@tailrec 如何工作 [英] How does @tailrec work

问题描述

我已经使用并阅读了关于 @tailrec 注释的尾递归方法.我已经浏览了许多解释它的链接.例如，它仅适用于自调用函数，不应被覆盖等.

I have used and read about @tailrec annotation to have a tail recursive method. I have gone through many links that explains it. For example it only works when for self-calling functions and shouldnot be overriden etc.

到处都提到编译器优化.但是编译器做了什么魔术/概念来使它尾递归.对于下面的一个简单函数，编译器做了什么:

Everywhere it is mentioned that the compiler optimizes. But what magic/concept does the compiler do to make it tail recursive. For a simple function below, what does the compiler do:

@tailrec def fact(acc: Int, n: Int): Int = {
  if (n <= 1) acc
  else fact(n * acc, n - 1)
}
fact(1,10)

我的意思是它是否将它转换成一个循环，重复调用它然后返回最终值?有没有解释它的论文链接

I mean does it convert it into a loop where it repeatedly calls it and then returns the final value? Is there any link to paper which explains it

推荐答案

除了我对您的问题的评论(在此处重新粘贴代码):

In addition to my comment on your question (repasting the code here):

  var acc = 1 
  var n = 10
start: 
  if (n <= 1) return acc 
  else { 
    acc = n * acc
    n = n - 1
    goto start
  }

我尝试使用我碰巧拥有的最近版本和 scalac -Xprint:all 编译 fact 方法，并且编译器以某种方式发出了 icode 文件.所以这真的说明了它是如何优化尾调用的:

I tried compiling the fact method with a recent build I just happened to have and with scalac -Xprint:all and somehow the compiler emitted an icode file. So this really illustrates how it optimizes the tail call:

  // methods
  def fact(acc: Int (INT), n: Int (INT)): Int {
  locals: value acc, value n, value _$this
  startBlock: 1
  blocks: [1,2,3,4,5]

  1: 
    2   JUMP 2

  2: // huynhjl's comment: IF condition is here
    3   LOAD_LOCAL(value n)
    3   CONSTANT(1)
    3   CJUMP (INT)LE ? 3 : 4

  3: // huynhjl's comment: first branch of IF, will return acc
    3   LOAD_LOCAL(value acc)
    3   JUMP 5

  5: 
    2   RETURN(INT)

  4: // huynhjl's comment: else branch of IF, update acc and n and jump back
    4   LOAD_LOCAL(value n)
    4   LOAD_LOCAL(value acc)
    4   CALL_PRIMITIVE(Arithmetic(MUL,INT))
    4   LOAD_LOCAL(value n)
    4   CONSTANT(1)
    4   CALL_PRIMITIVE(Arithmetic(SUB,INT))
    4   STORE_LOCAL(value n)
    4   STORE_LOCAL(value acc)
    4   JUMP 2

  }

这篇关于@tailrec 如何工作的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！