On Mon, 3 Nov 2025 20:45:44 GMT, jengebr <[email protected]> wrote:
> # JVM Collections Optimizations: Eliminating toArray() Performance Bottlenecks
>
> ## Summary
>
> This PR addresses performance bottlenecks in ArrayList.addAll() and
> Collections.SingletonSet.toArray() methods by implementing direct
> optimizations that bypass inefficient intermediate allocations and abstract
> implementations. The optimizations target high-frequency operations
> identified through profiling analysis, delivering 37% performance
> improvements for ArrayList operations and 17-43% performance improvements for
> SingletonSet operations under real-world conditions where multiple collection
> types are used.
>
> ## Problem Context
>
> ### ArrayList.addAll() Inefficiency
> ArrayList.addAll() currently calls `c.toArray()` on the source collection to
> avoid iterator-based copying, but this creates unnecessary intermediate array
> allocation when the source is also an ArrayList. The method performs:
>
> 1. Call `c.toArray()` - creates intermediate array
> 2. Call `System.arraycopy()` to copy from intermediate array to destination
> 3. Discard intermediate array
>
> When both source and destination are ArrayList instances, this can be
> optimized to direct array copying.
>
> ### Collections.SingletonSet toArray() Missing Implementation
> Collections.SingletonSet inherits the default `AbstractCollection.toArray()`
> implementation, which:
>
> 1. Creates an Object[] of the expected size
> 2. Iterates through the collection (1 element)
> 3. Ensures "expected" size is the actual size
> 4. Returns the array
>
> For a single-element collection, this overhead is disproportionate to the
> actual work needed. Additionally, this implementation is vulnerable to call
> site poisoning, showing 74-118% performance degradation under megamorphic
> conditions.
>
> ## Optimized Methods
>
> ### ArrayList
> - **`addAll(Collection<? extends E> c)`**: Added fast path for
> ArrayList-to-ArrayList copying using direct `System.arraycopy()` from
> source's internal `elementData` array, eliminating intermediate `toArray()`
> allocation
>
> ### Collections.SingletonSet
> - **`toArray()`**: Direct implementation returning `new Object[] {element}`
> - **`toArray(T[] a)`**: Direct implementation with proper array sizing and
> null termination per Collection contract
>
> ## Performance Impact
>
> | Class | Method | Size | Baseline | Optimized | Improvement |
> |-------|--------|------|----------|-----------|-------------|
> | ArrayList | addAll | 0 | 10.149 ns/op, 40 B/op | 3.929 ns/op, 24 B/op |
> **61% faster, 40% less allocation** |
> | ArrayList | addAll | 5 | 23.804 ns/op, 104 B...
src/java.base/share/classes/java/util/Collections.java line 5260:
> 5258: }
> 5259: @SuppressWarnings("unchecked")
> 5260: public <T> T[] toArray(T[] a) {
Should these have `@Override`?
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/28116#discussion_r2489724142
