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

[Quan Anh Mai]

On Fri, 27 Oct 2023 03:49:17 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>> 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...)

@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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1374370475