Ruby attr_accessor与getter / setter基准：为什么访问器更快？

class A
  # we define two R/W attributes with accessors
  attr_accessor :acc,:bcc

  # we define two attributes with getter/setter-functions
  def dirA=(d); @dirA=d; end
  def dirA; @dirA; end
  def dirB=(d); @dirB=d; end
  def dirB; @dirB; end
end

varA   = A.new
startT = 0
dirT   = 0
accT   = 0

# now we do 100 times the same benchmarking
# where we do the same assignment operation
# 50000 times
100.times do
  startT = Time.now.to_f
  50000.times do |i|
    varA.dirA = i
    varA.dirB = varA.dirA
  end
  dirT += (Time.now.to_f - startT)

  startT = Time.now.to_f
  50000.times do |i|
    varA.acc = i
    varA.bcc = varA.acc
  end
  accT += (Time.now.to_f - startT)
end

puts "direct:   %10.4fs" % (dirT/100)
puts "accessor: %10.4fs" % (accT/100)

程序输出是：

direct:       0.2640s
accessor:     0.1927s

所以attr_accessor明显更快.有人可以分享一些智慧,为什么这样呢？