为什么 JavaScript 代码在 Node.js 优化后运行变慢

Question

当我运行遇到一个有趣的问题时，我正在为 ICPC problem using JavaScript and Node.js 创建一个解决方案：在某些情况下，我的程序在同一数据集上运行会慢两倍。

我精简了它，直到我得到这个演示行为的最小示例：

function solve(arr) {
  const total = arr.reduce((a, c) => a + c, 0);
  const count = arr.length;
  for (let i = 0; i < total; i++) {
    for (let j = 0; j < count; j++) {
      // calculate some stuff
    }
  }
}

for (let i = 0; i < 10; i++) {
  // generate some sample data (array of 5000 random numbers 1-10)
  const data = [];
  for (let i = 0; i < 5000; i++) {
    data.push(Math.floor(Math.random() * 10) + 1);
  }

  const start = new Date();
  solve(data);  // run solve on the data
  console.log(`${i + 1}: ${new Date() - start}ms`);
}

这是运行ning node --trace-opt code.js 使用 Node v10.15.1:

的输出

[marking 0x005062b82521 <JSFunction solve (sfi = 000001DA56AD8CD9)> for optimized recompilation, reason: small function, ICs with typeinfo: 5/7 (71%), generic ICs: 0/7 (0%)]
[compiling method 0x005062b82521 <JSFunction solve (sfi = 000001DA56AD8CD9)> using TurboFan OSR]
[optimizing 0x005062b82521 <JSFunction solve (sfi = 000001DA56AD8CD9)> - took 1.453, 0.702, 0.082 ms]
1: 86ms
[marking 0x005062b82581 <JSFunction (sfi = 000001DA56AD8BD9)> for optimized recompilation, reason: hot and stable, ICs with typeinfo: 22/23 (95%), generic ICs: 1/23 (4%)]
[compiling method 0x005062b82521 <JSFunction solve (sfi = 000001DA56AD8CD9)> using TurboFan]
[optimizing 0x005062b82521 <JSFunction solve (sfi = 000001DA56AD8CD9)> - took 0.159, 0.632, 0.096 ms]
2: 82ms
3: 80ms
[compiling method 0x005062b82581 <JSFunction (sfi = 000001DA56AD8BD9)> using TurboFan OSR]
[optimizing 0x005062b82581 <JSFunction (sfi = 000001DA56AD8BD9)> - took 0.592, 2.312, 0.154 ms]
4: 245ms
5: 243ms
6: 236ms
7: 237ms
8: 240ms
9: 246ms
10: 239ms

在前三次迭代中，运行时间约为 80 毫秒，但就在第四次迭代之前 Node 重新编译并优化了一个方法，从那时起 代码运行慢了大约 3 倍。

通常当 Node 进行运行时间分析、重新编译和优化时，一切都更快运行秒。

谁能解释为什么 Node 在这种情况下优化会使事情变得更糟？

请注意，如果示例代码更改为通过迭代而不是使用 reduce 来计算 total，优化会按预期提高性能（运行时间下降到 60 毫秒左右）：

let total = 0;
for (let v of arr) total += v;

Answer 1

我提交了错误报告并得到了 following response from a Chromium 开发者：

Some array builtins used branch hints for loop bounds checks, causing all code after the inlined builtin to become deferred code. This is detrimental for performance.

事实证明这是 TurboFan 编译器和 a fix has been created 的一个已知问题，目前正在测试中：

This CL removes the hints, which improves code scheduling a lot, on the micro benchmark from the linked bug by 3x.

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为什么 JavaScript 代码在 Node.js 优化后运行变慢