RFR: JDK-8199216: Memory leak and quadratic layout time with nested nodes (hbox) and pseudo-class in style sheet [v3]

[Michael Strauß]

On Fri, 31 Mar 2023 21:56:08 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 incrementally with three additional commits since the last revision:
> 
>  - 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

modules/javafx.graphics/src/main/java/com/sun/javafx/css/ImmutablePseudoClassSetsCache.java line 53:

> 51:         return CACHE.computeIfAbsent(
> 52:             Objects.requireNonNull(pseudoClasses, "pseudoClasses cannot be null"),
> 53:             k -> Set.copyOf(pseudoClasses)

The call to `computeIfAbsent` will always do the following things:
1. Capture the `pseudoClasses` argument
2. Call `Map.get` internally
3. ...which calls `Set.hashCode`
4. ...which, for `AbstractSet.hashCode`, iterates over the entire set using an iterator and calls `hashCode` for each element.

Since there is no fast path, I wonder whether this may have negative performance implications for larger scene graphs.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1076#discussion_r1155524739