diff --git a/libcxxabi/include/__cxxabi_config.h b/libcxxabi/include/__cxxabi_config.h
index 6ec10e469387e..7dafa976d6de2 100644
--- a/libcxxabi/include/__cxxabi_config.h
+++ b/libcxxabi/include/__cxxabi_config.h
@@ -134,4 +134,19 @@
 #  define _LIBCXXABI_DISABLE_POINTER_FIELD_PROTECTION
 #endif
 
+// On Apple, Clang generates calls to _tlv_atexit and __cxa_thread_atexit is defined
+// in libc instead.
+//
+// AIX uses a different mechanism for registering thread local destructors (__pt_atexit_np).
+//
+// On Windows, Clang either doesn't use __cxa_thread_atexit at all (in the MSVC ABI),
+// or (on MinGW) the function is already provided by C runtime.
+//
+// On other platforms, libc++abi provides __cxa_thread_atexit.
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
+#  define _LIBCXXABI_DEFINE_THREAD_ATEXIT 0
+#else
+#  define _LIBCXXABI_DEFINE_THREAD_ATEXIT 1
+#endif
+
 #endif // ____CXXABI_CONFIG_H
diff --git a/libcxxabi/include/cxxabi.h b/libcxxabi/include/cxxabi.h
index 66301d308282d..5d0513c4ccd66 100644
--- a/libcxxabi/include/cxxabi.h
+++ b/libcxxabi/include/cxxabi.h
@@ -179,8 +179,8 @@ __cxa_decrement_exception_refcount(void *primary_exception) _LIBCXXABI_NOEXCEPT;
 extern _LIBCXXABI_FUNC_VIS bool __cxa_uncaught_exception() _LIBCXXABI_NOEXCEPT;
 extern _LIBCXXABI_FUNC_VIS unsigned int __cxa_uncaught_exceptions() _LIBCXXABI_NOEXCEPT;
 
-#if defined(__linux__) || defined(__Fuchsia__)
-// Linux and Fuchsia TLS support. Not yet an official part of the Itanium ABI.
+#if _LIBCXXABI_DEFINE_THREAD_ATEXIT
+// Register a thread local destructor. This is not yet an official part of the Itanium ABI.
 // https://sourceware.org/glibc/wiki/Destructor%20support%20for%20thread_local%20variables
 extern _LIBCXXABI_FUNC_VIS int __cxa_thread_atexit(void (*)(void *), void *,
                                                    void *) _LIBCXXABI_NOEXCEPT;
diff --git a/libcxxabi/src/CMakeLists.txt b/libcxxabi/src/CMakeLists.txt
index dd23e9bb5bf79..8f779d913e0a9 100644
--- a/libcxxabi/src/CMakeLists.txt
+++ b/libcxxabi/src/CMakeLists.txt
@@ -9,6 +9,7 @@ set(LIBCXXABI_SOURCES
   cxa_handlers.cpp
   cxa_vector.cpp
   cxa_virtual.cpp
+  cxa_thread_atexit.cpp
   # C++ STL files
   stdlib_exception.cpp
   stdlib_stdexcept.cpp
@@ -36,13 +37,6 @@ else()
   )
 endif()
 
-if (LIBCXXABI_ENABLE_THREADS AND (UNIX OR FUCHSIA) AND NOT (APPLE OR CYGWIN)
-    AND NOT ("${CMAKE_SYSTEM_NAME}" MATCHES "AIX"))
-  list(APPEND LIBCXXABI_SOURCES
-    cxa_thread_atexit.cpp
-  )
-endif()
-
 set(LIBCXXABI_HEADERS
   ../include/cxxabi.h
 )
diff --git a/libcxxabi/src/cxa_thread_atexit.cpp b/libcxxabi/src/cxa_thread_atexit.cpp
index 402a52c741012..366480f9437da 100644
--- a/libcxxabi/src/cxa_thread_atexit.cpp
+++ b/libcxxabi/src/cxa_thread_atexit.cpp
@@ -6,16 +6,18 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "abort_message.h"
 #include "cxxabi.h"
+
+#if !defined(_LIBCXXABI_HAS_NO_THREADS) && _LIBCXXABI_DEFINE_THREAD_ATEXIT
+
 #include <__thread/support.h>
-#ifndef _LIBCXXABI_HAS_NO_THREADS
+#include <stdlib.h>
+
+#include "abort_message.h"
+
 #if defined(__ELF__) && defined(_LIBCXXABI_LINK_PTHREAD_LIB)
 #pragma comment(lib, "pthread")
 #endif
-#endif
-
-#include <stdlib.h>
 
 namespace __cxxabiv1 {
 
@@ -73,7 +75,9 @@ namespace {
   // Used to trigger destructors on thread exit; value is ignored
   std::__libcpp_tls_key dtors_key;
 
-  void run_dtors(void*) {
+  // The calling convention of TLS destructors is dictated by the underlying
+  // threading API.
+  void _LIBCPP_TLS_DESTRUCTOR_CC run_dtors(void*) {
     while (auto head = dtors) {
       dtors = head->next;
       head->dtor(head->obj);
@@ -106,7 +110,6 @@ namespace {
 
 #endif // HAVE___CXA_THREAD_ATEXIT_IMPL
 
-#if defined(__linux__) || defined(__Fuchsia__)
 extern "C" {
 
   _LIBCXXABI_FUNC_VIS int __cxa_thread_atexit(Dtor dtor, void* obj, void* dso_symbol) throw() {
@@ -141,6 +144,9 @@ extern "C" {
     }
 #endif // HAVE___CXA_THREAD_ATEXIT_IMPL
   }
+
 } // extern "C"
-#endif // defined(__linux__) || defined(__Fuchsia__)
+
 } // namespace __cxxabiv1
+
+#endif // !defined(_LIBCXXABI_HAS_NO_THREADS) && _LIBCXXABI_DEFINE_THREAD_ATEXIT