Those are more like "allocating the stack in the GC nursery" than they are "allocating objects on the stack." They store the actual chain of function activation

Those are more like "allocating the stack in the GC nursery" than they are "allocating objects on the stack." They store the actual chain of function activation records as GC objects, and only "pop" the machine stack as part of running the GC. C2 has two separate spaces in memory- one for the stack and another for the GC nursery (and the rest of the heap). It pops the machine stack when a Java function returns, and clears out the nursery as part of running the GC. The difference is that the C2 approach to managing the stack (shared by C, etc.) loses some flexibility to increase performance. When the machine stack maps 1:1 with the call stack like that, objects that are allocated on the stack incur no GC cost- they are always freed automatically on function return simply by moving the stack pointer, never kept alive and promoted, the way they can be in Chicken Scheme's approach.