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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109164] wrong code with thread_local reference, loops and
 -ftree-pre
Date: Fri, 17 Mar 2023 10:12:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109164

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|tree-optimization           |c++
                 CC|                            |jason at gcc dot gnu.org

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
So we have

;; Function Struct& _ZTW16thread_local_ref() (null)
;; enabled by -tree-original


if (_ZTH16thread_local_ref !=3D 0B)
  {
    <<cleanup_point <<< Unknown tree: expr_stmt
      _ZTH16thread_local_ref () >>>>>;
  }
return <retval> =3D thread_local_ref;

which is eventually inlined.  But 'thread_local_ref' is TREE_READONLY and
thus may_be_aliased is false and the readonly-global exception triggers:

  /* If the reference is based on a decl that is not aliased the call
     cannot possibly clobber it.  */
  if (DECL_P (base)
      && !may_be_aliased (base)
      /* But local non-readonly statics can be modified through recursion
         or the call may implement a threading barrier which we must
         treat as may-def.  */
      && (TREE_READONLY (base)
          || !is_global_var (base)))
    return false;

I think it's a mistake for the frontend to set TREE_READONLY on the
decl if it requires initialization (we've been there for other runtime
initialized globals).

Thus -> C++ FE bug.=