SkOnce: 2 bytes -> 1 byte

include/private/SkOnce.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkOnce_DEFINED
 #define SkOnce_DEFINED
 
-#include "../private/SkSpinlock.h"
 #include <atomic>
 #include <utility>
+#include "SkTypes.h"
 
 // SkOnce provides call-once guarantees for Skia, much like std::once_flag/std::call_once().
 //
 // There should be no particularly error-prone gotcha use cases when using SkOnce.
 // It works correctly as a class member, a local, a global, a function-scoped static, whatever.
 
 class SkOnce {
 public:
     template <typename Fn, typename... Args>
     void operator()(Fn&& fn, Args&&... args) {
-        // Vanilla double-checked locking.
-        if (!fDone.load(std::memory_order_acquire)) {
-            fLock.acquire();
-            if (!fDone.load(std::memory_order_relaxed)) {
+        auto state = fState.load(std::memory_order_acquire);
+
+        if (state == Done) {
+            return;
+        }
+
+        if (state == NotStarted) {
+            // Try to claim the job of calling fn() by swapping from NotStarted to Calling.
+            // See [1] below for why we use std::memory_order_acquire instead of relaxed.
+            if (fState.compare_exchange_strong(state, Calling, std::memory_order_acquire)) {
+                // Claimed!  Call fn(), then mark this SkOnce as Done.
                 fn(std::forward<Args>(args)...);
-                fDone.store(true, std::memory_order_release);
+                return fState.store(Done, std::memory_order_release);
             }
-            fLock.release();
         }
+
+        while (state == Calling) {
+            // Some other thread is calling fn().  Wait for them to finish.
+            state = fState.load(std::memory_order_acquire);
+        }
+        SkASSERT(state == Done);
     }
 
 private:
-    std::atomic<bool> fDone{false};
-    SkSpinlock        fLock;
+    enum State : uint8_t { NotStarted, Calling, Done};
+    std::atomic<uint8_t> fState{NotStarted};
 };
 
+/* [1]  Why do we compare_exchange_strong() with std::memory_order_acquire instead of relaxed?
+ *
+ * If we succeed, we really only need a relaxed compare_exchange_strong()... we're the ones
+ * who are about to do a release store, so there's certainly nothing yet for an acquire to
+ * synchronize with.
+ *
+ * If that compare_exchange_strong() fails, we're either in Calling or Done state.
+ * Again, if we're in Calling state, relaxed would have been fine: the spin loop will
+ * acquire up to the Calling thread's release store.
+ *
+ * But if that compare_exchange_strong() fails and we find ourselves in the Done state,
+ * we've never done an acquire load to sync up to the store of that Done state.
+ *
+ * So on failure we need an acquire load.  Generally the failure memory order cannot be
+ * stronger than the success memory order, so we need acquire on success too.  The single
+ * memory order version of compare_exchange_strong() uses the same acquire order for both.
+ */
+
 #endif  // SkOnce_DEFINED