RFR: ZGC: C2: Don't split loadbarriers past self referencing phi nodes

[Nils Eliasson]

On 2018-05-04 17:37, Roland Westrelin wrote:
> Hi Nils,
>
>> I found a case when we are trying to split a loadbarrier past a phi
>> nodes which has one input that is itself. This is a workaround to just
>> skip those. Later we might want to try to remove these nodes earlier in
>> loop optimizations, but that is an upstream issue.
> I would have thought that PhiNode::Ideal() gets rid of them. AFAICT,
> that call would trigger that:
>
>    Node* uin = unique_input(phase, false);
>
> So I suppose the cycle is created by some other transformation during
> loop opts before the compiler has a chance to run a pass of IGVN?
Found it. IdealLoopTree::split_outer_loop, during beautify_loops, moves 
a phi to an outer loop and replaces the old in with self.

http://hg.openjdk.java.net/jdk/jdk/file/b99e90f885bf/src/hotspot/share/opto/loopnode.cpp#l1804

//Nils

>
>> http://cr.openjdk.java.net/~neliasso/zgc/dont_split/webrev.01/
> Looks ok to me.
>
> Roland.