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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.

1 comments

Do you often find yourself doing mutual recursion between functions crossing compilation units/modules? I'm not going to say it can't happen, I just don't see it as much of a problem.

The point is that the compiler has to deal with it and has to check if the function is exported or its address is taken. So it’s a problem when implementing the compiler and adjacent tooling, you can’t just add it naively. You also have to document the side conditions under which TCO will or won’t happen, which the programmer will have to take into account.

If on the other hand the regular calling convention is compatible with TCO, then everything becomes much simpler, because it fits in with the existing model.

>The point is that the compiler has to deal with it and has to check if the function is exported or its address is taken.

Like I said in a different comment below, these are not obstacles for TCO. The compiler can simply emit a second copy of the function that doesn't need to honor a calling convention.

>So it’s a problem when implementing the compiler and adjacent tooling

Yeah, implementing a compiler is difficult work. Who ever said otherwise? I originally responded to a comment talking about TCO being incompatible with certain calling conventions. I.e. if your platform uses a certain calling convention then TCO is impossible. That's what it means for two things to be incompatible: you can have either one or the other, but not both at the same time.