Function DoIsStringASCII

src/base/strings/string_util.cc:472–499

template <class Char>
inline bool DoIsStringASCII(const Char* characters, size_t length) {
  MachineWord all_char_bits = 0;
  const Char* end = characters + length;

  // Prologue: align the input.
  while (!IsAlignedToMachineWord(characters) && characters != end) {
    all_char_bits |= *characters;
    ++characters;
  }

  // Compare the values of CPU word size.
  const Char* word_end = AlignToMachineWord(end);
  const size_t loop_increment = sizeof(MachineWord) / sizeof(Char);
  while (characters < word_end) {
    all_char_bits |= *(reinterpret_cast<const MachineWord*>(characters));
    characters += loop_increment;
  }

  // Process the remaining bytes.
  while (characters != end) {
    all_char_bits |= *characters;
    ++characters;
  }

  MachineWord non_ascii_bit_mask =
      NonASCIIMask<sizeof(MachineWord), Char>::value();
  return !(all_char_bits & non_ascii_bit_mask);
}

bool IsStringASCII(const StringPiece& str) {
  return DoIsStringASCII(str.data(), str.length());