diff --git a/src/libsync/one.rs b/src/libsync/one.rs
index 7da6f39b840e5..388c63becbfd2 100644
--- a/src/libsync/one.rs
+++ b/src/libsync/one.rs
@@ -63,7 +63,17 @@ impl Once {
     ///
     /// When this function returns, it is guaranteed that some initialization
     /// has run and completed (it may not be the closure specified).
+    #[inline(always)]
     pub fn doit(&self, f: ||) {
+        // Optimize common path: load is much cheaper than fetch_add.
+        if self.cnt.load(atomics::SeqCst) < 0 {
+            return
+        }
+
+        self.doit_slow(f);
+    }
+
+    fn doit_slow(&self, f: ||) {
         // Implementation-wise, this would seem like a fairly trivial primitive.
         // The stickler part is where our mutexes currently require an
         // allocation, and usage of a `Once` should't leak this allocation.