[builtins] migrate C++ String Iterator builtins to baseline TurboFan

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 598 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());
-
-  int position = iterator->index();
-  int length = string->length();
-
-  if (position < length) {
-    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 surrogate pair code units
-        iterator->set_index(position + 2);
-        Handle<String> value =
-            isolate->factory()->NewSurrogatePairString(lead, trail);
-        return *isolate->factory()->NewJSIteratorResult(value, false);
-      }
-    }
-
-    // Return single code unit
-    iterator->set_index(position + 1);
-    Handle<String> value =
-        isolate->factory()->LookupSingleCharacterStringFromCode(lead);
-    return *isolate->factory()->NewJSIteratorResult(value, false);
-  }
-
-  iterator->set_string(isolate->heap()->empty_string());
-
-  return *isolate->factory()->NewJSIteratorResult(
-      isolate->factory()->undefined_value(), true);
+void Builtins::Generate_StringPrototypeIterator(CodeStubAssembler* assembler) {
+  typedef CodeStubAssembler::Label Label;
+  typedef compiler::Node Node;
+  typedef CodeStubAssembler::Variable Variable;
+
+  Variable var_string(assembler, MachineRepresentation::kTagged);
+  Variable var_index(assembler, MachineRepresentation::kWord32);
+
+  Variable* loop_inputs[] = {&var_string, &var_index};
+  Label loop(assembler, 2, loop_inputs);
+  Label allocate_iterator(assembler);
+
+  Node* receiver = assembler->Parameter(0);
+  Node* context = assembler->Parameter(3);
+
+  Node* string = assembler->ToThisString(context, receiver,
+                                         "String.prototype[Symbol.iterator]");
+  var_string.Bind(string);
+  var_index.Bind(assembler->Int32Constant(0));
+
+  assembler->Goto(&loop);
+  assembler->Bind(&loop);
+  {
+    // Load the instance type of the {string}.
+    Node* string_instance_type = assembler->LoadInstanceType(string);
+
+    // Check if the {string} is a SeqString.
+    Label if_stringisnotsequential(assembler);
+    assembler->Branch(assembler->Word32Equal(
+                          assembler->Word32And(string_instance_type,
+                                               assembler->Int32Constant(
+                                                   kStringRepresentationMask)),
+                          assembler->Int32Constant(kSeqStringTag)),
+                      &allocate_iterator, &if_stringisnotsequential);
+
+    assembler->Bind(&if_stringisnotsequential);
+    {
+      // Check if the {string} is a ConsString.
+      Label if_stringiscons(assembler), if_stringisnotcons(assembler);
+      assembler->Branch(
+          assembler->Word32Equal(
+              assembler->Word32And(
+                  string_instance_type,
+                  assembler->Int32Constant(kStringRepresentationMask)),
+              assembler->Int32Constant(kConsStringTag)),
+          &if_stringiscons, &if_stringisnotcons);
+
+      assembler->Bind(&if_stringiscons);
+      {
+        // Flatten cons-string and finish.
+        var_string.Bind(assembler->CallRuntime(
+            Runtime::kFlattenString, assembler->NoContextConstant(), string));
+        assembler->Goto(&allocate_iterator);
+      }
+
+      assembler->Bind(&if_stringisnotcons);
+      {
+        // Check if the {string} is an ExternalString.
+        Label if_stringisnotexternal(assembler);
+        assembler->Branch(
+            assembler->Word32Equal(
+                assembler->Word32And(
+                    string_instance_type,
+                    assembler->Int32Constant(kStringRepresentationMask)),
+                assembler->Int32Constant(kExternalStringTag)),
+            &allocate_iterator, &if_stringisnotexternal);
+
+        assembler->Bind(&if_stringisnotexternal);
+        {
+          // The {string} is a SlicedString, continue with its parent.
+          Node* index = var_index.value();
+          Node* string_offset = assembler->LoadAndUntagObjectField(
+              string, SlicedString::kOffsetOffset);
+          Node* string_parent =
+              assembler->LoadObjectField(string, SlicedString::kParentOffset);
+          var_index.Bind(assembler->IntPtrAdd(index, string_offset));
+          var_string.Bind(string_parent);
+          assembler->Goto(&loop);
+        }
+      }
+    }
+  }
+
+  assembler->Bind(&allocate_iterator);
+  {
+    Node* native_context = assembler->LoadNativeContext(context);
+    Node* index = assembler->Int32Constant(Context::STRING_ITERATOR_MAP_INDEX);
+    Node* map = assembler->LoadFixedArrayElement(
+        native_context, index, 0, CodeStubAssembler::INTPTR_PARAMETERS);
+    Node* iterator = assembler->Allocate(JSStringIterator::kSize);
+    assembler->StoreMapNoWriteBarrier(iterator, map);
+    assembler->StoreObjectFieldRoot(iterator, JSValue::kPropertiesOffset,
+                                    Heap::kEmptyFixedArrayRootIndex);
+    assembler->StoreObjectFieldRoot(iterator, JSObject::kElementsOffset,
+                                    Heap::kEmptyFixedArrayRootIndex);
+    assembler->StoreObjectField(iterator, JSStringIterator::kStringOffset,

[Benedikt Meurer, 2016/09/23 18:14:53]

This doesn't need a write barrier, since the object is newly allocated in new
space.

[caitp, 2016/09/23 19:14:16]

Done.

+                                var_string.value());
+
+    assembler->StoreObjectField(iterator, JSStringIterator::kNextIndexOffset,

[Benedikt Meurer, 2016/09/23 18:14:53]

This doesn't need a write barrier, you can safely skip it.

[caitp, 2016/09/23 19:14:16]

Done.

+                                assembler->SmiFromWord32(var_index.value()));
+    assembler->Return(iterator);
+  }
+}
+
+namespace {
+
+enum class UnicodeEncoding {
+  UTF16,  // high 16bits trail surrogate, low 16bits lead surrogate
+  UTF32
+};
+
+// Return the |String| containing a single {codepoint}, which may be encoded
+// as one of the following: a single UTF16 code unit, a |word32| with the
+// high 16 bits set to the trailing surrogate, and low 16 bits set to the lead
+// surrogate, or a single UTF32-encoded codepoint.
+static compiler::Node* CreateStringFromCodePointInternal(

[Benedikt Meurer, 2016/09/23 03:46:24]

You don't need static inside an anonymous namespace.
I'm also not sure about the encoding, why do you need that?
Because w/o the encoding I think this should be on the CodeStubAssembler as
StringFromCodePoint, which we can also utilize for String.fromCodePoint in the
future.

[caitp, 2016/09/23 10:14:28]

The UnicodeEncoding thing is there to control how the character is represented,
which would allow it to be used for String.prototype.codePointAt /
String.prototype.fromCodePoint. But that's the only reason

[caitp, 2016/09/23 19:14:16]

The UnicodeEncoding thing is there to control how the character is represented,
which would allow it to be used for String.prototype.codePointAt /
String.prototype.fromCodePoint.

[Benedikt Meurer, 2016/09/26 04:15:40]

Acknowledged.

+    CodeStubAssembler* assembler, compiler::Node* codepoint,
+    UnicodeEncoding encoding) {
+  typedef CodeStubAssembler::Label Label;
+  typedef compiler::Node Node;
+  typedef CodeStubAssembler::Variable Variable;
+
+  Variable var_result(assembler, MachineRepresentation::kTagged);
+  var_result.Bind(assembler->EmptyStringConstant());
+
+  Label if_isonebyte(assembler), if_istwobyte(assembler),
+      return_result(assembler);
+
+  assembler->Branch(
+      assembler->Uint32LessThan(codepoint, assembler->Int32Constant(0x100)),
+      &if_isonebyte, &if_istwobyte);
+
+  assembler->Bind(&if_isonebyte);
+  {
+    Node* value = assembler->AllocateSeqOneByteString(1);
+    assembler->StoreNoWriteBarrier(
+        MachineRepresentation::kWord8, value,
+        assembler->IntPtrConstant(SeqOneByteString::kHeaderSize -
+                                  kHeapObjectTag),
+        codepoint);
+    var_result.Bind(value);
+    assembler->Goto(&return_result);
+  }
+
+  assembler->Bind(&if_istwobyte);
+  {
+    Label if_isdouble(assembler), if_issingle(assembler);
+    assembler->Branch(
+        assembler->Uint32LessThan(codepoint, assembler->Int32Constant(0x10000)),
+        &if_issingle, &if_isdouble);
+
+    assembler->Bind(&if_issingle);
+    {
+      Node* value = assembler->AllocateSeqTwoByteString(1);
+      assembler->StoreNoWriteBarrier(
+          MachineRepresentation::kWord16, value,
+          assembler->IntPtrConstant(SeqTwoByteString::kHeaderSize -
+                                    kHeapObjectTag),
+          codepoint);
+      var_result.Bind(value);
+      assembler->Goto(&return_result);
+    }
+
+    assembler->Bind(&if_isdouble);
+    {
+      switch (encoding) {
+        case UnicodeEncoding::UTF16:
+          break;
+        case UnicodeEncoding::UTF32: {
+          // Convert UTF32 to UTF16 code units, and store as a 32 bit word.
+          Node* lead_offset =

[caitp, 2016/09/23 19:14:16]

This code here in particular should be very useful for `String.fromCodePoint()`,
that's what the `UnicodeEncoding` parameter was originally added for. Similarly,
`String.prototype.codePointAt()` is able to make use of the logic in
LoadSurrogatePairsInternal() with the UTF32 encoding.

Do you have an idea how to improve the "useful for other methods" aspect of
this?

+              assembler->Int32Constant(0xD800 - (0x10000 >> 10));
+
+          Node* lead = assembler->Int32Add(
+              assembler->WordShr(codepoint, assembler->Int32Constant(10)),
+              lead_offset);
+
+          Node* trail = assembler->Int32Add(
+              assembler->Word32And(codepoint, assembler->Int32Constant(0x3FF)),
+              assembler->Int32Constant(0xDC00));
+
+          codepoint = assembler->Word32Or(
+              assembler->WordShl(trail, assembler->Int32Constant(16)), lead);
+        }
+        default:
+          UNREACHABLE();
+      }
+
+      Node* value = assembler->AllocateSeqTwoByteString(2);
+      assembler->StoreNoWriteBarrier(
+          MachineRepresentation::kWord32, value,
+          assembler->IntPtrConstant(SeqTwoByteString::kHeaderSize -
+                                    kHeapObjectTag),
+          codepoint);
+      var_result.Bind(value);
+      assembler->Goto(&return_result);
+    }
+  }
+
+  assembler->Bind(&return_result);
+  return var_result.value();
+}
+
+static compiler::Node* CreateStringFromSurrogatePair(
+    CodeStubAssembler* assembler, compiler::Node* code_units) {
+  return CreateStringFromCodePointInternal(assembler, code_units,
+                                           UnicodeEncoding::UTF16);
+}
+
+// Return the |word32| codepoint at {index}. Supports SeqStrings and
+// ExternalStrings.
+static compiler::Node* LoadSurrogatePairInternal(CodeStubAssembler* assembler,
+                                                 compiler::Node* string,
+                                                 compiler::Node* length,
+                                                 compiler::Node* index,
+                                                 UnicodeEncoding encoding) {
+  typedef CodeStubAssembler::Label Label;
+  typedef compiler::Node Node;
+  typedef CodeStubAssembler::Variable Variable;
+  Label handle_surrogate_pair(assembler), return_result(assembler);
+  Variable var_result(assembler, MachineRepresentation::kWord32);
+  Variable var_trail(assembler, MachineRepresentation::kWord16);
+  var_result.Bind(assembler->Int32Constant(0));
+  var_trail.Bind(assembler->Int32Constant(0));
+
+  Node* string_instance_type = assembler->LoadInstanceType(string);
+
+  Label if_stringissequential(assembler), if_stringisexternal(assembler);
+  assembler->Branch(assembler->Word32Equal(
+                        assembler->Word32And(string_instance_type,
+                                             assembler->Int32Constant(
+                                                 kStringRepresentationMask)),
+                        assembler->Int32Constant(kSeqStringTag)),
+                    &if_stringissequential, &if_stringisexternal);
+
+  assembler->Bind(&if_stringissequential);
+  {
+    Label if_stringisonebyte(assembler), if_stringistwobyte(assembler);
+    assembler->Branch(
+        assembler->Word32Equal(
+            assembler->Word32And(string_instance_type,
+                                 assembler->Int32Constant(kStringEncodingMask)),
+            assembler->Int32Constant(kOneByteStringTag)),
+        &if_stringisonebyte, &if_stringistwobyte);
+
+    assembler->Bind(&if_stringisonebyte);
+    {
+      var_result.Bind(assembler->Load(
+          MachineType::Uint8(), string,
+          assembler->IntPtrAdd(
+              index, assembler->IntPtrConstant(SeqOneByteString::kHeaderSize -
+                                               kHeapObjectTag))));
+      assembler->Goto(&return_result);
+    }
+
+    assembler->Bind(&if_stringistwobyte);
+    {
+      Node* lead = assembler->Load(
+          MachineType::Uint16(), string,
+          assembler->IntPtrAdd(
+              assembler->WordShl(index, assembler->IntPtrConstant(1)),
+              assembler->IntPtrConstant(SeqTwoByteString::kHeaderSize -
+                                        kHeapObjectTag)));
+      var_result.Bind(lead);
+      Node* next_pos = assembler->Int32Add(index, assembler->Int32Constant(1));
+
+      Label if_isdoublecodeunit(assembler);
+      assembler->GotoIf(assembler->Int32GreaterThanOrEqual(next_pos, length),
+                        &return_result);
+      assembler->GotoIf(
+          assembler->Uint32LessThan(lead, assembler->Int32Constant(0xD800)),
+          &return_result);
+      assembler->Branch(
+          assembler->Uint32LessThan(lead, assembler->Int32Constant(0xDC00)),
+          &if_isdoublecodeunit, &return_result);
+
+      assembler->Bind(&if_isdoublecodeunit);
+      {
+        Node* trail = assembler->Load(
+            MachineType::Uint16(), string,
+            assembler->IntPtrAdd(
+                assembler->WordShl(next_pos, assembler->IntPtrConstant(1)),
+                assembler->IntPtrConstant(SeqTwoByteString::kHeaderSize -
+                                          kHeapObjectTag)));
+        assembler->GotoIf(
+            assembler->Uint32LessThan(trail, assembler->Int32Constant(0xDC00)),
+            &return_result);
+        assembler->GotoIf(assembler->Uint32GreaterThanOrEqual(
+                              trail, assembler->Int32Constant(0xE000)),
+                          &return_result);
+
+        var_trail.Bind(trail);
+        assembler->Goto(&handle_surrogate_pair);
+      }
+    }
+  }
+
+  assembler->Bind(&if_stringisexternal);
+  {
+    assembler->Assert(assembler->Word32Equal(
+        assembler->Word32And(
+            string_instance_type,
+            assembler->Int32Constant(kStringRepresentationMask)),
+        assembler->Int32Constant(kExternalStringTag)));
+    Label if_stringisshort(assembler), if_stringisnotshort(assembler);
+
+    assembler->Branch(assembler->Word32Equal(
+                          assembler->Word32And(string_instance_type,
+                                               assembler->Int32Constant(
+                                                   kShortExternalStringMask)),
+                          assembler->Int32Constant(0)),
+                      &if_stringisshort, &if_stringisnotshort);
+
+    assembler->Bind(&if_stringisshort);
+    {
+      // Load the actual resource data from the {string}.
+      Node* string_resource_data = assembler->LoadObjectField(
+          string, ExternalString::kResourceDataOffset, MachineType::Pointer());
+
+      Label if_stringistwobyte(assembler), if_stringisonebyte(assembler);
+      assembler->Branch(assembler->Word32Equal(
+                            assembler->Word32And(
+                                string_instance_type,
+                                assembler->Int32Constant(kStringEncodingMask)),
+                            assembler->Int32Constant(kTwoByteStringTag)),
+                        &if_stringistwobyte, &if_stringisonebyte);
+
+      assembler->Bind(&if_stringisonebyte);
+      {
+        var_result.Bind(
+            assembler->Load(MachineType::Uint8(), string_resource_data, index));
+        assembler->Goto(&return_result);
+      }
+
+      assembler->Bind(&if_stringistwobyte);
+      {
+        Label if_isdoublecodeunit(assembler);
+        Node* lead = assembler->Load(
+            MachineType::Uint16(), string_resource_data,
+            assembler->WordShl(index, assembler->IntPtrConstant(1)));
+        var_result.Bind(lead);
+        Node* next_pos =
+            assembler->Int32Add(index, assembler->Int32Constant(1));
+
+        assembler->GotoIf(assembler->Int32GreaterThanOrEqual(next_pos, length),
+                          &return_result);
+        assembler->GotoIf(
+            assembler->Uint32LessThan(lead, assembler->Int32Constant(0xD800)),
+            &return_result);
+        assembler->Branch(
+            assembler->Uint32LessThan(lead, assembler->Int32Constant(0xDC00)),
+            &if_isdoublecodeunit, &return_result);
+
+        assembler->Bind(&if_isdoublecodeunit);
+        {
+          Node* trail = assembler->Load(
+              MachineType::Uint16(), string,
+              assembler->IntPtrAdd(
+                  assembler->WordShl(next_pos, assembler->IntPtrConstant(1)),
+                  assembler->IntPtrConstant(SeqTwoByteString::kHeaderSize -
+                                            kHeapObjectTag)));
+          assembler->GotoIf(assembler->Uint32LessThan(
+                                trail, assembler->Int32Constant(0xDC00)),
+                            &return_result);
+          assembler->GotoIf(assembler->Uint32GreaterThanOrEqual(
+                                trail, assembler->Int32Constant(0xE000)),
+                            &return_result);
+
+          var_trail.Bind(trail);
+          assembler->Goto(&handle_surrogate_pair);
+        }
+      }
+    }
+
+    assembler->Bind(&if_stringisnotshort);
+    {
+      Label if_isdoublecodeunit(assembler);
+      Node* lead = assembler->SmiToWord32(assembler->CallRuntime(
+          Runtime::kExternalStringGetChar, assembler->NoContextConstant(),
+          string, assembler->SmiTag(index)));
+      var_result.Bind(lead);
+      Node* next_pos = assembler->Int32Add(index, assembler->Int32Constant(1));
+
+      assembler->GotoIf(assembler->Int32GreaterThanOrEqual(next_pos, length),
+                        &return_result);
+      assembler->GotoIf(
+          assembler->Uint32LessThan(lead, assembler->Int32Constant(0xD800)),
+          &return_result);
+      assembler->Branch(assembler->Uint32GreaterThanOrEqual(
+                            lead, assembler->Int32Constant(0xDC00)),
+                        &return_result, &if_isdoublecodeunit);
+
+      assembler->Bind(&if_isdoublecodeunit);
+      {
+        Node* trail = assembler->SmiToWord32(assembler->CallRuntime(
+            Runtime::kExternalStringGetChar, assembler->NoContextConstant(),
+            string, assembler->SmiTag(next_pos)));
+        assembler->GotoIf(
+            assembler->Uint32LessThan(trail, assembler->Int32Constant(0xDC00)),
+            &return_result);
+        assembler->GotoIf(assembler->Uint32GreaterThanOrEqual(
+                              trail, assembler->Int32Constant(0xE000)),
+                          &return_result);
+        var_trail.Bind(trail);
+        assembler->Goto(&handle_surrogate_pair);
+      }
+    }
+  }
+
+  assembler->Bind(&handle_surrogate_pair);
+  {
+    Node* lead = var_result.value();
+    Node* trail = var_trail.value();
+#ifdef ENABLE_SLOW_DCHECKS
+    // Check that this path is only taken if a surrogate pair is found
+    assembler->Assert(assembler->Uint32GreaterThanOrEqual(
+        lead, assembler->Int32Constant(0xD800)));
+    assembler->Assert(
+        assembler->Uint32LessThan(lead, assembler->Int32Constant(0xDC00)));
+    assembler->Assert(assembler->Uint32GreaterThanOrEqual(
+        trail, assembler->Int32Constant(0xDC00)));
+    assembler->Assert(
+        assembler->Uint32LessThan(trail, assembler->Int32Constant(0xE000)));
+#endif
+
+    switch (encoding) {
+      case UnicodeEncoding::UTF16:
+        var_result.Bind(assembler->WordOr(
+            assembler->WordShl(trail, assembler->Int32Constant(16)), lead));
+        break;
+
+      case UnicodeEncoding::UTF32: {
+        // Convert UTF16 surrogate pair into |word32| code point, encoded as
+        // UTF32.
+        Node* surrogate_offset =
+            assembler->Int32Constant(0x10000 - (0xD800 << 10) - 0xDC00);
+
+        // (lead << 10) + trail + SURROGATE_OFFSET
+        var_result.Bind(assembler->Int32Add(
+            assembler->WordShl(lead, assembler->Int32Constant(10)),
+            assembler->Int32Add(trail, surrogate_offset)));
+        break;
+      }
+    }
+    assembler->Goto(&return_result);
+  }
+
+  assembler->Bind(&return_result);
+  return var_result.value();
+}
+
+static compiler::Node* LoadSurrogatePairAt(CodeStubAssembler* assembler,
+                                           compiler::Node* string,
+                                           compiler::Node* length,
+                                           compiler::Node* index) {
+  return LoadSurrogatePairInternal(assembler, string, length, index,
+                                   UnicodeEncoding::UTF16);
+}
+
+}  // namespace
+
+void Builtins::Generate_StringIteratorPrototypeNext(
+    CodeStubAssembler* assembler) {
+  typedef CodeStubAssembler::Label Label;
+  typedef compiler::Node Node;
+  typedef CodeStubAssembler::Variable Variable;
+
+  Variable var_value(assembler, MachineRepresentation::kTagged);
+  Variable var_done(assembler, MachineRepresentation::kTagged);
+
+  var_value.Bind(assembler->UndefinedConstant());
+  var_done.Bind(assembler->BooleanConstant(true));
+
+  Label throw_bad_receiver(assembler), next_codepoint(assembler),
+      return_result(assembler);
+
+  Node* iterator = assembler->Parameter(0);
+  Node* context = assembler->Parameter(3);
+
+  assembler->GotoIf(assembler->WordIsSmi(iterator), &throw_bad_receiver);
+  assembler->GotoUnless(
+      assembler->WordEqual(assembler->LoadInstanceType(iterator),
+                           assembler->Int32Constant(JS_STRING_ITERATOR_TYPE)),
+      &throw_bad_receiver);
+
+  Node* string =
+      assembler->LoadObjectField(iterator, JSStringIterator::kStringOffset);
+  Node* position = assembler->LoadAndUntagObjectField(
+      iterator, JSStringIterator::kNextIndexOffset);
+  Node* length =
+      assembler->LoadAndUntagObjectField(string, String::kLengthOffset);
+
+  assembler->Branch(assembler->Int32LessThan(position, length), &next_codepoint,
+                    &return_result);
+
+  assembler->Bind(&next_codepoint);
+  {
+    Node* ch = LoadSurrogatePairAt(assembler, string, length, position);
+    Node* value = CreateStringFromSurrogatePair(assembler, ch);
+    var_value.Bind(value);
+    Node* length =
+        assembler->LoadAndUntagObjectField(value, String::kLengthOffset);
+    assembler->StoreObjectFieldNoWriteBarrier(
+        iterator, JSStringIterator::kNextIndexOffset,
+        assembler->SmiFromWord32(assembler->Int32Add(position, length)));
+    var_done.Bind(assembler->BooleanConstant(false));
+    assembler->Goto(&return_result);
+  }
+
+  assembler->Bind(&return_result);
+  {
+    Node* native_context = assembler->LoadNativeContext(context);
+    Node* map = assembler->LoadFixedArrayElement(
+        native_context,
+        assembler->IntPtrConstant(Context::ITERATOR_RESULT_MAP_INDEX), 0,
+        CodeStubAssembler::INTPTR_PARAMETERS);
+    Node* result = assembler->Allocate(JSIteratorResult::kSize);
+    assembler->StoreMapNoWriteBarrier(result, map);
+    assembler->StoreObjectFieldRoot(result, JSIteratorResult::kPropertiesOffset,
+                                    Heap::kEmptyFixedArrayRootIndex);
+    assembler->StoreObjectFieldRoot(result, JSIteratorResult::kElementsOffset,
+                                    Heap::kEmptyFixedArrayRootIndex);
+    assembler->StoreObjectField(result, JSIteratorResult::kValueOffset,

[Benedikt Meurer, 2016/09/23 18:14:53]

You can skip write barrier for these two stores because of newly allocated new
space object.

[caitp, 2016/09/23 19:14:16]

Done.

+                                var_value.value());
+    assembler->StoreObjectField(result, JSIteratorResult::kDoneOffset,
+                                var_done.value());
+    assembler->Return(result);
+  }
+
+  assembler->Bind(&throw_bad_receiver);
+  {
+    // The {receiver} is not a valid JSGeneratorObject.
+    Node* result = assembler->CallRuntime(
+        Runtime::kThrowIncompatibleMethodReceiver, context,
+        assembler->HeapConstant(assembler->factory()->NewStringFromAsciiChecked(
+            "String Iterator.prototype.next", TENURED)),
+        iterator);
+    assembler->Return(result);  // Never reached.
+  }
 }
 
 }  // namespace internal
 }  // namespace v8