RFR: 8294729: [s390] Implement nmethod entry barriers [v13]

[Tyler Steele]

On Thu, 27 Oct 2022 21:35:41 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

> I have to revoke my review. This does no longer look correct.

Noted. Since the platform is building again, it may be a good idea to follow your suggestion of pushing the changes up to your review, and then create a separate PR for the new changes. I suppose this depends on whether @fisk believes the previous changes (up to the time of their first comment)[[1]](https://github.com/openjdk/jdk/pull/10558#pullrequestreview-1157627111) to be complete.

> src/hotspot/cpu/s390/gc/shared/barrierSetAssembler_s390.cpp line 169:
> 
>> 167:   // Load class loader data to determine whether the method's holder is concurrently unloading.
>> 168:   __ load_method_holder(Z_R0_scratch, Z_method);
>> 169:   __ z_lg(Z_R0_scratch, in_bytes(InstanceKlass::class_loader_data_offset()), Z_R0_scratch);
> 
> I don't think R0 can be used for storage addressing.

Those cases where 'if you use R0 it really means 0 and not the contents of the register' are a real gotcha.

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