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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug ipa/100112] missed optimization for dead code elimination at
 -O3, -Os (vs. -O1, -O2)
Date: Tue, 20 Apr 2021 08:14:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100112

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|tree-optimization           |ipa
           Keywords|                            |missed-optimization
     Ever confirmed|0                           |1
                 CC|                            |marxin at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
            Version|unknown                     |11.0
   Last reconfirmed|                            |2021-04-20

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
It's very late eliminated at -O1 (DCE7) while with -O3 we still have

  <bb 2> [local count: 1073741824]:
  _2 =3D e;
  _3 =3D _2 + 1;
  e =3D _3;=20
  c.1_4 =3D c;
  if (c.1_4 =3D=3D 0B)
    goto <bb 3>; [48.88%]
  else
    goto <bb 4>; [51.12%]

  <bb 3> [local count: 524845000]:
  foo ();

there.  In the end it's some IPA issue, failing to make 'c' readonly at -O3
vs. -O1.=