RFR: 8254693: Add Panama feature to pass heap segments to native code [v12]

[Jorn Vernee]

On Thu, 9 Nov 2023 00:27:46 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> Even though it's straightforward to support on-heap accesses during critical function calls, object pinning would support that for non-critical function calls as well, but proposed API doesn't cover it and new API will be required. What's the plan here?

The issue is that most GCs don't support object pinning. I don't think we want an API that only works for some GCs. But, if we do, there's a better API that we can have for just pinning support, which is a `MemorySegment::pin(Arena)` method that returns a MemorySegment wrapping the pinned array. That would allow doing multiple native calls with just a single pin operation, and also allows embedding pointers to pinned segments in other data structures.

For the current approach where we make the array accessible for the duration of the native call: without pinning support, other GCs would have to use GCLocker. That means that the native call also has to be relatively short-lived, at which point I figured we might as well drop the thread state transition, since that has the same requirement. I.e. we detect that the call is short-lived, and do the optimization ourselves without the hint from the user (`critical`). This coincidentally also greatly simplifies the implementation. In a prior iteration I did have a separate `allowHeap` `Option` that implied `critical`. But it was suggested to just merge the two together in that case.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1388181983