Why is += (addition assignment, plus equal) so slow in node?

Question

In certain versions of node, a += b is significantly slower than a = a + b, but similar in the browser and later versions. What causes them to run so differently?

• node v6.10.0 (V8 5.1.281.93), 75% slower, or reverse is 4x faster
• node v8.0.0 (V8 5.8.283.41), 86% slower, or reverse is 7x faster
• node v8.2.1 (V8 5.8.283.41), 86% slower, or reverse is 7x faster
• node v8.3.0 (V8 6.0.286.52), similar
• node v8.7.0 (v8 6.1.534.42), similar
• node v8.9.2 (V8 6.1.534.48), similar
• chrome 62.0.3202.94 (V8 6.1.534.42), similar
• safari 11.0.1, similar

Edit: found that it's related to node's v8 engine update from 5 to 6.

Snippet:

const iter = 10 * 1000 * 1000

function one() {
  let i = 0
  let sum = 0
  while (i < iter) {
    sum += i
    i++
  }
}

function two() {
  let i = 0
  let sum = 0
  while (i < iter) {
    sum = sum + i
    i++
  }
}

let time
const tries = 10

time = Date.now()
for (let i = 0; i < tries; i++) {
  one()
}
console.log('one: ' + String(Date.now() - time))

time = Date.now()
for (let i = 0; i < tries; i++) {
  two()
}
console.log('two: ' + String(Date.now() - time))

Answer 1

This is not an issue in Node, but with the V8 engine (which Node happens to use). There's a list of "bailout reasons" on the github page (github.com/vhf/v8-bailout-reasons), which are language constructs that kill optimization. "Unsupported let compound assignment", that is, compound assignment with let-bound variables, is one of them. If you profile the code in Chromium's dev tools, you should be able to see "Not optimized: Unsupported let compound assignment": https://i.stack.imgur.com/qBlp3.png