> From: "r r" <grosi...@gmail.com> > To: "mechanical-sympathy" <mechanical-sympathy@googlegroups.com> > Sent: Saturday, February 5, 2022 12:20:12 PM > Subject: Megamorphic virtual call optimization in Java
> 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. I've recently implemented a small library for that kind of stuff. [ https://github.com/forax/macro | 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, RĂ©mi -- You received this message because you are subscribed to the Google Groups "mechanical-sympathy" group. To unsubscribe from this group and stop receiving emails from it, send an email to mechanical-sympathy+unsubscr...@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/mechanical-sympathy/1998782990.10616957.1644061571789.JavaMail.zimbra%40u-pem.fr.