RFR: 8371164: ArrayList.addAll() optimizations [v4]

[jengebr]

> # 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...

jengebr has updated the pull request incrementally with one additional commit since the last revision:

  Cleaning up tests per PR feedback

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28116/files
  - new: https://git.openjdk.org/jdk/pull/28116/files/7725441a..6c333bdc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28116&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28116&range=02-03

  Stats: 288 lines in 3 files changed: 36 ins; 250 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28116.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28116/head:pull/28116

PR: https://git.openjdk.org/jdk/pull/28116