RFR: 8373630: r18_tls should not be modified on Windows AArch64

[Saint Wesonga]

On Sat, 13 Dec 2025 05:15:17 GMT, Saint Wesonga <duke at openjdk.org> wrote:

>> Nice find.  It would be really useful to have a test case that reproduces the problem, and also some idea of how likely it is.  I bumped the bug to P1 for now.
>
>> Nice find. It would be really useful to have a test case that reproduces the problem, and also some idea of how likely it is. I bumped the bug to P1 for now.
> 
> The virtual threads MonitorEnterExit test has a 100% failure repro rate on Windows AArch64 without this change (but it does not fail on macosx-aarch64 without this change, even though x18 is also reserved on macosx-aarch64). I was specifically running the [testMutualExclusion](https://github.com/openjdk/jdk/blob/23c39757ecdc834c631f98f4487cfea21c9b948b/test/jdk/java/lang/Thread/virtual/MonitorEnterExit.java#L392-L425) parametized test with 0 platform threads and at least 2 virtual threads when investigating this behavior.

> What a bug! @swesonga, are you handling backports?

Yes, I'm preparing the jdk26u backport this afternoon

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PR Comment: https://git.openjdk.org/jdk/pull/28808#issuecomment-3667085205