Oh... math.log() has an optional second argument: base. math.log(x, 2). But it is equivalent as math.log(x) / math.log(2) in Python. math.log(x, 2) is implemented as:
  num=math.log(x)
  den=math.log(2)
  return num / den
where num and den are Python floats (64 bits).

So we don't benefit from 80 bits float used internally in x87.