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From: "agriff at tin dot it" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/108299] New: toplevel thread_local variables are not
 initialized if not referenced and initialized at wrong moment when referenced
Date: Thu, 05 Jan 2023 10:51:17 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.2.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108299

            Bug ID: 108299
           Summary: toplevel thread_local variables are not initialized if
                    not referenced and initialized at wrong moment when
                    referenced
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: agriff at tin dot it
  Target Milestone: ---

The standard mandates that thread_local variables at top level must be
initialized in case anything in the compilation unit is referred to. There =
is a
specific note on the fact that they must be initialized even if they're not
used in the program.
Also they must be initialized at thread start or before anything from the
compilation unit is accessed by the thread.

This code however shows that g++ doesn't follow this rule

    #include <stdio.h>

    thread_local int flag =3D ([](){
        printf("HERE!\n");
        return 1;
    })();

    int main() {
        printf("Hello, world.\n");
        return 0;
    }

In this version the thread local initialization is skipped.
Mover if after the printf the statement

    flag;

is added then the initialization is performed, but AFTER the "Hello, world"
message (another violation of the standard).

Trying on godbolt I saw the problem is present even in trunk and in clang
(works as expected in MSVC).

The problems are present both on the main implicit thread and with threads
started explicitly with std::thread.=