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 T.
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 *
45 * We may call Create more than once, but all threads will see the same pointer
46 * returned from get(). Any extra calls to Create will be cleaned up.
47 *
48 * These macros must be used in a global scope, not in function scope or as a class member.
49 */
50
51 #define SK_DECLARE_STATIC_LAZY_PTR(T, name, ...) \
52 namespace {} static Private::SkStaticLazyPtr<T, ##__VA_ARGS__> name
53
54 #define SK_DECLARE_STATIC_LAZY_PTR_ARRAY(T, name, N, ...) \
55 namespace {} static Private::SkStaticLazyPtrArray<T, N, ##__VA_ARGS__> name
56
57 // namespace {} forces these macros to only be legal in global scopes. Chrome has thread-safety
58 // problems with them in function-local statics because it uses -fno-threadsafe-statics, and even
59 // in builds with threadsafe statics, those threadsafe statics are just unnecessary overhead.
60
61 // Everything below here is private implementation details. Don't touch, don't even look.
62
63 #include "SkAtomics.h"
64
65 // See FIXME below.
66 class SkFontConfigInterfaceDirect;
67
68 namespace Private {
69
70 // Set *dst to ptr if *dst is NULL. Returns value of *dst, destroying ptr if not swapped in.
71 // Issues acquire memory barrier on failure, release on success.
72 template <typename P, void (*Destroy)(P)>
try_cas(P * dst,P ptr)73 static P try_cas(P* dst, P ptr) {
74 P prev = NULL;
75 if (sk_atomic_compare_exchange(dst, &prev, ptr,
76 sk_memory_order_release/*on success*/,
77 sk_memory_order_acquire/*on failure*/)) {
78 // We need a release barrier before returning ptr. The compare_exchange provides it.
79 SkASSERT(!prev);
80 return ptr;
81 } else {
82 Destroy(ptr);
83 // We need an acquire barrier before returning prev. The compare_exchange provided it.
84 SkASSERT(prev);
85 return prev;
86 }
87 }
88
sk_new()89 template <typename T> T* sk_new() { return SkNEW(T); }
sk_delete(T * ptr)90 template <typename T> void sk_delete(T* ptr) { SkDELETE(ptr); }
91
92 // We're basing these implementations here on this article:
93 // http://preshing.com/20140709/the-purpose-of-memory_order_consume-in-cpp11/
94 //
95 // Because the users of SkLazyPtr and SkLazyPtrArray will read the pointers
96 // _through_ our atomically set pointer, there is a data dependency between our
97 // atomic and the guarded data, and so we only need writer-releases /
98 // reader-consumes memory pairing rather than the more general write-releases /
99 // reader-acquires convention.
100 //
101 // This is nice, because a consume load is free on all our platforms: x86,
102 // ARM, MIPS. In contrast, an acquire load issues a memory barrier on non-x86.
103
104 template <typename T>
consume_load(T * ptr)105 T consume_load(T* ptr) {
106 #if defined(THREAD_SANITIZER)
107 // TSAN gets anxious if we don't tell it what we're actually doing, a consume load.
108 return sk_atomic_load(ptr, sk_memory_order_consume);
109 #else
110 // All current compilers blindly upgrade consume memory order to acquire memory order.
111 // For our purposes, though, no memory barrier is required, so we lie and use relaxed.
112 return sk_atomic_load(ptr, sk_memory_order_relaxed);
113 #endif
114 }
115
116 // This has no constructor and must be zero-initalized (the macro above does this).
117 template <typename T, T* (*Create)() = sk_new<T>, void (*Destroy)(T*) = sk_delete<T> >
118 class SkStaticLazyPtr {
119 public:
get()120 T* get() {
121 // If fPtr has already been filled, we need a consume barrier when loading it.
122 // If not, we need a release barrier when setting it. try_cas will do that.
123 T* ptr = consume_load(&fPtr);
124 return ptr ? ptr : try_cas<T*, Destroy>(&fPtr, Create());
125 }
126
127 private:
128 T* fPtr;
129 };
130
sk_new_arg(int i)131 template <typename T> T* sk_new_arg(int i) { return SkNEW_ARGS(T, (i)); }
132
133 // This has no constructor and must be zero-initalized (the macro above does this).
134 template <typename T, int N, T* (*Create)(int) = sk_new_arg<T>, void (*Destroy)(T*) = sk_delete<T> >
135 class SkStaticLazyPtrArray {
136 public:
137 T* operator[](int i) {
138 SkASSERT(i >= 0 && i < N);
139 // If fPtr has already been filled, we need an consume barrier when loading it.
140 // If not, we need a release barrier when setting it. try_cas will do that.
141 T* ptr = consume_load(&fArray[i]);
142 return ptr ? ptr : try_cas<T*, Destroy>(&fArray[i], Create(i));
143 }
144
145 private:
146 T* fArray[N];
147 };
148
149 } // namespace Private
150
151 // This version is suitable for use as a class member.
152 // It's much the same as above except:
153 // - it has a constructor to zero itself;
154 // - it has a destructor to clean up;
155 // - get() calls SkNew(T) to create the pointer;
156 // - get(functor) calls functor to create the pointer.
157 template <typename T, void (*Destroy)(T*) = Private::sk_delete<T> >
158 class SkLazyPtr : SkNoncopyable {
159 public:
SkLazyPtr()160 SkLazyPtr() : fPtr(NULL) {}
~SkLazyPtr()161 ~SkLazyPtr() { if (fPtr) { Destroy((T*)fPtr); } }
162
get()163 T* get() const {
164 T* ptr = Private::consume_load(&fPtr);
165 return ptr ? ptr : Private::try_cas<T*, Destroy>(&fPtr, SkNEW(T));
166 }
167
168 template <typename Create>
get(const Create & create)169 T* get(const Create& create) const {
170 T* ptr = Private::consume_load(&fPtr);
171 return ptr ? ptr : Private::try_cas<T*, Destroy>(&fPtr, create());
172 }
173
174 private:
175 mutable T* fPtr;
176 };
177
178
179 #endif//SkLazyPtr_DEFINED
180