RFR: 8282475: SafeFetch should not rely on existence of Thread::current [v7]

[Anton Kozlov]

On Fri, 11 Mar 2022 07:52:16 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:
> 
>   Remove two unnecessary lines

I looked on the patch again from the perspective of a pure refactoring. It looks fine except we lost one of the asserts.

src/hotspot/share/runtime/thread.cpp line 278:

> 276:   }
> 277: 
> 278:   MACOS_AARCH64_ONLY(DEBUG_ONLY(os::ThreadWX::init();))

This line meant the WX state is not initialized at this point (as a part of Thread constructor). Since there are a several places where the state is initialized and it was easy to miss one, I would like to preserve some assert that the state is initialized.

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Changes requested by akozlov (Committer).

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