Those are the same as the Linux kernel macros, and probably a bunch of
other projects. Usage:

    if (likely(condition))
        branch_likely_taken();

    if (unlikely(condition))
        branch_unlikely_taken();

Attention: those two macros convert the predicate to a boolean value.
Therefore you generally cannot use them for assignments like this:
    void *p_sys = likely (malloc (sizeof (*p_sys)));
    if (!p_sys)
        return VLC_ENOMEM;
Instead you can do this:
    void *p_sys = malloc (sizeof (*p_sys));
    if (unlikely (!p_sys))
        return VLC_ENOMEM;