为什么Ruby的loop命令比true慢? [英] Why is Ruby's loop command slower than while true?

问题描述

Ruby有一个内置的loop命令，该命令永远执行其后的块(或直到被break停止).但是，将其与功能相似的while true进行比较时，它的速度要慢得多:

Ruby has a built-in loop command that executes the block following it forever (or until stopped by break). However, when comparing it against the functionally similar while true, it is significantly slower:

require "benchmark/ips"

NUMBER = 100_000_000

def fast
  index = 0
  while true
    break if index > NUMBER
    index += 1
  end
end

def slow
  index = 0
  loop do
    break if index > NUMBER
    index += 1
  end
end

Benchmark.ips do |x|
  x.report("While Loop")  { fast }
  x.report("Kernel loop") { slow }
  x.compare!
end

在Ruby 2.4.1(p111(2017-03-22修订版58053)[x64-mingw32])下，区别非常明显:

Under Ruby 2.4.1 (p111 (2017-03-22 revision 58053) [x64-mingw32]), the difference is striking:

Warming up --------------------------------------
          While Loop     1.000  i/100ms
         Kernel loop     1.000  i/100ms
Calculating -------------------------------------
          While Loop      0.630  (± 0.0%) i/s -      4.000  in   6.350897s
         Kernel loop      0.190  (± 0.0%) i/s -      1.000  in   5.274249s

Comparison:
          While Loop:        0.6 i/s
         Kernel loop:        0.2 i/s - 3.32x  slower

为什么会有这样的性能差异?为什么单用途loop命令 比通用while更糟糕?

Why is there such a performance difference? And why is the single-purpose loop command worse at its job than the general-purpose while?

(从这里，根据CC-BY-SA许可)

(Benchmark copied from here, licensed under CC-BY-SA)

推荐答案

loop是采用block的内核方法.提醒一下，block 引入了新的局部变量作用域.

loop is a kernel method which takes a block. As a reminder, a block introduces new local variable scope.

例如:

loop do
 a = 2
 break
end
puts a

将返回错误，例如:" NameError:main:Object 的未定义局部变量或方法'a'" 另一方面:

Will return an error such as: "NameError: undefined local variable or method `a' for main:Object" On the other hand:

while true
 a = 2
 break
end
p a #=> return a = 2

因此，loop创建某种局部变量(例如用于break语句的局部变量)将在其范围之内，我不会感到惊讶.在每次迭代中创建/删除这些变量都会减慢该过程.

So I wouldn't be surprised that loop creates some sort of local variable(s) such as one for the break statement that is (are) going to be in its scope. Creating/deleting those variables at every iteration slow down the process.

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