Copy strings 1 word at a time when flattening etc.

src/utils.h

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 429 matching lines @@
 
   bool is_finalized() const { return position_ < 0; }
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(StringBuilder);
 };
 
 
 // Copy from ASCII/16bit chars to ASCII/16bit chars.
 template <typename sourcechar, typename sinkchar>
 static inline void CopyChars(sinkchar* dest, const sourcechar* src, int chars) {
-  while (chars--) {
+  sinkchar* limit = dest + chars;
+#ifdef CAN_READ_UNALIGNED
+  if (sizeof(*dest) == sizeof(*src)) {
+    // Number of characters in a uint32_t.
+    static const int kStepSize = sizeof(uint32_t) / sizeof(*dest);  // NOLINT
+    while (dest <= limit - kStepSize) {
+      *reinterpret_cast<uint32_t*>(dest) =
+          *reinterpret_cast<const uint32_t*>(src);
+      dest += kStepSize;
+      src += kStepSize;
+    }
+  }
+#endif
+  while (dest < limit) {
     *dest++ = static_cast<sinkchar>(*src++);
   }
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_UTILS_H_