[15] RFR(S): 8235332: TestInstanceCloneAsLoadsStores.java fails with -XX:+StressGCM

[Christian Hagedorn]

>>> If there's no GC barrier then:
>>>
>>> Node* control_proj_ac = bs->step_over_gc_barrier(proj_in->in(0));
>>>
>>> control_proj_ac is not a projection, it's proj_in->in(0) so the test
>>> below can never succeed?
>>
>> If there is no GC barrier, then proj_in is a membar with its control
>> input being a control projection from the ArrayCopyNode.
>>
>> As an example, I have taken the test case TestEliminatedCloneBadMemEdge
>> [1] which does not disable ReduceInitialCardMarks and has no GC barrier.
>> We have the following situation at [2]:
>>
>> proj_in is 132 MemBarCPUOrder. The original code then looked at the
>> MergeMem 100 input following in(Compile::AliasIdxRaw) which is 101 Proj
>> and its input 98 ArrayCopy. This passes the check.
>>
>> My fix now directly looks at the control input of 132 MemBarCPUOrder
>> which is the projection 99 Proj coming from the 98 ArrayCopy. This also
>> passes the check.
> 
> Ah! Right. I wonder if there's a reason why the current code follows the
> control graph (as you do) instead of the memory graph. I couldn't find
> one and your change looks good to me.

Thank you for your review Roland! Was wondering about that, too.

Best regards,
Christian