1 files changed, 65 insertions, 0 deletions
diff --git a/include/private/SkOncePtr.h b/include/private/SkOncePtr.h
new file mode 100644
index 0000000000..921c6a6534
--- /dev/null
+++ b/include/private/SkOncePtr.h
@@ -0,0 +1,65 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkOncePtr_DEFINED
+#define SkOncePtr_DEFINED
+
+#include "../private/SkAtomics.h"
+#include <memory>
+
+// Use this to create a global static pointer that's intialized exactly once when you call get().
+#define SK_DECLARE_STATIC_ONCE_PTR(type, name) namespace {} static SkBaseOncePtr<type> name;
+
+template <typename T>
+class SkBaseOncePtr {
+public:
+    template <typename F>
+    T* get(const F& f) const {
+        uintptr_t state = sk_atomic_load(&fState, sk_memory_order_acquire);
+        if (state < 2) {
+            if (state == 0) {
+                // It looks like no one has tried to create our pointer yet.
+                // We try to claim that task by atomically swapping our state from '0' to '1'.
+                // See SkOnce.h for why we use an acquire memory order here rather than relaxed.
+                if (sk_atomic_compare_exchange(
+                    &fState, &state, (uintptr_t)1, sk_memory_order_acquire, sk_memory_order_acquire)) {
+                    // We've claimed it.  Create our pointer and store it into fState.
+                    state = (uintptr_t)f();
+                    SkASSERT(state > 1);
+                    sk_atomic_store(&fState, state, sk_memory_order_release);
+                } else {
+                    // Someone else claimed it.
+                    // We fall through to the spin loop just below to wait for them to finish.
+                }
+            }
+
+            while (state == 1) {
+                // State '1' is our busy-but-not-done state.
+                // Some other thread has claimed the job of creating our pointer.
+                // We just need to wait for it to finish.
+                state = sk_atomic_load(&fState, sk_memory_order_acquire);
+            }
+
+            // We shouldn't be able to get here without having created our pointer.
+            SkASSERT(state > 1);
+        }
+        return (T*)state;
+    }
+
+    operator T*() const {
+        auto state = sk_atomic_load(&fState, sk_memory_order_acquire);
+        return state < 2 ? nullptr : (T*)state;
+        // TODO: If state == 1 spin until it's not?
+    }
+
+    // fState == 0 --> we have not created our ptr yet
+    // fState == 1 --> someone is in the middle of creating our ptr
+    // else        --> (T*)fState is our ptr
+    mutable uintptr_t fState;
+};
+
+#endif//SkOncePtr_DEFINED