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

[Kim Barrett]

On Sun, 17 Apr 2022 09:04:10 GMT, Doug Simon <dnsimon at openjdk.org> wrote:

>> src/hotspot/share/jvmci/jvmciRuntime.cpp line 1185:
>> 
>>> 1183: JNIEnv* JVMCIRuntime::init_shared_library_javavm() {
>>> 1184:   MutexLocker locker(_lock);
>>> 1185:   JavaVM* javaVM = (JavaVM*) _shared_library_javavm;
>> 
>> Pointless cast.
>
> Without that cast, I get this compiler error:

I forgot that `_shared_library_javavm` is declared `volatile JavaVM*`.  So what's needed is a const_cast to discard the volatile.  But why is it pointer to volatile?  I'm not seeing any reason for that; the out argument for `JNI_CreateJavaVM` doesn't specify volatile.  I wonder if it's supposed to instead be `JavaVM* volatile` to (attempt to, though maybe not correctly) support unlocked reads with concurrent locked writes?

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