1 files changed, 28 insertions, 0 deletions
diff --git a/llvm/lib/Support/Threading.cpp b/llvm/lib/Support/Threading.cpp
index ca7f3f64aa3..a4e1e44261b 100644
--- a/llvm/lib/Support/Threading.cpp
+++ b/llvm/lib/Support/Threading.cpp
@@ -16,6 +16,7 @@
 #include "llvm/Config/config.h"
 #include "llvm/Support/Atomic.h"
 #include "llvm/Support/Mutex.h"
+#include "llvm/Support/thread.h"
 #include <cassert>
 
 using namespace llvm;
@@ -110,3 +111,30 @@ void llvm::llvm_execute_on_thread(void (*Fn)(void*), void *UserData,
 }
 
 #endif
+
+void llvm::call_once(once_flag &flag, void (*fptr)(void)) {
+#if LLVM_THREADING_USE_STD_CALL_ONCE
+  std::call_once(flag, fptr);
+#else
+  // For other platforms we use a generic (if brittle) version based on our
+  // atomics.
+  sys::cas_flag old_val = sys::CompareAndSwap(&flag, Wait, Uninitialized);
+  if (old_val == Uninitialized) {
+    fptr();
+    sys::MemoryFence();
+    TsanIgnoreWritesBegin();
+    TsanHappensBefore(&flag);
+    flag = Done;
+    TsanIgnoreWritesEnd();
+  } else {
+    // Wait until any thread doing the call has finished.
+    sys::cas_flag tmp = flag;
+    sys::MemoryFence();
+    while (tmp != Done) {
+      tmp = flag;
+      sys::MemoryFence();
+    }
+  }
+  TsanHappensAfter(&flag);
+#endif
+}