Add subtract to atomics.

include/core/SkAtomics.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_DEFINED
 #define SkAtomics_DEFINED
 
@@ skipping 12 matching lines @@
 template <typename T>
 T sk_atomic_load(const T*, sk_memory_order = sk_memory_order_seq_cst);
 
 template <typename T>
 void sk_atomic_store(T*, T, sk_memory_order = sk_memory_order_seq_cst);
 
 template <typename T>
 T sk_atomic_fetch_add(T*, T, sk_memory_order = sk_memory_order_seq_cst);
 
 template <typename T>
+T sk_atomic_fetch_sub(T*, T, sk_memory_order = sk_memory_order_seq_cst);
+
+template <typename T>
 bool sk_atomic_compare_exchange(T*, T* expected, T desired,
                                 sk_memory_order success = sk_memory_order_seq_cst,
                                 sk_memory_order failure = sk_memory_order_seq_cst);
 
 template <typename T>
 T sk_atomic_exchange(T*, T, sk_memory_order = sk_memory_order_seq_cst);
 
 // A little wrapper class for small T (think, builtins: int, float, void*) to
 // ensure they're always used atomically.  This is our stand-in for std::atomic<T>.
 template <typename T>
 class SkAtomic : SkNoncopyable {
 public:
     SkAtomic() {}
     explicit SkAtomic(const T& val) : fVal(val) {}
 
     // It is essential we return by value rather than by const&.  fVal may change at any time.
     T load(sk_memory_order mo = sk_memory_order_seq_cst) const {
         return sk_atomic_load(&fVal, mo);
     }
 
     void store(const T& val, sk_memory_order mo = sk_memory_order_seq_cst) {
         sk_atomic_store(&fVal, val, mo);
     }
 
     T fetch_add(const T& val, sk_memory_order mo = sk_memory_order_seq_cst) {
         return sk_atomic_fetch_add(&fVal, val, mo);
     }
 
+    T fetch_sub(const T& val, sk_memory_order mo = sk_memory_order_seq_cst) {
+        return sk_atomic_fetch_sub(&fVal, val, mo);
+    }
+
     bool compare_exchange(T* expected, const T& desired,
                           sk_memory_order success = sk_memory_order_seq_cst,
                           sk_memory_order failure = sk_memory_order_seq_cst) {
         return sk_atomic_compare_exchange(&fVal, expected, desired, success, failure);
     }
 private:
     T fVal;
 };
 
 // IWYU pragma: begin_exports
@@ skipping 38 matching lines @@
 template <typename T>
 T sk_acquire_load(T* ptr) { return sk_atomic_load(ptr, sk_memory_order_acquire); }
 
 template <typename T>
 void sk_release_store(T* ptr, T val) { sk_atomic_store(ptr, val, sk_memory_order_release); }
 
 inline void sk_membar_acquire__after_atomic_dec() {}
 inline void sk_membar_acquire__after_atomic_conditional_inc() {}
 
 #endif//SkAtomics_DEFINED