1141471 - Array.prototype.slice() needs to be optimized

Status: NEW

(Core :: JavaScript Engine, enhancement)

Description

[石庭豐 (Seak, Teng-Fong)]

Everyone agrees that theoretically Array.prototype.slice() should be a better way to clone an array than doing a for-loop/while-loop to copy element by element.  However, according to this webpage on benchmark:
http://jsperf.com/new-array-vs-splice-vs-slice/76
slice() on my Firefox 35.0.1 is a LOT slower than while-loop.  More precisely,
slice: 434,633 ops/sec vs while: 1,889,386 ops/sec

Using the same benchmark test on IE 11 and Google Chrome ver 40 (on Win7 64-bit), slice is a lot FASTER!

I think slice method can be optimized in Firefox.  Memory block copy in native C++?

Comment 1

[Jan de Mooij [:jandem]]

Hm, the while-loop is about 3x slower on Nightly than in 35 here and barely faster than the slice. That needs investigation..

Comment 2

[Jan de Mooij [:jandem]]

Main problem for slice here is our old friend malloc, to allocate the dynamic slots. If I change the array so that it has 126 elements I get:

SM: 144 ms
d8: 178 ms

However, if I add one extra element, we no longer use nursery allocation and we get:

SM: 534 ms
d8: 180 ms

(In reply to 石庭豐 (Seak, Teng-Fong) from comment #0)
> I think slice method can be optimized in Firefox.  Memory block copy in
> native C++?

Well it's not that easy unfortunately. We need to fire post barriers for generational GC, need to track element type information etc.

Anyway, as I said above, the actual copy is not the problem here. Most of our time is spent allocating the new array. Your while-loop doesn't have that issue because it uses a single array instead of creating a new one each time.

Comment 3

[Jan de Mooij [:jandem]]

(In reply to Jan de Mooij [:jandem] from comment #2)
> Well it's not that easy unfortunately. We need to fire post barriers for
> generational GC, need to track element type information etc.

Er nvm this part, we do use a memcpy in this case with some extra work for barriers.

Comment 4

[Jan de Mooij [:jandem]]

Attachment: slice shell testcase

Comment 5

[Boris Zbarsky [:bzbarsky]]

The "while loop" test is just totally broken in http://jsperf.com/new-array-vs-splice-vs-slice/76

Someone created http://jsperf.com/new-array-vs-splice-vs-slice/77 recently which has that fixed, fwiw.

Comment 6

[石庭豐 (Seak, Teng-Fong)]

OK, here are the benchmark with "77"

        | FF 36.0.1 | Chrome 41 |  IE 11
--------+-----------+-----------+---------
slice   | 666,730   | 3,820,876 | 703,837
concat  | 625,986   | 3,272,164 | 718,593
while:  | 526,221   |   644,009 | 150,677
slice(0)| 704,095   | 3,105,080 | 696,559

Result for while in FF is indeed a lot less than before.

Comment 7

[Guilherme Lima]

WFM?
If I didn't make any mistake while testing this, I got the same results for Chrome and Nightly on the shell testcase and on jsperf (it's Back!!!).