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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103202] [12 regression] gcc miscompiles
 ed-1.17 since r12-3876-g4a960d548b7d7d94
Date: Fri, 12 Nov 2021 15:23:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103202

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |aldyh at gcc dot gn=
u.org

--- Comment #8 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
The problem here is this thread:

[5] Registering jump thread: (13, 11) incoming edge;  (11, 10) nocopy;=20

BB11 has this:

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D BB 11 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Imports: n_42=20=20
Exports: n_42=20=20
Relational : (n_45 < n_42)
    <bb 11> [local count: 118111614]:
    # np_43 =3D PHI <np_19(13), &buffer_head(4)>
    # n_42 =3D PHI <m_31(13), addr_14(D)(4)>
    # m_31 =3D PHI <0(13), m_16(4)>
    n_45 =3D n_42 + -1;
    if (n_42 !=3D 0)
      goto <bb 12>; [89.00%]
    else
      goto <bb 10>; [11.00%]

Because of the ordering of the import bitmap, we solve m_31 first to 0.  Th=
en,
when we solve n_42, we think we can use the m_31 in the cache, but the orde=
ring
is wrong.

PHIs must always be done first.  We went through a similar exercise to get
relationals right and somehow missed this.=