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Investigating CSS Performance

This patch instruments a bunch of key places and should give an estimate of the order of magnitude of the different parents.

Two counts are collected which allow for an estimation of the amount of work being done during restyle:

  • resolveStyleForCount
    • this is incremented everytime that we do style resolution on an element
  • ContentEnumFuncCount
    • this is incremented roughly for every rule that we test against

Time during restyle can be spent in a bunch of places. For example, HasStateDependentStyle will compute a hint that determines how many elements we'll restyle. If we get a eRestyle_Subtree we will restyle more elements.

This was developed while investigating Bug 1110625