[builtins] Improve StringPrototypeEndsWith performance by adding a fastpath.

src/builtins/builtins-string.cc

Index: src/builtins/builtins-string.cc
diff --git a/src/builtins/builtins-string.cc b/src/builtins/builtins-string.cc
index 4ccccbc85970cb89ab3977d9ac9c7791d918cdc6..4338676275f8828a4f0fe4a7b54cc1f7b9b0af8a 100644
--- a/src/builtins/builtins-string.cc
+++ b/src/builtins/builtins-string.cc
@@ -758,8 +758,28 @@ BUILTIN(StringPrototypeEndsWith) {
   int start = end - search_string->length();
   if (start < 0) return isolate->heap()->false_value();
 
-  FlatStringReader str_reader(isolate, String::Flatten(str));
-  FlatStringReader search_reader(isolate, String::Flatten(search_string));
+  str = String::Flatten(str);
+  search_string = String::Flatten(search_string);
+
+  DisallowHeapAllocation no_gc;  // ensure vectors stay valid
+  String::FlatContent str_content = str->GetFlatContent();
+  String::FlatContent search_content = search_string->GetFlatContent();
+
+  if (str_content.IsOneByte() && search_content.IsOneByte()) {
+    Vector<const uint8_t> str_vector = str_content.ToOneByteVector();
+    Vector<const uint8_t> search_vector = search_content.ToOneByteVector();
+    const char* str_ptr =
+        reinterpret_cast<const char*>(str_vector.start() + start);
+    const char* search_ptr =
+        reinterpret_cast<const char*>(search_vector.start());
+
+    if (strncmp(str_ptr, search_ptr, search_string->length()) == 0) {
+      return *isolate->factory()->true_value();

[Yang, 2016/11/16 12:13:24]

isolate->heap()->true_value()

also, why don't we return false_value if the compare fails? As in

return isolate->heap()->ToBoolean(strncmp(...) == 0)

[petermarshall, 2016/11/16 12:19:15]

Oops, yep.

+    }
+  }
+
+  FlatStringReader str_reader(isolate, str);
+  FlatStringReader search_reader(isolate, search_string);
 
   for (int i = 0; i < search_string->length(); i++) {
     if (str_reader.Get(start + i) != search_reader.Get(i)) {