RFR: 8200235: Generalize jniFastGetField jobject/jweak resolve

[David Holmes]

I should have looked closer first ...

On 18/04/2018 10:30 PM, David Holmes wrote:
> Hi Erik,
> 
> How does this affect performance? That's the only reason we have these 
> 'fast' functions.

Okay at the moment there's no affect as you've just added a potential 
for change in a future GC that needs it.

For the ports not addressed they continue to use whatever "hard-wired" 
approach they use.

David

> Thanks,
> David
> 
> On 18/04/2018 9:14 PM, Erik Österlund wrote:
>> Hi,
>>
>> The fantastic jniFastGetField optimization that we all know and love, 
>> resolves jobjects/jweaks in native, possibly concurrent with GC 
>> safepoints.
>> Currently it is assumed that it is "safe" to just unmask the potential 
>> jweak tag, and read the jobject/jweak oop and then speculatively read 
>> the integer value of that oop (resorting to the signal handler as plan 
>> B if the heap was concurrently unmapped in the safepoint).
>>
>> This happens to be safe with existing collectors, but ties very 
>> strongly to how these oops are processed (as it is normally strictly 
>> forbidden by mutators to resolve jobject/jweak in safepoints, except 
>> of course when using it to quickly read integers via JNI).
>>
>> My proposed solution is to add a try_resolve_jobject_in_native() 
>> function in the BarrierSetAssembler class, and allow it try to perform 
>> this resolution, but also give it an option to opt out should it find 
>> that this jobject/jweak really has to go through the proper VM 
>> transition. This allows a GC where this is not in general safe (but 
>> possibly mostly safe depending on GC-specific details such as pointer 
>> colouring) to override this special in-native jobject/jweak 
>> resolution. That should make everyone happy I hope.
>>
>> I provided changes for x86_64, SPARC and AArch64. PPC and S390 do not 
>> use jniFastGetField, and there are no current plans that I am aware of 
>> to introduce support for any new GC any time soon to the non-AArch64 
>> arm port.
>>
>> Webrev:
>> http://cr.openjdk.java.net/~eosterlund/8200235/webrev.00/
>>
>> Bug:
>> https://bugs.openjdk.java.net/browse/JDK-8200235
>>
>> Thanks,
>> /Erik