Implementing VarHandle

Remi Forax forax at univ-mlv.fr
Tue Apr 14 10:17:05 UTC 2015


On 04/14/2015 10:38 AM, MacGregor, Duncan (GE Energy Management) wrote:
> On 12/04/2015 15:54, "Remi Forax" <forax at univ-mlv.fr> wrote:
>> Hi guys,
>> I was about to write a blog post explaining why i don't like the way
>> VarHandle are currently implemented when it occurs to me that providing
>> another implementation may be a more efficient to discuss about
>> implementation.
>>
>> So my implementation is here,
>>    https://github.com/forax/varhandle2
>>
>> the API is typesafe, the implementation is runtime safe and i get mostly
>> the same assembly code using unsafe or this VarHandle API.
>>
>> The idea is that there is no need to add more polymorphic signature
>> methods, a good old Java method is enough if it using a method handle
>> under the hood.
>> All the logic is described using method handles and the only logic
>> written in Java is
>> 1) when to create such method handle (the first time a method is called),
>>      it works like invokedynamic but the method handles are stable
>> instead of being constant.
>> 2) how to link all things together to do nullcheck, atomic operation and
>> post operation (like fence or sum).
> Although I agree regarding the type safety of this being better than the
> polymorphic signatures of the current VarHandle API, this doesn¹t feel
> like it has the right shape. How would your API expand to handle user
> defined value types? I think if we want something type safe then
> specialisation over type variables would be a much better path forward for
> this, assuming the two can be lined up in when they might be delivered
> into the main JDK.
>
> Regards, Duncan.
>

Hi Duncan,
in my opinion, there are two different issues,
one is to provide a safe API for doing things people currently do with 
Unsafe,
the other is the support of value type for some of these operations.

The prototype i have written is good to do the former but not the later.

Supporting value type will require 3 things:
   - being able to declare a type variable as any, like you can do with 
the vahalla compiler
   - adding support of value types in the method handle API, exactly 
being able
     to invoke a method handle with a value type as parameter.
   - adding support of value types in Unsafe

If you know the answer to the last point, everything else will fold.

Before that, I think we can deliver an API that does a CAS in a safe way 
without directly using sun.misc.Unsafe.
BTW, perhaps we do not need a new API but just change the implementation 
of the classes of java.util.concurrent.atomic.

regards,
Rémi



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