Re: [PATCH] libsupc++: try cxa_thread_atexit_impl at runtime

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Wed, 6 Dec 2023 at 13:53, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On Wed, 6 Dec 2023 at 12:30, Thomas Schwinge <thomas@codesourcery.com> wrote:
> >
> > Hi Alexandre!
> >
> > On 2023-12-06T02:28:42-0300, Alexandre Oliva <oliva@adacore.com> wrote:
> > > libsupc++: try cxa_thread_atexit_impl at runtime
> > >
> > > g++.dg/tls/thread_local-order2.C fails when the toolchain is built for
> > > a platform that lacks __cxa_thread_atexit_impl, even if the program is
> > > built and run using that toolchain on a (later) platform that offers
> > > __cxa_thread_atexit_impl.
> > >
> > > This patch adds runtime testing for __cxa_thread_atexit_impl on select
> > > platforms (GNU variants, for starters) that support weak symbols.
> >
> > Need something like:
> >
> >     --- libstdc++-v3/libsupc++/atexit_thread.cc
> >     +++ libstdc++-v3/libsupc++/atexit_thread.cc
> >     @@ -164,2 +164,4 @@ __cxxabiv1::__cxa_thread_atexit (void (_GLIBCXX_CDTOR_CALLABI *dtor)(void *),
> >          return __cxa_thread_atexit_impl (dtor, obj, dso_handle);
> >     +#else
> >     +  (void) dso_handle;
> >      #endif
>
> I would prefer:
>
> --- a/libstdc++-v3/libsupc++/atexit_thread.cc
> +++ b/libstdc++-v3/libsupc++/atexit_thread.cc
> @@ -148,7 +148,7 @@ __cxa_thread_atexit_impl (void
> (_GLIBCXX_CDTOR_CALLABI *func) (void *),
>  // ??? We can't make it an ifunc, can we?
>  extern "C" int
>  __cxxabiv1::__cxa_thread_atexit (void (_GLIBCXX_CDTOR_CALLABI *dtor)(void *),
> -                                void *obj, void *dso_handle)
> +                                void *obj, [[maybe_unused]] void *dso_handle)
>    _GLIBCXX_NOTHROW
>  {
>  #if __GXX_WEAK__


The patch is OK with that change.


>
>
>
>
> >
> > ... to avoid:
> >
> >     [...]/source-gcc/libstdc++-v3/libsupc++/atexit_thread.cc: In function ‘int __cxxabiv1::__cxa_thread_atexit(void (*)(void*), void*, void*)’:
> >     [...]/source-gcc/libstdc++-v3/libsupc++/atexit_thread.cc:151:51: error: unused parameter ‘dso_handle’ [-Werror=unused-parameter]
> >       151 |                                  void *obj, void *dso_handle)
> >           |                                             ~~~~~~^~~~~~~~~~
> >     cc1plus: all warnings being treated as errors
> >     make[4]: *** [atexit_thread.lo] Error 1
> >
> > With that, GCC/nvptx then is back to:
> >
> >     UNSUPPORTED: g++.dg/tls/thread_local6.C  -std=c++98
> >     PASS: g++.dg/tls/thread_local6.C  -std=c++14 (test for excess errors)
> >     PASS: g++.dg/tls/thread_local6.C  -std=c++14 execution test
> >     PASS: g++.dg/tls/thread_local6.C  -std=c++17 (test for excess errors)
> >     PASS: g++.dg/tls/thread_local6.C  -std=c++17 execution test
> >     PASS: g++.dg/tls/thread_local6.C  -std=c++20 (test for excess errors)
> >     PASS: g++.dg/tls/thread_local6.C  -std=c++20 execution test
> >
> >
> > Grüße
> >  Thomas
> >
> >
> > > for  libstdc++-v3/ChangeLog
> > >
> > >       * config/os/gnu-linux/os_defines.h
> > >       (_GLIBCXX_MAY_HAVE___CXA_THREAD_ATEXIT_IMPL): Define.
> > >       * libsupc++/atexit_thread.cc [__GXX_WEAK__ &&
> > >       _GLIBCXX_MAY_HAVE___CXA_THREAD_ATEXIT_IMPL]
> > >       (__cxa_thread_atexit): Add dynamic detection of
> > >       __cxa_thread_atexit_impl.
> > > ---
> > >  libstdc++-v3/config/os/gnu-linux/os_defines.h |    5 +++++
> > >  libstdc++-v3/libsupc++/atexit_thread.cc       |   23 ++++++++++++++++++++++-
> > >  2 files changed, 27 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/libstdc++-v3/config/os/gnu-linux/os_defines.h b/libstdc++-v3/config/os/gnu-linux/os_defines.h
> > > index 87317031fcd71..a2e4baec069d5 100644
> > > --- a/libstdc++-v3/config/os/gnu-linux/os_defines.h
> > > +++ b/libstdc++-v3/config/os/gnu-linux/os_defines.h
> > > @@ -60,6 +60,11 @@
> > >  # define _GLIBCXX_HAVE_FLOAT128_MATH 1
> > >  #endif
> > >
> > > +// Enable __cxa_thread_atexit to rely on a (presumably libc-provided)
> > > +// __cxa_thread_atexit_impl, if it happens to be defined, even if
> > > +// configure couldn't find it during the build.
> > > +#define _GLIBCXX_MAY_HAVE___CXA_THREAD_ATEXIT_IMPL 1
> > > +
> > >  #ifdef __linux__
> > >  // The following libpthread properties only apply to Linux, not GNU/Hurd.
> > >
> > > diff --git a/libstdc++-v3/libsupc++/atexit_thread.cc b/libstdc++-v3/libsupc++/atexit_thread.cc
> > > index 9346d50f5dafe..aa4ed5312bfe3 100644
> > > --- a/libstdc++-v3/libsupc++/atexit_thread.cc
> > > +++ b/libstdc++-v3/libsupc++/atexit_thread.cc
> > > @@ -138,11 +138,32 @@ namespace {
> > >    }
> > >  }
> > >
> > > +#if __GXX_WEAK__ && _GLIBCXX_MAY_HAVE___CXA_THREAD_ATEXIT_IMPL
> > > +extern "C"
> > > +int __attribute__ ((__weak__))
> > > +__cxa_thread_atexit_impl (void (_GLIBCXX_CDTOR_CALLABI *func) (void *),
> > > +                       void *arg, void *d);
> > > +#endif
> > > +
> > > +// ??? We can't make it an ifunc, can we?
> > >  extern "C" int
> > >  __cxxabiv1::__cxa_thread_atexit (void (_GLIBCXX_CDTOR_CALLABI *dtor)(void *),
> > > -                              void *obj, void */*dso_handle*/)
> > > +                              void *obj, void *dso_handle)
> > >    _GLIBCXX_NOTHROW
> > >  {
> > > +#if __GXX_WEAK__ && _GLIBCXX_MAY_HAVE___CXA_THREAD_ATEXIT_IMPL
> > > +  if (__cxa_thread_atexit_impl)
> > > +    // Rely on a (presumably libc-provided) __cxa_thread_atexit_impl,
> > > +    // if it happens to be defined, even if configure couldn't find it
> > > +    // during the build.  _GLIBCXX_MAY_HAVE___CXA_THREAD_ATEXIT_IMPL
> > > +    // may be defined e.g. in os_defines.h on platforms where some
> > > +    // versions of libc have a __cxa_thread_atexit_impl definition,
> > > +    // but whose earlier versions didn't.  This enables programs build
> > > +    // by toolchains compatible with earlier libc versions to still
> > > +    // benefit from a libc-provided __cxa_thread_atexit_impl.
> > > +    return __cxa_thread_atexit_impl (dtor, obj, dso_handle);
> > > +#endif
> > > +
> > >    // Do this initialization once.
> > >    if (__gthread_active_p ())
> > >      {
> > -----------------
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>