Class uint128_t

cpp/src/arrow/util/basic_decimal.cc:505–528

// multiplication operations when there may or may not be native support.
#ifdef ARROW_USE_NATIVE_INT128
struct uint128_t {
  uint128_t() {}
  uint128_t(uint64_t hi, uint64_t lo) : val_((static_cast<__uint128_t>(hi) << 64) | lo) {}
  explicit uint128_t(const BasicDecimal128& decimal) {
    val_ = (static_cast<__uint128_t>(decimal.high_bits()) << 64) | decimal.low_bits();
  }

  explicit uint128_t(uint64_t value) : val_(value) {}

  uint64_t hi() { return val_ >> 64; }
  uint64_t lo() { return val_ & kInt64Mask; }

  uint128_t& operator+=(const uint128_t& other) {
    val_ += other.val_;
    return *this;
  }

  uint128_t& operator*=(const uint128_t& other) {
    val_ *= other.val_;
    return *this;
  }

  __uint128_t val_;
};

#else
// Multiply two 64 bit word components into a 128 bit result, with high bits