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

[Roland Westrelin]

> Can you create new webrev which includes everything (aarch64)?

Here it is:
http://cr.openjdk.java.net/~roland/8087341/webrev.01/

Roland.

> And I am satisfied with your answers to my objections.
> 
> Thanks,
> Vladimir
> 
> On 2/12/16 4:36 AM, Roland Westrelin wrote:
>> Hi Andrew,
>> 
>>> A patch for the AArch64 C2 volatile/CAS generation code which deals with
>>> the effects of your proposed C2 patch is available as a webrev
>>> 
>>> http://cr.openjdk.java.net/~adinn/8087341-aarch64/webrev.00/
>> 
>> Thanks for putting that together. I didn’t expect that simple change to cause so much trouble.
>> 
>>> n.b. I have /not/ created a separate issue for the AArch64 part of this
>>> fix. I am not sure whether you want to combine it with your patch or
>>> push it as a separate stage.
>> 
>> I can push everything together and list you as a contributor (in the contributed-by field) if that works for you.
>> 
>> Vladimir, can you take another look at this? Your two objections were:
>> 
>>> 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.
>> 
>> Mikael said it’s not ok to eliminate the memory barrier if we leave the gc barrier.
>> 
>> Roland.
>>