On Mon, 17 Nov 2025 15:42:35 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 | >> **... > > jengebr has updated the pull request incrementally with one additional commit > since the last revision: > > Removing explicit classname
Marked as reviewed by smarks (Reviewer). ------------- PR Review: https://git.openjdk.org/jdk/pull/28116#pullrequestreview-3478995078
