RFR: 8375643: G1: Convert G1RegionMarkStatsCache to use Atomic<T>

[Aleksey Shipilev]

On Tue, 20 Jan 2026 10:16:18 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this change to make `G1RegionMarkStatsCache` use `Atomic<T>`.
>> 
>> The only unusual part is that I added two getters to that class to avoid the repeated `.load_relaxed()` typing.
>> 
>> Testing: gha
>> 
>> Thanks,
>>   Thomas
>
> src/hotspot/share/gc/g1/g1RegionMarkStatsCache.hpp line 119:
> 
>> 117:   void add_live_words(uint region_idx, size_t live_words) {
>> 118:     G1RegionMarkStatsCacheEntry* const cur = find_for_add(region_idx);
>> 119:     cur->_stats._live_words.store_relaxed(cur->_stats.live_words() + live_words);
> 
> Is this just `add_then_fetch` with `memory_order_relaxed` then? Or are we trying to dodge some performance penalty avoiding the atomic RMW instructions? I suspect x86 still implicitly does CAS memory effects when doing `cmpxchg` on memory location; `memory_order_relaxed` would not help there.

Actually, what _are_ the atomicity requirements here? Is this code supposed to be called on same region stats by multiple threads? Or is this single threaded?

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PR Review Comment: https://git.openjdk.org/jdk/pull/29303#discussion_r2707705008