Recommit introducing short external strings.

src/objects.cc

 // Copyright 2011 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 934 matching lines @@
     ASSERT(static_cast<size_t>(this->length()) == resource->length());
     ScopedVector<uc16> smart_chars(this->length());
     String::WriteToFlat(this, smart_chars.start(), 0, this->length());
     ASSERT(memcmp(smart_chars.start(),
                   resource->data(),
                   resource->length() * sizeof(smart_chars[0])) == 0);
   }
 #endif  // DEBUG
   Heap* heap = GetHeap();
   int size = this->Size();  // Byte size of the original string.
-  if (size < ExternalString::kSize) {
+  if (size < ExternalString::kShortSize) {
     return false;
   }
-  ASSERT(size >= ExternalString::kSize);
   bool is_ascii = this->IsAsciiRepresentation();
   bool is_symbol = this->IsSymbol();
-  int length = this->length();
-  int hash_field = this->hash_field();
 
   // Morph the object to an external string by adjusting the map and
   // reinitializing the fields.
-  this->set_map(is_ascii ?
-                heap->external_string_with_ascii_data_map() :
-                heap->external_string_map());
+  if (size >= ExternalString::kSize) {
+    this->set_map(
+        is_symbol
+            ? (is_ascii ?  heap->external_symbol_with_ascii_data_map()
+                        :  heap->external_symbol_map())
+            : (is_ascii ?  heap->external_string_with_ascii_data_map()
+                        :  heap->external_string_map()));
+  } else {
+    this->set_map(
+        is_symbol
+            ? (is_ascii ?  heap->short_external_symbol_with_ascii_data_map()
+                        :  heap->short_external_symbol_map())
+            : (is_ascii ?  heap->short_external_string_with_ascii_data_map()
+                        :  heap->short_external_string_map()));
+  }
   ExternalTwoByteString* self = ExternalTwoByteString::cast(this);
-  self->set_length(length);
-  self->set_hash_field(hash_field);
   self->set_resource(resource);
-  // Additionally make the object into an external symbol if the original string
-  // was a symbol to start with.
-  if (is_symbol) {
-    self->Hash();  // Force regeneration of the hash value.
-    // Now morph this external string into a external symbol.
-    this->set_map(is_ascii ?
-                  heap->external_symbol_with_ascii_data_map() :
-                  heap->external_symbol_map());
-  }
+  if (is_symbol) self->Hash();  // Force regeneration of the hash value.
 
   // Fill the remainder of the string with dead wood.
   int new_size = this->Size();  // Byte size of the external String object.
   heap->CreateFillerObjectAt(this->address() + new_size, size - new_size);
   if (Marking::IsBlack(Marking::MarkBitFrom(this))) {
     MemoryChunk::IncrementLiveBytes(this->address(), new_size - size);
   }
   return true;
 }
 
 
 bool String::MakeExternal(v8::String::ExternalAsciiStringResource* resource) {
 #ifdef DEBUG
   if (FLAG_enable_slow_asserts) {
     // Assert that the resource and the string are equivalent.
     ASSERT(static_cast<size_t>(this->length()) == resource->length());
     ScopedVector<char> smart_chars(this->length());
     String::WriteToFlat(this, smart_chars.start(), 0, this->length());
     ASSERT(memcmp(smart_chars.start(),
                   resource->data(),
                   resource->length() * sizeof(smart_chars[0])) == 0);
   }
 #endif  // DEBUG
   Heap* heap = GetHeap();
   int size = this->Size();  // Byte size of the original string.
-  if (size < ExternalString::kSize) {
+  if (size < ExternalString::kShortSize) {
     return false;
   }
-  ASSERT(size >= ExternalString::kSize);
   bool is_symbol = this->IsSymbol();
-  int length = this->length();
-  int hash_field = this->hash_field();
 
   // Morph the object to an external string by adjusting the map and
-  // reinitializing the fields.
-  this->set_map(heap->external_ascii_string_map());
+  // reinitializing the fields.  Use short version if space is limited.
+  if (size >= ExternalString::kSize) {
+    this->set_map(is_symbol ? heap->external_ascii_symbol_map()
+                            : heap->external_ascii_string_map());
+  } else {
+    this->set_map(is_symbol ? heap->short_external_ascii_symbol_map()
+                            : heap->short_external_ascii_string_map());
+  }
   ExternalAsciiString* self = ExternalAsciiString::cast(this);
-  self->set_length(length);
-  self->set_hash_field(hash_field);
   self->set_resource(resource);
-  // Additionally make the object into an external symbol if the original string
-  // was a symbol to start with.
-  if (is_symbol) {
-    self->Hash();  // Force regeneration of the hash value.
-    // Now morph this external string into a external symbol.
-    this->set_map(heap->external_ascii_symbol_map());
-  }
+  if (is_symbol) self->Hash();  // Force regeneration of the hash value.
 
   // Fill the remainder of the string with dead wood.
   int new_size = this->Size();  // Byte size of the external String object.
   heap->CreateFillerObjectAt(this->address() + new_size, size - new_size);
   if (Marking::IsBlack(Marking::MarkBitFrom(this))) {
     MemoryChunk::IncrementLiveBytes(this->address(), new_size - size);
   }
-
   return true;
 }
 
 
 void String::StringShortPrint(StringStream* accumulator) {
   int len = length();
   if (len > kMaxShortPrintLength) {
     accumulator->Add("<Very long string[%u]>", len);
     return;
   }
@@ skipping 11468 matching lines @@
   if (break_point_objects()->IsUndefined()) return 0;
   // Single break point.
   if (!break_point_objects()->IsFixedArray()) return 1;
   // Multiple break points.
   return FixedArray::cast(break_point_objects())->length();
 }
 #endif  // ENABLE_DEBUGGER_SUPPORT
 
 
 } }  // namespace v8::internal