[9] RFR (S): 8153540: C2 intrinsic for Unsafe.allocateInstance doesn't properly filter out array classes

[Vladimir Ivanov]

>> Additional flag in a mirror (j.l.Class) which marks instance klasses could help here, but I'm still not sure it's worth the effort.
>>
>> Ideally, something like [1] (which requires 2 new intrinsics):
>
> I would advise against that.  We are fixing a long-standing bug here and although we see a regression the code we produced before was just wrong.  Comparing against something that was wrong in the first place is moot.

It wasn't intended as a call for action, but more like a backup plan if 
there's a need to speed up the reflection case.

I'd like to keep the fix simple and current version looks good enough:
   http://cr.openjdk.java.net/~vlivanov/8153540/webrev.00

Any Reviews, please?

Best regards,
Vladimir Ivanov

>
> Take the hit; I doubt it will show up at customer applications.
>
>>
>>   * isFastAllocatable() performs all necessary checks: null checks on cls, not primitive, not array, not interface, not abstract, fully initialized, no finalizers;
>>
>>   * allocateInstanceSlow() handles all cases the intrisic doesn't handle: either throws IE or does necessary operations (e.g., initialize the class or register a finalizer) when instantiating an object.
>>
>> Best regards,
>> Vladimir Ivanov
>>
>> [1]
>>     @ForceInline
>>     public Object allocateInstance(Class<?> cls) throws InstantiationException {
>>         // Interfaces and abstract classes are handled by the intrinsic.
>>         if (isFastAllocatable(cls)) {
>>             return allocateInstance0(cls);
>>         } else {
>>             return allocateInstanceSlow(cls);
>>         }
>>     }
>>
>>     @HotSpotIntrinsicCandidate
>>     private native boolean isFastAllocatable(Class<?> cls);
>>
>>     @HotSpotIntrinsicCandidate
>>     private native Object allocateInstance0(Class<?> cls) throws InstantiationException;
>>
>>     // Calls into modified OptoRuntime::new_instance_C
>>     @HotSpotIntrinsicCandidate
>>     private native Object allocateInstanceSlow(Class<?> cls) throws InstantiationException;
>>
>>
>>>
>>>> [1] http://cr.openjdk.java.net/~vlivanov/8153540/AllocInstance.java
>>>
>>> Suggestions to improve fidelity:
>>>    * Run allocation benchmarks with -Xmx1g -Xms1g; this improves variance
>>>    * Add @CompilerControl(CompilerControl.Mode.DONT_INLINE) on
>>> @Benchmarks if you want to use -prof perfasm
>>>
>>> Thanks,
>>> -Aleksey
>>>
>>>
>