public static void premain(String args, Instrumentation I) {
		instrumentation = I;
	}
Just a hook so we can call {@link java.lang.instrument.Instrumentation}
 methods like sizeof().  Start the Java VM with -javaagent instrumentor.jar
 if instrumentor.jar is where you put the .class file for this code.
 MANIFEST.MF for that jar must have "Premain-Class:Instrumentor".

 I'm not using at moment but I'm adding in case.
runtime-testsuite/test/org/antlr/v4/test/runtime/java/api/perf/Instrumentor.java