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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug bootstrap/102681] [12 Regression] AArch64 bootstrap failure
Date: Fri, 22 Oct 2021 05:42:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102681

--- Comment #15 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So the major difference comes from mark_stack_region_used.
We have a missing jump thread in ethread.

Before the patch, ethread was able to jump thread all the way through:
  if (_13 !=3D 0)
    goto <bb 3>; [5.50%]
  else
    goto <bb 4>; [94.50%]

  <bb 3> :
  # _22 =3D PHI <0(2)>
  goto <bb 7>; [INV]

  <bb 4> :
  # _18 =3D PHI <1(2)>
  _15 =3D upper_bound.coeffs[0];
  goto <bb 6>; [100.00%]

  <bb 5> :

But after we get:

  <bb 2> :
  _13 =3D upper_bound.coeffs[1];
  if (_13 !=3D 0)
    goto <bb 3>; [5.50%]
  else
    goto <bb 4>; [94.50%]

  <bb 3> :
  # _22 =3D PHI <0(2)>
  goto <bb 5>; [100.00%]

  <bb 4> :
  # _9 =3D PHI <1(2)>
  _15 =3D upper_bound.coeffs[0];

  <bb 5> :
  # _16 =3D PHI <0(3), 1(4)>
  # const_upper_20 =3D PHI <const_upper_21(D)(3), _15(4)>
  if (_16 !=3D 0)
    goto <bb 7>; [INV]
  else
    goto <bb 8>; [INV]

We totally missed the jump threading of 3->5->7 path and the 4->5->8 path.

Aldy,
  Can you look into why there is a missing jump threading there?=