1408039 - Spread ...args are 99% slower than arguments

Status: RESOLVED WORKSFORME

(Core :: JavaScript Engine, defect, P2)

Description

[glex.spb]

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36

Steps to reproduce:

Benchmarked these three functions:

  function sumArguments(a, b) {
    if (arguments.length !== 2) return -100;
    return a + b;
  }
  
  function sumSpreadDestructure(...args) {
    if (args.length !== 2) return -100;
    let [a, b] = args;
    return a + b;
  }
  
  function sumSpreadIndex(...args) {
    if (args.length !== 2) return -100;
    let a = args[0], b = args[1];
    return a + b;
  }

https://jsperf.com/arity-checking-spread-vs-arguments


Actual results:

The ...args versions are 99% slower than the arguments version in both Firefox and Nightly (Windows 10).


Expected results:

All three functions perform roughly the same (+- 30%). E.g. in Chrome all 3 are fast, and the ...args versions are ~15% faster than the arguments one.

Comment 1

[Kagami Rosylight [:saschanaz] (they/them)]

Tried running the benchmark and the performance is all similar now. (Values are 1,814,783,470 / 1,802,237,367 / 1,759,341,094 respectively.)

Comment 2

[Nicolas B. Pierron [:nbp]]

I tried as well and got similar result.
This issue seems to be fixed now.