[jdk16] RFR: 8258393: Shenandoah: "graph should be schedulable" assert failure

[Roland Westrelin]

This is a shenandoah bug but the fix is in shared code.

At barrier expansion time, we need the raw memory state for the
control at which the barrier is expanded. Before expansion, the memory
graph is traversed and memory state for each control is recorded. In
that process, if opportunities to improve the memory graph are found,
the graph is modified. In the case of the failure, a raw memory load
was assigned a call projection as control but for that control, no
memory state was recorded and the fall back is to use the call's input
memory state. As a consequence the load's memory input is updated to a
wrong memory state. This causes the assert failure.

The call has 2 memory projections: one for the fallthrough and one for
the exception path. One projection is the memory state for the
CatchProj for the fallthrough, the other one for the CatchProj of the
exception path. For the control projection out of the call, there's no
memory state that makes sense and assigning the control projection of
the call as control for the load is the root cause of the
problem. This happens because anti-dependence analysis is too
conservative: a memory Phi that merges states from the exception and
fallthrough path is considered a dependency and that pushed the load
right after the call.

I propose to be less conservative in anti-dependence analysis for
Phis. For a Phi, when computing the LCA, I think it's sufficient to
only consider region's inputs that we actually reach by following the
memory edges and that's what I propose here.

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Commit messages:
 - test & fix

Changes: https://git.openjdk.java.net/jdk16/pull/41/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=41&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258393
  Stats: 110 lines in 3 files changed: 103 ins; 2 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/41.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/41/head:pull/41

PR: https://git.openjdk.java.net/jdk16/pull/41