Introduce read buffer for external strings when using charAt (ia32).

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 0ecc2867ac0b4706c209f3fa57b5fec1754caaa2..0aa0806b866349aa64750c184a6b7e1d5d27e746 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -3007,7 +3007,12 @@ MaybeObject* Heap::AllocateExternalStringFromAscii(
     return Failure::OutOfMemoryException();
   }
 
-  Map* map = external_ascii_string_map();
+  Map* map;
+  if (length < static_cast<size_t>(ExternalString::kMinBufferedStringLength)) {
+    map = external_ascii_string_map();
+  } else {
+    map = external_buffered_ascii_string_map();
+  }
   Object* result;
   { MaybeObject* maybe_result = Allocate(map, NEW_SPACE);
     if (!maybe_result->ToObject(&result)) return maybe_result;
@@ -3017,7 +3022,9 @@ MaybeObject* Heap::AllocateExternalStringFromAscii(
   external_string->set_length(static_cast<int>(length));
   external_string->set_hash_field(String::kEmptyHashField);
   external_string->set_resource(resource);
-
+  if (external_string->IsBuffered()) {
+    external_string->set_buffer_index(ExternalString::kInvalidBufferIndex);
+  }
   return result;
 }
 
@@ -3035,8 +3042,14 @@ MaybeObject* Heap::AllocateExternalStringFromTwoByte(
   static const size_t kAsciiCheckLengthLimit = 32;
   bool is_ascii = length <= kAsciiCheckLengthLimit &&
       String::IsAscii(resource->data(), static_cast<int>(length));
-  Map* map = is_ascii ?
-      external_string_with_ascii_data_map() : external_string_map();
+  Map* map;
+  if (length < static_cast<size_t>(ExternalString::kMinBufferedStringLength)) {
+    map = is_ascii ? external_string_with_ascii_data_map()
+                   : external_string_map();
+  } else {
+    map = is_ascii ? external_buffered_string_with_ascii_data_map()
+                   : external_buffered_string_map();
+  }
   Object* result;
   { MaybeObject* maybe_result = Allocate(map, NEW_SPACE);
     if (!maybe_result->ToObject(&result)) return maybe_result;
@@ -3046,7 +3059,9 @@ MaybeObject* Heap::AllocateExternalStringFromTwoByte(
   external_string->set_length(static_cast<int>(length));
   external_string->set_hash_field(String::kEmptyHashField);
   external_string->set_resource(resource);
-
+  if (external_string->IsBuffered()) {
+    external_string->set_buffer_index(ExternalString::kInvalidBufferIndex);
+  }
   return result;
 }