make callonce a header library

Author: SchrodingerZhu

libc/src/__support/threads/linux/CMakeLists.txt

@@ -97,10 +97,8 @@ add_object_library(
                             # value other than 0 is dangerous. We know.
 )
 
-add_object_library(
+add_header_library(
   callonce
-  SRCS
-    callonce.cpp
   HDRS
     ../callonce.h
     callonce.h

libc/src/__support/threads/linux/callonce.cpp

@@ -26,6 +26,31 @@ static constexpr FutexWordType FINISH = 0x33;
 LIBC_INLINE bool callonce_fastpath(CallOnceFlag *flag) {
   return flag->load(cpp::MemoryOrder::RELAXED) == FINISH;
 }
+
+[[gnu::noinline]]
+LIBC_INLINE int callonce_slowpath(CallOnceFlag *flag, void (*func)(void)) {
+  auto *futex_word = reinterpret_cast<Futex *>(flag);
+
+  FutexWordType not_called = NOT_CALLED;
+
+  // The call_once call can return only after the called function |func|
+  // returns. So, we use futexes to synchronize calls with the same flag value.
+  if (futex_word->compare_exchange_strong(not_called, START)) {
+    func();
+    auto status = futex_word->exchange(FINISH);
+    if (status == WAITING)
+      futex_word->notify_all();
+    return 0;
+  }
+
+  FutexWordType status = START;
+  if (futex_word->compare_exchange_strong(status, WAITING) ||
+      status == WAITING) {
+    futex_word->wait(WAITING);
+  }
+
+  return 0;
+}
 } // namespace callonce_impl
 
 } // namespace LIBC_NAMESPACE_DECL