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轮回展开是一种程序变换，经由过程增添每次迭代盘算的元素的数目，削减轮回的迭代次数。

用代码来申明就是将

for (i = 0; i < len; i++) {
    sum += arry[i]
}

替换为

for (i = 0; i < len; i += 2) {
    newSum += arry[i] + arry[i + 1]
}

轮回展开关于算术运算来讲，优化的作用是很大的。我分别对整数运算和浮点数运算作了屡次测试，得出表格以下：

测试环境

• cpu:i5-7400
• 浏览器: chrome 70.0.3538.110

运算是用了1万万个数，取值是运转十次测试得出的平均数。附上加法测试的代码

const arry = []
let num = 10000000
while (num) {
    arry.push(num)
    num--
}

let sum = 0
let last = new Date()
let i 
let len = arry.length
for (i = 0; i < len; i++) {
    sum += arry[i]
}
let now = new Date()

console.log(now - last)

let newSum = 0
last = new Date()
for (i = 0; i < len; i += 2) {
    newSum += arry[i] + arry[i + 1]
}
now = new Date()

console.log(now - last)
console.log(sum, newSum)