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

[Thomas Schwinge <thomas@codesourcery.com>]

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

... 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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