[9] RFR(XS): 8176513 Poor code quality for ByteBuffers

[Vladimir Ivanov]

> Good for JDK 9. Since Vladimir I. suggested the fix I assume he reviewer
> too.
>
> Roland, did you get ByteBuffers some performance back?
>
> I will need to run pre-integration testing before pushing.

FYI I've just submitted the test batch.

Slightly adjusted the fix as well:

-          need_mem_bar = mismatched || can_access_non_heap;
+          need_mem_bar = mismatched && !adr_type->isa_aryptr();

Best regards,
Vladimir Ivanov

> On 3/15/17 2:48 AM, Roland Westrelin wrote:
>>
>> http://cr.openjdk.java.net/~roland/8176513/webrev.01/
>>
>> This changes relaxes the condition under which MemBarCPUOrder nodes are
>> added around unsafe accesses. If the object being accessed is an array
>> and the access is mismatched, the current code add useless barriers
>> (there's only one memory slice for the entire array so no risk of
>> breaking alias analysis). If the base is not known to be not null,
>> membars are added as well. These membars are not added for arrays in 8u
>> and that hasn't caused any known problem. So in order to keep this
>> change simple, we propose removing that condition (this was suggested by
>> Vladimir Ivanov in the comments for this bug).
>>
>> Roland.
>>