RFR: 8371164: ArrayList.addAll() optimizations [v2]

[jengebr]

On Tue, 4 Nov 2025 19:44:01 GMT, Johannes Döbler <duke at openjdk.org> wrote:

>> I think peeling off and outlining distinct paths in a larger method to separate, simpler methods usually reduces distracting noise and improves readability, but I guess that's subjective territory.
>
>> cl4es: Consider simplifying this to reduce code duplication:
> 
> or even simpler?
> 
> 
> Object[] a = c.getClass() == ArrayList.class ? ((ArrayList<?>)c).elementData : a.toArray();
> int newNum = a.length;
> ...
> 
> 
> 
>> jengebr: Interestingly, this hurt the fast-path about 5%, but the control case about 35%.
> 
> what is the control case, LinkedList.addAll() ?

No, ArrayList.addAll(LinkedList).  This is the slow path that ideally will perform the same as before this change - and the initial change achieves that.  Rewriting to reduce duplication increased runtime of both the optimized and control.

I'll benchmark your suggestion as well.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28116#discussion_r2491902162