How to hint to GCC that a line should be unreachable at compile time?

Question

It's common for compilers to provide a switch to warn when code is unreachable. I've also seen macros for some libraries, that provide assertions for unreachable code.

Is there a hint, such as through a pragma, or builtin that I can pass to GCC (or any other compilers for that matter), that will warn or error during compilation if it's determined that a line expected to be unreachable can actually be reached?

Here's an example:

    if (!conf->devpath) {
        conf->devpath = arg;
        return 0;
    } // pass other opts into fuse
    else {
        return 1;
    }
    UNREACHABLE_LINE();

The value of this is in detecting, after changes in conditions above the expected unreachable line, that the line is in fact reachable.

Answer 1

gcc 4.5 supports the __builtin_unreachable() compiler inline, combining this with -Wunreachable-code might do what you want, but will probably cause spurious warnings

Answer 2

With gcc 4.4.0 Windows cross compiler to PowerPC compiling with -O2 or -O3 the following works for me:

#define unreachable asm("unreachable\n")

The assembler fails with unknown operation if the compiler doesn't optimise it away because it has concluded that it is unreachable.

Yes, it is quite probably `highly unpredictable under different optimization options', and likely to break when I finally update the compiler, but for the moment it's better then nothing.