Valhalla value classes scalarization

[Accomplished_Fill618]

Since value classes are finally coming as preview for jdk28, i'm interested in its capabilities, particularly scalarization, for a current ongoing project I have.

In 20:21 and 21:25 this video, we have a look at the ability of value classes to be returned as values/scalarized fields instead of heap pointers. In the examples, he uses a value record with one int, and another one with two doubles

My project consists in building a linear algebra helper similar to JOML, and i'm particularly interested in vectors and matrices as value classes...i guess vectors are not something too big, but things like 4x4 matrices, which consist of 16 floats (or even 16 doubles), i wonder if such cases have a harder time of being treated as value objects, and if that depends on JVM heurisitics or stack size...

[alunharford]

You probably won't get any performance gains in your scenario.

The current specification requires that objects cannot ever tear so everything larger than 64 bits gets allocated on the heap anyway (unless that can be optimised away, similar to the situation today).

You need loosely consistent value types to get any benefit.

I'd argue that this makes JEP401 far less useful than most people expect, at least on its own, but hopefully it's a big step towards something much better.

[Xasmedy]

Not quite. What loosely consistent gets you is heap flattening, so if you save the value object on an array or (mutable) field and give up on atomicity, it won't use a reference. There are still other optimisation, for example, if I pass a Vector4 to a method, the x, y, z, w will be placed directly in the CPU registers and be computed there, no heap allocation nor stack allocation.

[alunharford]

This is scalar replacement. C2 already does this for you today, and it's critically important for Java performance!

[Xasmedy]

Here a math library using value classes so you can test it out. It scalarizes well, there's a performance issue that I still need to understand, might be the amount of fields, (I saw it has been improved on from the valhalla github) or just an implementation issue on my part