RFR: 8320370: NMT: Change MallocMemorySnapshot to simplify code. [v2]

[Thomas Stuefe]

On Mon, 20 Nov 2023 13:35:54 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> Hi,
>> 
>> `MallocMemorySnapshot` used to be initialized in a quite non-standard way, using global placement new on an array of `size_t` of the size required to fit `MallocMemorySnapshot`. This looks like it was intended to circumvent some `ResourceObj` internals, but I'm unsure of its purpose. This change does what you expect a regular initialization of a global variable to look like.
>> 
>> ~Currently running through tier1.~ GHA passed and so did tier1 on Oracle CI.
>
> Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Thomas comments

I believe the reason for the original shape of this code was to avoid initialization altogether if NMT was disabled. @zhengyu123 ?

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PR Comment: https://git.openjdk.org/jdk/pull/16724#issuecomment-1819087047