In-place shrinking of descriptor arrays with non-live transitions.

src/factory.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 921 matching lines @@
   CALL_HEAP_FUNCTION(isolate(),
                      isolate()->heap()->LookupSymbol(*value), String);
 }
 
 
 Handle<DescriptorArray> Factory::CopyAppendCallbackDescriptors(
     Handle<DescriptorArray> array,
     Handle<Object> descriptors) {
   v8::NeanderArray callbacks(descriptors);
   int nof_callbacks = callbacks.length();
+  int descriptor_count = array->number_of_descriptors();
   Handle<DescriptorArray> result =
-      NewDescriptorArray(array->number_of_descriptors() + nof_callbacks);
-
-  // Number of descriptors added to the result so far.
-  int descriptor_count = 0;
+      NewDescriptorArray(descriptor_count + nof_callbacks);
 
   // Ensure that marking will not progress and change color of objects.
   DescriptorArray::WhitenessWitness witness(*result);
 
   // Copy the descriptors from the array.
-  for (int i = 0; i < array->number_of_descriptors(); i++) {
-    if (!array->IsNullDescriptor(i)) {
-      DescriptorArray::CopyFrom(result, descriptor_count++, array, i, witness);
-    }
+  for (int i = 0; i < descriptor_count; i++) {
+    DescriptorArray::CopyFrom(result, i, array, i, witness);
   }
 
-  // Number of duplicates detected.
-  int duplicates = 0;
-
   // Fill in new callback descriptors.  Process the callbacks from
   // back to front so that the last callback with a given name takes
   // precedence over previously added callbacks with that name.
   for (int i = nof_callbacks - 1; i >= 0; i--) {
     Handle<AccessorInfo> entry =
         Handle<AccessorInfo>(AccessorInfo::cast(callbacks.get(i)));
     // Ensure the key is a symbol before writing into the instance descriptor.
     Handle<String> key =
         SymbolFromString(Handle<String>(String::cast(entry->name())));
     // Check if a descriptor with this name already exists before writing.
     if (result->LinearSearch(EXPECT_UNSORTED, *key, descriptor_count) ==
         DescriptorArray::kNotFound) {
       CallbacksDescriptor desc(*key, *entry, entry->property_attributes());
       result->Set(descriptor_count, &desc, witness);
       descriptor_count++;
-    } else {
-      duplicates++;
     }
   }
 
   // If duplicates were detected, allocate a result of the right size
   // and transfer the elements.
-  if (duplicates > 0) {
-    int number_of_descriptors = result->number_of_descriptors() - duplicates;
-    Handle<DescriptorArray> new_result =
-        NewDescriptorArray(number_of_descriptors);
-    for (int i = 0; i < number_of_descriptors; i++) {
+  if (descriptor_count < result->length()) {
+    Handle<DescriptorArray> new_result = NewDescriptorArray(descriptor_count);
+    for (int i = 0; i < descriptor_count; i++) {
       DescriptorArray::CopyFrom(new_result, i, result, i, witness);
     }
     result = new_result;
   }
 
   // Sort the result before returning.
   result->Sort(witness);
   return result;
 }
 
@@ skipping 511 matching lines @@
 
 
 Handle<Object> Factory::ToBoolean(bool value) {
   return Handle<Object>(value
                         ? isolate()->heap()->true_value()
                         : isolate()->heap()->false_value());
 }
 
 
 } }  // namespace v8::internal