[builtins] move String.prototype[@@iterator] to C++ builtin

src/builtins/builtins-string.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/builtins/builtins.h"
 #include "src/builtins/builtins-utils.h"
 
 #include "src/code-factory.h"
 
 namespace v8 {
@@ skipping 589 matching lines @@
   typedef compiler::Node Node;
 
   Node* receiver = assembler->Parameter(0);
   Node* context = assembler->Parameter(3);
 
   Node* result = assembler->ToThisValue(
       context, receiver, PrimitiveType::kString, "String.prototype.valueOf");
   assembler->Return(result);
 }
 
+BUILTIN(StringPrototypeIterator) {
+  HandleScope scope(isolate);
+  TO_THIS_STRING(object, "String.prototype[Symbol.iterator]");
+
+  Handle<String> string;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, string,
+                                     Object::ToString(isolate, object));
+
+  return *isolate->factory()->NewJSStringIterator(string);
+}
+
+BUILTIN(StringIteratorPrototypeNext) {
+  HandleScope scope(isolate);
+
+  if (!args.receiver()->IsJSStringIterator()) {
+    Handle<String> reason = isolate->factory()->NewStringFromAsciiChecked(
+        "String Iterator.prototype.next");
+    THROW_NEW_ERROR_RETURN_FAILURE(
+        isolate,
+        NewTypeError(MessageTemplate::kIncompatibleMethodReceiver, reason));
+  }
+  Handle<JSStringIterator> iterator =
+      Handle<JSStringIterator>::cast(args.receiver());
+  Handle<String> string(iterator->string());
+
+  if (*string != isolate->heap()->empty_string()) {
+    int position;
+    int length = string->length();
+
+    do {
+      if (!iterator->index()->ToUint32(reinterpret_cast<uint32_t*>(&position)))
+        break;
+      if (position >= length) break;
+
+      uint16_t lead = string->Get(position);
+      if (lead >= 0xD800 && lead <= 0xDBFF && position + 1 < length) {
+        uint16_t trail = string->Get(position + 1);
+        if (V8_LIKELY(trail >= 0xDC00 && trail <= 0xDFFF)) {
+          // Return combined code units
+          iterator->set_index(Smi::FromInt(position + 2));
+          Handle<String> value = isolate->factory()->NewProperSubString(

[caitp, 2016/09/15 19:02:39]

I was expecting NewProperSubString to create SlicedStrings, but it looks like it
doesn't. There might be a smarter/faster way to do this instead.

[Benedikt Meurer, 2016/09/16 03:16:37]

Sliced strings would be really a waste of time/space for single char strings.
You should just look up the single char string in the cache and/or create one;
the CodeStubAssembler (once we go to TurboFan) has a StringFromCharCode that
does that.

+              string, position, position + 2);
+          return *isolate->factory()->NewJSIteratorResult(value, false);
+        }
+      }
+
+      // Return single code unit
+      iterator->set_index(Smi::FromInt(position + 1));
+      Handle<String> value = isolate->factory()->NewProperSubString(
+          string, position, position + 1);
+      return *isolate->factory()->NewJSIteratorResult(value, false);
+    } while (false);
+
+    iterator->set_string(isolate->heap()->empty_string());
+  }
+
+  return *isolate->factory()->NewJSIteratorResult(
+      isolate->factory()->undefined_value(), true);
+}
+
 }  // namespace internal
 }  // namespace v8