RFR: 8371164: ArrayList.addAll() optimizations

[Oli Gillespie]

On Mon, 3 Nov 2025 20:45:44 GMT, jengebr <duke at openjdk.org> 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`?

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PR Review Comment: https://git.openjdk.org/jdk/pull/28116#discussion_r2489724142