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

@@ -1438,17 +1438,9 @@ 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);
+    InstallFunction(string_iterator_prototype, "next", JS_OBJECT_TYPE,
+                    JSObject::kHeaderSize, MaybeHandle<JSObject>(),
+                    Builtins::kStringIteratorPrototypeNext);
 
     Handle<JSFunction> string_iterator_function = CreateFunction(
         isolate, factory->NewStringFromAsciiChecked("StringIterator"),

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);
 }