1 /*
2  * Copyright 2015 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 SkOncePtr_DEFINED
9 #define SkOncePtr_DEFINED
10 
11 #include "../private/SkAtomics.h"
12 #include "SkUniquePtr.h"
13 
14 template <typename T> class SkBaseOncePtr;
15 
16 // Use this to create a global static pointer that's intialized exactly once when you call get().
17 #define SK_DECLARE_STATIC_ONCE_PTR(type, name) namespace {} static SkBaseOncePtr<type> name;
18 
19 // Use this for a local or member pointer that's initialized exactly once when you call get().
20 template <typename T, typename Delete = skstd::default_delete<T>>
21 class SkOncePtr : SkNoncopyable {
22 public:
SkOncePtr()23     SkOncePtr() { sk_bzero(this, sizeof(*this)); }
~SkOncePtr()24     ~SkOncePtr() {
25         if (T* ptr = (T*)*this) {
26             Delete()(ptr);
27         }
28     }
29 
30     template <typename F>
get(const F & f)31     T* get(const F& f) const {
32         return fOnce.get(f);
33     }
34 
35     operator T*() const {
36         return (T*)fOnce;
37     }
38 
39 private:
40     SkBaseOncePtr<T> fOnce;
41 };
42 
43 // If you ask for SkOncePtr<T[]>, we'll clean up with delete[] by default.
44 template <typename T>
45 class SkOncePtr<T[]> : public SkOncePtr<T, skstd::default_delete<T[]>> {};
46 
47 /* TODO(mtklein): in next CL
48 typedef SkBaseOncePtr<void> SkOnceFlag;
49 #define SK_DECLARE_STATIC_ONCE(name) namespace {} static SkOnceFlag name
50 
51 template <typename F>
52 inline void SkOnce(SkOnceFlag* once, const F& f) {
53     once->get([&]{ f(); return (void*)2; });
54 }
55 */
56 
57 // Implementation details below here!  No peeking!
58 
59 template <typename T>
60 class SkBaseOncePtr {
61 public:
62     template <typename F>
get(const F & f)63     T* get(const F& f) const {
64         uintptr_t state = sk_atomic_load(&fState, sk_memory_order_acquire);
65         if (state < 2) {
66             if (state == 0) {
67                 // It looks like no one has tried to create our pointer yet.
68                 // We try to claim that task by atomically swapping our state from '0' to '1'.
69                 if (sk_atomic_compare_exchange(
70                     &fState, &state, (uintptr_t)1, sk_memory_order_relaxed, sk_memory_order_relaxed)) {
71                     // We've claimed it.  Create our pointer and store it into fState.
72                     state = (uintptr_t)f();
73                     SkASSERT(state > 1);
74                     sk_atomic_store(&fState, state, sk_memory_order_release);
75                 } else {
76                     // Someone else claimed it.
77                     // We fall through to the spin loop just below to wait for them to finish.
78                 }
79             }
80 
81             while (state == 1) {
82                 // State '1' is our busy-but-not-done state.
83                 // Some other thread has claimed the job of creating our pointer.
84                 // We just need to wait for it to finish.
85                 state = sk_atomic_load(&fState, sk_memory_order_acquire);
86             }
87 
88             // We shouldn't be able to get here without having created our pointer.
89             SkASSERT(state > 1);
90         }
91         return (T*)state;
92     }
93 
94     operator T*() const {
95         auto state = sk_atomic_load(&fState, sk_memory_order_acquire);
96         return state < 2 ? nullptr : (T*)state;
97         // TODO: If state == 1 spin until it's not?
98     }
99 
100     // fState == 0 --> we have not created our ptr yet
101     // fState == 1 --> someone is in the middle of creating our ptr
102     // else        --> (T*)fState is our ptr
103     mutable uintptr_t fState;
104 };
105 
106 #endif//SkOncePtr_DEFINED
107