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1 /* | |
2 * Copyright 2013 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 SkOnce_DEFINED | |
9 #define SkOnce_DEFINED | |
10 | |
11 // Before trying SkOnce, see if SkLazyPtr or SkLazyFnPtr will work for you. | |
12 // They're smaller and faster, if slightly less versatile. | |
13 | |
14 | |
15 // SkOnce.h defines SK_DECLARE_STATIC_ONCE and SkOnce(), which you can use | |
16 // together to create a threadsafe way to call a function just once. E.g. | |
17 // | |
18 // static void register_my_stuff(GlobalRegistry* registry) { | |
19 // registry->register(...); | |
20 // } | |
21 // ... | |
22 // void EnsureRegistered() { | |
23 // SK_DECLARE_STATIC_ONCE(once); | |
24 // SkOnce(&once, register_my_stuff, GetGlobalRegistry()); | |
25 // } | |
26 // | |
27 // No matter how many times you call EnsureRegistered(), register_my_stuff will
be called just once. | |
28 // OnceTest.cpp also should serve as a few other simple examples. | |
29 | |
30 #include "../private/SkAtomics.h" | |
31 #include "SkSpinlock.h" | |
32 | |
33 // This must be used in a global scope, not in function scope or as a class memb
er. | |
34 #define SK_DECLARE_STATIC_ONCE(name) namespace {} static SkOnceFlag name | |
35 | |
36 class SkOnceFlag; | |
37 | |
38 inline void SkOnce(SkOnceFlag* once, void (*f)()); | |
39 | |
40 template <typename Arg> | |
41 inline void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg); | |
42 | |
43 // If you've already got a lock and a flag to use, this variant lets you avoid a
n extra SkOnceFlag. | |
44 template <typename Lock> | |
45 inline void SkOnce(bool* done, Lock* lock, void (*f)()); | |
46 | |
47 template <typename Lock, typename Arg> | |
48 inline void SkOnce(bool* done, Lock* lock, void (*f)(Arg), Arg arg); | |
49 | |
50 // ---------------------- Implementation details below here. -----------------
------------ | |
51 | |
52 // This class has no constructor and must be zero-initialized (the macro above d
oes this). | |
53 class SkOnceFlag { | |
54 public: | |
55 bool* mutableDone() { return &fDone; } | |
56 | |
57 void acquire() { fSpinlock.acquire(); } | |
58 void release() { fSpinlock.release(); } | |
59 | |
60 private: | |
61 bool fDone; | |
62 SkPODSpinlock fSpinlock; | |
63 }; | |
64 | |
65 // We've pulled a pretty standard double-checked locking implementation apart | |
66 // into its main fast path and a slow path that's called when we suspect the | |
67 // one-time code hasn't run yet. | |
68 | |
69 // This is the guts of the code, called when we suspect the one-time code hasn't
been run yet. | |
70 // This should be rarely called, so we separate it from SkOnce and don't mark it
as inline. | |
71 // (We don't mind if this is an actual function call, but odds are it'll be inli
ned anyway.) | |
72 template <typename Lock, typename Arg> | |
73 static void sk_once_slow(bool* done, Lock* lock, void (*f)(Arg), Arg arg) { | |
74 lock->acquire(); | |
75 if (!sk_atomic_load(done, sk_memory_order_relaxed)) { | |
76 f(arg); | |
77 // Also known as a store-store/load-store barrier, this makes sure that
the writes | |
78 // done before here---in particular, those done by calling f(arg)---are
observable | |
79 // before the writes after the line, *done = true. | |
80 // | |
81 // In version control terms this is like saying, "check in the work up | |
82 // to and including f(arg), then check in *done=true as a subsequent cha
nge". | |
83 // | |
84 // We'll use this in the fast path to make sure f(arg)'s effects are | |
85 // observable whenever we observe *done == true. | |
86 sk_release_store(done, true); | |
87 } | |
88 lock->release(); | |
89 } | |
90 | |
91 // This is our fast path, called all the time. We do really want it to be inlin
ed. | |
92 template <typename Lock, typename Arg> | |
93 inline void SkOnce(bool* done, Lock* lock, void (*f)(Arg), Arg arg) { | |
94 // When *done == true: | |
95 // Also known as a load-load/load-store barrier, this acquire barrier make
s | |
96 // sure that anything we read from memory---in particular, memory written
by | |
97 // calling f(arg)---is at least as current as the value we read from done. | |
98 // | |
99 // In version control terms, this is a lot like saying "sync up to the | |
100 // commit where we wrote done = true". | |
101 // | |
102 // The release barrier in sk_once_slow guaranteed that done = true | |
103 // happens after f(arg), so by syncing to done = true here we're | |
104 // forcing ourselves to also wait until the effects of f(arg) are readble. | |
105 // | |
106 // When *done == false: | |
107 // We'll try to call f(arg) in sk_once_slow. | |
108 // If we get the lock, great, we call f(arg), release true into done, and
drop the lock. | |
109 // If we race and don't get the lock first, we'll wait for the first guy t
o finish. | |
110 // Then lock acquire() will give us at least an acquire memory barrier to
get the same | |
111 // effect as the acquire load in the *done == true fast case. We'll see *
done is true, | |
112 // then just drop the lock and return. | |
113 if (!sk_atomic_load(done, sk_memory_order_acquire)) { | |
114 sk_once_slow(done, lock, f, arg); | |
115 } | |
116 } | |
117 | |
118 template <typename Arg> | |
119 inline void SkOnce(SkOnceFlag* once, void (*f)(Arg), Arg arg) { | |
120 return SkOnce(once->mutableDone(), once, f, arg); | |
121 } | |
122 | |
123 // Calls its argument. | |
124 // This lets us use functions that take no arguments with SkOnce methods above. | |
125 // (We pass _this_ as the function and the no-arg function as its argument. Cut
e eh?) | |
126 static void sk_once_no_arg_adaptor(void (*f)()) { | |
127 f(); | |
128 } | |
129 | |
130 inline void SkOnce(SkOnceFlag* once, void (*func)()) { | |
131 return SkOnce(once, sk_once_no_arg_adaptor, func); | |
132 } | |
133 | |
134 template <typename Lock> | |
135 inline void SkOnce(bool* done, Lock* lock, void (*func)()) { | |
136 return SkOnce(done, lock, sk_once_no_arg_adaptor, func); | |
137 } | |
138 | |
139 #endif // SkOnce_DEFINED | |
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