RFR: 8365656: [ubsan] G1CSetCandidateGroup::liveness() reports division by 0 [v2]

[Thomas Schatzl]

On Fri, 22 Aug 2025 09:07:31 GMT, Francesco Andreuzzi <duke at openjdk.org> wrote:

>> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   * walulyai review
>
> src/hotspot/share/gc/g1/g1ConcurrentMark.cpp line 3165:
> 
>> 3163:   G1CollectedHeap* g1h = G1CollectedHeap::heap();
>> 3164: 
>> 3165:   if (g1h->young_regions_cset_group()->length() != 0) {
> 
> Would it make sense to have this check inside `log_cset_candidate_group_add_total` instead?

Actually this hunk has been committed wrongly, as I did some testing, but thanks for the discussion:

I am fairly unsure where to best put that check

- for a card group with a length of 0 the liveness does not make sense, but since it is that special young card group does exist and always takes some memory, printing it is useful.

  We want its memory usage to show up in the total memory consumption too.

  Adding this check in `log_cset_candidate_group_add_total` to (as far as I can guess from your suggestion) just not print the log line for this is also confusing: total memory consumption will be != 0, but no separate line for that card group printed...

- So the initial change that @walulyai did not like put that check into the argument of the log processing, so it would print the line, and we would have an assert in `liveness_percent()` to not try to get that value for such card groups otherwise.

Thoughts?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26888#discussion_r2293327982