RFR: 8285984: G1: Use standard idiom for inlined payload in G1MonotonicArena::Segment [v2]

[Leo Korinth]

On Fri, 12 Sep 2025 13:37:10 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Leo Korinth has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   may fail on 32 bit machines, added explicit alignas
>
> src/hotspot/share/gc/g1/g1MonotonicArena.hpp line 113:
> 
>> 111: };
>> 112: 
>> 113: static constexpr uint SegmentAlignment = 8;
> 
> Maybe this should be called `SegmentMaxAlignment`, or `SegmentPayloadMaxAlignment`.
> 
> And have a comment describing that we align the Segment class to this alignment to guarantee that the payload is aligned to all powers of 2 less than or equal to this value.

`SegmentPayloadMaxAlignment`  is okay with me. `SegmentMaxAlignment` sounds wrong, as it is actually the minimum alignment for the Segment.

> src/hotspot/share/gc/g1/g1MonotonicArena.hpp line 239:
> 
>> 237:     assert(_max_num_slots > 0, "Must be");
>> 238:     assert(_slot_alignment > 0, "Must be");
>> 239:     assert(_slot_alignment <= SegmentAlignment, "Must be");
> 
> Not sure if it should be clarified that this assert is sufficient because we only ever use allow alignments which are a power of two.

Do you wish me to assert that it is a power of two as well?

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PR Review Comment: https://git.openjdk.org/jdk/pull/27258#discussion_r2344340038
PR Review Comment: https://git.openjdk.org/jdk/pull/27258#discussion_r2344348392