what makes TCO so difficult to implement? it feels like it should be a very simple "if the final instruction before RET is CALL, then eliminate the call"
but clearly I'm missing something
but clearly I'm missing something
If you scroll up you'll see a discussion of how the caller does the popping of arguments it pushed, so it has to be the case that if a different number of arguments were needed for a tail call then the caller will still pop the correct number of arguments, and that is where the complexity lies: because the caller does not actually know anything about the called function's tail call details, so how does one cause the correct thing to happen? One way is by changing the calling conventions radically to ensure that either the called function cleans up the arguments before returning, or that the number of bytes to pop is effectively part of the return signature of the function (with the caller somehow being careful to check that the advertised number wouldn't destroy its frame), or just arrange to leave exactly the number of bytes on the stack that the caller expects even if one tail-calls a function that would leave a different number of bytes.