Modernize and trim down SkOnce.

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
 
-// Before trying SkOnce, see if SkLazyPtr or SkLazyFnPtr will work for you.
-// They're smaller and faster, if slightly less versatile.
+#include "../private/SkSpinlock.h"
+#include <atomic>
+#include <utility>
 
+// 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.
 
-// 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 {
+class SkOnce {
 public:
-    bool* mutableDone() { return &fDone; }
-
-    void acquire() { fSpinlock.acquire(); }
-    void release() { fSpinlock.release(); }
+    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)) {
+                fn(std::forward<Args>(args)...);
+                fDone.store(true, std::memory_order_release);

[herb_g, 2016/04/18 14:29:51]

Should the memory release from the spinlock cover this release?

[mtklein, 2016/04/18 14:41:23]

As I understand it, the synchronizes-with edge here is only guaranteed if we
acquire and release on the same atomic object, here fDone.  The fast path reader
does not read or write the atomic fLock object, so there's no way to use that to
create the synchronizes-with edge.

Just keep in mind everything inside the first if is done approximately once, and
everything inside the second if is done exactly once.  There's not much benefit
to tweaking the stock double-checked locking formula except if we can find a way
to remove the outermost acquire.

[herb_g, 2016/04/18 14:53:09]

Acknowledged.

+            }
+            fLock.release();
+        }
+    }
 
 private:
-    bool fDone;
-    SkSpinlock fSpinlock;
+    std::atomic<bool> fDone{false};
+    SkSpinlock        fLock;
 };
 
-// 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_atomic_store(done, true, sk_memory_order_release);
-    }
-    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