RFR: 8294729: [s390] Implement nmethod entry barriers

[Tyler Steele]

On Tue, 4 Oct 2022 14:27:09 GMT, Tyler Steele <tsteele at openjdk.org> wrote:

> This draft PR implements native method barriers on s390. When complete, this will fix the build, and bring the other benefits of [JDK-8290025](https://bugs.openjdk.org/browse/JDK-8290025) to that platform.

Thanks for your answers :slightly_smiling_face: 

1. I wanted to make sure there weren't conflicts with using R0 in s390.ad. Switching to R0 seems to have resolved the issue.
2. Thanks for clarifying. Yes, it seems they are c-style imports, but wrapped in `source %{ ... }` sections, not at the beginning of the file where I was expecting them. This issue is fixed as well.
3. I see why you may be thinking that R2 is the problem, but I don't see where the issue is. AFIK, R2 should be restored to its original value before the jump to wrong_method_stub.
   - The hs_err file is certainly useful. But, I wasn't able to pass the -XX args you mentioned because I encounter the issue during the optimized build phase. Is there a way to pass those args to the build jvm?

Switching to a PR as the implementation is now complete. The tests in `hotspot/jtreg/gc` all pass, and I am running T1 currently. The pre-test failures appear to be unrelated to the change.

Thanks again to @TheRealMDoerr and @RealLucy for their prodigious help 💯

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PR: https://git.openjdk.org/jdk/pull/10558