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

[Aleksey Shipilev]

On Mon, 19 Jan 2026 13:32:46 GMT, Thomas Schatzl <tschatzl 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

Can go a bit further:

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.

src/hotspot/share/gc/g1/g1HeapRegionManager.cpp line 740:

> 738: bool G1HeapRegionClaimer::claim_region(uint region_index) {
> 739:   assert(region_index < _n_regions, "Invalid index.");
> 740:   uint old_val = _claims[region_index].compare_exchange(Unclaimed, Claimed);

Same thing here, it could be just `return _claims[region_index].compare_set(Unclaimed, Claimed)`.

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PR Review: https://git.openjdk.org/jdk/pull/29301#pullrequestreview-3681362712
PR Review Comment: https://git.openjdk.org/jdk/pull/29301#discussion_r2707640538
PR Review Comment: https://git.openjdk.org/jdk/pull/29301#discussion_r2707646539