Add CodeStubAssembler::FlattenString

src/code-stub-assembler.cc

Index: src/code-stub-assembler.cc
diff --git a/src/code-stub-assembler.cc b/src/code-stub-assembler.cc
index f23b82829ecc3d8dbc81cf1a7622977a6a67bdca..8e129d4b23e70fd772fce29d2b83a51315bd6b6d 100644
--- a/src/code-stub-assembler.cc
+++ b/src/code-stub-assembler.cc
@@ -3610,15 +3610,44 @@ Node* CodeStubAssembler::ToString(Node* context, Node* input) {
   }
 
   Bind(&runtime);
-  {
-    result.Bind(CallRuntime(Runtime::kToString, context, input));
-    Goto(&done);
-  }
+  result.Bind(CallRuntime(Runtime::kToString, context, input));

[danno, 2016/10/20 08:35:52]

After our discussion, not sure if you want this whitespace change.

+  Goto(&done);
 
   Bind(&done);
   return result.value();
 }
 
+Node* CodeStubAssembler::FlattenString(Node* string) {
+  Variable var_result(this, MachineRepresentation::kTagged);
+  var_result.Bind(string);
+
+  Node* instance_type = LoadInstanceType(string);
+
+  // Check if the {string} is not a ConsString (i.e. already flat).
+  Label is_flat_in_cons(this), end(this);
+  GotoUnless(Word32Equal(Word32And(instance_type,
+                                   Int32Constant(kStringRepresentationMask)),
+                         Int32Constant(kConsStringTag)),
+             &end);
+
+  // Check whether the right hand side is the empty string (i.e. if
+  // this is really a flat string in a cons string).
+  Node* rhs = LoadObjectField(string, ConsString::kSecondOffset);
+  GotoIf(WordEqual(rhs, EmptyStringConstant()), &is_flat_in_cons);
+
+  // Bail out to the runtime.

[danno, 2016/10/20 08:35:52]

Make this label deferred. You can make that efficient by making the above
statement a GotoUnless to the runtime label, which you explicitly declare as
deferred.

+  var_result.Bind(
+      CallRuntime(Runtime::kFlattenString, NoContextConstant(), string));
+  Goto(&end);
+
+  Bind(&is_flat_in_cons);
+  var_result.Bind(LoadObjectField(string, ConsString::kFirstOffset));
+  Goto(&end);
+
+  Bind(&end);
+  return var_result.value();
+}
+
 Node* CodeStubAssembler::JSReceiverToPrimitive(Node* context, Node* input) {
   STATIC_ASSERT(LAST_JS_RECEIVER_TYPE == LAST_TYPE);
   Label if_isreceiver(this, Label::kDeferred), if_isnotreceiver(this);