When Short Methods Pay Off: JIT Inlining

Among all Just-In-Time Java compiler optimizations, inlining methods are a powerful approach. Usually, when we write code following good object-oriented practices, we end up having lots of small objects with well-encapsulated attributes – most of them accessible via getters. But there is an overhead of making additional calls and increasing the callstack. Fortunately, with JIT inlining, we can follow good practices and benefit from performant code.

A method is eligible for inlining if:

• It’s small – the bytecode size is smaller than 35 bytes (can be overridden by the -XX:MaxInlineSize=X flag).
• It’s called frequently (it’s hot) and it’s smaller than 325 bytes (can be overridden by the -XX:MaxFreqInlineSize=X flag).

Consider the following snippet:

public class JitInlining {
    public static void main(String args[]) {
        Position position = new Position(25);
        for (int i = 0; i < 1000000; ++i) {
            int x = position.getX();
            position.setX(x + 1);
        }
        System.out.println(position.getX());
    }
}

class Position {
    private int x;

    public Position(int x) {
        this.x = x;
    }

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }
}

When inlining is applied, the position.setX(x + 1) operation will look like this:

position.x = x + 1;

To make sure that a particular piece of code is inlined, you can use the -XX:+PrintInlining flag (together with -XX:+PrintCompilation and -XX:+UnlockDiagnosticVMOptions):

java -XX:+PrintCompilation -XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining com.performantcode.JitInlining

The result will be something like that:

111   19 %     4       com.performantcode.JitInlining::main @ 12 (47 bytes)

                      @ 19   com.performantcode.Position::getX (5 bytes)   accessor

                      @ 27   com.performantcode.Position::setX (6 bytes)   inline (hot)

Please notice that inline (hot) is displayed next to setX.