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

[Martin Doerr]

On Fri, 27 Oct 2023 10:23:43 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Question: `r12` does not need to remember?
>> 
>> According to [CallingSequences in OpenJDK Wiki](https://wiki.openjdk.org/display/HotSpot/CallingSequences), `r12` may be reserved for HeapBase if COOP is enabled.  
>> (`r12` is also used in another places in downcallLinker_x86_64.cpp without restoring...)
>
> @YaSuenag `r12` is restored in `reinit_heapbase()` if needed and no, `r12` does not need remembering because it is a constant and can be restored from somewhere else.

I think your code is fine. Restoring `r12_heapbase` at this point is not bad because `runtime_call` is only for slow paths. I don't think it should be moved.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1374436854