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1 #ifndef SkLazyFnPtr_DEFINED | |
2 #define SkLazyFnPtr_DEFINED | |
3 | |
4 /** Declare a lazily-chosen static function pointer of type F. | |
5 * | |
6 * Example usage: | |
7 * | |
8 * typedef int (*FooImpl)(int, int); | |
9 * | |
10 * static FooImpl choose_foo() { return ... }; | |
11 * | |
12 * int Foo(int a, int b) { | |
13 * SK_DECLARE_STATIC_LAZY_FN_PTR(FooImpl, choice); | |
14 * return choice.get(choose_foo)(a, b); | |
15 * } | |
16 * | |
17 * You can think of SK_DECLARE_STATIC_LAZY_FN_PTR as a cheaper specialization o
f SkOnce. | |
18 * There is no mutex, and in the fast path, no memory barriers are issued. | |
19 * | |
20 * This must be used in a global or function scope, not as a class member. | |
21 */ | |
22 #define SK_DECLARE_STATIC_LAZY_FN_PTR(F, name) static Private::SkLazyFnPtr<F> na
me = { NULL } | |
23 | |
24 | |
25 // Everything below here is private implementation details. Don't touch, don't
even look. | |
26 | |
27 #include "SkDynamicAnnotations.h" | |
28 #include "SkThread.h" | |
29 | |
30 namespace Private { | |
31 | |
32 // This has no constructor and is link-time initialized, so its members must be
public. | |
33 template <typename F> | |
34 struct SkLazyFnPtr { | |
35 F get(F (*choose)()) { | |
36 // First, try reading to see if it's already set. | |
37 F fn = (F)SK_ANNOTATE_UNPROTECTED_READ(fPtr); | |
38 if (fn != NULL) { | |
39 return fn; | |
40 } | |
41 | |
42 // We think it's not already set. | |
43 fn = choose(); | |
44 | |
45 // No particular memory barriers needed; we're not guarding anything but
the pointer itself. | |
46 F prev = (F)sk_atomic_cas(&fPtr, NULL, (void*)fn); | |
47 | |
48 // If prev != NULL, someone snuck in and set fPtr concurrently. | |
49 // If prev == NULL, we did write fn to fPtr. | |
50 return prev != NULL ? prev : fn; | |
51 } | |
52 | |
53 void* fPtr; | |
54 }; | |
55 | |
56 } // namespace Private | |
57 | |
58 #endif//SkLazyFnPtr_DEFINED | |
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