I'm currently working on optimizing a particular method, which is unfortunately inlined by the JVM, which prevents it from being properly vectorized. I've noticed that there is an annotation to forbid inlining, namely jdk.internal.vm.annotation.DontInline . However, it cannot be accessed from the default module.
Is there a clean way of gaining access to this annotation or to prevent the inlining of the offending method some other way?
DontInline, ForceInline, etc. are JDK internal annotations, they cannot be applied to user code. Even if you somehow manage to open these annotations, HotSpot JVM has an explicit check to disallow them for non-privileged classes.
The reasons are understandable. These annotations are the implementation detail of the particular JVM version; JDK developers are free to add/remove/change meaning of such annotations without notice, even in a minor JDK update.
Using @DontInline to force vectorization does not seem a good approach anyway. In general, inlining should not prevent from other optimizations. If you encounter such problem, it's better to report an issue on hotspot-compiler-dev mailing list.
Now the good news.
Since JDK 9, there is a public supported API to manually tune JIT compiler. This is JEP 165: Compiler Control.
The idea is to provide compiler directives in a separate JSON file, and start the JVM with -XX:CompilerDirectivesFile=<file> option. If your application is sensitive to certain compiler decisions, you may provide the directives file along with the application.
{
match: "*::*",
inline: "-org/package/MyClass::hotMethod"
}
It is even possible to apply compiler directives programmatically in runtime using DiagnosticCommand API:
ManagementFactory.getPlatformMBeanServer().invoke(
new ObjectName("com.sun.management:type=DiagnosticCommand"),
"compilerDirectivesAdd",
new Object[]{new String[]{"compiler.json"}},
new String[]{"[Ljava.lang.String;"}
);
By the way, there is Vectorize: true option among the directives list, which may probably help in vectorizing the particular method.