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      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 // This has no constructor and must be zero-initalized (the macro above does this).
     92 template <typename T, T* (*Create)() = sk_new<T>, void (*Destroy)(T*) = sk_delete<T> >
     93 class SkLazyPtr {
     94 public:
     95     T* get() {
     96         // If fPtr has already been filled, we need an acquire barrier when loading it.
     97         // If not, we need a release barrier when setting it.  try_cas will do that.
     98         T* ptr = (T*)sk_acquire_load(&fPtr);
     99         return ptr ? ptr : try_cas<T*, Destroy>(&fPtr, Create());
    100     }
    101 
    102 #ifdef SK_DEVELOPER
    103     // FIXME: We know we leak refs on some classes.  For now, let them leak.
    104     void cleanup(SkFontConfigInterfaceDirect*) {}
    105     template <typename U> void cleanup(U* ptr) { Destroy(ptr); }
    106 
    107     ~SkLazyPtr() {
    108         this->cleanup((T*)fPtr);
    109         fPtr = NULL;
    110     }
    111 #endif
    112 
    113 private:
    114     void* fPtr;
    115 };
    116 
    117 template <typename T> T* sk_new_arg(int i) { return SkNEW_ARGS(T, (i)); }
    118 
    119 // This has no constructor and must be zero-initalized (the macro above does this).
    120 template <typename T, int N, T* (*Create)(int) = sk_new_arg<T>, void (*Destroy)(T*) = sk_delete<T> >
    121 class SkLazyPtrArray {
    122 public:
    123     T* operator[](int i) {
    124         SkASSERT(i >= 0 && i < N);
    125         // If fPtr has already been filled, we need an acquire barrier when loading it.
    126         // If not, we need a release barrier when setting it.  try_cas will do that.
    127         T* ptr = (T*)sk_acquire_load(&fArray[i]);
    128         return ptr ? ptr : try_cas<T*, Destroy>(&fArray[i], Create(i));
    129     }
    130 
    131 #ifdef SK_DEVELOPER
    132     ~SkLazyPtrArray() {
    133         for (int i = 0; i < N; i++) {
    134             Destroy((T*)fArray[i]);
    135             fArray[i] = NULL;
    136         }
    137     }
    138 #endif
    139 
    140 private:
    141     void* fArray[N];
    142 };
    143 
    144 }  // namespace Private
    145 
    146 #endif//SkLazyPtr_DEFINED
    147