1199054 - TryToStartImageLoad gets called way too often for CSS variable values that resolve to images

Status: NEW

(Core :: CSS Parsing and Computation, defect)

Description

[Seth Fowler [:seth] [:s2h]]

It's not clear why, but bug 1181907 revealed that we are calling TryToStartImageLoad very frequently for CSS variable values (i.e., CSS property values that contain variables). Although bug 1181907 ensures that we won't leak ImageValues as a result of this, it still costs us in terms of performance, so we should fix it.

The actual problem is probably more fundamentally that we are recomputing computed styles way too often for CSS variable values. (Forgive my no-doubt-terribly-correct terminology there.)

Comment 1

[Seth Fowler [:seth] [:s2h]]

(Terribly-incorrect, of course.)

Comment 2

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

I talked about this with seth a bit.  Perhaps we need to cache the substituted token stream, and only reparse and recompute it when the substituted token stream changes.  (I don't think we have anything that does that now.)

Comment 3

[Cameron McCormack (:heycam)]

Since any use of a variable causes the struct to be uncacheable in the rule tree, I think we can just store the cached result of the resolved property value on the nsCSSTokenStream itself.  (Or an array of values, for when we had a shorthand with a variable reference.)

Comment 4

[Cameron McCormack (:heycam)]

> store the cached result of the resolved property value on the nsCSSTokenStream itself

Not really, unless we keyed those cached results off the individual style contexts.  I guess they need to go on the style context somewhere.