* @throws IllegalArgumentException if {@code x <= 0.0}, {@code x} is NaN, or {@code x} is
   *     infinite
   */
  @GwtIncompatible // java.lang.Math.getExponent, com.google.common.math.DoubleUtils
  @SuppressWarnings("fallthrough")
  public static int log2(double x, RoundingMode mode) {
    checkArgument(x > 0.0 && isFinite(x), "x must be positive and finite");
    int exponent = getExponent(x);
    if (!isNormal(x)) {