692960 - Add Array.concat stub for concatenating two known dense arrays

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Brian Hackett [Laid off!]]

Attachment: patch against 15bfad783467

Another behavior in PeaceKeeper 2.0's arrayCombined where we are absurdly slow compared to V8.  While there is ongoing work to add dense array paths to Array.concat (bug 609896), it will be faster yet to call a stub which just does a malloc + 2 memcpys.  This can only be done in limited circumstances (type effects of the call have been accounted for, statically known dense arrays, ...) so does not subsume other concat improvements.

With the attached patch:

function foo(x, y) {
  for (var i = 0; i < 500000; i++)
    x.concat(y);
}
var x = [];
var y = [];
for (var i = 0; i < 100; i++)
  y[i] = i;
foo(x, y);

js -m -n (old): 701
js -m -n (new): 3550
js -m -j -p: 2725
d8: 71

We still get clobbered by V8 here due to a combination of a lack of generational collection, allocating array buffers with malloc, and fat values.  Not really sure which, as tweaking the constants gives pretty nonlinear behavior.

Comment 1

[Brian Hackett [Laid off!]]

Uh, transpose '(old)' and '(new)' in comment 0.

Comment 2

[Brian Hackett [Laid off!]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/c537139643a9

Comment 3

[Marco Bonardo [:mak] (Away Apr 25 - May 5)]

https://hg.mozilla.org/mozilla-central/rev/c537139643a9