inline - Java: JIT method inlining

Question

When does Java JIT inline a method call? Is it based on #times the caller method is called (if yes, what would that number be?), or some other criteria (and what would that be?)

I've read that JIT can inline 'final' methods, but it also inlines nonfinal methods based on runtime statistics, so want to know what is that triggering criteria.

I guess the answers would differ based on JVM implementation, but maybe there's something common across all of them?

Answer 1

The short answer is whenever it wants.

Very often a JITC will inline small final or pseudo-final methods automatically, without first gathering any stats. This is because it's easy to see that the inlining actually saves code bytes vs coding the call (or at least that it's nearly a "wash").

Inlining truly non-final methods is not usually done unless stats suggest it's worthwhile, since inlined non-finals must be "guarded" somehow in case an unexpected subclass comes through.

As to the number of times something may be called before it's JITCed or inlined, that's highly variable, and is likely to vary even within a running JVM.