RFR: 8338780: GenShen: Fix up some comments

[Y. Srinivas Ramakrishna]

On Thu, 22 Aug 2024 17:29:50 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> In https://github.com/openjdk/jdk/pull/20395 Aleksey provided some suggestions for improving various comments. This ticket gathers together several of those changes.
>> 
>> **Testing:**
>> - [x] GHA
>> - [x] SPECjbb w/GenShen
>> - [x] jtreg:tier1
>> - [ ] Codepipeline testing
>
> src/hotspot/share/gc/shenandoah/heuristics/shenandoahGlobalHeuristics.cpp line 115:
> 
>> 113:     ShenandoahHeapRegion* r = data[idx].get_region();
>> 114:     if (cset->is_preselected(r->index())) {
>> 115:       assert(false, "There should be no preselected regions during GLOBAL GC");
> 
> If this is a true invariant (which it is in testing to date), then it must be the case that the call at line 55 above to `add_preselected_regions_to_collection_set()` doesn't do anything, and indeed returns `0` into `cur_young_garbage` there.
> 
> Clearly, I am missing something here. @kdnilsen ?

I'll run some testing with another assert at the code at line 55 to check my (mis)understanding.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/484#discussion_r1727562566