Modernize and trim down SkOnce.

The API and implementation are very much simplified.
You may not want to bother reading the diff.

As is our trend, SkOnce now uses <atomic> directly.

Member initialization means we don't need SK_DECLARE_STATIC_ONCE.
SkSpinlock already works this same way.

All uses of the old API taking an external bool* and Lock* were pessimal,
so I have not carried this sort of API forward.  It's simpler, faster,
and more space-efficient to always use this single SkOnce class interface.

SkOnce weighs 2 bytes: a done bool and an SkSpinlock, also a bool internally.
This API refactoring opens up the opportunity to fuse those into a single
three-state byte if we'd like.

No public API changes.
TBR=reed@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1894893002

Review URL: https://codereview.chromium.org/1894893002

1 files changed, 21 insertions, 120 deletions

diff --git a/include/private/SkOnce.h b/include/private/SkOnce.h
index 34eb79cd77..d83b63f283 100644
--- a/include/private/SkOnce.h
+++ b/include/private/SkOnce.h
@@ -8,132 +8,33 @@
 #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"
+#include <atomic>
+#include <utility>
 
-// 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. -----------------------------
+// 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.
 
-// 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);
+            }
+            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