Comment by cogman10

> There’s a catch worth knowing about here, though: flattened data has to be readable and writable atomically (otherwise it risks “tearing” under concurrent access).

I really hope they give an escape hatch for this. It will make it really hard to extract a lot of the benefit of valhala if you can't make a thread unsafe value class. It's also one of those problems that will be quite hard to run into. You basically need something like this

    class Bar {
      static Foo value[] = new Foo[10];
      static void setFooFromManyThreads(Foo foo) {
        value[0] = foo;
      }
      
      value record Foo(int x, int y, int z) {};
    }

Not something you typically run into and generally already a thread safety problem.

The solution is also simple, a `synchronized{}` block will fix it if you need to have a tearable class that's written from multiple threads.

But the other thing is that for SIMD operations, you really need flattening, and that really does typically mean having something like `Foo(double x, double y, double z)` in play. It'd be a shame if the way we have to do this is a struct of arrays.