- Add AtomicOperations::CompareAndSwapWord

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 39714)
+++ runtime/vm/object.h	(working copy)
@@ -230,11 +230,18 @@
   }
 
   void set_tags(intptr_t value) const {
+    ASSERT(!IsNull());
     // TODO(asiva): Remove the capability of setting tags in general. The mask
     // here only allows for canonical and from_snapshot flags to be set.

[koda, 2014/09/02 04:18:31]

Before masking (i.e., ignoring part of the input...), please assert that the old
and new tag value only differ on those flags:
ASSERT(((value ^ tags) & ~static_cast<uword>(0xc)) == 0);

Also, please put 0x0...c into a uword-typed local constant and file an issue for
the TODO.

-    ASSERT(!IsNull());
-    uword tags = raw()->ptr()->tags_ & ~0x0000000c;
-    raw()->ptr()->tags_ = tags | (value & 0x0000000c);
+    value = value & 0x0000000c;
+    uword tags = raw()->ptr()->tags_;
+    uword old_tags;
+    do {
+      old_tags = tags;
+      uword new_tags = (old_tags & ~0x0000000c) | value;
+      tags = AtomicOperations::CompareAndSwapWord(
+          &raw()->ptr()->tags_, old_tags, new_tags);
+    } while (tags != old_tags);
   }
   void SetCreatedFromSnapshot() const {
     ASSERT(!IsNull());