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From: "law at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/96015] [10/11 Regression] gcc-10.1.0 miscompiles Python
 on hppa
Date: Wed, 01 Jul 2020 16:35:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96015

--- Comment #12 from Jeffrey A. Law <law at redhat dot com> ---
The block in question goes away because it serves no purpose:

  <bb 5> [local count: 242478389]:
  _13 =3D *self_11(D);
  _16 =3D *other_12(D);
  sign_17 =3D _13 - _16;
  if (sign_17 =3D=3D 0)
    goto <bb 13>; [34.00%]
  else
    goto <bb 6>; [66.00%]

  <bb 6> [local count: 160035736]:
  goto <bb 13>; [100.00%]


Note that bb6 just transfers control to bb13 with no other side effects.  A=
s a
result bb5 is equivalent to:

  <bb 5> [local count: 242478389]:
  _13 =3D *self_11(D);
  _16 =3D *other_12(D);
  sign_17 =3D _13 - _16;
  if (sign_17 =3D=3D 0)
    goto <bb 13>; [34.00%]
  else
    goto <bb 13>; [66.00%]


With both arms of the conditional going to the same place and no other uses=
 of
sign_17 the whole block just turns into

  goto <bb13>;

I see nothing wrong with what was done by DCE.  The problem must be earlier=
 in
the optimizer pipeline.=