1504361 - Add logging to warn when flex items need to be reflowed multiple times

Status: RESOLVED FIXED

(Core :: Layout: Flexbox, enhancement, P3)

Description

[Daniel Holbert [:dholbert]]

To assist in validating that bug 1376536 is (soon) fixed, and to assist in diagnosing flexbox-related perf issues in the future: let's add some logging to warn for cases when flex items are reflowed multiple times.

These may be somewhat spammy, so we can make them opt-in using the exiting MOZ_LOG stuff in nsFlexContainerFrame.cpp.

Comment 1

[Daniel Holbert [:dholbert]]

BTW, just to illustrate how this looks in the wild -- if I apply this patch and run
> MOZ_LOG="nsFlexContainerFrame:5" ./mach run

...then I see lines like this go by while we're doing flex reflow for WhatsApp Web (the scenario in bug 1376536):

> [Child 5560: Main Thread]: D/nsFlexContainerFrame [perf] MeasureAscentAndBSizeForFlexItem rejected cached value
> [Child 5560: Main Thread]: D/nsFlexContainerFrame [perf] Flex item needed both a measuring reflow and a final reflow

And when I apply one of the WIP patches on bug 1503660, the former line ("rejected cached value") goes away, as does a bunch of other logging for flex reflow inside the subtree that we no longer have to remeasure.

The latter line ("needed both") still gets logged -- though that one's not really problematic in this case, I think, because the "measuring reflow" is actually a no-op because it's benefiting from having a cached value which we can use.  (It's still worth it to report this second line though, IMO, because it's a signal that we *might* be reflowing an item multiple times.)

Comment 2

[Daniel Holbert [:dholbert]]

Attachment: Bug 1504361: Add MOZ_LOG logging to note when we reflow a flex item extra times. r?emilio

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/e9ac4160d051
Add MOZ_LOG logging to note when we reflow a flex item extra times. r=emilio

Comment 4

[Stefan Hindli [:stefan_hindli]]

https://hg.mozilla.org/mozilla-central/rev/e9ac4160d051