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

[Doug Simon]

On Mon, 18 Apr 2022 20:55:40 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> It is declared volatile for correct double checked locking in the code prior to this PR:
>> 
>> JNIEnv* JVMCIRuntime::init_shared_library_javavm() {
>>   JavaVM* javaVM = (JavaVM*) _shared_library_javavm;
>>   if (javaVM == NULL) {
>>     MutexLocker locker(JVMCI_lock);
>>     // Check again under JVMCI_lock
>>     javaVM = (JavaVM*) _shared_library_javavm;
>>     if (javaVM != NULL) {
>>       return NULL;
>>     }
>> 
>> However, now that `JVMCIRuntime::_lock` is acquired at the entry to `JVMCIRuntime::init_shared_library_javavm` there is indeed no longer a need for it to be volatile.
>
> The declaration was wrong for that usage - it should have been "volatile pointer to X" (`JavaVM* volatile`) but was declared "pointer to volatile X" (`volatile JavaVM*`).  That also wasn't a correct implementation of DCLP.  Good thing you are changing this.  So yes, the volatile should be nuked and this cast and the similar one in destroy_shared_library_javavm can be removed.

Thanks for the correction. I have removed `volatile` altogether now. It's also a good reminder of the subtleties in C++ that do not exist in Java.

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