RFR: 8342730: Get rid of SummaryDiff in VMATree [v2]

[Paul Hübner]

On Tue, 16 Sep 2025 09:03:57 GMT, Afshin Zafari <azafari at openjdk.org> wrote:

>> `SummaryDiff` result of any `register_mapping()` call, is passed into the function rather than returned. This enables avoiding copying the structure to/from the stack.
>> 
>> Tests:
>> local linux-x64-debug: NMT gtests and JTREGs passed.
>
> Afshin Zafari has updated the pull request incrementally with one additional commit since the last revision:
> 
>   better naming for diff

Marked as reviewed by phubner (Author).

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PR Review: https://git.openjdk.org/jdk/pull/26761#pullrequestreview-3228689604