RFR: 8287788: Implement a better allocator for downcalls [v16]

[Maurizio Cimadamore]

On Thu, 23 Jan 2025 12:53:08 GMT, Matthias Ernst <duke at openjdk.org> wrote:

> We could, in the sense that an allocation in a lower stack frame seems suspicious, but technically it is completely legal. The frame has been allocated and is sliced to the requested size, and is guaranteed as long as the Frame's arena hasn't been closed, no matter whether other frames are on top:
> 
> ```
> frame1 = pushFrame(256);
> frame2 = pushFrame(256);
> <<frame1 can safely allocate up to 256>>
> ```

Ok, in this particular design it's ok because you allocate up front. So each frame allocate in its own space.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23142#discussion_r1927062606