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 SkLazyPtr_DEFINED 9 #define SkLazyPtr_DEFINED 10 11 /** Declare a lazily-chosen static pointer (or array of pointers) of type F. 12 * 13 * Example usage: 14 * 15 * Foo* GetSingletonFoo() { 16 * SK_DECLARE_STATIC_LAZY_PTR(Foo, singleton); // Created with SkNEW, destroyed with SkDELETE. 17 * return singleton.get(); 18 * } 19 * 20 * These macros take an optional T* (*Create)() and void (*Destroy)(T*) at the end. 21 * If not given, we'll use SkNEW and SkDELETE. 22 * These options are most useful when T doesn't have a public constructor or destructor. 23 * Create comes first, so you may use a custom Create with a default Destroy, but not vice versa. 24 * 25 * Foo* CustomCreate() { return ...; } 26 * void CustomDestroy(Foo* ptr) { ... } 27 * Foo* GetSingletonFooWithCustomCleanup() { 28 * SK_DECLARE_STATIC_LAZY_PTR(Foo, singleton, CustomCreate, CustomDestroy); 29 * return singleton.get(); 30 * } 31 * 32 * If you have a bunch of related static pointers of the same type, you can 33 * declare an array of lazy pointers together, and we'll pass the index to Create(). 34 * 35 * Foo* CreateFoo(int i) { return ...; } 36 * Foo* GetCachedFoo(Foo::Enum enumVal) { 37 * SK_DECLARE_STATIC_LAZY_PTR_ARRAY(Foo, Foo::kEnumCount, cachedFoos, CreateFoo); 38 * return cachedFoos[enumVal]; 39 * } 40 * 41 * 42 * You can think of SK_DECLARE_STATIC_LAZY_PTR as a cheaper specialization of 43 * SkOnce. There is no mutex or extra storage used past the pointer itself. 44 * In debug mode, each lazy pointer will be cleaned up at process exit so we 45 * can check that we've not leaked or freed them early. 46 * 47 * We may call Create more than once, but all threads will see the same pointer 48 * returned from get(). Any extra calls to Create will be cleaned up. 49 * 50 * These macros must be used in a global or function scope, not as a class member. 51 */ 52 53 #define SK_DECLARE_STATIC_LAZY_PTR(T, name, ...) \ 54 static Private::SkLazyPtr<T, ##__VA_ARGS__> name 55 56 #define SK_DECLARE_STATIC_LAZY_PTR_ARRAY(T, name, N, ...) \ 57 static Private::SkLazyPtrArray<T, N, ##__VA_ARGS__> name 58 59 60 61 // Everything below here is private implementation details. Don't touch, don't even look. 62 63 #include "SkDynamicAnnotations.h" 64 #include "SkThread.h" 65 #include "SkThreadPriv.h" 66 67 // See FIXME below. 68 class SkFontConfigInterfaceDirect; 69 70 namespace Private { 71 72 // Set *dst to ptr if *dst is NULL. Returns value of *dst, destroying ptr if not swapped in. 73 // Issues the same memory barriers as sk_atomic_cas: acquire on failure, release on success. 74 template <typename P, void (*Destroy)(P)> 75 static P try_cas(void** dst, P ptr) { 76 P prev = (P)sk_atomic_cas(dst, NULL, ptr); 77 78 if (prev) { 79 // We need an acquire barrier before returning prev, which sk_atomic_cas provided. 80 Destroy(ptr); 81 return prev; 82 } else { 83 // We need a release barrier before returning ptr, which sk_atomic_cas provided. 84 return ptr; 85 } 86 } 87 88 template <typename T> T* sk_new() { return SkNEW(T); } 89 template <typename T> void sk_delete(T* ptr) { SkDELETE(ptr); } 90 91 // We're basing these implementations here on this article: 92 // http://preshing.com/20140709/the-purpose-of-memory_order_consume-in-cpp11/ 93 // 94 // Because the users of SkLazyPtr and SkLazyPtrArray will read the pointers 95 // _through_ our atomically set pointer, there is a data dependency between our 96 // atomic and the guarded data, and so we only need writer-releases / 97 // reader-consumes memory pairing rather than the more general write-releases / 98 // reader-acquires convention. 99 // 100 // This is nice, because a sk_consume_load is free on all our platforms: x86, 101 // ARM, MIPS. In contrast, sk_acquire_load issues a memory barrier on non-x86. 102 103 // This has no constructor and must be zero-initalized (the macro above does this). 104 template <typename T, T* (*Create)() = sk_new<T>, void (*Destroy)(T*) = sk_delete<T> > 105 class SkLazyPtr { 106 public: 107 T* get() { 108 // If fPtr has already been filled, we need a consume barrier when loading it. 109 // If not, we need a release barrier when setting it. try_cas will do that. 110 T* ptr = (T*)sk_consume_load(&fPtr); 111 return ptr ? ptr : try_cas<T*, Destroy>(&fPtr, Create()); 112 } 113 114 #ifdef SK_DEVELOPER 115 // FIXME: We know we leak refs on some classes. For now, let them leak. 116 void cleanup(SkFontConfigInterfaceDirect*) {} 117 template <typename U> void cleanup(U* ptr) { Destroy(ptr); } 118 119 ~SkLazyPtr() { 120 this->cleanup((T*)fPtr); 121 fPtr = NULL; 122 } 123 #endif 124 125 private: 126 void* fPtr; 127 }; 128 129 template <typename T> T* sk_new_arg(int i) { return SkNEW_ARGS(T, (i)); } 130 131 // This has no constructor and must be zero-initalized (the macro above does this). 132 template <typename T, int N, T* (*Create)(int) = sk_new_arg<T>, void (*Destroy)(T*) = sk_delete<T> > 133 class SkLazyPtrArray { 134 public: 135 T* operator[](int i) { 136 SkASSERT(i >= 0 && i < N); 137 // If fPtr has already been filled, we need an consume barrier when loading it. 138 // If not, we need a release barrier when setting it. try_cas will do that. 139 T* ptr = (T*)sk_consume_load(&fArray[i]); 140 return ptr ? ptr : try_cas<T*, Destroy>(&fArray[i], Create(i)); 141 } 142 143 #ifdef SK_DEVELOPER 144 ~SkLazyPtrArray() { 145 for (int i = 0; i < N; i++) { 146 Destroy((T*)fArray[i]); 147 fArray[i] = NULL; 148 } 149 } 150 #endif 151 152 private: 153 void* fArray[N]; 154 }; 155 156 } // namespace Private 157 158 #endif//SkLazyPtr_DEFINED 159
