Hi Jonathan,
Just wanted to gently ping to see if you've had a chance to take a look.
Thanks,
Keno
On Thu, Oct 16, 2025 at 3:15 PM Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
> 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