why is this code running time different?

[Jeong HyunJun]

hello, I've seen a performance difference depending on whether or not a variable is used in the loop.

let longString = '';

for (let i=0;i<10000000;i++) {
longString += `${i}`;
}

console.log(longString.length);

let cnt = 0;
const st = Date.now();

for (let i=0;i<longString.length - 1;i++) {
  if (longString[1] === longString[0]) {

  /// if (longString[i] === longString[0]) {
    cnt ++;
  }
}

console.log(Date.now() - st, cnt);

It is twice as slow with longString[i] than with longString[1].

What optimizations are causing these results?

It doesn't seem to have anything to do with ConsString flattening.

[Jakob Kummerow]

`longString[1]` is loop-independent and can hence be hoisted out of the loop. `longString[i]` actually has to be loaded from memory in each iteration.

(I haven't spent the time to check whether that's really what's happening. Using `--print-opt-code`, or `--trace-turbo` and Turbolizer, you can check on your own.)

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[Jeong HyunJun]

sorry,  longString[i] is faster than longString[1]

i checked with `--print-opt-code`, but it was too hard to see the difference

2023년 3월 23일 목요일 오후 11시 11분 44초 UTC+9에 Jakob Kummerow님이 작성: