Unreachable is mainly used as an optimization hint. For instance if you put an unreachable into the default branch of a continuous and non-exhaustive (from the pov of the compiler) switch-case statement, the compiler will not emit a range check for the jump table lookup.

It helps optimization. One example is if you have code like this:

    if(condition) {
       error_stuff()
       abort();
    }
    normal_stuff();

If the compiler doesn't know that abort exits the program, they have to compile the normal_stuff path under the assumption that the error path might have run before it. This might result in suboptimal code.

Currently, many compilers support annotations such as __attribute__(noreturn) and __builtin_unreachable() to manually indicate that a code path is unreachable. C23 is now standardizing these features (with a slight tweak to the syntax).

You can for example use it to give hints to the compiler that allows for optimisations, that it couldn't do otherwise.

Described e.g. here https://web.archive.org/web/20160508051118/http://blog.regeh...

Github https://github.com/preames/llvm-assume-hack

> Why would you want that?

To aid with optimisation, it basically lets you ask the compiler to remove branches, and provide constraints to the same.

An implementation might trap in debug code, but given no context would be provided you'd likely avoid this and would instead use your own wrapper macro to output a message of some sort in that case.