1/*
2 * Copyright 2014 Google Inc.
3 *
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
6 */
7
8#ifndef SkLazyFnPtr_DEFINED
9#define SkLazyFnPtr_DEFINED
10
11/** Declare a lazily-chosen static function pointer of type F.
12 *
13 *  Example usage:
14 *
15 *  typedef int (*FooImpl)(int, int);
16 *
17 *  static FooImpl choose_foo() { return ... };
18 *
19 *  int Foo(int a, int b) {
20 *     SK_DECLARE_STATIC_LAZY_FN_PTR(FooImpl, foo, choose_foo);
21 *     return foo.get()(a, b);
22 *  }
23 *
24 *  You can think of SK_DECLARE_STATIC_LAZY_FN_PTR as a cheaper specialization of SkOnce.
25 *  There is no mutex, and in the fast path, no memory barriers are issued.
26 *
27 *  This must be used in a global or function scope, not as a class member.
28 */
29#define SK_DECLARE_STATIC_LAZY_FN_PTR(F, name, Choose) static Private::SkLazyFnPtr<F, Choose> name
30
31
32// Everything below here is private implementation details.  Don't touch, don't even look.
33
34#include "SkDynamicAnnotations.h"
35#include "SkThreadPriv.h"
36
37namespace Private {
38
39// This has no constructor and must be zero-initialized (the macro above does this).
40template <typename F, F (*Choose)()>
41class SkLazyFnPtr {
42public:
43    F get() {
44        // First, try reading to see if it's already set.
45        F fn = (F)SK_ANNOTATE_UNPROTECTED_READ(fPtr);
46        if (fn != NULL) {
47            return fn;
48        }
49
50        // We think it's not already set.
51        fn = Choose();
52
53        // No particular memory barriers needed; we're not guarding anything but the pointer itself.
54        F prev = (F)sk_atomic_cas(&fPtr, NULL, (void*)fn);
55
56        // If prev != NULL, someone snuck in and set fPtr concurrently.
57        // If prev == NULL, we did write fn to fPtr.
58        return prev != NULL ? prev : fn;
59    }
60
61private:
62    void* fPtr;
63};
64
65}  // namespace Private
66
67#endif//SkLazyFnPtr_DEFINED
68