C# stackalloc returns a ‘ref struct’ which has certain restrictions and would be a Q-world type.

Java chose to go with an L-world implementation where everything is still a reference type on the heap, but memory management is more efficient via flattening. That’s why it’s a ‘value class’ and not a ‘struct’.

Primitive wrappers are scalarized into registers.

Stack allocated value types would have different semantics and is incompatible with the L-world implementation. Sure, they could implement it, but it would be the Q-world implementation that they decided to forego.

They can do automatic stack allocation for optimization via escape analysis as you mentioned. But, stackalloc is user allocated.

https://learn.microsoft.com/en-us/dotnet/csharp/language-ref...