RFR: 8294729: [s390] Implement nmethod entry barriers

[Lutz Schmidt]

On Fri, 7 Oct 2022 15:18:27 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

>> @TheRealMDoerr Yes, z_larl() will always work within the bounds of CodeCache. But it only gives you the address where the address is stored. You need another z_lg() to get the branch address. And then there is no benefit anymore compared to two 4-byte immediate loads.
>
> The branch target is constant once the stub is created. We don't need to load it from memory.
> I meant `__ z_larl(Z_R1_scratch, StubRoutines::zarch::nmethod_entry_barrier());`

Good idea! One instruction instead of two, plus avoiding a data interlock when loading the constant address.

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