1991101 - Slow getter calls after stub folding on postcss-wtb

Status: RESOLVED FIXED

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

Description

[Jan de Mooij [:jandem]]

I noticed slow getter calls from GetPropMaybeCached on postcss-wtb in JetStream 3. What seems to happen is this:

1. We have a getter call with > 16 receiver shapes but the shapes have a shared proto with the getter property.
2. We can use stub folding for the first 16 shapes and after that we stop folding and add a few more single-shape IC stubs.
3. In Warp we see we have multiple Baseline stubs so we use an IonGetPropertyIC.
4. The IonGetPropertyIC eventually uses MegamorphicLoadSlotPermissiveResult.
5. Because this is IC code, we don't support calling the getter directly from JIT code and instead always go through C++.

We could support calling the getter from IC code, but then we still have an Ion IC instead of being able to use the transpiler. Maybe we should discard stubs and mark the IC megamorphic when we reach the stub folding limit.

Below is a micro-benchmark that's 2.5x faster with --no-ion or 5-6x faster with TryFoldingStubs disabled.

var objs = [];
var proto = {get getter() { return 1; }};
for (var i = 0; i < 18; i++) {
  var obj = Object.create(proto);
  obj["x" + i] = 1;
  objs.push(obj);
}
function f(objs) {
  var res = 0;
  var t = Date.now();
  for (var i = 0; i < 10_000_000; i++) {
    var o = objs[i % 18];
    res += o.getter;
  }
  print(Date.now() - t);
  return res;
}
f(objs);

Comment 1

[Iain Ireland [:iain]]

Yeah, it seems reasonable to go megamorphic here.

Comment 2

[Iain Ireland [:iain]]

Attachment: Bug 1991101: Transition to megamorphic when stub folding limit is reached r=jandem

This makes the microbenchmark in this comment ~5x faster.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/3622a329e9ed
https://hg.mozilla.org/integration/autoland/rev/7b3df8ade384
Transition to megamorphic when stub folding limit is reached r=jandem

Comment 4

[Atila Butkovits]

https://hg.mozilla.org/mozilla-central/rev/7b3df8ade384

Comment 5

[Jan de Mooij [:jandem]]

This improved our postcss-wtb score by 10-20% (depending on platform and metric) and closed about half the gap with chrome.

Comment 6

[Acasandrei Beatrice (needinfo me)]

(In reply to Pulsebot from comment #3)

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/3622a329e9ed
https://hg.mozilla.org/integration/autoland/rev/7b3df8ade384
Transition to megamorphic when stub folding limit is reached r=jandem

Perfherder has detected a devtools performance change from push 7b3df8ade38494b6847c71a7582bf25afd642d43.

If you have any questions, please reach out to a performance sheriff. Alternatively, you can find help on Slack by joining #perf-help, and on Matrix you can find help by joining #perftest.

Improvements:

Ratio	Test	Platform	Options	Absolute values (old vs new)
7%	damp custom.netmonitor.manyrequests.togglepanel	linux1804-64-shippable-qr	e10s fission stylo webrender	482.45 -> 448.66

Details of the alert can be found in the alert summary, including links to graphs and comparisons for each of the affected tests.

If you need the profiling jobs you can trigger them yourself from treeherder job view or ask a performance sheriff to do that for you.

You can run all of these tests on try with ./mach try perf --alert 47092

The following documentation link provides more information about this command.

Comment 7

[Andra Esanu (needinfo me)]

(In reply to Pulsebot from comment #3)

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/3622a329e9ed
https://hg.mozilla.org/integration/autoland/rev/7b3df8ade384
Transition to megamorphic when stub folding limit is reached r=jandem

Perfherder has detected a browsertime performance change from push 7b3df8ade38494b6847c71a7582bf25afd642d43.

If you have any questions, please reach out to a performance sheriff. Alternatively, you can find help on Slack by joining #perf-help, and on Matrix you can find help by joining #perftest.

Improvements:

Ratio	Test	Platform	Options	Absolute values (old vs new)	Performance Profiles
3%	speedometer3 NewsSite-Nuxt/NavigateToUS/Async	linux1804-64-nightlyasrelease-qr	fission webrender	41.11 -> 40.04	Before/After

Details of the alert can be found in the alert summary, including links to graphs and comparisons for each of the affected tests.

If you need the profiling jobs you can trigger them yourself from treeherder job view or ask a performance sheriff to do that for you.

You can run all of these tests on try with ./mach try perf --alert 47054

The following documentation link provides more information about this command.