Megamorphic virtual call optimization in Java

[r r]

Hello,

we know that there are some techniques that make virtual calls not so expensive in JVM like Inline Cache or Polymorphic Inline Cache. 

Let's consider the following situation:

Base is an interface. 

public void f(Base[] b) {

    for(int i = 0; i < b.length; i++) {

          b[i].m();   

    }

}

I see from my profiler that calling virtual (interface) method m is relatively expensive.

f is on the hot path and it was compiled to machine code (C2) but I see that call to m is a real virtual call. It means that it was not optimised by JVM. 

The question is, how to deal with a such situation? Obviously, I cannot make the method m not virtual here because it requires a serious redesign. 

Can I do anything or I have to accept it? I was thinking how to "force" or "convince" a JVM to 

1. use polymorphic inline cache here - the number of different types in b is quite low - between 4-5 types.

2. to unroll this loop - length of b is also relatively small. After an unroll it is possible that Inline Cache will be helpful here.

Thanks in advance for any advices.

Regards,

[Greg Young]

which jvm?

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Studying for the Turing test

[r r]

JVM 11+ (OpenJDK / Zulu)

[Remi Forax]

I've recently implemented a small library for that kind of stuff.

https://github.com/forax/macro

The idea is to see b[i].m() or f(Base[] b) as a method call and extract constants from that method call, the library allows you to choose if a value is a constant or f(value) is a constant (in case of the array Base[].length is a constant) and to decide the policy when it's not a real constant (fail, use a monomorphic inlining cache or use a polymorphic inlining cache).

It uses java.lang.invoke under the hood so everything tend to be inlined.

Thanks in advance for any advices.

Regards,

regards,

Rémi

[Gil Tene]

You might want to give it a spin on Prime (Zing) 11 and see what you get, the Falcon JIT in Prime will do polymorphic guarded in-line caching for up to 6 implementers by default, I believe (as is configurable to higher if needed). This is exactly the sort of thing that capability is meant for.

[r r]

Thanks for your responses :)