RFR: 8254693: Add Panama feature to pass heap segments to native code [v2]

[Maurizio Cimadamore]

On Wed, 18 Oct 2023 09:57:25 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> I'm not sure what to write to make this clearer. The address that is exposed to the native target function is indeed a temporary address that is constructed from the oop and offset. It is temporary because after the native call, the GC might move the object around, which invalidates the address.
>> 
>> This part is meant to document that native code should not be holding on to the address until after the call completes. This also includes returning the address back to Java. The address would be invalid the moment the function returns.
>> 
>> Would it help if this said: `The memory region inside the Java heap is exposed to the native target function through a temporary native address`?
>
> I'm considering to leave `As such, these temporary addresses...` out. You already said that the address "is valid for the duration of the call". I'm not sure the subsequent sentence adds much. If you want to make a concrete example of something that should not be done, that would be better. But it seems to me that the client is as in charge as this text suggests - (e.g. if a native lib decides to hold onto an address, the client can't do much about it).

So, summing up - either we write more and spell what are the things to be on the lookout for, or we leave it out.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16201#discussion_r1363595149