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

[Andrew Haley]

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

On 3/11/22 10:12, Thomas Stuefe wrote:
>     We could also redefine SafeFetch on MacOS/AArch64 to not need WX. We could do this by statically generating SafeFetch on that platform, and it wouldn't be in the JIT region at all. Why not just do that?
> 
> Do you mean using inline assembly?

I'd use out-of-line assembly, as I do for atomic compare-and-swap
on linux:

https://github.com/openjdk/jdk/blob/master/src/hotspot/os_cpu/linux_aarch64/atomic_linux_aarch64.S

But I guess inline would work.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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PR: https://git.openjdk.java.net/jdk/pull/7727