RFR (S): 8033380: Experimental VM flag to enforce access atomicity

[Aleksey Shipilev]

Thanks Dean!

The study of the compiler code, and the generated assembly, and the
empirical tests seems to indicate double accesses are atomic by
construction.

Either way, we mark the C2 Store/Load nodes to require atomic access.
Should there be platform which ignores that flag for doubles, that would
mean volatile double access atomicity is broken there as well. This
makes the ground for the VM fix, not the additional work for the
experimental feature.

-Aleksey.

On 02/12/2014 01:08 AM, Dean Long wrote:
> It looks to me that C2 will only look at the atomic flag for longs, not
> doubles, so if you need
> atomic access to doubles, I think more work needs to be done.
> 
> dl
> 
> On 2/11/2014 3:02 AM, Aleksey Shipilev wrote:
>> Hi,
>>
>> I understand we are still closed for integration?
>>
>> Please review this small feature meanwhile:
>>    https://bugs.openjdk.java.net/browse/JDK-8033380
>>    http://cr.openjdk.java.net/~shade/8033380/webrev.02/
>>
>> TL;DR: JMM 9 may need to extend the access atomicity over longs and
>> doubles. Luckily, our logic in emitting the relevant access-atomic
>> instructions is decoupled from memory barriers logic, and so we can
>> "just" unconditionally go through needs_atomic_access where appropriate.
>>
>> Testing:
>>   - full cycle JPRT
>>   - targeted microbenchmarks on x86/ARM/PPC
>>
>> Thanks,
>> -Aleksey
>