Implement faster splitting with empty string as pattern. Add special native for computing substring…

runtime/lib/string.cc

Index: runtime/lib/string.cc
===================================================================
--- runtime/lib/string.cc	(revision 15545)
+++ runtime/lib/string.cc	(working copy)
@@ -51,7 +51,7 @@
 
 
 DEFINE_NATIVE_ENTRY(StringBase_substringUnchecked, 3) {
-  GET_NATIVE_ARGUMENT(String, receiver, arguments->NativeArgAt(0));
+  const String& receiver = String::CheckedHandle(arguments->NativeArgAt(0));
   GET_NATIVE_ARGUMENT(Smi, start_obj, arguments->NativeArgAt(1));
   GET_NATIVE_ARGUMENT(Smi, end_obj, arguments->NativeArgAt(2));
 
@@ -61,6 +61,18 @@
 }
 
 
+DEFINE_NATIVE_ENTRY(OneByteString_substringUnchecked, 3) {
+  const String& receiver = String::CheckedHandle(arguments->NativeArgAt(0));
+  ASSERT(receiver.IsOneByteString());
+  GET_NATIVE_ARGUMENT(Smi, start_obj, arguments->NativeArgAt(1));
+  GET_NATIVE_ARGUMENT(Smi, end_obj, arguments->NativeArgAt(2));
+
+  const intptr_t start = start_obj.Value();
+  const intptr_t end = end_obj.Value();
+  return OneByteString::New(receiver, start, end - start, Heap::kNew);
+}
+
+
 DEFINE_NATIVE_ENTRY(String_getHashCode, 1) {
   const String& receiver = String::CheckedHandle(arguments->NativeArgAt(0));
   intptr_t hash_val = receiver.Hash();