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* Copyright 2014 Google Inc.
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
#ifndef SkLazyFnPtr_DEFINED
#define SkLazyFnPtr_DEFINED
/** Declare a lazily-chosen static function pointer of type F.
* Example usage:
* typedef int (*FooImpl)(int, int);
* static FooImpl choose_foo() { return ... };
* int Foo(int a, int b) {
* SK_DECLARE_STATIC_LAZY_FN_PTR(FooImpl, foo, choose_foo);
* return foo.get()(a, b);
* }
* You can think of SK_DECLARE_STATIC_LAZY_FN_PTR as a cheaper specialization of SkOnce.
* There is no mutex, and in the fast path, no memory barriers are issued.
* This must be used in a global or function scope, not as a class member.
#define SK_DECLARE_STATIC_LAZY_FN_PTR(F, name, Choose) static Private::SkLazyFnPtr<F, Choose> name
// Everything below here is private implementation details. Don't touch, don't even look.
#include "SkDynamicAnnotations.h"
#include "SkThreadPriv.h"
namespace Private {
// This has no constructor and must be zero-initialized (the macro above does this).
template <typename F, F (*Choose)()>
class SkLazyFnPtr {
F get() {
// First, try reading to see if it's already set.
if (fn != NULL) {
return fn;
// We think it's not already set.
fn = Choose();
// No particular memory barriers needed; we're not guarding anything but the pointer itself.
F prev = (F)sk_atomic_cas(&fPtr, NULL, (void*)fn);
// If prev != NULL, someone snuck in and set fPtr concurrently.
// If prev == NULL, we did write fn to fPtr.
return prev != NULL ? prev : fn;
void* fPtr;
} // namespace Private