When you exit a function, you must clean up local stack allocations. It doesn't matter if you exit with a return or a jump. It doesn't matter where you jump. If you leave the stack with temporary allocations in place, they'll leak space.

(A compiler pipeline may, at some point, observe that releasing stack space, jumping, then claiming the same amount of stack space could be optimised to just a jump, but it may equally observe that releasing, jumping, and claiming a different amount could be optimised to either releasing or claiming the difference, then jumping, too - there's still no difference between recursive, co-recursive, and non-recursive tail calls.)

If A has local variables, the compiler will insert an instruction to increment the SP before the final JMP, which cleans up the space for the locals.

[This requires stack frames to push local variables after pushing the return address (if any), but that's easy to arrange.]

The point is that the compiler can make the TCO decision statically just by inspecting the function in question. Dynamic analysis of who-calls- who is not necessary.