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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/99101] optimization bug with -ffinite-loops
Date: Mon, 15 Feb 2021 10:46:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99101

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #4)
> If there are EH edges, doesn't it fit then the -ffinite-loops description:
> '-ffinite-loops'
>      Assume that a loop with an exit will eventually take the exit and
>      not loop indefinitely.  This allows the compiler to remove loops
>      that otherwise have no side-effects, not considering eventual
>      endless looping as such.
>=20
>      This option is enabled by default at '-O2' for C++ with -std=3Dc++11
>      or higher.
> ?  EH is an exit as well...

Note the exit isn't an EH edge (sorry for misleading), it's a conditional
controlled exit to noreturn std::__throw_bad_cast ():

  <bb 5> [local count: 11633210387]:
  comp_md_.at_eof_ =3D 1;
  _14 =3D std::basic_ostream<char>::operator<< (&cout, 1);
  _15 =3D _14->_vptr.basic_ostream;
  _16 =3D MEM[(long int *)_15 + -24B];
  _17 =3D (sizetype) _16;
  _18 =3D _14 + _17;
  _23 =3D MEM[(const struct __ctype_type * *)_18 + 240B];
  if (_23 =3D=3D 0B)
    goto <bb 6>; [0.04%]
  else
    goto <bb 7>; [99.96%]

  <bb 6> [local count: 4653337]:
  std::__throw_bad_cast ();

and handling this call first in the post-dom DFS when walking not
reverse reachable blocks makes control dependence behave wrong (or
CD-DCEs expectation is).

Note _not_ treating EH or abnormals as exit makes us more conservative
(we consider the loop to be _not_ finite then).

Again, sorry for misleading.=