Turn visitor_id into a byte field and free back one word in Map.

Review URL: http://codereview.chromium.org/3364007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5414 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/objects-inl.h

@@ -2109,7 +2109,16 @@ void ExternalFloatArray::set(int index, float value) {
 }
 
 
-INT_ACCESSORS(Map, visitor_id, kScavengerCallbackOffset)
+int Map::visitor_id() {
+  return READ_BYTE_FIELD(this, kVisitorIdOffset);
+}
+
+
+void Map::set_visitor_id(int id) {
+  ASSERT(0 <= id && id < 256);
+  WRITE_BYTE_FIELD(this, kVisitorIdOffset, static_cast<byte>(id));
+}
+
 
 int Map::instance_size() {
   return READ_BYTE_FIELD(this, kInstanceSizeOffset) << kPointerSizeLog2;

src/objects-visiting.h

@@ -106,6 +106,9 @@ class StaticVisitorBase : public AllStatic {
     kMinObjectSizeInWords = 2
   };
 
+  // Visitor ID should fit in one byte.
+  STATIC_ASSERT(kVisitorIdCount <= 256);
+
   // Determine which specialized visitor should be used for given instance type
   // and instance type.
   static VisitorId GetVisitorId(int instance_type, int instance_size);

src/objects.cc

@@ -2126,9 +2126,6 @@ Object* NormalizedMapCache::Get(JSObject* obj, PropertyNormalizationMode mode) {
         // The cached map should match newly created normalized map bit-by-bit.
         Object* fresh = fast->CopyNormalized(mode);
         if (!fresh->IsFailure()) {
-          // Copy the unused byte so that the assertion below works.
-          Map::cast(fresh)->address()[Map::kUnusedOffset] =
-              Map::cast(result)->address()[Map::kUnusedOffset];
           ASSERT(memcmp(Map::cast(fresh)->address(),
                         Map::cast(result)->address(),
                         Map::kSize) == 0);

src/objects.h

@@ -3245,8 +3245,7 @@ class Map: public HeapObject {
   static const int kInstanceDescriptorsOffset =
       kConstructorOffset + kPointerSize;
   static const int kCodeCacheOffset = kInstanceDescriptorsOffset + kPointerSize;
-  static const int kScavengerCallbackOffset = kCodeCacheOffset + kPointerSize;
-  static const int kPadStart = kScavengerCallbackOffset + kPointerSize;
+  static const int kPadStart = kCodeCacheOffset + kPointerSize;
   static const int kSize = MAP_POINTER_ALIGN(kPadStart);
 
   // Layout of pointer fields. Heap iteration code relies on them
@@ -3263,9 +3262,8 @@ class Map: public HeapObject {
   static const int kPreAllocatedPropertyFieldsByte = 2;
   static const int kPreAllocatedPropertyFieldsOffset =
       kInstanceSizesOffset + kPreAllocatedPropertyFieldsByte;
-  // The byte at position 3 is not in use at the moment.
-  static const int kUnusedByte = 3;
-  static const int kUnusedOffset = kInstanceSizesOffset + kUnusedByte;
+  static const int kVisitorIdByte = 3;
+  static const int kVisitorIdOffset = kInstanceSizesOffset + kVisitorIdByte;
 
   // Byte offsets within kInstanceAttributesOffset attributes.
   static const int kInstanceTypeOffset = kInstanceAttributesOffset + 0;