Fix compile for -DLIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY

[Nicu]

Summary

Compilation with -DLIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY was failing due to missing declarations of functions used in libcxx/include/atomic. The lines this commit affects are the places where those functions are defined, now moved to be always defined.

[ldionne]

Out of curiosity, can you explain how you're using LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY? It is documented as being a feature only for libc++ developers.

The change LGTM though.

[miyuki]

David is working with me on improving the testing of external threading configurations.

[ldionne]

In D80372#2048930, @miyuki wrote:

David is working with me on improving the testing of external threading configurations.

I'm curious to understand what your setup is.

Also, please run arcfilter next time you commit a patch to libcxx! Thanks!

[miyuki]

I'm curious to understand what your setup is.

Internally we use _LIBCPP_HAS_THREAD_API_EXTERNAL in our downstream libc++ fork intended for the bare-metal environment. We aim to provide testing of this configuration and also the _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL one. The plan is to set up public build bots for testing them. Unfortunately, this work is not high-priority, so I cannot make any commitments w.r.t. to it.

libcxx/include/__threading_support

 _LIBCPP_THREAD_ABI_VISIBILITY
 void *__libcpp_tls_get(__libcpp_tls_key __key);
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_tls_set(__libcpp_tls_key __key, void *__p);
 
 #endif // !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 
-#if (!defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
-     defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL))
-
 struct __libcpp_timed_backoff_policy {
   _LIBCPP_THREAD_ABI_VISIBILITY
   bool operator()(chrono::nanoseconds __elapsed) const;
 };
 
+inline _LIBCPP_INLINE_VISIBILITY
+bool __libcpp_timed_backoff_policy::operator()(chrono::nanoseconds __elapsed) const
+{
+    if(__elapsed > chrono::milliseconds(128))
+        __libcpp_thread_sleep_for(chrono::milliseconds(8));
+    else if(__elapsed > chrono::microseconds(64))
+        __libcpp_thread_sleep_for(__elapsed / 2);
+    else if(__elapsed > chrono::microseconds(4))
+      __libcpp_thread_yield();
+    else
+      ; // poll
+    return false;
+}
+
+static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;
+
 template<class _Fn, class _BFn>
 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
 bool __libcpp_thread_poll_with_backoff(
-  _Fn && __f, _BFn && __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero());
+  _Fn && __f, _BFn && __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero())
+{
+    auto const __start = chrono::high_resolution_clock::now();
+    for(int __count = 0;;) {
+      if(__f())
+        return true; // _Fn completion means success
+      if(__count < __libcpp_polling_count) {
+        __count += 1;
+        continue;
+      }
+      chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start;
+      if(__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed)
+          return false; // timeout failure
+      if(__bf(__elapsed))
+        return false; // _BFn completion means failure
+    }
+}
+
+#if (!defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
+     defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL))
+
 
 namespace __thread_detail {
 
 inline __libcpp_timespec_t __convert_to_timespec(const chrono::nanoseconds& __ns)
 {
   using namespace chrono;
   seconds __s = duration_cast<seconds>(__ns);
   __libcpp_timespec_t __ts;

 
 int __libcpp_tls_set(__libcpp_tls_key __key, void *__p)
 {
   return tss_set(__key, __p) == thrd_success ? 0 : EINVAL;
 }
 
 #endif
 
-inline _LIBCPP_INLINE_VISIBILITY
-bool __libcpp_timed_backoff_policy::operator()(chrono::nanoseconds __elapsed) const
-{
-    if(__elapsed > chrono::milliseconds(128))
-        __libcpp_thread_sleep_for(chrono::milliseconds(8));
-    else if(__elapsed > chrono::microseconds(64))
-        __libcpp_thread_sleep_for(__elapsed / 2);
-    else if(__elapsed > chrono::microseconds(4))
-      __libcpp_thread_yield();
-    else
-      ; // poll
-    return false;
-}
-
-static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;
-
-template<class _Fn, class _BFn>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
-bool __libcpp_thread_poll_with_backoff(_Fn && __f, _BFn && __bf, chrono::nanoseconds __max_elapsed)
-{
-    auto const __start = chrono::high_resolution_clock::now();
-    for(int __count = 0;;) {
-      if(__f())
-        return true; // _Fn completion means success
-      if(__count < __libcpp_polling_count) {
-        __count += 1;
-        continue;
-      }
-      chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start;
-      if(__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed)
-          return false; // timeout failure
-      if(__bf(__elapsed))
-        return false; // _BFn completion means failure
-    }
-}
 
 #endif // !_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL || _LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL
 
 class _LIBCPP_TYPE_VIS thread;
 class _LIBCPP_TYPE_VIS __thread_id;
 
 namespace this_thread
 {