RFR: 8242440: use separate, destroyable JavaVM instances per libgraal compiler thread

[Kim Barrett]

On Sun, 17 Apr 2022 08:58:23 GMT, Doug Simon <dnsimon at openjdk.org> wrote:

>> src/hotspot/share/jvmci/jvmciRuntime.cpp line 976:
>> 
>>> 974: JVMCIRuntime::JVMCIRuntime(JVMCIRuntime* next, int id, bool for_compile_broker) {
>>> 975:   _init_state = uninitialized;
>>> 976:   _shared_library_javavm = NULL;
>> 
>> Prefer nullptr to NULL in new code.
>
> Thanks for the reminder. I'm tempted to now change `NULL` to `nullptr` in all `jvmci/*` files. What's the policy/recommendation on that? Don't bother/separate PR/piggy-back on this PR?

I think most folks have been taking a nullptr in new code approach, without anyone doing substantial conversions.  Though Leo did look at a wholesale conversion at one point (it turned out to be a little premature, since we didn't really have nullptr yet).  There are some places where there are strange overload games being played because NULL is an unadorned `0` on some platforms, and I think Leo found some places where the selected overload was even wrong because of that.  I certainly wouldn't a wholesale conversion of JVMCI as part of this PR.

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PR: https://git.openjdk.java.net/jdk/pull/8262