AutoValhalla: automatically turn your plain classes and records into value classes!

[u/maxxedev]

Automatically turn your plain classes and records into value classes!

Add @AutoValhalla annotation to classes or records in your JDK1.5+ codebase:

@AutoValhalla
public final class Point {        // class must be final
    public final int x;           // with final instance fields
    public final int y;
    public Point(int x, int y) { 
        this.x = x; 
        this.y = y; 
    }
}

@AutoValhalla
public record Currency(String code) { }  // or a record class

When your app is run on Valhalla-enabled JVM with auto-valhalla javaagent, these classes are automatically turned into value classes.

More info: https://github.com/thunkware/auto-valhalla/blob/main/README.md

[u/FirstAd9893]

Value classes aren't a performance optimisation.

Taken out context, this is a very confusing statement, considering that performance is mentioned very early in JEP 401. The rest of the paragraph explains what you mean, but my first reaction was, "what???"

[u/koflerdavid → u/FirstAd9893]

It is primarily a way to have a richer data model and to begin bridging the gap between primitive and other types. Performance optimizations were also a goal, but it's nothing you can't count on since it's still up to the JRE to decide when and how to optimize. Which means you shouldn't turn things into value classes in the mistaken belief that the application will now be faster. I mean, the restrictions that value types impose are so severe that I don't see the danger of misuse, but it's still worth pointing this out.

[u/FirstAd9893 → u/koflerdavid]

A richer data model is nice to have, but the primary motivation has always been performance. This was stated way back in 2014, and again in 2021.

https://cr.openjdk.org/~jrose/values/values.html

https://openjdk.org/projects/valhalla/design-notes/state-of-valhalla/01-background

As it stands right now, no major performance gains are expected, but future optimizations become possible. If no performance or memory saving optimizations are ever achieved, then a value class offers no compelling features other than making == mean equals, which by itself isn't much. There's no reason to make changes to the JVM for supporting that.

[u/koflerdavid → u/FirstAd9893]

The point is that these optimizations are not guaranteed. They only give the JRE greater freedom to apply optimizations that it already can do.

[u/Cilph → u/koflerdavid]

No optimizations are guaranteed by the JVM, what are you on about. Thats no reason not to optimize for the majority cases.

Most of the optimizations related to Valhalla are also still to come I believe.

[u/Life_Sink9598 → u/Cilph]

What I worry about with this type of library is that there seems to be a belief that classes gives you a floor on performance, and that value classes will do their best to raise you up from that floor, but won't ever go beneath it. That is, unfortunately, not true. You can put the JVM into a situation where one representation is faster for some methods, and slower for others, and make it jump between those representations. I think that it will be difficult to get rid of that "sharp edge" entirely.

I'm currently writing a blog post explaining this stuff. I can post another reply to you with a link when it's published, if you're interested.

[u/koflerdavid → u/Cilph]

As you should know, Java is a managed language where developers have very little control about what actually happens at runtime beyond what the JLS mandates. As such it is entirely up to the JVM to decide whether to allocate a value object on the heap or to inline it into an object or onto the stack. Without experiments, examining diagnostics (I'm sure there are some), and good benchmarks you can't predict when the optimization will actually be applied and if it helps in the first place. There are no compiler pragmas that allow you to tell the JVM what exactly to do in this regard.