1421310 - stylo: Investigate why Stylo fails to skip some restyle which Gecko is able to skip

Status: RESOLVED INVALID

(Core :: CSS Parsing and Computation, defect, P3)

Description

[Xidorn Quan [:xidorn] UTC+10]

Attachment: comparison

See the attached comparison on the profile https://perfht.ml/2BkLPgd (Gecko) and https://perfht.ml/2BkKtlD (Stylo).

Despite all the difference length of restyle showing in the chart, it can be seen that the third cyan bar in Stylo's profile doesn't have anything corresponding in Gecko's profile at all.

From the profile, the notification flush is triggered from Element::GetScrollFrame. It's probably worth knowing why Gecko can skip this restyle, and only do a short reflow.

Comment 1

[Emilio Cobos Álvarez (:emilio)]

This is probably related to bug 1417991.

I was waiting to https://github.com/devtools-html/perf.html/issues/352 to dig into it...

Comment 2

[Xidorn Quan [:xidorn] UTC+10]

So, after some investigation, this seems to be invalid, because Gecko does do a style flush. It doesn't show up in the profile because it is a sync restyle via RebuildAllStyleData from nsPresContext::MediaFeatureValuesChanged, while Stylo simply post a restyle event.

Synchronous restyle doesn't show up on the timeline because the timeline marker only tracks ProcessPendingRestyles() directly called from PresShell::DoFlushPendingNotifications.

Regardless of this, we are still much slower than Gecko on the full document rebuild.