8365656: [ubsan] G1CSetCandidateGroup::liveness() reports division by 0

[tschatzl]

Hi all,

please review this change that fixes a div-by-zero in calculating liveness for logging.

It is possible that the young gen group cardset is empty (has no regions assigned to it as no eden region has retired yet) when the Cleanup pause starts, making the liveness calculation divide by zero.

The change special-cases this case when printing - I decided to always print the group in this case, regardless of length, just fudging the output value a bit. An alternative would be not printing it at all in this case. Feel free to argue for that solution. Also the printed value for liveness (0.0%) is up for discussion.

Testing: failing test case does not fail any more now and then, gha, tier1-4

Thanks,
Thomas

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Issue

• JDK-8365656: [ubsan] G1CSetCandidateGroup::liveness() reports division by 0 (Bug - P4)

Reviewers

• Ivan Walulya (@walulyai - Reviewer)

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8365656: [ubsan] G1CSetCandidateGroup::liveness() reports division by 0

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At the time when this comment was updated there had been 11 new commits pushed to the master branch:

• e1c58f8: 8360540: nmethod entry barriers of new nmethods should be disarmed
• f0498c2: 8364764: java/nio/channels/vthread/BlockingChannelOps.java subtests timed out
• 8e44856: 8365180: Remove sun.awt.windows.WInputMethod.finalize()
• ... and 8 more: https://git.openjdk.org/jdk/compare/fb651fd6d246e69b42363e050eb8d96afb633eed...master

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