Convert SkRefCnt to std::atomic.

include/private/SkWeakRefCnt.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkWeakRefCnt_DEFINED
 #define SkWeakRefCnt_DEFINED
 
 #include "SkRefCnt.h"
-#include "../private/SkAtomics.h"
+#include <atomic>
 
 /** \class SkWeakRefCnt
 
     SkWeakRefCnt is the base class for objects that may be shared by multiple
     objects. When an existing strong owner wants to share a reference, it calls
     ref(). When a strong owner wants to release its reference, it calls
     unref(). When the shared object's strong reference count goes to zero as
     the result of an unref() call, its (virtual) weak_dispose method is called.
     It is an error for the destructor to be called explicitly (or via the
     object going out of scope on the stack or calling delete) if
@@ skipping 32 matching lines @@
         The strong references collectively hold one weak reference. When the
         strong reference count goes to zero, the collectively held weak
         reference is released.
     */
     SkWeakRefCnt() : SkRefCnt(), fWeakCnt(1) {}
 
     /** Destruct, asserting that the weak reference count is 1.
     */
     virtual ~SkWeakRefCnt() {
 #ifdef SK_DEBUG
-        SkASSERT(fWeakCnt == 1);
-        fWeakCnt = 0;
+        SkASSERT(getWeakCnt() == 1);
+        fWeakCnt.store(0, std::memory_order_relaxed);
 #endif
     }
 
-    /** Return the weak reference count.
-    */
-    int32_t getWeakCnt() const { return fWeakCnt; }
+#ifdef SK_DEBUG
+    /** Return the weak reference count. */
+    int32_t getWeakCnt() const {
+        return fWeakCnt.load(std::memory_order_relaxed);
+    }
 
-#ifdef SK_DEBUG
     void validate() const {
         this->INHERITED::validate();
-        SkASSERT(fWeakCnt > 0);
+        SkASSERT(getWeakCnt() > 0);
     }
 #endif
 
+private:
+    /** If fRefCnt is 0, returns 0.
+     *  Otherwise increments fRefCnt, acquires, and returns the old value.
+     */
+    int32_t atomic_conditional_acquire_strong_ref() const {
+        int32_t prev = fRefCnt.load(std::memory_order_relaxed);
+        do {
+            if (0 == prev) {
+                break;
+            }
+        } while(!fRefCnt.compare_exchange_weak(prev, prev+1, std::memory_order_acquire,
+                                                             std::memory_order_relaxed));
+        return prev;
+    }
+
+public:
     /** Creates a strong reference from a weak reference, if possible. The
         caller must already be an owner. If try_ref() returns true the owner
         is in posession of an additional strong reference. Both the original
         reference and new reference must be properly unreferenced. If try_ref()
         returns false, no strong reference could be created and the owner's
         reference is in the same state as before the call.
     */
     bool SK_WARN_UNUSED_RESULT try_ref() const {
-        if (sk_atomic_conditional_inc(&fRefCnt) != 0) {
+        if (atomic_conditional_acquire_strong_ref() != 0) {
             // Acquire barrier (L/SL), if not provided above.
             // Prevents subsequent code from happening before the increment.
-            sk_membar_acquire__after_atomic_conditional_inc();
             return true;
         }
         return false;
     }
 
     /** Increment the weak reference count. Must be balanced by a call to
         weak_unref().
     */
     void weak_ref() const {
-        SkASSERT(fRefCnt > 0);
-        SkASSERT(fWeakCnt > 0);
-        sk_atomic_inc(&fWeakCnt);  // No barrier required.
+        SkASSERT(getRefCnt() > 0);
+        SkASSERT(getWeakCnt() > 0);
+        // No barrier required.
+        (void)fWeakCnt.fetch_add(+1, std::memory_order_relaxed);
     }
 
     /** Decrement the weak reference count. If the weak reference count is 1
         before the decrement, then call delete on the object. Note that if this
         is the case, then the object needs to have been allocated via new, and
         not on the stack.
     */
     void weak_unref() const {
-        SkASSERT(fWeakCnt > 0);
-        // Release barrier (SL/S), if not provided below.
-        if (sk_atomic_dec(&fWeakCnt) == 1) {
-            // Acquire barrier (L/SL), if not provided above.
-            // Prevents code in destructor from happening before the decrement.
-            sk_membar_acquire__after_atomic_dec();
+        SkASSERT(getWeakCnt() > 0);
+        // A release here acts in place of all releases we "should" have been doing in ref().
+        if (1 == fWeakCnt.fetch_add(-1, std::memory_order_acq_rel)) {
+            // Like try_ref(), the acquire is only needed on success, to make sure
+            // code in internal_dispose() doesn't happen before the decrement.
 #ifdef SK_DEBUG
             // so our destructor won't complain
-            fWeakCnt = 1;
+            fWeakCnt.store(1, std::memory_order_relaxed);
 #endif
             this->INHERITED::internal_dispose();
         }
     }
 
     /** Returns true if there are no strong references to the object. When this
         is the case all future calls to try_ref() will return false.
     */
     bool weak_expired() const {
-        return fRefCnt == 0;
+        return fRefCnt.load(std::memory_order_relaxed) == 0;
     }
 
 protected:
     /** Called when the strong reference count goes to zero. This allows the
         object to free any resources it may be holding. Weak references may
         still exist and their level of allowed access to the object is defined
         by the object's class.
     */
     virtual void weak_dispose() const {
     }
 
 private:
     /** Called when the strong reference count goes to zero. Calls weak_dispose
         on the object and releases the implicit weak reference held
         collectively by the strong references.
     */
     void internal_dispose() const override {
         weak_dispose();
         weak_unref();
     }
 
     /* Invariant: fWeakCnt = #weak + (fRefCnt > 0 ? 1 : 0) */
-    mutable int32_t fWeakCnt;
+    mutable std::atomic<int32_t> fWeakCnt;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif