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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() {
// 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 "SkAtomics.h"
#include "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 {
bool* mutableDone() { return &fDone; }
void acquire() { fSpinlock.acquire(); }
void release() { fSpinlock.release(); }
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) {
if (!sk_atomic_load(done, sk_memory_order_relaxed)) {
// 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);
// 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)()) {
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