[14] RFR 8230765: Implement nmethod barrier for x86_32 platforms

[Zhengyu Gu]

Hi Erik,

Thanks for the reviewing and suggestion.

On 11/26/19 5:32 AM, erik.osterlund at oracle.com wrote:
> Hi Zhengyu,
> 
> Nice to see nmethod entry barriers added one more platform configuration.
> So now you use a global instead of thread-local on 32 bit x64 as it 
> doesn't have a Thread register.
> That makes sense. I wonder though if that difference has spread too far 
> into the runtime code. It does
> sting a bit in my eyes to read the seemingly unnecessary #ifdef _LP64 
> macros in BarrierSetNMethod.
> 
> In particular, when computing the BarrierSetNMethod::disarmed_value(), 
> it seems like it would work
> for everyone to simply read the global value, instead of doing it only 
> sometimes, and sometimes read
> the thread-local.
> 
> Here is a patch with my proposed cleanup:
> http://cr.openjdk.java.net/~eosterlund/8230765/webrev.02/
> 
> Incremental:
> http://cr.openjdk.java.net/~eosterlund/8230765/webrev.01_02/
> 

Yes, this indeed a much cleaner approach. I will take your proposed 
cleanup and run through submit.

-Zhengyu


> Thanks,
> /Erik
> 
> On 11/25/19 9:35 PM, Zhengyu Gu wrote:
>> Hi all,
>>
>> Please review this implementation of nmethod barrier for x86_32 
>> platforms.
>>
>> x86_32 implementation mirrors x86_64's. The only difference is where 
>> it reads nmethod disarmed value.
>>
>> Unlike 64-bits, 32-bits platform does not have a dedicated register 
>> for current thread. So that it is cheaper to read disarmed value from 
>> global location than from per-thread GC data.
>>
>> Currently, only Shenandoah GC uses the implementation for its 
>> concurrent class unloading. This implementation, along with Shenandoah 
>> concurrent class unloading, has been baked in shenandoah/jdk repo for 
>> some time now,  they are ready for integration.
>>
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8230765
>> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8230765/webrev.01/
>>
>>
>> Test:
>>   hotspot_gc with x86_64 and x86_32 JVM on Linux
>>   Submit test.
>>
>> Thanks,
>>
>> -Zhengyu
>>
>