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

[Ivan Walulya]

On Thu, 21 Aug 2025 16:13:10 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> 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 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
> 
> Thanks,
>   Thomas

Changes requested by iwalulya (Reviewer).

src/hotspot/share/gc/g1/g1ConcurrentMark.cpp line 3125:

> 3123:                           group->length(),
> 3124:                           group->gc_efficiency(),
> 3125:                           group->length() > 0 ? group->liveness_percent() : 0.0f,

Why not move this logic into `liveness_percent()` (i.e returns 0.0f if length() == 0)?

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PR Review: https://git.openjdk.org/jdk/pull/26888#pullrequestreview-3141593342
PR Review Comment: https://git.openjdk.org/jdk/pull/26888#discussion_r2291667171