RFR: 8282475: SafeFetch should not rely on existence of Thread::current [v6]

[Anton Kozlov]

On Wed, 9 Mar 2022 08:35:41 GMT, Johannes Bechberger <duke at openjdk.java.net> wrote:

>> The WXMode for the current thread (on MacOS aarch64) is currently stored in the thread class which is unnecessary as the WXMode is bound to the current OS thread, not the current instance of the thread class.
>> This pull request moves the storage of the current WXMode into a thread local global variable in `os` and changes all related code. SafeFetch depended on the existence of a thread object only because of the WXMode. This pull request therefore removes the dependency, making SafeFetch usable in more contexts.
>
> Johannes Bechberger has updated the pull request incrementally with one additional commit since the last revision:
> 
>   current_thread_wx -> ThreadWX

https://github.com/openjdk/jdk/compare/master...478ec1a7ca2c72e5916b28613a4875aa2ee1a793 touches more places than a targeted change in ThreadWXEnable... I'm not sure the real nesting is required for a thread that is not registered properly with VM. The initial state is always assumed for the NULL Thread. The SafeFetch assembly does not do up-calls to VM. I don't see why we'd need runtime tracking of WX state. The state is either WXExec for SafeFetch assembly, or unknown -- which we assume to be WXWrite regardless of approach taken.

Nesting was implemented to reduce the amount of changes in JVM (yes, WX code scattered around the VM less than it could be :)), but it is possible to avoid runtime WX tracking if you always know the state, like we do if Thread == NULL.

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PR: https://git.openjdk.java.net/jdk/pull/7727