code: @Beta
    public static long floorPowerOfTwo(long x) {
      checkPositive("x", x);
  
      // Long.highestOneBit was buggy on GWT.  We've fixed it, but I'm not certain when the fix will
      // be released.
      return 1L << ((Long.SIZE - 1) - Long.numberOfLeadingZeros(x));
    }
docs: Returns the largest power of two less than or equal to {@code x}. This is equivalent to {@code
  checkedPow(2, log2(x, FLOOR))}.
  
  @throws IllegalArgumentException if {@code x <= 0}
  @since 20.0