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+/*
+ * 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
+
+// Before trying SkOnce, see if SkLazyPtr or SkLazyFnPtr will work for you.
+// They're smaller and faster, if slightly less versatile.
+
+
+// SkOnce.h defines SK_DECLARE_STATIC_ONCE and SkOnce(), which you can use
+// together to create a threadsafe way to call a function just once.  E.g.
+//
+// static void register_my_stuff(GlobalRegistry* registry) {
+//     registry->register(...);
+// }
+// ...
+// void EnsureRegistered() {
+//     SK_DECLARE_STATIC_ONCE(once);
+//     SkOnce(&once, register_my_stuff, GetGlobalRegistry());
+// }
+//
+// No matter how many times you call EnsureRegistered(), register_my_stuff will be called just once.
+// OnceTest.cpp also should serve as a few other simple examples.
+
+#include "SkDynamicAnnotations.h"
+#include "SkThread.h"
+#include "SkTypes.h"
+
+// This must be used in a global or function scope, not as a class member.
+#define SK_DECLARE_STATIC_ONCE(name) static SkOnceFlag name
+
+class SkOnceFlag;
+
+inline void SkOnce(SkOnceFlag* once, void (*f)());
+
+template <typename Arg>
+inline void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg);
+
+// If you've already got a lock and a flag to use, this variant lets you avoid an extra SkOnceFlag.
+template <typename Lock>
+inline void SkOnce(bool* done, Lock* lock, void (*f)());
+
+template <typename Lock, typename Arg>
+inline void SkOnce(bool* done, Lock* lock, void (*f)(Arg), Arg arg);
+
+//  ----------------------  Implementation details below here. -----------------------------
+
+// This class has no constructor and must be zero-initialized (the macro above does this).
+class SkOnceFlag {
+public:
+    bool* mutableDone() { return &fDone; }
+
+    void acquire() {
+        // To act as a mutex, this needs an acquire barrier on success.
+        // sk_atomic_cas doesn't guarantee this ...
+        while (!sk_atomic_cas(&fSpinlock, 0, 1)) {
+            // spin
+        }
+        // ... so make sure to issue one of our own.
+        SkAssertResult(sk_acquire_load(&fSpinlock));
+    }
+
+    void release() {
+        // To act as a mutex, this needs a release barrier.  sk_atomic_cas guarantees this.
+        SkAssertResult(sk_atomic_cas(&fSpinlock, 1, 0));
+    }
+
+private:
+    bool fDone;
+    int32_t fSpinlock;
+};
+
+// We've pulled a pretty standard double-checked locking implementation apart
+// into its main fast path and a slow path that's called when we suspect the
+// one-time code hasn't run yet.
+
+// This is the guts of the code, called when we suspect the one-time code hasn't been run yet.
+// This should be rarely called, so we separate it from SkOnce and don't mark it as inline.
+// (We don't mind if this is an actual function call, but odds are it'll be inlined anyway.)
+template <typename Lock, typename Arg>
+static void sk_once_slow(bool* done, Lock* lock, void (*f)(Arg), Arg arg) {
+    lock->acquire();
+    if (!*done) {
+        f(arg);
+        // Also known as a store-store/load-store barrier, this makes sure that the writes
+        // done before here---in particular, those done by calling f(arg)---are observable
+        // before the writes after the line, *done = true.
+        //
+        // In version control terms this is like saying, "check in the work up
+        // to and including f(arg), then check in *done=true as a subsequent change".
+        //
+        // We'll use this in the fast path to make sure f(arg)'s effects are
+        // observable whenever we observe *done == true.
+        sk_release_store(done, true);
+    }
+    lock->release();
+}
+
+// This is our fast path, called all the time.  We do really want it to be inlined.
+template <typename Lock, typename Arg>
+inline void SkOnce(bool* done, Lock* lock, void (*f)(Arg), Arg arg) {
+    if (!SK_ANNOTATE_UNPROTECTED_READ(*done)) {
+        sk_once_slow(done, lock, f, arg);
+    }
+    // Also known as a load-load/load-store barrier, this acquire barrier makes
+    // sure that anything we read from memory---in particular, memory written by
+    // calling f(arg)---is at least as current as the value we read from done.
+    //
+    // In version control terms, this is a lot like saying "sync up to the
+    // commit where we wrote done = true".
+    //
+    // The release barrier in sk_once_slow guaranteed that done = true
+    // happens after f(arg), so by syncing to done = true here we're
+    // forcing ourselves to also wait until the effects of f(arg) are readble.
+    SkAssertResult(sk_acquire_load(done));
+}
+
+template <typename Arg>
+inline void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg) {
+    return SkOnce(once->mutableDone(), once, f, arg);
+}
+
+// Calls its argument.
+// This lets us use functions that take no arguments with SkOnce methods above.
+// (We pass _this_ as the function and the no-arg function as its argument.  Cute eh?)
+static void sk_once_no_arg_adaptor(void (*f)()) {
+    f();
+}
+
+inline void SkOnce(SkOnceFlag* once, void (*func)()) {
+    return SkOnce(once, sk_once_no_arg_adaptor, func);
+}
+
+template <typename Lock>
+inline void SkOnce(bool* done, Lock* lock, void (*func)()) {
+    return SkOnce(done, lock, sk_once_no_arg_adaptor, func);
+}
+
+#endif  // SkOnce_DEFINED