I haven't digested just how it works, but "if constexpr" is used to choose different, more efficient instructions, when "special" permutations are chosen. Here is the implementation for a 128-bit permutation: https://github.com/vectorclass/version2/blob/master/vectorf1...

Implementation and comments related to "perm_flags" function here: https://github.com/vectorclass/version2/blob/6b16b1aaa388067...