RFR: 8312585: Rename DisableTHPStackMitigation flag to THPStackMitigation

[Aleksey Shipilev]

On Mon, 24 Jul 2023 09:02:59 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> The flag newly added by [JDK-8312182](https://bugs.openjdk.org/browse/JDK-8312182) is prompting the use of `-XX:+DisableTHPStackMitigation` to disable the THP stack mitigation, thus allowing THP in thread stacks. This double negation does not read well, and not in line with other mitigation flags like `IntelJccErratumMitigation`.
> 
> It would be better to rename the flag to avoid double-negation, before it proliferates to other JDK releases.
> 
> (I would have the same comment during the original review, but missed it :P)
> 
> Additional testing: 
>  - [ ] GHA
>  - [x] Linux x86_64 fastdebug, affected test

It was confusing at least for me, because I had to pause and think what exactly `-XX:-DisableTHPStackMitigation` would mean. It is disabling (`-`) the disablement (`Disable`) of the mitigation, so it actually enables it? And yes, I do dislike `Use*`, `Disable*` and `Enable*` prefixes for something that is already covered by binary `(+|-)` because of collisions like these.

In my opinion, `(+|-)IntelJccErratumMitigation` did it right, and we should follow that one.

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PR Comment: https://git.openjdk.org/jdk/pull/14992#issuecomment-1647913643