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From: "avieira at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/112282] [14 Regression] wrong code (generated
 code hangs) at -O3 on x86_64-linux-gnu since r14-4777-g88c27070c25309
Date: Mon, 30 Oct 2023 20:21:47 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112282

avieira at gcc dot gnu.org changed:

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

--- Comment #9 from avieira at gcc dot gnu.org ---
So I had a look at this and this is as far as I got.
It seems to get stuck in the 'for (u =3D -22; u < 2; ++u)' loop. It looks l=
ike
the loop IV never gets updated and it keeps looping.

Looking at the codegen it seems that cunroll decides to remove A LOT of code
and there is now:
bb 4:
..
# ivtmp_1055 =3D PHI <ivtmp_1056(24), 24(2)>
..
bb 24:
...
ivtmp_1056 =3D ivtmp_1055 - 1;
  goto <bb 4>; [100.00%]

I've not yet been able to figure out why this happens, the dumps weren't ve=
ry
helpful. So I tried -fdisable-tree-cunroll, it was still failing. So I look=
ed
at the dumps to try and see what was turning this loop into an infinite loop
and vrp2 shows me:
Global Exported: _19 =3D [irange] int [-21, 0]
Folding predicate _19 !=3D 2 to 1

and in the dump before vrp2 we see:
 <bb 4> [local count: 7354175]:
  # u.13_485 =3D PHI <_19(105), -22(3)>
  # u_lsm.72_510 =3D PHI <_19(105), _497(D)(3)>
  # u_lsm_flag.73_235 =3D PHI <1(105), 0(3)>
...
  <bb 27> [local count: 6634488]:
  al =3D{v} {CLOBBER(eol)};
  _19 =3D u.13_485 + 1;
  if (_19 !=3D 2)
    goto <bb 105>; [96.34%]
  else
    goto <bb 28>; [3.66%]

  <bb 105> [local count: 6391666]:
  goto <bb 4>; [100.00%]

Something to point out here, that u_lsm.72_510 seems odd. It is used to set
global 'u', but its initialized with _497(D) which is undefined... So that
itself seems wrong to me too... I'll try and find out what's causing that
codegen next. Maybe that can explain why the irange for _19 is so wrong her=
e.=