[RFC PATCH] libstdc++: Provide ABI compatibility for _GLIBCXX_HAVE_TLS

Thu Oct 16 19:15:02 GMT 2025

I went travelling for a month on the day you sent the first patch and so I
missed it. I'll take a look ASAP.

On Thu, 16 Oct 2025, 19:39 Keno Fischer, <keno@juliahub.com> wrote:

> Hello all,
>
> My apologies for not following up on the below earlier, but we're getting
> additional user complaints about this ABI break, so it's back on my radar.
> Were there any comments or concerns on my RFC patch below?
>
> Keno
>
> On Tue, Jul 15, 2025 at 1:37 AM Keno Fischer <keno@juliahub.com> wrote:
> >
> > The _GLIBCXX_HAVE_TLS flag is an ABI-breaking flag - setting it removes
> the
> > definitions for `set_lock_ptr` (as well as the comptability symbols for
> > even older ABIs). Try to improve this situation by changing the flag to
> > retain the old symbols, falling back to the previous behavior if
> __once_call
> > is not set. These symbols are used to forward closure data for the
> > initialization
> > from whichever thread ends up winning the pthread_once. The __once_call
> > is set immediately before the pthread_once and unset (in a destructor)
> > immediately after. To avoid the case where there may be a nested
> __once_proxy
> > call that uses the old ABI, we also unset __once_call in the success path
> > after retrieving the pointer.
> >
> > Signed-off-by: Keno Fischer <keno@juliahub.com>
> > ---
> >
> > Greetings from the Julia Language infrastructure team. For background
> > behind this patch, we maintain a large number of pre-built binaries for
> > multiple operating systems, including windows. On windows, we generally
> > follow the MSYS2-provided mingw ABI for our binaries (even though we
> > maintain our own toolchains to actually generate the binaries).
> > At some point recently, msys2 switched to building gcc with --enable-tls.
> > This caused an ABI break for us, making our old binaries no longer
> > work in newer msys2 environments because. I have two distinct questions
> > here:
> >
> > 1. Is there something obviously wrong with this approach that I'm
> >    missing?
> >
> > 2. Is this appropriate for upstream inclusion?
> >
> > We're fine if the answer to #2 is no - in which case we'd ship a patched
> > libstdc++ version with this patch for a while until all binaries that
> > have shipped with the old ABI have filtered out as part of our usual
> > replacement cycle. Of course, if this approach doesn't work, we'd
> > appreciate any thoughts on how to maintain ABI compatibility here, even
> > temporarily.
> >
> > (I think the diffstat looks messier than it is because of the indentation
> > changes - the basic changes is just to merge the two __once_proxy
> > calls together, checking if __once_call is set, using it if so and
> falling
> > back to the old ABI if not).
> >
> >  libstdc++-v3/src/c++11/mutex.cc | 111 +++++++++++++++++---------------
> >  1 file changed, 59 insertions(+), 52 deletions(-)
> >
> > diff --git a/libstdc++-v3/src/c++11/mutex.cc
> b/libstdc++-v3/src/c++11/mutex.cc
> > index d5da5c66ae9..4be64633315 100644
> > --- a/libstdc++-v3/src/c++11/mutex.cc
> > +++ b/libstdc++-v3/src/c++11/mutex.cc
> > @@ -23,6 +23,7 @@
> >  // <http://www.gnu.org/licenses/>.
> >
> >  #include <mutex>
> > +#include <bits/std_function.h>  // std::function
> >
> >  #ifdef _GLIBCXX_HAS_GTHREADS
> >
> > @@ -30,30 +31,17 @@ namespace std _GLIBCXX_VISIBILITY(default)
> >  {
> >  _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >
> > -#ifdef _GLIBCXX_HAVE_TLS
> > -  __thread void* __once_callable;
> > -  __thread void (*__once_call)();
> > +// Explicit instantiation due to -fno-implicit-instantiation.
> > +template class function<void()>;
> >
> > -  extern "C" void __once_proxy()
> > -  {
> > -    // The caller stored a function pointer in __once_call. If it
> requires
> > -    // any state, it gets it from __once_callable.
> > -    __once_call();
> > -  }
> > +function<void()> __once_functor;
> >
> > -#else // ! TLS
> > -
> > -  // Explicit instantiation due to -fno-implicit-instantiation.
> > -  template class function<void()>;
> > -
> > -  function<void()> __once_functor;
> > -
> > -  mutex&
> > -  __get_once_mutex()
> > -  {
> > -    static mutex once_mutex;
> > -    return once_mutex;
> > -  }
> > +mutex&
> > +__get_once_mutex()
> > +{
> > +  static mutex once_mutex;
> > +  return once_mutex;
> > +}
> >
> >  namespace
> >  {
> > @@ -66,41 +54,60 @@ namespace
> >    }
> >  }
> >
> > -  // code linked against ABI 3.4.12 and later uses this
> > -  void
> > -  __set_once_functor_lock_ptr(unique_lock<mutex>* __ptr)
> > -  {
> > -    (void) set_lock_ptr(__ptr);
> > -  }
> > +// code linked against ABI 3.4.12 and later uses this
> > +void
> > +__set_once_functor_lock_ptr(unique_lock<mutex>* __ptr)
> > +{
> > +  (void) set_lock_ptr(__ptr);
> > +}
> >
> > -  // unsafe - retained for compatibility with ABI 3.4.11
> > -  unique_lock<mutex>&
> > -  __get_once_functor_lock()
> > -  {
> > -    static unique_lock<mutex> once_functor_lock(__get_once_mutex(),
> > defer_lock);
> > -    return once_functor_lock;
> > -  }
> > +// unsafe - retained for compatibility with ABI 3.4.11
> > +unique_lock<mutex>&
> > +__get_once_functor_lock()
> > +{
> > +  static unique_lock<mutex> once_functor_lock(__get_once_mutex(),
> defer_lock);
> > +  return once_functor_lock;
> > +}
> >
> > -  // This is called via pthread_once while __get_once_mutex() is locked.
> > -  extern "C" void
> > -  __once_proxy()
> > -  {
> > -    // Get the callable out of the global functor.
> > -    function<void()> callable = std::move(__once_functor);
> > -
> > -    // Then unlock the global mutex
> > -    if (unique_lock<mutex>* lock = set_lock_ptr(nullptr))
> > -    {
> > -      // Caller is using the new ABI and provided a pointer to its lock.
> > -      lock->unlock();
> > +#ifdef _GLIBCXX_HAVE_TLS
> > +  __thread void* __once_callable;
> > +  __thread void (*__once_call)();
> > +#endif
> > +
> > +extern "C" void
> > +__once_proxy()
> > +{
> > +#ifdef _GLIBCXX_HAVE_TLS
> > +    if (__once_call) {
> > +      void (*this_once_call)() = __once_call;
> > +      // Reset __once_call early in case of a nested call to
> __once_proxy
> > +      // with the old ABI.
> > +      __once_call = NULL;
> > +      return this_once_call();
> >      }
> > -    else
> > -      __get_once_functor_lock().unlock();  // global lock
> >
> > -    // Finally, invoke the callable.
> > -    callable();
> > +    // For compatibility with callers linked against
> non-_GLIBCXX_HAVE_TLS ABI,
> > +    // we fall through here to the old behavior.
> > +#endif
> > +
> > +  // If the caller is not using _GLIBCXX_HAVE_TLS, this was called
> > +  // via pthread_once while __get_once_mutex() is locked.
> > +
> > +  // Get the callable out of the global functor.
> > +  function<void()> callable = std::move(__once_functor);
> > +
> > +  // Then unlock the global mutex
> > +  if (unique_lock<mutex>* lock = set_lock_ptr(nullptr))
> > +  {
> > +    // Caller is using the new ABI and provided a pointer to its lock.
> > +    lock->unlock();
> >    }
> > -#endif // ! TLS
> > +  else
> > +    __get_once_functor_lock().unlock();  // global lock
> > +
> > +  // Finally, invoke the callable.
> > +  callable();
> > +}
> >
> >  _GLIBCXX_END_NAMESPACE_VERSION
> >  } // namespace std
> > --
> > 2.43.0
>
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