RFR: 8350567: NMT: update VMATree::register_mapping to copy the existing tag of the region [v2]

[Afshin Zafari]

On Sun, 2 Mar 2025 20:12:41 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> Afshin Zafari has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - removed extra whitespace.
>>  - unit test added.
>
> src/hotspot/share/nmt/vmatree.cpp line 80:
> 
>> 78:       MemTag tag = leqA_n->val().out.mem_tag();
>> 79:       stA.out.set_tag(tag);
>> 80:       LEQ_A.state.out.set_tag(tag);
> 
> Shouldn't `LEQ_A.state.out`'s tag already be `tag`? Is this line really necessary?

It's not. Removed.

> src/hotspot/share/nmt/vmatree.cpp line 210:
> 
>> 208: 
>> 209:   // Finally, we can register the new region [A, B)'s summary data.
>> 210:   MemTag tag_to_change = use_tag_inplace ? stA.out.mem_tag() : metadata.mem_tag;
> 
> Do we ever change the `stA.out.mem_tag()` ? Can't it always be `stA.out.mem_tag()`?

Yes, it is changed in line 79 above.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23771#discussion_r1977351075
PR Review Comment: https://git.openjdk.org/jdk/pull/23771#discussion_r1977350774