[foreign-memaccess] RFR 8224993: Add Unsafe support for MemoryAddress (again)

[Maurizio Cimadamore]

The way I see this, is that we need some place where to put potentially 
unsafe API points.

jdk.internal is not an acceptable answer, as that's clearly a 
non-exported module.

sun.misc.Unsafe is not great either, as that's something that we want to 
discourage - but it's the only vehicle we've got right now, so I'm not 
too worried about having it there, since this is still a prototype.

Moving forward I have some ideas on how to do this more cleanly (clearly 
this is a problem that will show up again, and magnified, once we start 
talking SystemABI). The basic idea is to put the unsafe parts of the API 
in a separate module (e.g. jdk.foreign.unsafe) and mark that module as 
'non resolvable by default', which means that developers can only 'see' 
that module if they pass an explicit flag to javac/jvm (--add-modules).

This seems like a robust way to add unsafe functionalities, as there 
will be a clear opt-in from the user. Whether this is a viable 
alternative, time will tell - we're in the process of discussing these 
points internally.

Maurizio

On 30/05/2019 19:10, Jorn Vernee wrote:
> Thanks, I understood it so far. But, I was wondering if new APIs 
> should be added to sun.misc.Unsafe as well. sun.misc.Unsafe didn't for 
> instance get the rename from get/setObjectXXX to get/setReferenceXXX 
> either.
>
> From what I understood, sun.misc.Unsafe is only there to allow code 
> that relied on it previously to keep working under Java 9+, since 
> there is currently no public alternative for some of the 
> functionality. But, in general we'd want to discourage use of 
> sun.misc.Unsafe since it's an internal API.
>
> Jorn
>
> Maurizio Cimadamore schreef op 2019-05-30 19:12:
>> My understanding is that sun.misc.Unsafe is available via the
>> jdk.unsupported module - which people can rely upon when writing
>> modular code. The jdk.internal Unsafe is just not exported, so you
>> will have to burst through module boundary checks in order to access
>> it from outside the JDK.
>>
>> Maurizio
>>
>> On 30/05/2019 17:09, Jorn Vernee wrote:
>>> I wanted to ask about the addition of the APIs to sun.misc.Unsafe 
>>> last time, but got side-tracked by the test build error and then 
>>> forgot :/. I had assumed sun.misc.Unsafe would not be updated going 
>>> forward? What's the policy on this?