Recommit introducing short external strings.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 2ebeb655f28d210a2a6905147457461bf47a51c6..f3fe4cff921b187d3bd588110e28bd190df19fad 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -952,33 +952,32 @@ bool String::MakeExternal(v8::String::ExternalStringResource* resource) {
 #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.
@@ -1004,28 +1003,23 @@ bool String::MakeExternal(v8::String::ExternalAsciiStringResource* resource) {
 #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.
@@ -1033,7 +1027,6 @@ bool String::MakeExternal(v8::String::ExternalAsciiStringResource* resource) {
   if (Marking::IsBlack(Marking::MarkBitFrom(this))) {
     MemoryChunk::IncrementLiveBytes(this->address(), new_size - size);
   }
-
   return true;
 }