1362930 - JavaScript var vs let (de)optimization/slowdown issue in FF/SpiderMonkey

Status: RESOLVED FIXED

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

Description

[olvap80]

Attachment: FF_let_Slowdown.png

User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:53.0) Gecko/20100101 Firefox/53.0
Build ID: 20170504105526

Steps to reproduce:

I've switched to modern JavaScript in my projects and noticed that new "let" statement sometimes slows down my application loops drastically:
For minimal working example please see https://jsfiddle.net/5yxytm9z/ 
and compare loop times.


Actual results:

On FF 53.0.2 (32-bit) https://jsfiddle.net/5yxytm9z/ produces:
without_let: 93.81ms
let_below: 98.19ms
let_above_and_in_loop: 894.19ms <---- Slowdown!
let_in_loop: 99.63ms
all_let: 878.05ms <---- Slowdown!


Expected results:

All the loops shall perform similarly, without slowdowns.

Comment 1

[Tom S [:evilpie]]

let should already be faster in Firefox 54 (currently Beta, I think), because of bug 1273858. But there is definitely still remove for improvements.

Comment 2

[Tom S [:evilpie]]

s/remove/room/ ..

Comment 3

[Guilherme Lima]

Mozilla/5.0 (Windows NT 6.1; WOW64; rv:54.0) Gecko/20100101 Firefox/54.0

without_let: 137.3ms
let_below: 137.48ms
let_above_and_in_loop: 137.38ms
let_in_loop: 136.4ms
all_let: 136.23ms

This bug can be closed, right?

Comment 4

[Witold Baryluk]

In Firefox 66

=========================
without_let: 95ms
let_below: 94ms
let_above_and_in_loop: 297ms
let_in_loop: 94ms
all_let: 298ms
=========================
without_let: 95ms
let_below: 94ms
let_above_and_in_loop: 296ms
let_in_loop: 94ms
all_let: 299ms



so there is still some issue here.

Comment 5

[Tom S [:evilpie]]

I looked into this again. We should certainly allocate the lexical environment object in JIT code. For this micro benchmark at least we probably can't do any scalar replacement, because we have the OSR entry.

Comment 6

[Witold Baryluk]

Linux amd64

Firefox 68.5.0esr

without_let: 100ms
let_below: 98ms
let_above_and_in_loop: 303ms
let_in_loop: 96ms
all_let: 304ms

Firefox Nightly 75.0a1 (2020-03-06)

without_let: 101ms
let_below: 96ms
let_above_and_in_loop: 96ms
let_in_loop: 98ms
all_let: 101ms

So it looks to be fixed in Nightly now.

Comment 7

[matthew.w.fearnley]

Hi.. I've just tested this in 85.0.2 (64-bit), and unfortunately it's still (or again?) an issue:
without_let: 105ms
let_below: 101ms
let_above_and_in_loop: 262ms
let_in_loop: 101ms
all_let: 260ms

Comment 8

[André Bargull [:anba]]

(In reply to matthew.w.fearnley from comment #7)

Hi.. I've just tested this in 85.0.2 (64-bit), and unfortunately it's still (or again?) an issue:

Thanks for the heads-up! It regressed with the new compiler. I've filed bug 1692855 and bug 1692856 to investigate this further.