This telemetry was trying to measure three things:
* The time spent during style / layout in a tick. This is useful.
* The times SetNeed{Style,Layout} flush have been called. This is not a
particularly useful metric, there's not much to learn or change from
it. Any call is basically free as long as they're consecutive /
there's no flush in between.
* The number of times layout / style has been flushed during a tick.
This is more useful than the above but, similarly, there's not too
much to be learned here. Number of flushes can be completely
orthogonal to how expensive it has been.
It also had a number of flaws:
* It is recorded per-PresShell, so we get multiple pings per tick.
* Similarly, if a sub-frame dies before the tick, we just never report
that time.
Instead, refactor it to make it simpler, and just record the first
metric reliably. This simplifies the code substantially, and I don't
think we're losing much.
The histograms are also expired. We might want to renew this, I'll file
a follow-up to either properly renew this if we plan to look at it, or
remove the code entirely from the tree.
Differential Revision: https://phabricator.services.mozilla.com/D210907
Summary of the changes/reasons:
- LayoutTelemetryTools.h directly uses several types whose headers it needs to
include. (These includes were present in its .cpp file; I'm migrating them
from there to the .h file, and I'm adding a new include for "Saturate.h" to
provide the SaturateUint8 type.)
- LayoutTelemetryTools.cpp needs an include for MainThreadUtils.h, to provide
NS_IsMainThread().
- StaticPresData.cpp needs an include for ServoUtils.h, to provide
AssertIsMainThreadOrServoFontMetricsLocked().
- ZoomConstraintsClient.h needs a forward-decl for mozilla::dom::Document since
it uses a pointer of that type in a function-decl.
- ScrollSnap.h needs forward-decls of nsPoint/nsRect for some references to
those types in a method signature.
- nsGridContainerFrame.cpp needs an include for nsBoxLayoutState.h since it
uses that type (it instantiates a nsBoxLayoutState instance).
- nsPlaceholderFrame.cpp needs a "using" decl for the mozilla::dom namespace in
order to use the un-namespace-prefixed "Element" type.
Differential Revision: https://phabricator.services.mozilla.com/D61603
Emilio Cobos Álvarez