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#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 "SkThread.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
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