511147 - Optimize a > b ~ c as well as a b ~ c in SelectorMatchesTree

Status: RESOLVED FIXED

Description

[Zack Weinberg (:zwol)]

I noticed this while working on bug 508466.  In one branch of SelectorMatchesTree, there is this logic:

      // to avoid greedy matching, we need to recur if this is a
      // descendant or general sibling combinator and the next
      // combinator is different, but we can make an exception for
      // sibling, then parent, since a sibling's parent is always the
      // same.
      if ((NS_IS_GREEDY_OPERATOR(selector->mOperator)) &&
          (selector->mNext) &&
          (selector->mNext->mOperator != selector->mOperator) &&
          !(selector->mOperator == '~' &&
            selector->mNext->mOperator == PRUnichar(0))) {

The last part of the condition exempts 'a b ~ c' from recursion, but the rationale in the comment applies equally to 'a > b ~ c'.  In testing, adding an additional exemption for selector->mNext->mOperator == '>' seems to be safe.

Patch to follow shortly.  Blocking 508466 only because both this and that need to modify this part of the code.

Comment 1

[Zack Weinberg (:zwol)]

Created attachment 395098 [details] [diff] [review]
patch

Comment 2

[David Baron :dbaron: 🏴󠁵󠁳󠁣󠁡󠁿 ⌚UTC-7]

Comment on attachment 395098 [details] [diff] [review]
patch

r=dbaron

Comment 3

[David Baron :dbaron: 🏴󠁵󠁳󠁣󠁡󠁿 ⌚UTC-7]

I'll land this later today.

Comment 4

[David Baron :dbaron: 🏴󠁵󠁳󠁣󠁡󠁿 ⌚UTC-7]

http://hg.mozilla.org/mozilla-central/rev/2c20ec236783