Index: src/core/SkLazyFnPtr.h |
diff --git a/src/core/SkLazyFnPtr.h b/src/core/SkLazyFnPtr.h |
new file mode 100644 |
index 0000000000000000000000000000000000000000..94594b4aeb8ead707ad5066219957654e18798d9 |
--- /dev/null |
+++ b/src/core/SkLazyFnPtr.h |
@@ -0,0 +1,58 @@ |
+#ifndef SkLazyFnPtr_DEFINED |
+#define SkLazyFnPtr_DEFINED |
+ |
+/** Declare a lazily-chosen static function pointer of type F. |
+ * |
+ * Example usage: |
+ * |
+ * typedef int (*FooImpl)(int, int); |
+ * |
+ * static FooImpl choose_foo() { return ... }; |
+ * |
+ * int Foo(int a, int b) { |
+ * SK_DECLARE_STATIC_LAZY_FN_PTR(FooImpl, choice); |
+ * return choice.get(choose_foo)(a, b); |
+ * } |
+ * |
+ * You can think of SK_DECLARE_STATIC_LAZY_FN_PTR as a cheaper specialization of SkOnce. |
+ * There is no mutex, and in the fast path, no memory barriers are issued. |
+ * |
+ * This must be used in a global or function scope, not as a class member. |
+ */ |
+#define SK_DECLARE_STATIC_LAZY_FN_PTR(F, name) static Private::SkLazyFnPtr<F> name = { NULL } |
+ |
+ |
+// Everything below here is private implementation details. Don't touch, don't even look. |
+ |
+#include "SkDynamicAnnotations.h" |
+#include "SkThreadPriv.h" |
+ |
+namespace Private { |
+ |
+// This has no constructor and is link-time initialized, so its members must be public. |
+template <typename F> |
+struct SkLazyFnPtr { |
+ F get(F (*choose)()) { |
+ // First, try reading to see if it's already set. |
+ F fn = (F)SK_ANNOTATE_UNPROTECTED_READ(fPtr); |
+ if (fn != NULL) { |
+ return fn; |
+ } |
+ |
+ // We think it's not already set. |
+ fn = choose(); |
+ |
+ // No particular memory barriers needed; we're not guarding anything but the pointer itself. |
+ F prev = (F)sk_atomic_cas(&fPtr, NULL, (void*)fn); |
+ |
+ // If prev != NULL, someone snuck in and set fPtr concurrently. |
+ // If prev == NULL, we did write fn to fPtr. |
+ return prev != NULL ? prev : fn; |
+ } |
+ |
+ void* fPtr; |
+}; |
+ |
+} // namespace Private |
+ |
+#endif//SkLazyFnPtr_DEFINED |