RFR: 8349146: [REDO] Implement a better allocator for downcalls [v8]

[Per Minborg]

On Fri, 2 May 2025 11:56:31 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> And, if you do that, you then don't need the `of` factory -- clients can just use `new`
>
>> any reason why you didn't use a lambda here?
> 
> I also think that CleanupAction should be changed to lambda

Using an anonymous class for the cleanup action had very adverse effects on performance. I didn't want to use a lambda for startup performance reasons.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24829#discussion_r2071674052