[SystemZ][z/OS] Account for no __cxa_thread_atexit_impl on z/OS

[muiez]

Summary

The following patch resolves a build error and accounts for no cxa_thread_atexit_impl on z/OS. It avoids using thread since thread-local storage is not supported. It should be noted that the fallback implementation uses TLS, which is also not supported on z/OS.

[muiez]

Abort with message instead of silently returning 0.

[zibi]

LGTM,
ideally we should have macro for __thread which would define to nothing for z/OS.

[ldionne]

How is this code going to be correct if there is no thread-local storage? z/OS does support multiple threads, so does this mean that this is now a race condition?

libcxxabi/src/cxa_thread_atexit.cpp

25–28

Why doesn't that handle the z/OS case?

libcxxabi/src/cxa_thread_atexit.cpp

 
 #include <stdlib.h>
 
 namespace __cxxabiv1 {
 
   using Dtor = void(*)(void*);
 
   extern "C"
 #ifndef HAVE___CXA_THREAD_ATEXIT_IMPL
   // A weak symbol is used to detect this function's presence in the C library
   // at runtime, even if libc++ is built against an older libc
   _LIBCXXABI_WEAK
 #endif
+#ifdef __MVS__
+  // FIXME: No __cxa_thread_atexit_impl on z/OS.
+  int __cxa_thread_atexit_impl(Dtor, void*, void*) {
+    abort_message("__cxa_thread_atexit() is not implemented on z/OS");
+    return -1;
+  }
+#else
   int __cxa_thread_atexit_impl(Dtor, void*, void*);
+#endif
 
 #ifndef HAVE___CXA_THREAD_ATEXIT_IMPL
 
 namespace {
   // This implementation is used if the C library does not provide
   // __cxa_thread_atexit_impl() for us.  It has a number of limitations that are
   // difficult to impossible to address without ..._impl():
   //
namespace {
 
   struct DtorList {
     Dtor dtor;
     void* obj;
     DtorList* next;
   };
 
   // The linked list of thread-local destructors to run
-  __thread DtorList* dtors = nullptr;
+#ifndef __MVS__
+  __thread
+#endif
+  DtorList* dtors = nullptr;
   // True if the destructors are currently scheduled to run on this thread
-  __thread bool dtors_alive = false;
+#ifndef __MVS__
+  __thread
+#endif
+  bool dtors_alive = false;
   // Used to trigger destructors on thread exit; value is ignored
   std::__libcpp_tls_key dtors_key;
 
   void run_dtors(void*) {
     while (auto head = dtors) {
       dtors = head->next;
       head->dtor(head->obj);
       ::free(head);