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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/112616] [11/12/13/14 Regression] wrong code at -O{s,2,3} on
 x86_64-linux-gnu
Date: Mon, 20 Nov 2023 09:55:30 +0000
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X-Bugzilla-Component: ipa
X-Bugzilla-Version: 13.2.1
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112616

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |jamborm at gcc dot gnu.org
            Version|unknown                     |13.2.1

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
IPA-SRA at work.  With -fno-inline added you see

-long int p ()
+void p.isra ()
 {
...
@@ -106,13 +82,13 @@
   i.2_2 =3D i;
   h.3_3 =3D h;
   *i.2_2 =3D h.3_3;
-  _4 =3D o ();
+  o.isra ();

   <bb 4> [local count: 1073741824]:
-  # q_11 =3D PHI <0B(2), _4(3)>
-  _5 =3D *q_11;
-  _10 =3D (long int) _5;
-  return _10;
+  # q_5 =3D PHI <0B(2), removed_return.17_12(D)(3)>
+  _6 =3D *q_5;
+  _7 =3D (long int) _6;
+  return;


Note the issue is that we end up with the following after inlining:

  <bb 5> [local count: 1073741824]:
  # q_11 =3D PHI <0B(2), removed_return.14_14(D)(4), removed_return.14_14(D=
)(3)>
  _12 =3D *q_11;

and CCP will optimistically simplify q_11 to 0B because accessing the
removed_return.14_14(D) would be undefined behavior.

That means substituting a default-def isn't correct - it exposes undefined
behavior on valid paths of the program.=