RFR(S): 8087341: C2 doesn't optimize redundant memory operations with G1

[Mikael Gerdin]

Hi,

On 2016-01-29 15:04, Roland Westrelin wrote:
> Hi Vladimir,
>
> Thanks for looking at this.
>
>> G1 barrier was added by Mikael Gerdin from GC. He should also look on this change.
>>
>> https://bugs.openjdk.java.net/browse/JDK-8014555
>>
>> Also we have specialized insert_mem_bar_volatile() if we don't want wide memory affect. Why not use it?
>
> The membar in the change takes the entire memory state as input but only changes raw memory. I don’t think that can be achieved with insert_mem_bar_volatile(). As explained by Mikael, the membar is here to force ordering between the oop store and the card table load. That’s why I think the membar’s inputs and outputs should be set up that way.
>
>> And we need to keep precedent edge link to oop store in case EA eliminates related allocation.
>
> I missed that, indeed. Mikael, can you confirm if this is ok (eliminating the barrier if the object being stored to doesn’t escape)?

The barrier can only be removed if the object is never allocated.
If the object is allocated and the field store is performed then the 
barrier must be preserved, even if the compiler can prove that no 
application thread can see the objects, a concurrent gc thread may still 
see these objects.

/Mikael

>
> Roland.
>
>>
>> Thanks,
>> Vladimir
>>
>> On 1/28/16 4:49 AM, Roland Westrelin wrote:
>>> http://cr.openjdk.java.net/~roland/8087341/webrev.00/
>>>
>>> C2 currently doesn’t optimize the field load in the following code:
>>>
>>>      static Object field;
>>>
>>>      static Object m(Object o) {
>>>          field = o;
>>>          return field;
>>>      }
>>>
>>> It should return o but instead loads the value back from memory. The reason it misses such simple optimization is that the G1 post barrier has a memory barrier with a wide effect on the memory state. C2 doesn’t optimize this either:
>>>
>>>          object.field = other_object;
>>>          object.field = other_object;
>>>
>>> Same applies to -XX:+UseConcMarkSweepGC -XX:+UseCondCardMark
>>>
>>> That memory barrier was added to have a memory barrier instruction and doesn’t have to have a wide memory effect.
>>>
>>> Roland.
>>>
>