If the callee is an exported symbol, the compiler has no choice but to adhere to the calling convention. For the C model of translation units that's the default, only local (declared "static") functions are exempted, and usually also only if their address is not taken. More generally, when the tail call crosses the boundaries of modularization that are supported by separate compilation, a recompilation step at the module-linking level would be required. The other complication is function pointers, which assume a specific calling convention, so either you have to have different function-pointer types with different calling conventions, or the compiler has to generate thunks or similar that translate between different calling conventions.

Of course, a language implementation can arrange for all that; but clearly, calling conventions are relevant here.

> But the compiler controls both the caller and callee.

Why? In functional languages, it's common for an exported function from one compilation context to tail call into an exported function from another.

It is usually assumed that it does not control the callee and the jump has to preserve the calling convention for a call.