blob: 921c6a6534b83de2f95a97e836f5d71dba61bc2f (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
|
/*
* Copyright 2015 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef SkOncePtr_DEFINED
#define SkOncePtr_DEFINED
#include "../private/SkAtomics.h"
#include <memory>
// Use this to create a global static pointer that's intialized exactly once when you call get().
#define SK_DECLARE_STATIC_ONCE_PTR(type, name) namespace {} static SkBaseOncePtr<type> name;
template <typename T>
class SkBaseOncePtr {
public:
template <typename F>
T* get(const F& f) const {
uintptr_t state = sk_atomic_load(&fState, sk_memory_order_acquire);
if (state < 2) {
if (state == 0) {
// It looks like no one has tried to create our pointer yet.
// We try to claim that task by atomically swapping our state from '0' to '1'.
// See SkOnce.h for why we use an acquire memory order here rather than relaxed.
if (sk_atomic_compare_exchange(
&fState, &state, (uintptr_t)1, sk_memory_order_acquire, sk_memory_order_acquire)) {
// We've claimed it. Create our pointer and store it into fState.
state = (uintptr_t)f();
SkASSERT(state > 1);
sk_atomic_store(&fState, state, sk_memory_order_release);
} else {
// Someone else claimed it.
// We fall through to the spin loop just below to wait for them to finish.
}
}
while (state == 1) {
// State '1' is our busy-but-not-done state.
// Some other thread has claimed the job of creating our pointer.
// We just need to wait for it to finish.
state = sk_atomic_load(&fState, sk_memory_order_acquire);
}
// We shouldn't be able to get here without having created our pointer.
SkASSERT(state > 1);
}
return (T*)state;
}
operator T*() const {
auto state = sk_atomic_load(&fState, sk_memory_order_acquire);
return state < 2 ? nullptr : (T*)state;
// TODO: If state == 1 spin until it's not?
}
// fState == 0 --> we have not created our ptr yet
// fState == 1 --> someone is in the middle of creating our ptr
// else --> (T*)fState is our ptr
mutable uintptr_t fState;
};
#endif//SkOncePtr_DEFINED
|
