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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/99880] [10/11 Regression] ICE in
 maybe_set_vectorized_backedge_value, at tree-vect-loop.c:9161 since
 r10-3711-g69f8c1aef5cdcc54
Date: Tue, 06 Apr 2021 11:11:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99880

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
So we're having a PHI latch value vectorized because it's used by the outer
loop (vect_used_in_outer_by_reduction), but the vect_nested_cycle def PHI
in the inner loop isn't relevant since it's vect_unused_in_scope:

  <bb 5> [local count: 11811160]:
  # g_36 =3D PHI <g_21(11), -8(15)>
  # c_lsm.16_20 =3D PHI <_12(11), c_lsm.16_29(15)>
  # ivtmp_18 =3D PHI <ivtmp_1(11), 1717986920(15)>
  g_21 =3D g_36 + 5;
  vect_cst__28 =3D { 0, 0, 0, 0 };

  <bb 6> [local count: 118111600]:
  # h_14 =3D PHI <10(5), h_24(12)>
  # c_lsm.16_22 =3D PHI <c_lsm.16_20(5), _10(12)>
  # ivtmp_30 =3D PHI <10(5), ivtmp_31(12)>
  h_24 =3D h_14 + -1;
  _7 =3D d.6_6 <=3D c_lsm.16_22;
  _8 =3D (int) _7;
  vect__10.19_33 =3D vect_cst__28;
  _10 =3D 0;
  ivtmp_31 =3D ivtmp_30 - 1;
  if (ivtmp_31 !=3D 0)
    goto <bb 12>; [90.00%]
  else
    goto <bb 9>; [10.00%]

  <bb 12> [local count: 106300440]:
  goto <bb 6>; [100.00%]

  <bb 9> [local count: 11811160]:
  # _27 =3D PHI <_10(6)>
  _12 =3D _27 - e.10_11;

it's the c_lsm.16_22 PHI and the _10 backedge def (note how we failed to
constant propagate the 10 ...).  The fix is simple and hopefully it doesn't
break other stuff.=