Comment by OskarS

The C programming language does not provide any guarantees that tail calls will be optimized out in this way. Compilers are free to do it if they want to and it doesn't violate the "as-if" rule, but they are neither required to do it, nor required not to do it.

However, if you can statically GUARANTEE that this "optimization" happens (which is what this new clang feature does), it opens up a new kind of programming pattern that was simply not possible in C before, it was only really possible in assembly. The reason you couldn't do it in C before is that if the compiler decides (which it is free to do) not to optimize the tail call, it adds to the stack, and you will get stack overflow pretty much instantly.

This MUSTTAIL thing is basically an extension to the C programming language itself: if you can guarantee that the tail call happens, you can structure your code differently because there's no risk of blowing the stack in this case. Another language that actually does this is Scheme, in which the language guarantees that tail calls are eliminated, for the very same reason. If a scheme interpreter doesn't do this, it's not a valid Scheme interpreter.

"Debug mode" vs. "Release mode" doesn't really enter into it: if this optimization doesn't happen, the program is no longer correct, regardless of what optimization level you've selected. Asking "why is the compiler missing this obvious optimization in production mode" is missing the point: you have to guarantee that it always happens in all compilation modes, otherwise the code doesn't work at all.