RFR: JDK-8299229: Allow UseZGC with JVMCI and enable nmethod entry barrier support

[Erik Österlund]

On Fri, 13 Jan 2023 21:08:08 GMT, Tom Rodriguez <never at openjdk.org> wrote:

> Ok.  I wondered why it was still around since it seemed pretty obviously dead.  I'll restore that code and expose the extra state needed to support code generation for it.  Are 3 different strategies really required?

We are right at a transition point. We need the STW strategy to not regress for the normal GCs. The conc data + instruction approach, however, is strictly better than the conc data approach. It performs better and allows modifying both data and instructions. But it isn't directly compatible with mainline ZGC, as it kind of wants the colour bits to be enclosed in the low order 32 bits of the thread-local disarm guard value. It is in generational ZGC, but not in mainline ZGC. I suppose it is possible to poke the mainline ZGC code to conform to the new requirements, but as generational ZGC is around the corner, we were sort out waiting for that instead. Presumably something similar would have to be done to Shenandoah as well to migrate to the new better barrier.

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PR: https://git.openjdk.org/jdk/pull/11996