RFR: 8257146: C2: extend the scope of StressGCM

[Tobias Hartmann]

On Thu, 26 Nov 2020 20:36:53 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

> Extend the scope of StressGCM by allowing it to move instructions into basic blocks with worse frequency. This should improve StressGCM's ability to expose bugs where C2 relies on GCM heuristics for correctness.
> 
> Tested with StressGCM enabled on `hs-tier1-3` and a single repetition, the following test cases fail on `linux-aarch64-debug` and require further investigation:
> - `serviceability/sa/ClhsdbJstackXcompStress.java`
> - `compiler/c2/cr6663848/Tester.java`
> 
> Also tested with StressGCM disabled (default)  on `hs-tier1-3`, all tests pass.

Looks good to me but the test failures that you've found when running lower tiers should be investigated and fixed before integrating this to avoid frequent failures in the CI runs. I would suggest to defer this to JDK 17.

Also, as Vladimir already said, more testing should be executed. Please run the higher tiers as well.

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Changes requested by thartmann (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1469