SkAtomic: always use std::atomic

include/ports/SkAtomics_sync.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 SkAtomics_sync_DEFINED
-#define SkAtomics_sync_DEFINED
-
-// This file is mostly a shim.  We'd like to delete it.  Please don't put much
-// effort into maintaining it, and if you find bugs in it, the right fix is to
-// delete this file and upgrade your compiler to something that supports
-// __atomic builtins or std::atomic.
-
-static inline void barrier(sk_memory_order mo) {
-    asm volatile("" : : : "memory");  // Prevents the compiler from reordering code.
-    #if SK_CPU_X86
-        // On x86, we generally don't need an extra memory barrier for loads or stores.
-        if (sk_memory_order_seq_cst == mo) { __sync_synchronize(); }
-    #else
-        // On other platforms (e.g. ARM) we do unless the memory order is relaxed.
-        if (sk_memory_order_relaxed != mo) { __sync_synchronize(); }
-    #endif
-}
-
-// These barriers only support our majority use cases: acquire and relaxed loads, release stores.
-// For anything more complicated, please consider deleting this file and upgrading your compiler.
-
-template <typename T>
-T sk_atomic_load(const T* ptr, sk_memory_order mo) {
-    T val = *ptr;
-    barrier(mo);
-    return val;
-}
-
-template <typename T>
-void sk_atomic_store(T* ptr, T val, sk_memory_order mo) {
-    barrier(mo);
-    *ptr = val;
-}
-
-template <typename T>
-T sk_atomic_fetch_add(T* ptr, T val, sk_memory_order) {
-    return __sync_fetch_and_add(ptr, val);
-}
-
-template <typename T>
-T sk_atomic_fetch_sub(T* ptr, T val, sk_memory_order) {
-    return __sync_fetch_and_sub(ptr, val);
-}
-
-template <typename T>
-bool sk_atomic_compare_exchange(T* ptr, T* expected, T desired, sk_memory_order, sk_memory_order) {
-    T prev = __sync_val_compare_and_swap(ptr, *expected, desired);
-    if (prev == *expected) {
-        return true;
-    }
-    *expected = prev;
-    return false;
-}
-
-template <typename T>
-T sk_atomic_exchange(T* ptr, T val, sk_memory_order) {
-    // There is no __sync exchange.  Emulate it with a CAS loop.
-    T prev;
-    do {
-        prev = sk_atomic_load(ptr);
-    } while(!sk_atomic_compare_exchange(ptr, &prev, val));
-    return prev;
-}
-
-#endif//SkAtomics_sync_DEFINED