Speed up charCodeAt on very large cons strings, by insisting on

flattening the strings and not trying too hard to traverse a big
cons tree from generated code.
Review URL: http://codereview.chromium.org/402008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3317 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/ia32/codegen-ia32.cc

@@ -4831,6 +4831,12 @@ void CodeGenerator::GenerateFastCharCodeAt(ZoneList<Expression*>* args) {
   __ j(not_equal, &slow_case);
 
   // ConsString.
+  // Check that the right hand side is the empty string (ie if this is really a
+  // flat string in a cons string).  If that is not the case we would rather go
+  // to the runtime system now, to flatten the string.
+  __ mov(temp.reg(), FieldOperand(object.reg(), ConsString::kSecondOffset));
+  __ cmp(Operand(temp.reg()), Immediate(Handle<String>(Heap::empty_string())));
+  __ j(not_equal, &slow_case);
   // Get the first of the two strings.
   __ mov(object.reg(), FieldOperand(object.reg(), ConsString::kFirstOffset));
   __ jmp(&try_again_with_new_string);

src/runtime.cc

@@ -1274,7 +1274,9 @@ static Object* CharCodeAt(String* subject, Object* index) {
   // Flatten the string.  If someone wants to get a char at an index
   // in a cons string, it is likely that more indices will be
   // accessed.
-  subject->TryFlattenIfNotFlat();
+  Object* flat = subject->TryFlatten();
+  if (flat->IsFailure()) return flat;
+  subject = String::cast(flat);
   if (i >= static_cast<uint32_t>(subject->length())) {
     return Heap::nan_value();
   }

src/x64/codegen-x64.cc

@@ -3778,6 +3778,12 @@ void CodeGenerator::GenerateFastCharCodeAt(ZoneList<Expression*>* args) {
   __ j(not_equal, &slow_case);
 
   // ConsString.
+  // Check that the right hand side is the empty string (ie if this is really a
+  // flat string in a cons string).  If that is not the case we would rather go
+  // to the runtime system now, to flatten the string.
+  __ movq(temp.reg(), FieldOperand(object.reg(), ConsString::kSecondOffset));
+  __ CompareRoot(temp.reg(), Heap::kEmptyStringRootIndex);
+  __ j(not_equal, &slow_case);
   // Get the first of the two strings.
   __ movq(object.reg(), FieldOperand(object.reg(), ConsString::kFirstOffset));
   __ jmp(&try_again_with_new_string);