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

[Jason Mehrens]

On Tue, 4 Nov 2025 20:38:35 GMT, jengebr <duke at openjdk.org> wrote:

>> That variant is incorrect since `size <= a.length`, potentially adding a surprising tail of `null` elements to the list.
>
> Thank you both!  @cl4es splitting it out had zero performance impact (per the benchmark).  As you said, it's a bit subjective... how strongly do you feel?  Other opinions are welcome too.

I would think that you need to check `src.modCount` has same value at start and end of the method.


final int expectedSrcModCount = src.modCount;
//access src internals....
checkForComodification(expectedSrcModCount)


Looks like ArrayList::toArray is not doing this check either.  So perhaps it is not needed.  I would have thought mod check would have been needed in toArray to mimic iterator behavior.  Therefore needed in addAll.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28116#discussion_r2492622937