Valhalla and the vector API for SIMD, yes. In theory, at that point, the only major performance difference between them would be GC vs manual memory management. Also: code size.
One interesting experiment that'd be worth trying is generating value types that wrap memory segments, as that's the new manual memory allocation API. You can do pretty sophisticated manual allocation with arenas and stuff, but you can't allocate Java objects into those segments. What you can do though, is create a "struct" using VarHandles that you can cast the segment to which then reads/writes through to the segment. This leads to the question of whether you can make a nice wrapper around such value types that yield C++ style manual memory allocation.
One interesting experiment that'd be worth trying is generating value types that wrap memory segments, as that's the new manual memory allocation API. You can do pretty sophisticated manual allocation with arenas and stuff, but you can't allocate Java objects into those segments. What you can do though, is create a "struct" using VarHandles that you can cast the segment to which then reads/writes through to the segment. This leads to the question of whether you can make a nice wrapper around such value types that yield C++ style manual memory allocation.