RFR: 8297718: Make NMT free:ing protocol more granular [v2]

[David Holmes]

On Wed, 7 Dec 2022 16:44:29 GMT, Gerard Ziemski <gziemski at openjdk.org> wrote:

>> Johan Sjölen has updated the pull request incrementally with 10 additional commits since the last revision:
>> 
>>  - Add test that asserts block integrity after revival
>>  - Make MallocHeader NONCOPYABLE
>>  - Rest of it
>>  - Adjust comments
>>  - Put back comment, suitably rewritten
>>  - Note reversibility of mark_block_as_dead
>>  - Rename record_free_block to record_free
>>  - Rename record_free to deaccount
>>  - Move FreePackage into MallocHeader and rename
>>  - Rename as revive, delete copy ctr
>
> src/hotspot/share/runtime/os.cpp line 732:
> 
>> 730:       // We have however marked it as dead, revert this change.
>> 731:       header->revive();
>> 732:       return nullptr;
> 
> This is the very first usage of `nullptr` instead of `NULL`, and we now have a mix of both.
> 
> Was this a deliberate choice? Should we then switch all `NULLs` to `nullptr` in here?

We should try to use `nullptr` in new code and change from NULL within those methods for consistency, but a broader cleanup should be done as a separate cleanup RFE.

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PR: https://git.openjdk.org/jdk/pull/11390