[jdk16] RFR: 8258393: Shenandoah: "graph should be schedulable" assert failure [v2]

[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.

Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - review
 - test & fix

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Changes: https://git.openjdk.java.net/jdk16/pull/41/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=41&range=01
  Stats: 114 lines in 3 files changed: 107 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