RFR: 8365656: [ubsan] G1CSetCandidateGroup::liveness() reports division by 0 [v4]

[Thomas Schatzl]

> 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

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  * make liveness_percent() fail on an empty group, check before use

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26888/files
  - new: https://git.openjdk.org/jdk/pull/26888/files/496d9bb4..3730e498

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26888&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26888&range=02-03

  Stats: 5 lines in 2 files changed: 0 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26888.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26888/head:pull/26888

PR: https://git.openjdk.org/jdk/pull/26888