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

runtime/vm/atomic.h

Index: runtime/vm/atomic.h
diff --git a/runtime/vm/atomic.h b/runtime/vm/atomic.h
index 769aad67ae4106596dcac56018bc3282cbd7d78c..30bf94403ea0ec06fd65113902937c9877a46f09 100644
--- a/runtime/vm/atomic.h
+++ b/runtime/vm/atomic.h
@@ -21,14 +21,14 @@ class AtomicOperations : public AllStatic {
   // that are accessed by generated code
   static uintptr_t FetchAndIncrement(uintptr_t* p);
 
-  static intptr_t FetchAndAdd(intptr_t* p, intptr_t delta);
+  static uintptr_t FetchAndDecrement(uintptr_t* p);

[Ivan Posva, 2015/08/13 01:43:21]

This also needs the comment from above. Especially the NOTE about being accessed
by generated code.

[koda, 2015/08/13 16:22:37]

Done.

 
   static uword CompareAndSwapWord(uword* ptr, uword old_value, uword new_value);

[Ivan Posva, 2015/08/13 01:43:21]

Please add a comment here as well plus the NOTE that CompareAndSwapWord is safe
for use with generated code.

[koda, 2015/08/13 16:22:37]

Done.

 
+  // 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);
   }
 };