsm.21_25 =3D PHI <_20(D)(6), _15(8)>
>   # a_lsm_flag.22_8 =3D PHI <0(6), 1(8)>
>   # c_lsm.23_22 =3D PHI <0(6), _5(8)>
>   if (c_lsm.23_22 <=3D 2)
>     goto <bb 17>; [94.50%]
>   else
>     goto <bb 10>; [5.50%]
>=20
>   <bb 10> [local count: 1928248]:
>   # a_lsm_flag.22_14 =3D PHI <a_lsm_flag.22_8(9)>
>   # a_lsm.21_28 =3D PHI <a_lsm.21_25(9)>
>   c_lsm.23_27 =3D 3;
>   if (a_lsm_flag.22_14 !=3D 0)
>     goto <bb 11>; [66.67%]
>   else
>     goto <bb 12>; [33.33%]
>=20
>   <bb 11> [local count: 1285499]:
>   c =3D c_lsm.23_27;
>=20
>   <bb 12> [local count: 1285499]:
>   if (a_lsm_flag.22_14 !=3D 0)
>     goto <bb 13>; [66.67%]
>   else
>     goto <bb 14>; [33.33%]
>=20
>   <bb 13> [local count: 856999]:
>   a =3D a_lsm.21_28;
>=20
>   <bb 14> [local count: 1928248]:

That's likely a missed threading / header copying, the stores are condition=
al
now and thus need protecting against store data races.

What unroll-and-jam does is make the inner loop enter always, only consider=
ing
the loop header check for the second iteration and also fails to include the
increment.  That's likely a latent issue, maybe because the latch of the
outer loop is not empty?

Testcase that fails with -O2 -floop-unroll-and-jam:

int a, b[2], c, d, e, f;
int g(int h, int i) { return !i || h && i =3D=3D 1 ? 0 : h % i; }
static void j() {
  while (1)
    while (1) {
      if (d)
      L:
        if (f)
          break;
      if (e)
        goto L;
      return;
    }
}
int main() {
  j();
  for (c =3D 0; c < 3; c++)
    for (a =3D 0; a < 2; a++)
      if (g(0, b[a]++))
        while (1)
          ;
  if (b[1] !=3D 3)
    __builtin_abort();
  return 0;
}

Micha?=