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

[Yasumasa Suenaga]

On Wed, 25 Oct 2023 13:18:33 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - a -> an
>>  - add note to downcallHandle about passing heap segments by-reference
>
> src/hotspot/cpu/x86/downcallLinker_x86_64.cpp line 110:
> 
>> 108:   __ mov(rsp, r12); // restore sp
>> 109:   __ reinit_heapbase();
>> 110: }
> 
> This is a minor cleanup to share this code for the three use sites below.

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...)

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PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1374042333