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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104543] [12 Regression] wrong code at -O3 on
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Date: Tue, 15 Feb 2022 10:02:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104543

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot =
gnu.org

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue is that we call do_rpo_vn without having SSA form updated and thus
we see

<bb 5> [local count: 90350471]:
# c.4_22 =3D PHI <0(4), _40(24)>
_8 =3D a[c.4_22];
_9 =3D _8 ^ 1;
a[c.4_22] =3D _9;
_10 =3D c.4_22 + 1;

<bb 6> [local count: 11166912]:
_16 =3D 3;
_12 =3D b.6_23 + 1;

<bb 22> [local count: 11166912]:
# b.6_33 =3D PHI <_12(20)>

<bb 23> [local count: 90350471]:
# c.4_35 =3D PHI <c.4_22(22)>
_37 =3D a[c.4_35];
_38 =3D _37 ^ 1;
a[c.4_35] =3D _38;
_40 =3D c.4_35 + 1;
if (_40 <=3D 2)
  goto <bb 24>; [89.00%]
else
  goto <bb 25>; [11.00%]

but I think the PHI use in bb 23 will be _10 after updating SSA.  Though
need_ssa_update_p is false - tree_unroll_loop will call update SSA.

Ah, after unroll but before fuse we have

<bb 5> [local count: 90350471]:
# c.4_22 =3D PHI <_10(10), 0(4)>

and

<bb 23> [local count: 90350471]:
# c.4_35 =3D PHI <0(22), _40(24)>

we've changed how CFG copying may alter PHI args.  fuse_loops does

      /* The PHI nodes of the second body (single-argument now)
         need adjustments to use the right values: either directly
         the value of the corresponding PHI in the first copy or
         the one leaving the first body which unrolling did for us.

         See also unroll_jam_possible_p() for further possibilities.  */
      gphi_iterator psi_first, psi_second;
      e =3D single_pred_edge (next->header);
      for (psi_first =3D gsi_start_phis (loop->header),
           psi_second =3D gsi_start_phis (next->header);
           !gsi_end_p (psi_first);
           gsi_next (&psi_first), gsi_next (&psi_second))
        {

that we can iterate PHI args in lock-step this way is not guaranteed.=