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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/108552] Linux i386 kernel 5.14 memory
 corruption for pre_compound_page() when gcov is enabled
Date: Fri, 27 Jan 2023 12:51:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108552

--- Comment #19 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
Some further analysis:

  pretmp_94 =3D __gcov0.prep_compound_page[7];  <--
  _179 =3D pretmp_94 + 1;                       <--
  ivtmp.1725_211 =3D (unsigned long long) _179;

  _135 =3D (unsigned int) nr_pages_11;
  _134 =3D _135 + 4294967294;             <--
  _132 =3D (unsigned long long) _134;     <--
  _89 =3D (unsigned long long) pretmp_94; <--
  _76 =3D _89 + 2;       <-
  _19 =3D _76 + _132;    <-


And the loop exit condition is:

  # ivtmp.1725_77 =3D PHI <ivtmp.1725_69(9), ivtmp.1725_211(8)>
  ...
  ivtmp.1725_69 =3D ivtmp.1725_77 + 1;

And the loop exit condition is:

  if (_19 !=3D ivtmp.1725_69)


So, both ivtmp and _19 are calculated from the value at
__gcov0.prep_compound_page. But as shown in Comment #15, we have two separa=
te
reads from the location, the compiler assumes that the value there is
invariant, which is probably not the case.=