diff --git a/include/llvm/System/Atomic.h b/include/llvm/System/Atomic.h
index adbb975298e..c4049d40da7 100644
--- a/include/llvm/System/Atomic.h
+++ b/include/llvm/System/Atomic.h
@@ -26,6 +26,7 @@ namespace llvm {
                             cas_flag old_value);
     cas_flag AtomicIncrement(volatile cas_flag* ptr);
     cas_flag AtomicDecrement(volatile cas_flag* ptr);
+    cas_flag AtomicAdd(volatile cas_flag* ptr, cas_flag val);
   }
 }
 
diff --git a/lib/System/Atomic.cpp b/lib/System/Atomic.cpp
index 416f981df82..6e751a30d4d 100644
--- a/lib/System/Atomic.cpp
+++ b/lib/System/Atomic.cpp
@@ -78,4 +78,17 @@ sys::cas_flag sys::AtomicDecrement(volatile sys::cas_flag* ptr) {
 #endif
 }
 
+sys::cas_flag sys::AtomicAdd(volatile sys::cas_flag* ptr, sys::cas_flag val) {
+#if LLVM_MULTITHREADED==0
+  *ptr += val;
+  return *ptr;
+#elif defined(__GNUC__)
+  return __sync_add_and_fetch(ptr, val);
+#elif defined(_MSC_VER)
+  return InterlockedAdd(ptr, val);
+#else
+#  error No atomic add implementation for your platform!
+#endif
+}
+