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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/85563] [10/11/12/13 regression]
 -Wmaybe-uninitialized false alarm regression with __builtin_unreachable and
 GCC 8
Date: Thu, 30 Mar 2023 13:23:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D85563

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amacleod at redhat dot com,
                   |                            |rguenth at gcc dot gnu.org
   Last reconfirmed|2021-04-15 00:00:00         |2023-3-30

--- Comment #23 from Richard Biener <rguenth at gcc dot gnu.org> ---
Re-confirmed.  On the testcase in comment#6 we have before EVRP

  <bb 2> :
  Vframe_list.0_1 =3D Vframe_list;
  a.1_17 =3D (long int) Vframe_list.0_1;
  _15 =3D (unsigned int) a.1_17;
  _13 =3D _15 & 7;
  _10 =3D _13 =3D=3D 3;
  if (_13 !=3D 3)
    goto <bb 3>; [INV]
  else
    goto <bb 4>; [INV]

  <bb 3> :
  __builtin_unreachable ();

  <bb 4> :
  # frame1_6 =3D PHI <frame1_12(D)(2), frame1_16(5)>
  # tail_7 =3D PHI <Vframe_list.0_1(2), tail_18(5)>
  a.1_19 =3D (long int) tail_7;
  _20 =3D (unsigned int) a.1_19;
  _21 =3D _20 & 7;
  _22 =3D _21 =3D=3D 3;
  if (_21 =3D=3D 3)
    goto <bb 5>; [INV]

  else
    goto <bb 6>; [INV]

  <bb 5> :
  _25 =3D tail_7 + 18446744073709551613;
  _26 =3D __builtin_assume_aligned (_25, 8);
  frame1_16 =3D _26->u.s.car;
  tail_18 =3D _26->u.s.u.cdr;
  goto <bb 4>; [INV]


  <bb 6> :
  do_switch_frame (frame1_6);

and we fail to realize we are never exiting the loop in the first iteration.
The diagnostic happens because we run into loop header copying which
then results in a jump around the loop with an obviously uninitialized
argument to frame1_6.  And that's because we have thrown away the
__builtin_unreachable before loop header copying (in VRP1, after the first
threadfull).

Re-ordering CH before VRP fails to optimize even when I also put a copyprop
between CH and VRP.  We then see

  <bb 2> [local count: 118111600]:
  # PT =3D nonlocal escaped null
  Vframe_list.0_1 =3D Vframe_list;
  a.1_12 =3D (long int) Vframe_list.0_1;
  _10 =3D (unsigned int) a.1_12;
  # RANGE [irange] Lisp_Type [0, 7] NONZERO 0x7
  _8 =3D _10 & 7;
  if (_8 !=3D 3)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [100.00%]

  <bb 3> [count: 0]:
  __builtin_unreachable ();

  <bb 4> [local count: 118111600]:
  a.1_21 =3D (long int) Vframe_list.0_1;
  _22 =3D (unsigned int) a.1_21;
  # RANGE [irange] Lisp_Type [0, 7] NONZERO 0x7
  _23 =3D _22 & 7;
  if (_23 =3D=3D 3)

but for some reason VRP doesn't optimize the second compare.  We'd probably
need to value-number here (loop header copying does, but only parts of the
function and not including this leading block).

So somewhat of a pass ordering issue.  The next CSE is DOM and then PRE.=