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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109491] [13 Regression] Segfault in
 tree-ssa-sccvn.cc:expressions_equal_p()
Date: Thu, 13 Apr 2023 12:07:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109491

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
So we have a NULL op, in this case

(gdb) p *vro1
$4 =3D {opcode =3D TARGET_MEM_REF, clique =3D 0, base =3D 0, reverse =3D 0,=
 align =3D 0,=20
  off =3D {coeffs =3D {-1}}, type =3D <real_type 0x7ffff7309930 float>,=20
  op0 =3D <tree 0x0>, op1 =3D <tree 0x0>, op2 =3D <integer_cst 0x7ffff12cd1=
38>}
(gdb) p *vro2
$5 =3D {opcode =3D TARGET_MEM_REF, clique =3D 0, base =3D 0, reverse =3D 0,=
 align =3D 0,=20
  off =3D {coeffs =3D {-1}}, type =3D <real_type 0x7ffff7309930 float>,=20
  op0 =3D <ssa_name 0x7ffff656f120 8261>, op1 =3D <tree 0x0>,=20
  op2 =3D <integer_cst 0x7ffff6119528>}

I think we're lucky to not hit this more often (good hash!) and unlucky
here (bad hash!).  We used to have

  /* If only one of them is null, they cannot be equal.  */
  if (!e1 || !e2)
    return false;

but r11-4982-g4d6b8d4213376e removed that.  r11-5047-gaaccdb9cec423e fixed
one case where that was previously needed.

Now, for TARGET_MEM_REF a NULL operand is different from an operand
with no effect (a zero) since the addressing mode is different.  So
canonicalization in copy_reference_ops_from_ref is unwanted (we'd use
that representation for PRE insertion for example).

The safest option is to restore the above check.=