There are some methods to get around this. For example, there's an ELF extension called STT_GNU_IFUNC. It allows a symbol to be resolved at load time using a custom resolver function. This avoids the problem of figuring out which code-path to use on every invocation.

For example, you could have a function

    void hash(char *out, const char *in);

with two different possible implementations: a slow one using common instructions, and a fast one using exotic instructions. You can then can have a resolver like this:

    void fast_hash(char *out, const char *in);
    void slow_hash(char *out, const char *in);

    void (*resolve_hasher(void))(char *, const char *)
    {
        if (cpuSupportsFancyInstructions()) {
            return &fast_hash;
        } else {
            return &slow_hash;
        }
    }