RFR: 8371164: ArrayList.addAll() optimizations [v3]

[jengebr]

On Wed, 5 Nov 2025 05:23:37 GMT, Stuart Marks <smarks at openjdk.org> wrote:

>> 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.
>
> I'd have to see the code with the split methods to know for sure, but if it involves duplicating code, then I'd prefer to try harder to avoid the duplication. What's the main optimization here? Direct access to the argument ArrayList's array, avoiding an intermediate copy? If this is the case, then it's hard to believe that it's necessary to duplicate the remainder of the method, which does the growth and copying, which I think aren't changed by this optimization.

I just addressed this in a higher-level comment on this PR.  I cannot reproduce the negative result, so proceeded with the simpler and less-duplicative code.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28116#discussion_r2500662816