[builtins] Move StringStartsWith to a C++ builtin.

BUG=v8:5364

Review-Url: https://codereview.chromium.org/2407173002
Cr-Commit-Position: refs/heads/master@{#40165}

src/bootstrapper.cc

@@ -1453,6 +1453,8 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
                           2, true);
     SimpleInstallFunction(prototype, "substring",
                           Builtins::kStringPrototypeSubstring, 2, true);
+    SimpleInstallFunction(prototype, "startsWith",
+                          Builtins::kStringPrototypeStartsWith, 1, false);
     SimpleInstallFunction(prototype, "toString",
                           Builtins::kStringPrototypeToString, 0, true);
     SimpleInstallFunction(prototype, "trim", Builtins::kStringPrototypeTrim, 0,

src/builtins/builtins-string.cc

@@ -1202,6 +1202,55 @@ void Builtins::Generate_StringPrototypeSubstring(CodeStubAssembler* a) {
   }
 }
 
+BUILTIN(StringPrototypeStartsWith) {
+  HandleScope handle_scope(isolate);
+  TO_THIS_STRING(str, "String.prototype.startsWith");
+
+  // Check if the search string is a regExp and fail if it is.
+  Handle<Object> search = args.atOrUndefined(isolate, 1);
+  Maybe<bool> is_reg_exp = Object::IsRegExp(isolate, search);
+  if (is_reg_exp.IsNothing()) {
+    DCHECK(isolate->has_pending_exception());
+    return isolate->heap()->exception();
+  }
+  if (is_reg_exp.FromJust()) {
+    THROW_NEW_ERROR_RETURN_FAILURE(
+        isolate, NewTypeError(MessageTemplate::kFirstArgumentNotRegExp,
+                              isolate->factory()->NewStringFromStaticChars(
+                                  "String.prototype.startsWith")));
+  }
+  Handle<String> search_string;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, search_string,
+                                     Object::ToString(isolate, search));
+
+  Handle<Object> position = args.atOrUndefined(isolate, 2);
+  int start;
+
+  if (position->IsUndefined(isolate)) {
+    start = 0;
+  } else {
+    ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, position,
+                                       Object::ToInteger(isolate, position));
+    double index = std::max(position->Number(), 0.0);
+    index = std::min(index, static_cast<double>(str->length()));
+    start = static_cast<uint32_t>(index);
+  }
+
+  if (start + search_string->length() > str->length()) {
+    return *isolate->factory()->false_value();
+  }
+
+  FlatStringReader str_reader(isolate, String::Flatten(str));
+  FlatStringReader search_reader(isolate, String::Flatten(search_string));
+
+  for (int i = 0; i < search_string->length(); i++) {
+    if (str_reader.Get(start + i) != search_reader.Get(i)) {
+      return *isolate->factory()->false_value();
+    }
+  }
+  return *isolate->factory()->true_value();
+}
+
 // ES6 section 21.1.3.25 String.prototype.toString ()
 void Builtins::Generate_StringPrototypeToString(CodeStubAssembler* assembler) {
   typedef compiler::Node Node;

src/builtins/builtins.h

@@ -632,6 +632,9 @@ namespace internal {
   TFJ(StringPrototypeSubstr, 3)                                               \
   /* ES6 section 21.1.3.19 String.prototype.substring ( start, end ) */       \
   TFJ(StringPrototypeSubstring, 3)                                            \
+  /* ES6 section 21.1.3.20 */                                                 \
+  /* String.prototype.startsWith ( searchString [ , position ] ) */           \
+  CPP(StringPrototypeStartsWith)                                              \
   /* ES6 section 21.1.3.25 String.prototype.toString () */                    \
   TFJ(StringPrototypeToString, 1)                                             \
   CPP(StringPrototypeTrim)                                                    \

src/js/string.js

@@ -380,32 +380,6 @@ function StringRepeat(count) {
 }
 
 
-// ES6 draft 04-05-14, section 21.1.3.18
-function StringStartsWith(searchString, position) {  // length == 1
-  CHECK_OBJECT_COERCIBLE(this, "String.prototype.startsWith");
-
-  var s = TO_STRING(this);
-
-  if (IsRegExp(searchString)) {
-    throw %make_type_error(kFirstArgumentNotRegExp, "String.prototype.startsWith");
-  }
-
-  var ss = TO_STRING(searchString);
-  var pos = TO_INTEGER(position);
-
-  var s_len = s.length;
-  var start = MinSimple(MaxSimple(pos, 0), s_len);
-  var ss_len = ss.length;
-  if (ss_len + start > s_len) {
-    return false;
-  }
-
-  return %_SubString(s, start, start + ss_len) === ss;
-}
-
-%FunctionSetLength(StringStartsWith, 1);
-
-
 // ES6 Draft 05-22-2014, section 21.1.3.3
 function StringCodePointAt(pos) {
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.codePointAt");
@@ -469,7 +443,6 @@ utils.InstallFunctions(GlobalString.prototype, DONT_ENUM, [
   "search", StringSearch,
   "slice", StringSlice,
   "split", StringSplitJS,
-  "startsWith", StringStartsWith,
   "toLowerCase", StringToLowerCaseJS,
   "toLocaleLowerCase", StringToLocaleLowerCase,
   "toUpperCase", StringToUpperCaseJS,

test/mjsunit/es6/string-startswith.js

@@ -34,6 +34,7 @@ assertFalse(testString.startsWith("hello"));
 assertFalse(testString.startsWith("Hello World!"));
 assertFalse(testString.startsWith(null));
 assertFalse(testString.startsWith(undefined));
+assertFalse(testString.startsWith());
 
 assertTrue("null".startsWith(null));
 assertTrue("undefined".startsWith(undefined));