- Add AtomicOperations::CompareAndSwapWord

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
===================================================================
--- runtime/vm/raw_object.h	(revision 39714)
+++ runtime/vm/raw_object.h	(working copy)
@@ -6,6 +6,7 @@
 #define VM_RAW_OBJECT_H_
 
 #include "platform/assert.h"
+#include "vm/atomic.h"
 #include "vm/globals.h"
 #include "vm/token.h"
 #include "vm/snapshot.h"
@@ -329,7 +330,13 @@
   void ClearMarkBit() {
     ASSERT(IsMarked());
     uword tags = ptr()->tags_;
-    ptr()->tags_ = MarkBit::update(false, tags);
+    uword old_tags;
+    do {
+      old_tags = tags;
+      uword new_tags = MarkBit::update(false, old_tags);

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

All five changes in this file (and probably some in the other files too) can be
handled by a single template method:

template<typename BitFieldType, typename ValueType>
void RawObject::AtomicUpdateTagField(ValueType new_value) {
  uword tags = ptr()->tags_;
  uword old_tags;
  do {
    old_tags = tags;
    uword new_tags = BitFieldType::update(new_value, old_tags);
    tags = AtomicOperations::CompareAndSwapWord(
        &ptr()->tags_, old_tags, new_tags);
  } while (tags != old_tags);
}

void ClearMarkBit() {
  ASSERT(IsMarked()); 
  AtomicUpdateTagField<MarkBit, bool>(false);
}

etc.

I think this is safer than copy-pasting, especially for synchronization code,
which I find notoriously bug-prone.

Also consider forcing inlining of these methods.

+      tags = AtomicOperations::CompareAndSwapWord(
+          &ptr()->tags_, old_tags, new_tags);
+    } while (tags != old_tags);
   }
 
   // Support for GC watched bit.
@@ -353,14 +360,26 @@
   }
   void SetCanonical() {
     uword tags = ptr()->tags_;
-    ptr()->tags_ = CanonicalObjectTag::update(true, tags);
+    uword old_tags;
+    do {
+      old_tags = tags;
+      uword new_tags = CanonicalObjectTag::update(true, old_tags);
+      tags = AtomicOperations::CompareAndSwapWord(
+          &ptr()->tags_, old_tags, new_tags);
+    } while (tags != old_tags);
   }
   bool IsCreatedFromSnapshot() const {
     return CreatedFromSnapshotTag::decode(ptr()->tags_);
   }
   void SetCreatedFromSnapshot() {
     uword tags = ptr()->tags_;
-    ptr()->tags_ = CreatedFromSnapshotTag::update(true, tags);
+    uword old_tags;
+    do {
+      old_tags = tags;
+      uword new_tags = CreatedFromSnapshotTag::update(true, old_tags);
+      tags = AtomicOperations::CompareAndSwapWord(
+          &ptr()->tags_, old_tags, new_tags);
+    } while (tags != old_tags);
   }
 
   // Support for GC remembered bit.
@@ -370,11 +389,23 @@
   void SetRememberedBit() {
     ASSERT(!IsRemembered());
     uword tags = ptr()->tags_;
-    ptr()->tags_ = RememberedBit::update(true, tags);
+    uword old_tags;
+    do {
+      old_tags = tags;
+      uword new_tags = RememberedBit::update(true, old_tags);
+      tags = AtomicOperations::CompareAndSwapWord(
+          &ptr()->tags_, old_tags, new_tags);
+    } while (tags != old_tags);
   }
   void ClearRememberedBit() {
     uword tags = ptr()->tags_;
-    ptr()->tags_ = RememberedBit::update(false, tags);
+    uword old_tags;
+    do {
+      old_tags = tags;
+      uword new_tags = RememberedBit::update(false, old_tags);
+      tags = AtomicOperations::CompareAndSwapWord(
+          &ptr()->tags_, old_tags, new_tags);
+    } while (tags != old_tags);
   }
 
   bool IsDartInstance() {