Bug 283118 – for-of loops noticeably slower than alternatives, including map

• Status: NEW
• Component: JavaScriptCore

[Anthony Ricaud]

Created attachment 473227 [details]
Results in Safari, Firefox and Chrome

On the linked benchmark, a simple for-of loop performs noticeably worse than a forEach loop or a classic for loop.

It even performs worse than the equivalent map transformation which I find surprising because map has to do extra work (destructure the array, create a new one and replace it in the existing array).

Firefox and Chrome results are roughly what I would expect: map is the slowest, classic loop is the fastest, forEach and for-of are in between.

[Keith Miller]

Hi thanks for the report. I just tried running

```
let dataPoints = []
for (let i=0; i <= 200_000; i++) {
	dataPoints[i] = [1, 2]
}

let start = performance.now();
function test() {
	for (const dataPoint of dataPoints) {
		dataPoint[1] *= 8
	}

	// for (let i = 0; i < dataPoints.length; i++) {
	//     dataPoints[i][1] *= 8
	// }
}
test();

let end = performance.now();
print(end - start);
``` 

100 times. Then commenting out the top and uncommenting the bottom and running another 100 times. I got an average of 4.817399999999999 for the first set and 4.576200000000002 for the second set. This seems like a 5% difference. Maybe something about how jsperf.app works is impacting the results?

[Anthony Ricaud]

How can I run that script locally?

And yes, this could be because of the benchmarking does its work.
- https://jsben.ch/bU9pT gives the kind of results I was expecting
- https://jsbench.me/9rm3hwmqsn/1 seems to exhibit a similar issue as jsperf.app

[Keith Miller]

(In reply to Anthony Ricaud from comment #2)
> How can I run that script locally?
> 
> And yes, this could be because of the benchmarking does its work.
> - https://jsben.ch/bU9pT gives the kind of results I was expecting
> - https://jsbench.me/9rm3hwmqsn/1 seems to exhibit a similar issue as
> jsperf.app

It seems like you're on a Mac so you can do copy into a file then do: `/System/Library/Frameworks/JavaScriptCore.framework/Versions/A/Helpers/jsc file.js`

[Anthony Ricaud]

Thanks.

I'm getting this error though:
Exception: ReferenceError: Can't find variable: performance

[Keith Miller]

(In reply to Anthony Ricaud from comment #4)
> Thanks.
> 
> I'm getting this error though:
> Exception: ReferenceError: Can't find variable: performance

Ah yeah, I ran on ToT. `performance` was added to the CLI somewhat recently, you could just use `Date.now()` although it's less accurate.

[Anthony Ricaud]

I have STP installed, can I access its JSC CLI ?

[Keith Miller]

(In reply to Anthony Ricaud from comment #6)
> I have STP installed, can I access its JSC CLI ?

Uhh, probably but I don't have one installed right now. I would just download a "nightly" from nightly.webkit.org and run the jsc in there.

e.g. `DYLD_FRAMEWORK_PATH=286636@main/Release ./286636@main/Release/jsc`

[Radar WebKit Bug Importer]

<rdar://problem/140349974>