>In a callee cleanup convention, a tail caller could adjust the stack and jump to an unaware tail callee.

You can still do that with a caller-cleanup convention. Suppose you have a convention like

* Set up stack

* Call

* Clean up stack

and you have functions f(), g(), and h(), where g() and h() use this convention and f() calls into g(), and g() into h(). The sequence of instructions from f() to h() without TCO would be

* f: Set up stack for g()

* f: Call g()

* g: Do work

* g: Set up stack for h()

* g: Call h()

* h: Do work

* h: Return

* g: Clean up stack

* g: Return

* f: Clean up stack

And with TCO:

* f: Set up stack for g()

* f: Call g()

* g: Do work

* g: Move things around on the stack so that h()'s arguments are written where g()'s were. This may require a temporary stack allocation that's released before the next step.

* g: Jump to h()

(At this point it looks as if f() called h() directly.)

* h: Do work

* h: Return

* f: Clean up stack

This is always possible as long as h()'s caller-managed stack allocation is no bigger than g()'s.