[gcc(refs/users/aoliva/heads/testme)] link pthread_join from std::thread ctor

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

https://gcc.gnu.org/g:2a01a08b7dfb4102d5f86149536e5fc238b3e53d

commit 2a01a08b7dfb4102d5f86149536e5fc238b3e53d
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Fri Mar 3 02:14:53 2023 -0300

    link pthread_join from std::thread ctor
    
    Like pthread_create, pthread_join may fail to be statically linked in
    absent strong uses, so add to user code strong references to both when
    std::thread objects are created.
    
    
    for  libstdc++-v3/ChangeLog
    
            * include/bits/std_thread.h (thread::_M_thread_deps): New
            static inline function.
            (std::thread template ctor): Pass it to _M_start_thread.
            * sr/c++11/thread.cc (thread::_M_start_thread): Name depend
            parameter, force it live on entry.

Diff:
---
 libstdc++-v3/include/bits/std_thread.h | 51 +++++++++++++++++++++++++++++-----
 libstdc++-v3/src/c++11/thread.cc       | 10 +++++--
 2 files changed, 52 insertions(+), 9 deletions(-)

diff --git a/libstdc++-v3/include/bits/std_thread.h b/libstdc++-v3/include/bits/std_thread.h
index adbd3928ff7..3ffd2a823a6 100644
--- a/libstdc++-v3/include/bits/std_thread.h
+++ b/libstdc++-v3/include/bits/std_thread.h
@@ -132,6 +132,49 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     thread() noexcept = default;
 
 #ifdef _GLIBCXX_HAS_GTHREADS
+  private:
+    // This adds to user code that creates std:thread objects (because
+    // it is called by the template ctor below) strong references to
+    // pthread_create and pthread_join, which ensures they are both
+    // linked in even during static linking.  We can't depend on
+    // gthread calls to bring them in, because those may use weak
+    // references.
+    static void
+    _M_thread_deps_never_run() {
+#ifdef GTHR_ACTIVE_PROXY
+#if 0
+      static auto const __attribute__ ((__used__)) _M_create = pthread_create;
+      static auto const __attribute__ ((__used__)) _M_join = pthread_join;
+#elif 0
+      pthread_t thr;
+      pthread_create (&thr, nullptr, nullptr, nullptr);
+      pthread_join (thr, nullptr);
+#elif 0
+      asm goto ("" : : : : _M_never_run);
+      if (0)
+	{
+	_M_never_run:
+	  pthread_t thr;
+	  pthread_create (&thr, nullptr, nullptr, nullptr);
+	  pthread_join (thr, nullptr);
+	}
+#elif 0
+      bool _M_skip_always = false;
+      asm ("" : "+rm" (_M_skip_always));
+      if (__builtin_expect (_M_skip_always, false))
+	{
+	  pthread_t thr;
+	  pthread_create (&thr, nullptr, nullptr, nullptr);
+	  pthread_join (thr, nullptr);
+	}
+#else
+      reinterpret_cast<void (*)(void)>(&pthread_create)();
+      reinterpret_cast<void (*)(void)>(&pthread_join)();
+#endif
+#endif
+    }
+
+  public:
     template<typename _Callable, typename... _Args,
 	     typename = _Require<__not_same<_Callable>>>
       explicit
@@ -142,18 +185,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  "std::thread arguments must be invocable after conversion to rvalues"
 	  );
 
-#ifdef GTHR_ACTIVE_PROXY
-	// Create a reference to pthread_create, not just the gthr weak symbol.
-	auto __depend = reinterpret_cast<void(*)()>(&pthread_create);
-#else
-	auto __depend = nullptr;
-#endif
 	using _Wrapper = _Call_wrapper<_Callable, _Args...>;
 	// Create a call wrapper with DECAY_COPY(__f) as its target object
 	// and DECAY_COPY(__args)... as its bound argument entities.
 	_M_start_thread(_State_ptr(new _State_impl<_Wrapper>(
 	      std::forward<_Callable>(__f), std::forward<_Args>(__args)...)),
-	    __depend);
+	    _M_thread_deps_never_run);
       }
 #endif // _GLIBCXX_HAS_GTHREADS
 
diff --git a/libstdc++-v3/src/c++11/thread.cc b/libstdc++-v3/src/c++11/thread.cc
index 2d5ffaf678e..c91f7b02e1f 100644
--- a/libstdc++-v3/src/c++11/thread.cc
+++ b/libstdc++-v3/src/c++11/thread.cc
@@ -154,8 +154,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   }
 
   void
-  thread::_M_start_thread(_State_ptr state, void (*)())
+  thread::_M_start_thread(_State_ptr state, void (*depend)())
   {
+    // Make sure it's not optimized out, not even with LTO.
+    asm ("" : : "rm" (depend));
+
     if (!__gthread_active_p())
       {
 #if __cpp_exceptions
@@ -190,8 +193,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   }
 
   void
-  thread::_M_start_thread(__shared_base_type __b, void (*)())
+  thread::_M_start_thread(__shared_base_type __b, void (*depend)())
   {
+    // Make sure it's not optimized out, not even with LTO.
+    asm ("" : : "rm" (depend));
+
     auto ptr = __b.get();
     // Create a reference cycle that will be broken in the new thread.
     ptr->_M_this_ptr = std::move(__b);