Allow race-full access of map instance size when sweeping concurrently.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 978704ac54286aff224e370390121e305cd45100..fbd7af2a036d13a6287ed4c14ae3a8e42ae9f5e0 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -4000,7 +4000,7 @@ int Map::GetInObjectPropertyOffset(int index) {
 
 
 int HeapObject::SizeFromMap(Map* map) {
-  int instance_size = map->instance_size();
+  int instance_size = map->nobarrier_instance_size();
   if (instance_size != kVariableSizeSentinel) return instance_size;
   // Only inline the most frequent cases.
   int instance_type = static_cast<int>(map->instance_type());
@@ -4051,6 +4051,35 @@ void Map::set_instance_size(int value) {
 }
 
 
+int Map::nobarrier_instance_size() {
+  // Our atomics API does not support byte accessors. Hence, we have to manually
+  // read out atomically the whole word and mask out the upper bytes.
+  ASSERT(kInstanceSizeOffset % kPointerSize == 0);
+  AtomicWord value = reinterpret_cast<AtomicWord>(
+      NO_BARRIER_READ_FIELD(this, kInstanceSizeOffset));
+  return (value & 0xff) << kPointerSizeLog2;
+}
+
+
+void Map::nobarrier_set_instance_size(int value) {
+  ASSERT_EQ(0, value & (kPointerSize - 1));
+  value >>= kPointerSizeLog2;
+  ASSERT(0 <= value && value < 256);
+
+  // Our atomics API does not support byte accessors. Hence, we have to manually
+  // read out atomically the whole word, update the last byte and write back the
+  // modified word.
+  ASSERT(kInstanceSizeOffset % kPointerSize == 0);
+  AtomicWord size_field = reinterpret_cast<AtomicWord>(
+      NO_BARRIER_READ_FIELD(this, kInstanceSizeOffset));
+  size_field >>= 8;

[Hannes Payer (out of office), 2014/04/07 12:31:13]

Do you prefer masking ifdef style?

+  size_field <<= 8;
+  size_field |= value;
+  NO_BARRIER_WRITE_FIELD(
+      this, kInstanceSizeOffset, reinterpret_cast<Object*>(size_field));
+}
+
+
 void Map::set_inobject_properties(int value) {
   ASSERT(0 <= value && value < 256);
   WRITE_BYTE_FIELD(this, kInObjectPropertiesOffset, static_cast<byte>(value));