[builtins] Improve StringPrototypeEndsWith performance by adding a fastpath.

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"
 #include "src/regexp/regexp-utils.h"
 
@@ skipping 740 matching lines @@
     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()));
     end = static_cast<uint32_t>(index);
   }
 
   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();
+
+    return isolate->heap()->ToBoolean(memcmp(str_vector.start() + start,
+                                             search_vector.start(),
+                                             search_string->length()) == 0);
+  }
+
+  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)) {
       return isolate->heap()->false_value();
     }
   }
   return isolate->heap()->true_value();
 }
 
 // ES6 section 21.1.3.7
@@ skipping 654 matching lines @@
         Runtime::kThrowIncompatibleMethodReceiver, context,
         assembler->HeapConstant(assembler->factory()->NewStringFromAsciiChecked(
             "String Iterator.prototype.next", TENURED)),
         iterator);
     assembler->Return(result);  // Never reached.
   }
 }
 
 }  // namespace internal
 }  // namespace v8