Better verification of remset

[kdnilsen]

We do not need a remembered set at the start of a Global GC because Global GC scans all of old and all of young. Because it is not necessary, the implementation of System.gc() may jump directly into marking without finishing the evacuation or coalescing and filling of previously identified mixed-evacuation candidates. This means the remembered set may be in an incoherent state at the start of Global GC. If this occurs, the verifier should not attempt to verify the remembered set at the start of Global GC.

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Progress

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$ git fetch https://git.openjdk.org/shenandoah.git pull/423/head:pull/423
$ git checkout pull/423

Update a local copy of the PR:
$ git checkout pull/423
$ git pull https://git.openjdk.org/shenandoah.git pull/423/head

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$ git pr checkout 423

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https://git.openjdk.org/shenandoah/pull/423.diff

[kdnilsen]

I'll leave this in draft mode until I have finished more extensive testing.

[bridgekeeper[bot]]

:wave: Welcome back kdnilsen! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk[bot]]

❗ This change is not yet ready to be integrated. See the Progress checklist in the description for automated requirements.

[kdnilsen]

@earthling-amzn : I'm pondering your suggestion to not swap remset if global() and ShenandoahVerify. This PR, as currently drafted, will still verify remset at start of global if we are NOT EVACUATING and NOT FILLING. The verification is not "required" because we're not going to use the remset during GLOBAL. But verifying the remset here may help us more quickly identify a bug if one gets introduced...

I could easily be persuaded to always NOT verify remset if cycle is GLOBAL, and then I would be very comfortable not swapping remset at start of GLOBAL GC.

[openjdk[bot]]

⚠️ @kdnilsen This pull request contains merges that bring in commits not present in the target repository. Since this is not a "merge style" pull request, these changes will be squashed when this pull request in integrated. If this is your intention, then please ignore this message. If you want to preserve the commit structure, you must change the title of this pull request to Merge <project>:<branch> where <project> is the name of another project in the OpenJDK organization (for example Merge jdk:master).

[bridgekeeper[bot]]

@kdnilsen This pull request has been inactive for more than 8 weeks and will be automatically closed if another 8 weeks passes without any activity. To avoid this, simply add a new comment to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

[openjdk[bot]]

@kdnilsen this pull request can not be integrated into master due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout better-verification-of-remset
git fetch https://git.openjdk.org/shenandoah.git master
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge master"
git push

[bridgekeeper[bot]]

@kdnilsen This pull request has been inactive for more than 8 weeks and will be automatically closed if another 8 weeks passes without any activity. To avoid this, simply add a new comment to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!