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From: "zhendong.su at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102830] New: wrong code at -Os and above on
 x86_64-linux-gnu
Date: Tue, 19 Oct 2021 08:35:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102830

            Bug ID: 102830
           Summary: wrong code at -Os and above on x86_64-linux-gnu
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhendong.su at inf dot ethz.ch
  Target Milestone: ---

It appears to be a regression from GCC 8.* and affect GCC 9.* and later.
At -O3, the generated code segfaults, while at -Os and -O2, it hangs.


[655] % gcctk -v
Using built-in specs.
COLLECT_GCC=3Dgcctk
COLLECT_LTO_WRAPPER=3D/local/suz-local/software/local/gcc-trunk/libexec/gcc=
/x86_64-pc-linux-gnu/12.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --disable-bootstrap
--prefix=3D/local/suz-local/software/local/gcc-trunk --enable-languages=3Dc=
,c++
--disable-werror --enable-multilib --with-system-zlib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.0.0 20211019 (experimental) [master r12-4491-gf45610a4523] (=
GCC)=20
[656] %=20
[656] % gcctk -O0 small.c; ./a.out
[657] %=20
[657] % gcctk -O3 small.c
[658] % ./a.out
Segmentation fault
[659] %=20
[659] % cat small.c
int main() {
  unsigned a =3D 0, b;
  for (; a < 2; a++) {
    b =3D ~(~a - 1 - a);
    if (a > b)
      break;
  }
  return 0;
}=