The JVMCIRuntime::_shared_library_javavm_id field is initialized to a jlong in libgraal[1] and so it's C++ type in HotSpot should match.

[1] https://github.com/oracle/graal/blob/d544bbe3fe416d39e9e5b8fc645a67a36a5d7c07/substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jni/functions/JNIInvocationInterface.java#L396-L397