1383364 - for..of significantly slower in certain testcases

Status: RESOLVED INCOMPLETE

(Core :: JavaScript Engine: JIT, defect, P3)

Description

[YF (Yang)]

For example:

https://jsperf.com/pk-for-of-vs-for-each
For Of
30,885
±0.61%
97% slower

For Each
1,183,367
±0.32%
fastest


https://jsperf.com/for-of-vs-for-short-arr
for
515,049
±4.44%
fastest

for..of
21,891
±6.85%
96% slower

Comment 1

[Tooru Fujisawa [:arai]]

> https://jsperf.com/pk-for-of-vs-for-each

this case ignores the return value of the function call, and looks like the whole loop gets optimized away in forEach case.
(between "func05-pass15-De-Beta-mir.gv" and "func05-pass16-RA check UCE-mir.gv", in iongraph)

but the loop is still there in for-of case.

Comment 2

[Matthew Gaudet (he/him) [:mgaudet]]

Unfortunately the test case has been lost to the sands of time, rendering this bug unactionable, and our performance efforts are currently focused elsewhere.

If this test case can be recovered, we could re-check the current state of the world and re-open.

We've done work in this area of the last few years, and it may well be improved.