Re: [libstdc++] Use __gthread_join in jthread/95989

[Alexandre Oliva <oliva@adacore.com>]

On Mar  3, 2023, Jonathan Wakely <jwakely@redhat.com> wrote:

> On Fri, 3 Mar 2023 at 18:12, Jonathan Wakely <jwakely@redhat.com> wrote:

>> And those expressions aren't ever optimized away as unused?

> Oh, I missed that they're called after casting them

*nod*

> That would be UB to call them through the wrong pointer type, so the
> compiler could decide they're unreachable

Ugh, indeed.  We can drop the cast and add the required parameters if it
ever becomes an issue.  Here's what I've just installed.


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

	PR libstdc++/104852
	PR libstdc++/95989
	PR libstdc++/52590
	* include/bits/std_thread.h (thread::_M_thread_deps): New
	static implicitly-inline member function.
	(std::thread template ctor): Pass it to _M_start_thread.
	* src/c++11/thread.cc (thread::_M_start_thread): Name depend
	parameter, force it live on entry.
---
 libstdc++-v3/include/bits/std_thread.h |   24 +++++++++++++++++-------
 libstdc++-v3/src/c++11/thread.cc       |   10 ++++++++--
 2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/libstdc++-v3/include/bits/std_thread.h b/libstdc++-v3/include/bits/std_thread.h
index adbd3928ff783..e88c07fbd4f0f 100644
--- a/libstdc++-v3/include/bits/std_thread.h
+++ b/libstdc++-v3/include/bits/std_thread.h
@@ -132,6 +132,22 @@ _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
+      reinterpret_cast<void (*)(void)>(&pthread_create)();
+      reinterpret_cast<void (*)(void)>(&pthread_join)();
+#endif
+    }
+
+  public:
     template<typename _Callable, typename... _Args,
 	     typename = _Require<__not_same<_Callable>>>
       explicit
@@ -142,18 +158,12 @@ _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 2d5ffaf678e97..c91f7b02e1f3f 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);


-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice
but very few check the facts.  Ask me about <https://stallmansupport.org>