review for 7032963: StoreCM shouldn't participate in store elimination

[Vladimir Kozlov]

An other problem if n is on a branch and you could eliminate dominated StoreCM 
which above the split point resulting in not having StoreCM on opposite branch.

Vladimir

Tom Rodriguez wrote:
> On Apr 1, 2011, at 3:47 PM, Vladimir Kozlov wrote:
> 
>> You may put n->in(MemNode::Address) and n->in(MemNode::ValueIn) into locals before the loop. Also you need to kill the node explicitly otherwise it still be connected to its inputs:
>>
>> +         // Eliminate the previous StoreCM
>> +         prev->set_req(MemNode::Memory, mem->in(MemNode::Memory));
>> +         assert(mem->outcnt() == 0, "should be dead");
>> +         mem->disconnect_inputs(NULL);
> 
> I'll have to rework the mem traversal a little.  Actually I think there might have been a bug with the old code since it always updated prev.  I believe this is correct:
> 
>         // Eliminate the previous StoreCM                                                                                                             
>         prev->set_req(MemNode::Memory, mem->in(MemNode::Memory));
>         assert(mem->outcnt() == 0, "should be dead");
>         mem->disconnect_inputs(NULL);
>       } else {                                                                                                                                       
>         prev = mem;                                                                                                                                   
>       }
>       mem = prev->in(MemNode::Memory);
>     }
> 
> I think I'll put together a little test case to make sure this is working correctly.
> 
> tom
> 
>> Vladimir
>>
>> Tom Rodriguez wrote:
>>> I could push this to hotspot-gc so it gets more CMS testing .
>>> http://cr.openjdk.java.net/~never/7032963
>>> 7032963: StoreCM shouldn't participate in store elimination
>>> Reviewed-by:
>>> StoreCM shouldn't participate in redundant store elimination since
>>> that could violate the requirement that a StoreCM must be strictly
>>> after a field update.  This results in a large number of redundant
>>> StoreCMs being emitted for blocks of fields updates, so I added an
>>> optimization to fold them up safely.  Previously the extra dependence
>>> was converted into a precedence edge just before register allocation
>>> but I moved this logic into final_graph_reshape.  I then added logic
>>> to search through chains of StoreCMs to eliminate earlier redundant
>>> ones and transfer their precedence edges to the one that is kept.
>>> This ensures that they are scheduled properly.  This actually
>>> eliminates duplicates that were previously missed so the code quality
>>> is slightly better.  Tested by inspecting code generation with script
>>> to identify duplicates.  Also ran CTW with -XX:+UseCondCardMark and
>>> -XX:+UseG1GC.
>