RFR: JDK-8199216: Quadratic layout time with nested nodes and pseudo-class in style sheet [v8]

[Johan Vos]

On Fri, 9 Jun 2023 12:45:02 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> This fix introduces immutable sets of `PseudoClass` almost everywhere, as they are rarely modified.  These are re-used by caching them in a new class `ImmutablePseudoClassSetsCache`.
>> 
>> In order to make this work, `BitSet` had to be cleaned up.  It made assumptions about the collections it is given (which may no longer always be another `BitSet`).  I also added the appropriate null checks to ensure there weren't any other bugs lurking.
>> 
>> Then there was a severe bug in `toArray` in both the subclasses that implement `BitSet`.
>> 
>> The bug in `toArray` was incorrect use of the variable `index` which was used for both advancing the pointer in the array to be generated, as well as for the index to the correct `long` in the `BitSet`.  This must have resulted in other hard to reproduce problems when dealing with `Set<PseudoClass>` or `Set<StyleClass>` if directly or indirectly calling `toArray` (which is for example used by `List.of` and `Set.of`) -- I fixed this bug because I need to call `Set.copyOf` which uses `toArray` -- as the same bug was also present in `StyleClassSet`, I fixed it there as well.
>> 
>> The net result of this change is that there are far fewer `PseudoClassState` objects created; the majority of these are never modified, and the few that are left are where you'd expect to see them modified.
>> 
>> A test with 160 nested HBoxes which were given the hover state shows a 99.99% reduction in `PseudoClassState` instances and a 70% reduction in heap use (220 MB -> 68 MB), see the linked ticket for more details.
>> 
>> Although the test case above was extreme, this change should have positive effects for most applications.
>
> John Hendrikx has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 16 commits:
> 
>  - Merge branch 'master' of https://git.openjdk.org/jfx into
>    feature/immutable-pseudoclassstate
>  - Merge remote-tracking branch 'upstream/master' into feature/immutable-pseudoclassstate
>  - Avoid using Lambda in ImmutablePseudoClassSetsCache.of()
>  - Merge branch 'master' of https://git.openjdk.org/jfx into
>    feature/immutable-pseudoclassstate
>  - Fix another edge case in BitSet equals
>    
>    When arrays are not the same size, but there are no set bits in the ones
>    the other set doesn't have, two bit sets can still be considered equal
>  - Take element type into account for BitSet.equals()
>  - Base BitSet on AbstractSet to inherit correct equals/hashCode/toArray
>    
>    - Removed faulty toArray implementations in PseudoClassState and
>    StyleClassSet
>    - Added test that verifies equals/hashCode for PseudoClassState respect
>    Set contract now
>    - Made getBits package private so it can't be inherited
>  - Remove unused code
>  - Ensure Match doesn't allow modification
>  - Simplify ImmutablePseudoClassSetsCache and avoid an unnecessary copy
>  - ... and 6 more: https://git.openjdk.org/jfx/compare/9ad0e908...7975ae99

The caching means a huge performance improvement in real-world applications. This looks like one of the most relevant performance enhancements we can achieve.
I tried hard but could not spot regression. I'd love to see this PR merged and used in ea-releases so that we get hopefully feedback from real-world libraries and applications soon.

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Marked as reviewed by jvos (Reviewer).

PR Review: https://git.openjdk.org/jfx/pull/1076#pullrequestreview-1611779656