RFR: 8294729: [s390] Implement nmethod entry barriers [v14]

[Martin Doerr]

On Thu, 27 Oct 2022 23:46:43 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.
>
> Tyler Steele has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Changes
>   
>   - Adds labels to registers used in c2i_entry_barrier
>   - Removes use of R2
>   - Removes erroneous uses of R0
>   - Adds nm->c2i_entry_barrier in gen_i2c2i_adapters

src/hotspot/cpu/s390/gc/shared/barrierSetAssembler_s390.cpp line 164:

> 162:   Rtmp1 = Z_R1_scratch;
> 163:   Rtmp2 = Z_R7;
> 164:   Rtmp3 = Z_R8;

Registers other than R0 and R1 should not be hardcoded, here. Please pass them by argument and manage them one level above. We should avoid distributing register selection for maintainablily reasons.

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