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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/100923] [9/10/11/12 Regression] wrong code at
 -O2 and above on x86_64-linux-gnu
Date: Mon, 07 Jun 2021 10:20:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100923

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
The bug is that PTA computes

  <bb 3> :
  *j_15(D) =3D &l;
  # PT =3D null
  _2 =3D *j_15(D);
  # PT =3D null
  _3 =3D *_2;

and VN (in)correctly value-numbers the later load from 'l' to _3, pulling
the points-to info into the alias test.  PTA does not generate a constraint
for

  *j_15(D) =3D &l;

where *null =3D &l is thrown away early and

  _2 =3D *j_15(D);

_2 =3D *null as well.  So _2 remains empty thus _3 as well.

We thus enter the alias-oracle with a valueized ref that looks like
__MEM (_3) via ao_ref_init_from_vn_reference which fails to consider
availability here.

Not sure why -fno-strict-aliasing "works", it looks like we fail to
value-number the load from 'l' for some reason.

It's going to be a bit awkward to fix, but maybe it's not too bad.=