[gcc r14-6261] libsupc++: try cxa_thread_atexit_impl at runtime

[Alexandre Oliva <aoliva@gcc.gnu.org>]

https://gcc.gnu.org/g:3d0f3382fa7b5677f35a9becf75ac436cd7eda7b

commit r14-6261-g3d0f3382fa7b5677f35a9becf75ac436cd7eda7b
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Thu Dec 7 00:38:14 2023 -0300

    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.
    
    
    for  libstdc++-v3/ChangeLog
    
            PR libstdc++/112858
            * 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.

Diff:
---
 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 87317031fcd..a2e4baec069 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 9346d50f5da..28423344a0f 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, [[maybe_unused]] 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 ())
     {