Revert of [builtins] migrate C++ String Iterator builtins to baseline TurboFan...

Revert of [builtins] migrate C++ String Iterator builtins to baseline TurboFan (patchset #13 id:260001 of https://codereview.chromium.org/2358263002/ )

Reason for revert:
Introduces an infinite loop (see comment).

Original issue's description:
> [builtins] migrate C++ String Iterator builtins to baseline TurboFan
>
> Migrate newly added C++ String Iterator builtins to TFJ builtins, per
> step 4. of the String Iterator Baseline Implementation section of the design doc
>
> BUG=v8:5388
> R=bmeurer@chromium.org, mstarzinger@chromium.org
>
> Committed: https://crrev.com/f9a2c8b1112c4e915df8bc5f7ea1fccdf7a33ff8
> Cr-Commit-Position: refs/heads/master@{#39765}

TBR=bmeurer@chromium.org,mstarzinger@chromium.org,caitp@igalia.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=v8:5388

Review-Url: https://codereview.chromium.org/2374123005
Cr-Commit-Position: refs/heads/master@{#39870}

src/bootstrapper.cc

@@ -1438,18 +1438,10 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
         factory->NewStringFromAsciiChecked("String Iterator"),
         static_cast<PropertyAttributes>(DONT_ENUM | READ_ONLY));
 
-    Handle<JSFunction> next =
     InstallFunction(string_iterator_prototype, "next", JS_OBJECT_TYPE,
                     JSObject::kHeaderSize, MaybeHandle<JSObject>(),
                     Builtins::kStringIteratorPrototypeNext);
 
-    // Set the expected parameters for %StringIteratorPrototype%.next to 0 (not
-    // including the receiver), as required by the builtin.
-    next->shared()->set_internal_formal_parameter_count(0);
-
-    // Set the length for the function to satisfy ECMA-262.
-    next->shared()->set_length(0);
-
     Handle<JSFunction> string_iterator_function = CreateFunction(
         isolate, factory->NewStringFromAsciiChecked("StringIterator"),
         JS_STRING_ITERATOR_TYPE, JSStringIterator::kSize,

src/builtins/builtins-string.cc

@@ -1028,443 +1028,58 @@ void Builtins::Generate_StringPrototypeValueOf(CodeStubAssembler* assembler) {
   assembler->Return(result);
 }
 
-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::kTagged);
-
-  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->SmiConstant(Smi::FromInt(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);
-      }
+BUILTIN(StringPrototypeIterator) {
+  HandleScope scope(isolate);
+  TO_THIS_STRING(object, "String.prototype[Symbol.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);
+  Handle<String> string;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, string,
+                                     Object::ToString(isolate, object));
 
-        assembler->Bind(&if_stringisnotexternal);
-        {
-          // The {string} is a SlicedString, continue with its parent.
-          Node* index = var_index.value();
-          Node* string_offset =
-              assembler->LoadObjectField(string, SlicedString::kOffsetOffset);
-          Node* string_parent =
-              assembler->LoadObjectField(string, SlicedString::kParentOffset);
-          var_index.Bind(assembler->SmiAdd(index, string_offset));
-          var_string.Bind(string_parent);
-          assembler->Goto(&loop);
-        }
-      }
-    }
-  }
-
-  assembler->Bind(&allocate_iterator);
-  {
-    Node* native_context = assembler->LoadNativeContext(context);
-    Node* map = assembler->LoadFixedArrayElement(
-        native_context,
-        assembler->IntPtrConstant(Context::STRING_ITERATOR_MAP_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->StoreObjectFieldNoWriteBarrier(
-        iterator, JSStringIterator::kStringOffset, var_string.value());
-
-    assembler->StoreObjectFieldNoWriteBarrier(
-        iterator, JSStringIterator::kNextIndexOffset, var_index.value());
-    assembler->Return(iterator);
-  }
+  return *isolate->factory()->NewJSStringIterator(string);
 }
 
-namespace {
-
-// Return the |word32| codepoint at {index}. Supports SeqStrings and
-// ExternalStrings.
-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);
-        }
-      }
-    }
+BUILTIN(StringIteratorPrototypeNext) {
+  HandleScope scope(isolate);
 
-    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);
-      }
-    }
+  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());
 
-  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);
+  int position = iterator->index();
+  int length = string->length();
 
-        // (lead << 10) + trail + SURROGATE_OFFSET
-        var_result.Bind(assembler->Int32Add(
-            assembler->WordShl(lead, assembler->Int32Constant(10)),
-            assembler->Int32Add(trail, surrogate_offset)));
-        break;
-      }
+  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);
       }
-    assembler->Goto(&return_result);
     }
 
-  assembler->Bind(&return_result);
-  return var_result.value();
-}
-
-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->LoadObjectField(iterator, JSStringIterator::kNextIndexOffset);
-  Node* length = assembler->LoadObjectField(string, String::kLengthOffset);
-
-  assembler->Branch(assembler->SmiLessThan(position, length), &next_codepoint,
-                    &return_result);
-
-  assembler->Bind(&next_codepoint);
-  {
-    Node* ch =
-        LoadSurrogatePairAt(assembler, string, assembler->SmiUntag(length),
-                            assembler->SmiUntag(position));
-    Node* value = assembler->StringFromCodePoint(ch, UnicodeEncoding::UTF16);
-    var_value.Bind(value);
-    Node* length = assembler->LoadObjectField(value, String::kLengthOffset);
-    assembler->StoreObjectFieldNoWriteBarrier(
-        iterator, JSStringIterator::kNextIndexOffset,
-        assembler->SmiAdd(position, length));
-    var_done.Bind(assembler->BooleanConstant(false));
-    assembler->Goto(&return_result);
+    // Return single code unit
+    iterator->set_index(position + 1);
+    Handle<String> value =
+        isolate->factory()->LookupSingleCharacterStringFromCode(lead);
+    return *isolate->factory()->NewJSIteratorResult(value, false);
   }
 
-  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->StoreObjectFieldNoWriteBarrier(
-        result, JSIteratorResult::kValueOffset, var_value.value());
-    assembler->StoreObjectFieldNoWriteBarrier(
-        result, JSIteratorResult::kDoneOffset, var_done.value());
-    assembler->Return(result);
-  }
+  iterator->set_string(isolate->heap()->empty_string());
 
-  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.
-  }
+  return *isolate->factory()->NewJSIteratorResult(
+      isolate->factory()->undefined_value(), true);
 }
 
 }  // namespace internal

src/builtins/builtins.h

@@ -563,10 +563,10 @@ namespace internal {
   /* ES6 section 21.1.3.28 String.prototype.valueOf () */                     \
   TFJ(StringPrototypeValueOf, 1)                                              \
   /* ES6 #sec-string.prototype-@@iterator */                                  \
-  TFJ(StringPrototypeIterator, 1)                                             \
+  CPP(StringPrototypeIterator)                                                \
                                                                               \
   /* StringIterator */                                                        \
-  TFJ(StringIteratorPrototypeNext, 1)                                         \
+  CPP(StringIteratorPrototypeNext)                                            \
                                                                               \
   /* Symbol */                                                                \
   CPP(SymbolConstructor)                                                      \

src/code-stub-assembler.cc

@@ -2734,58 +2734,6 @@ Node* CodeStubAssembler::SubString(Node* context, Node* string, Node* from,
   return var_result.value();
 }
 
-Node* CodeStubAssembler::StringFromCodePoint(compiler::Node* codepoint,
-                                             UnicodeEncoding encoding) {
-  Variable var_result(this, MachineRepresentation::kTagged);
-  var_result.Bind(EmptyStringConstant());
-
-  Label if_isword16(this), if_isword32(this), return_result(this);
-
-  Branch(Uint32LessThan(codepoint, Int32Constant(0x10000)), &if_isword16,
-         &if_isword32);
-
-  Bind(&if_isword16);
-  {
-    var_result.Bind(StringFromCharCode(codepoint));
-    Goto(&return_result);
-  }
-
-  Bind(&if_isword32);
-  {
-    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 = Int32Constant(0xD800 - (0x10000 >> 10));
-
-        // lead = (codepoint >> 10) + LEAD_OFFSET
-        Node* lead =
-            Int32Add(WordShr(codepoint, Int32Constant(10)), lead_offset);
-
-        // trail = (codepoint & 0x3FF) + 0xDC00;
-        Node* trail = Int32Add(Word32And(codepoint, Int32Constant(0x3FF)),
-                               Int32Constant(0xDC00));
-
-        // codpoint = (trail << 16) | lead;
-        codepoint = Word32Or(WordShl(trail, Int32Constant(16)), lead);
-        break;
-      }
-    }
-
-    Node* value = AllocateSeqTwoByteString(2);
-    StoreNoWriteBarrier(
-        MachineRepresentation::kWord32, value,
-        IntPtrConstant(SeqTwoByteString::kHeaderSize - kHeapObjectTag),
-        codepoint);
-    var_result.Bind(value);
-    Goto(&return_result);
-  }
-
-  Bind(&return_result);
-  return var_result.value();
-}
-
 Node* CodeStubAssembler::StringToNumber(Node* context, Node* input) {
   Label runtime(this, Label::kDeferred);
   Label end(this);

src/code-stub-assembler.h

@@ -19,12 +19,6 @@ class StubCache;
 
 enum class PrimitiveType { kBoolean, kNumber, kString, kSymbol };
 
-enum class UnicodeEncoding {
-  // Different unicode encodings in a |word32|:
-  UTF16,  // hi 16bits -> trailing surrogate or 0, low 16bits -> lead surrogate
-  UTF32,  // full UTF32 code unit / Unicode codepoint
-};
-
 // Provides JavaScript-specific "macro-assembler" functionality on top of the
 // CodeAssembler. By factoring the JavaScript-isms out of the CodeAssembler,
 // it's possible to add JavaScript-specific useful CodeAssembler "macros"
@@ -478,9 +472,6 @@ class CodeStubAssembler : public compiler::CodeAssembler {
   compiler::Node* SubString(compiler::Node* context, compiler::Node* string,
                             compiler::Node* from, compiler::Node* to);
 
-  compiler::Node* StringFromCodePoint(compiler::Node* codepoint,
-                                      UnicodeEncoding encoding);
-
   // Type conversion helpers.
   // Convert a String to a Number.
   compiler::Node* StringToNumber(compiler::Node* context,

src/objects-debug.cc

@@ -785,8 +785,7 @@ void JSWeakMap::JSWeakMapVerify() {
 void JSStringIterator::JSStringIteratorVerify() {
   CHECK(IsJSStringIterator());
   JSObjectVerify();
-  CHECK(string()->IsSeqString() || string()->IsExternalString());
-
+  CHECK(string()->IsString());
   CHECK_GE(index(), 0);
   CHECK_LE(index(), String::kMaxLength);
 }