Reland of SkOncePtr -> SkOnce

include/private/SkOncePtr.h

-/*
- * Copyright 2015 Google Inc.
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-#ifndef SkOncePtr_DEFINED
-#define SkOncePtr_DEFINED
-
-#include "../private/SkAtomics.h"
-#include <memory>
-
-// Use this to create a global static pointer that's intialized exactly once when you call get().
-#define SK_DECLARE_STATIC_ONCE_PTR(type, name) namespace {} static SkBaseOncePtr<type> name;
-
-template <typename T>
-class SkBaseOncePtr {
-public:
-    template <typename F>
-    T* get(const F& f) const {
-        uintptr_t state = sk_atomic_load(&fState, sk_memory_order_acquire);
-        if (state < 2) {
-            if (state == 0) {
-                // It looks like no one has tried to create our pointer yet.
-                // We try to claim that task by atomically swapping our state from '0' to '1'.
-                // See SkOnce.h for why we use an acquire memory order here rather than relaxed.
-                if (sk_atomic_compare_exchange(
-                    &fState, &state, (uintptr_t)1, sk_memory_order_acquire, sk_memory_order_acquire)) {
-                    // We've claimed it.  Create our pointer and store it into fState.
-                    state = (uintptr_t)f();
-                    SkASSERT(state > 1);
-                    sk_atomic_store(&fState, state, sk_memory_order_release);
-                } else {
-                    // Someone else claimed it.
-                    // We fall through to the spin loop just below to wait for them to finish.
-                }
-            }
-
-            while (state == 1) {
-                // State '1' is our busy-but-not-done state.
-                // Some other thread has claimed the job of creating our pointer.
-                // We just need to wait for it to finish.
-                state = sk_atomic_load(&fState, sk_memory_order_acquire);
-            }
-
-            // We shouldn't be able to get here without having created our pointer.
-            SkASSERT(state > 1);
-        }
-        return (T*)state;
-    }
-
-    operator T*() const {
-        auto state = sk_atomic_load(&fState, sk_memory_order_acquire);
-        return state < 2 ? nullptr : (T*)state;
-        // TODO: If state == 1 spin until it's not?
-    }
-
-    // fState == 0 --> we have not created our ptr yet
-    // fState == 1 --> someone is in the middle of creating our ptr
-    // else        --> (T*)fState is our ptr
-    mutable uintptr_t fState;
-};
-
-#endif//SkOncePtr_DEFINED