[14] RFR(S): 8228772: C2 compilation fails due to unschedulable graph if DominatorSearchLimit is reached

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8228772
http://cr.openjdk.java.net/~thartmann/8228772/webrev.00/

With my fix for 8224957 [1] that is out for review, we bail out of Node::dominates when encountering
a RegionNode with more than 3 input paths. While testing, I've encountered a rare case where this
triggered an "unschedulable graph" assert in block local scheduling. While investigating, I found
that this issue also reproduces without [1] when we hit the DominatorSearchLimit. I was able to
create a regression test for both scenarios (see test1 and test2/3 in TestAntiDependentMembar).

The problem is that without LoadNode::split_through_phi, a LoadNode (5198) that is control dependent
on a membar (34) is not split through it's memory phi (5194) and therefore ends up being
anti-dependent because both the membar and the load use the same memory (through a MergeMem):
https://bugs.openjdk.java.net/secure/attachment/83831/8228772_graph.png

If PhaseCFG::insert_anti_dependences then adds a precedence edge between the membar and the load,
the graph becomes unschedulable (I strengthened the asserts to detect this already during GCM).

We should ignore the membar if the load is already control dependent on it.

Thanks,
Tobias

[1] http://cr.openjdk.java.net/~thartmann/8224957/webrev.00/