RFR: 8375438: G1: Convert G1HeapRegion related classes to use Atomic<T>

[Thomas Schatzl]

On Tue, 20 Jan 2026 10:04:46 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review conversion of G1HeapRegion related classes to use Atomic<T>.
>> 
>> Testing: tier1, tier4, tier5
>> 
>> (The PipelineLeaksFD failure in gha is a known issue)
>> 
>> Thanks,
>>   Thomas
>
> src/hotspot/share/gc/g1/g1HeapRegion.inline.hpp line 196:
> 
>> 194:     if (want_to_allocate >= min_word_size) {
>> 195:       HeapWord* new_top = obj + want_to_allocate;
>> 196:       HeapWord* result = _top.compare_exchange(obj, new_top);
> 
> Since you are not using `result` beyond checking for CAS completion, you can probably do `if (_top.compare_set(obj_top)) { ... `  directly. This would also obviate the need for the comment on what CAE return value is supposed to signify.

I rewrote the loop to avoid the reload of `top()`, using the result of the `compare_exchange`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/29301#discussion_r2707795071