Finalization and dead references: another proposal

[Peter Levart]

On 12/07/2017 07:22 PM, Peter Levart wrote:
>
>
> On 12/07/2017 06:46 PM, Vitaly Davidovich wrote:
>>
>>     So no magic here. Just API.
>>
>> This is an API version of Hans’s #3 approach. As he said, there’s 
>> performance overhead and nothing guarantees that the referent is kept 
>> alive - that’s an implementation artifact.
>>
>> I think without the VM knowing about these things intrinsically it’s 
>> not a 100% reliable solution because it’s not concretely requesting a 
>> certain behavior.
>
> I would say that for JNI "there’s performance overhead XOR nothing 
> guarantees that the referent is kept alive", because the overhead is 
> caused by reference parameter management that guarantees that the 
> referents are kept alive while the native method executes and may 
> access them. Can we live with such overhead? I don't know. Would have 
> to measure if it really presents a problem.
>
> The magic would have to be performed by intrinsified method(s) (Unsafe 
> for example), but they are just few and developed by select group of 
> developers.
>
> The main problem I think is not the overhead of passing referents 
> together with addresses to JNI functions and their overheads, but lack 
> of enforcement for this to be performed consistently. It's just too 
> easy to split the native resource from the referent that keeps it into 
> two parts with each having separate lifetime. API with value types 
> could enforce such pairs (address, referent) to be kept together until 
> they hit the final leaf operation which is typically a native method 
> where the referent(s) are either automatically kept alive or would 
> have to be manually, but such methods are in minority.
>
> Regards, Peter
>

There's a quick solution to get rid of that overhead. Each native method 
(or intrinsified leaf method such as those on Unsafe) can have a java 
front end. They are in minority and kept under control. Similar to 
argument validation front-ends for intrinsified methods. For example:

public class Unsafe {

     public Address allocateMemory(long bytes, Object referent) {
         return Address.create(allocateMemory(bytes), referent);
     }

     public void setMemory(Address address, long bytes, byte value) {
         setMemory(/* access check absent equivalent of */ 
address.address, bytes, value);
         Reference.reachabilityFence(/* access check absent equivalent 
of */ address.referent);
     }
     ...


Regards, Peter