90871 - Checking for firstline and firstletter is expensive

Status: NEW

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

Description

[Daniel Bratell]

It's too expensive to look for the firstline a firstletter styles. In the table 
stress case those "simple tests" are more than a 1% of the page load time in 
the table stress case. 

nsCSSFrameConstructor does the check in 
nsCSSFrameConstructor::HaveSpecialBlockStyle which is called during most 
construction (> 20000 calls for the table stress case). This is 0.78% of the 
total page load time.

The nsBlockFrame again checks for FirstLetter in 
nsBlockFrame::SetInitialChildList which is called from 
nsCSSFrameConstructor::SetInitialChildList. (The relevant function here is 
nsBlockFrame::GetFirstLetterStyle) That is 0.38% of the page load time.

So 1.16% of the page load time is spent calling 
nsPresContext::ProbePseudoStyleContextFor just checking for the two special 
styles.

Comment 1

[Pierre Saslawsky]

nsCSSFrameConstructor::HaveSpecialBlockStyle() seems to be called 10K times, not 
20K times, but I agree: a small but not negligible amount of time is spent there.

Comment 2

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

Assigning pierre's remaining Style System-related bugs to myself.

Comment 3

[Boris Zbarsky [:bzbarsky]]

David has mentioned in the past hanging the pseudo-style-contexts off the main
style context.  

In particular, that would mean that there would be a one-time cost to resolve
the pseudo-context; after that it would be either there or not.  And since there
is no way to trigger a reresolve of the pseudos without triggering a reresolve
of the parent, this should work....