RFR 8047737 Move array component mirror to instance of java/lang/Class

[Peter Levart]

On 07/02/2014 08:26 AM, Mandy Chung wrote:
>
> On 6/30/2014 9:51 PM, Christian Thalinger wrote:
>> On Jun 30, 2014, at 5:50 PM, Coleen Phillimore 
>> <coleen.phillimore at oracle.com> wrote:
>>
>>
>> On 6/30/14, 3:50 PM, Christian Thalinger wrote:
>>>>       private Class(ClassLoader loader) {
>>>>           // Initialize final field for classLoader.  The 
>>>> initialization value of non-null
>>>>           // prevents future JIT optimizations from assuming this 
>>>> final field is null.
>>>>           classLoader = loader;
>>>> +        componentType = null;
>>>>       }
>>>>
>>>> Are we worried about the same optimization?
>>> Hi,  I've decided to make them consistent and add another parameter 
>>> to the Class constructor.
>>>
>>> http://cr.openjdk.java.net/~coleenp/8047737_jdk_2/
>
> The jdk change looks okay while I am beginning to think whether we 
> really want to keep expanding this constructor to deal with this 
> future JIT optimization (you will be moving more fields out from the 
> VM to java.lang.Class).
>
> There are places in JDK initializing the final fields to null while 
> the final field value is overridden via native/VM - e.g. System.in, 
> System.out, etc.  I would prefer reverting the classLoader constructor 
> change to expanding the constructor for any new field being added.  
> Handle it (and other places in JDK) when such JIT optimization comes.
>
> Mandy
>

What about:


     private Class() {
         classLoader = none();
         componentType = none();
         ...
     }

     private <T> T none() { throw new Error(); }


I think this should be resistant to future optimizations.

Regards, Peter