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

[Thomas Schatzl]

On Fri, 22 Aug 2025 10:18:56 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> 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?

since it's "just" logging anyway @walulyai's suggestion is maybe easiest and acceptable
.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26888#discussion_r2293350379