Compatible atomics: volatile for LoadRelaxed, replace FetchAndAdd with FetchAndDecrement.

runtime/vm/atomic.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ATOMIC_H_
 #define VM_ATOMIC_H_
 
 #include "platform/globals.h"
 
 #include "vm/allocation.h"
 #include "vm/simulator.h"
 
 namespace dart {
 
 class AtomicOperations : public AllStatic {
  public:
   // Atomically fetch the value at p and increment the value at p.
   // Returns the original value at p.
   //
   // NOTE: Not to be used for any atomic operations involving memory locations
   // that are accessed by generated code
   static uintptr_t FetchAndIncrement(uintptr_t* p);
 
-  static intptr_t FetchAndAdd(intptr_t* p, intptr_t delta);
+  // Atomically fetch the value at p and decrement the value at p.
+  // Returns the original value at p.
+  //
+  // NOTE: Not to be used for any atomic operations involving memory locations
+  // that are accessed by generated code
+  static uintptr_t FetchAndDecrement(uintptr_t* p);
 
+  // Atomically compare *ptr to old_value, and if equal, store new_value.
+  // Returns the original value at ptr.
+  //
+  // NOTE: OK to use with memory locations that are accessed by generated code
   static uword CompareAndSwapWord(uword* ptr, uword old_value, uword new_value);
 
+  // Performs a load of a word from 'ptr', but without any guarantees about
+  // memory order (i.e., no load barriers/fences).
   static uword LoadRelaxed(uword* ptr) {
-    uword result;
-    memcpy(&result, ptr, sizeof(result));
-    return result;
+    return *static_cast<volatile uword*>(ptr);
   }
 };
 
 
 }  // namespace dart
 
 // We need to use the simulator to ensure that atomic operations are observed
 // both in C++ and in generated code if the simulator is active.
 #include "vm/atomic_simulator.h"
 
 #if defined(TARGET_OS_ANDROID)
 #include "vm/atomic_android.h"
 #elif defined(TARGET_OS_LINUX)
 #include "vm/atomic_linux.h"
 #elif defined(TARGET_OS_MACOS)
 #include "vm/atomic_macos.h"
 #elif defined(TARGET_OS_WINDOWS)
 #include "vm/atomic_win.h"
 #else
 #error Unknown target os.
 #endif
 
 #endif  // VM_ATOMIC_H_