RFR: 8297487: G1 Remark: no need to keep alive oop constants of nmethods on stack

[Richard Reingruber]

On Fri, 25 Nov 2022 15:21:08 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> One other note: since the ARM32 port does not have this nmethod walk safety net during Remark pause anymore and it does not implement nmethod barriers (at least that's what that `nullptr` return value by that removed comment indicates to me) it may be good to talk to ARM32 maintainers about this change. I do not know who maintains ARM32 code.
> 
> We at Oracle do not support ARM32 so it should be good, but it may help ARM32 maintainers to keep this now removed code after all.

Good point. It is myy understanding (also stated in the JBS item) that G1 concurrent marking requires the keep alive of oop constants by the nmethod entry barriers for SATB correctness. So without the entry barriers ARM32 has an issue there already because the keep alive during the remark pause is not sufficient, is it?

There's a JBS item [JDK-8291302](https://bugs.openjdk.org/browse/JDK-8291302) to implement the nmethod entry barrier assigned to @bulasevich. Boris are there plans to implement the nmethod entry barriers on ARM32 in the near future given that they are required for G1 correctness?

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PR: https://git.openjdk.org/jdk/pull/11314