Move Mutexy things to private.

There is no API change.

TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1368333004

1 files changed, 139 insertions, 0 deletions

diff --git a/include/private/SkOnce.h b/include/private/SkOnce.h
new file mode 100644
index 0000000000..a3ef4d6713
--- /dev/null
+++ b/include/private/SkOnce.h
@@ -0,0 +1,139 @@
+/*
+ * 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 "../private/SkAtomics.h"
+#include "../private/SkSpinlock.h"
+
+// This must be used in a global scope, not in function scope or as a class member.
+#define SK_DECLARE_STATIC_ONCE(name) namespace {} 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() { fSpinlock.acquire(); }
+    void release() { fSpinlock.release(); }
+
+private:
+    bool fDone;
+    SkPODSpinlock 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 (!sk_atomic_load(done, sk_memory_order_relaxed)) {
+        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) {
+    // When *done == true:
+    //   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.
+    //
+    // When *done == false:
+    //   We'll try to call f(arg) in sk_once_slow.
+    //   If we get the lock, great, we call f(arg), release true into done, and drop the lock.
+    //   If we race and don't get the lock first, we'll wait for the first guy to finish.
+    //   Then lock acquire() will give us at least an acquire memory barrier to get the same
+    //   effect as the acquire load in the *done == true fast case.  We'll see *done is true,
+    //   then just drop the lock and return.
+    if (!sk_atomic_load(done, sk_memory_order_acquire)) {
+        sk_once_slow(done, lock, f, arg);
+    }
+}
+
+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