RFR: 8307532: Implement LM_LIGHTWEIGHT for Zero [v2]

[Roman Kennke]

On Tue, 1 Oct 2024 14:51:56 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

> > The `LM_LIGHTWEIGHT` changes looks good to me.
> 
> > 
> 
> > This also enables `UseObjectMonitorTable`. I believe `BasicLock::object_monitor_cache()` must be modified as well. Assuming that both `ZERO` and a supported platform (e.g. `X86`) can and will be defined in the same build.
> 
> > 
> 
> > `ZERO` should return `nullptr` in `BasicLock::object_monitor_cache()` as well.
> 
> 
> 
> I might be missing something, but Zero cannot be combined with another VM variant in the configure step, so the risk of a CPU arch being defined at the same time as Zero cannot happen (The build system treats Zero a little strangely)

No, this doesn't seem to be the case. I placed a STATIC_ASSERT in the #if defined ... block and it fired with ZERO. This means that yes, we *do* get both ZERO and (e.g.) X86 defined at the same time. And it makes sense, too. ZERO is a JVM variant and X86, AARCH64, etc are CPU arches. It is entirely possible and reasonable to build zero JVM on x86 or aarch64.

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PR Comment: https://git.openjdk.org/jdk/pull/21220#issuecomment-2386348384