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Comment by IgorPartola | Hacker News Reader

I sort of see what you are getting at but I am still a bit confused:

If I have a program that based on the input given to it runs some number of recursions of a function and two compilers of the language, can I compile the program using both of them if compiler A has PTC and compiler B does not no matter what the actual program is? As in, is the only difference that you won’t get a runtime error if you exceed the max stack size?

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That is correct, the difference is only visible at runtime. So is the difference between garbage collection (whether tracing or reference counting) and lack thereof: you can write a long-lived C program that calls malloc() throughout its lifetime but never free(), but you’re not going to have a good time executing it. Unless you compile it with Fil-C, in which case it will work (modulo the usual caveats regarding syntactic vs semantic garbage).

I think features of the language can make it much easier (read: possible) for the compiler to recognize when a function is tail call optimizable. Not every recursion will be, so it matters greatly what the actual program is.

It is a feature of the language (with proper tail calls) that a certain class of calls defined in the spec must be TCOd, if you want to put things that way. It’s not just that it’s easier for the compiler to recognize them, it’s that it has to.

(The usual caveats about TCO randomly not working are due to constraints imposed by preexisting ABIs or VMs; if you don’t need to care about those, then the whole thing is quite straightforward.)

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