IdentitySet#clear

class IdentitySet
  primitive_nobridge '__removeAll', 'removeAll:'

  def clear
    __removeAll(self)   # This idiom optimized in C code.
  end
end

idset = IdentitySet.new
1_000_000.times { |i| idset << i }
p idset.size
idset.clear
p idset.size