Add Net.*Addr#is_cidr method

[rye]

• Implement the function on relevant types like so. (Add Net.*Addr#is_cidr method #82)
• Adjust the Display trait to give /-notation.
  • What comes after the / is <max> - count_ones.

[rye]

It turns out that I'm doing this already, who knew?

The basic algorithm: take the mask as a u32, count the ones in the mask (up to 32), then let cidr_mask = u32::max_value().checked_shl(32 - ones).unwrap_or(0). If cidr_mask == mask, then the mask is a standard CIDR mask and you're done.

The problem is that the time to finish the check here is the same regardless of whether or not the mask is CIDR; ideally I'd like CIDR masks to take a lot less time to return a result than non-CIDR masks, if that's possible.

[rye]

Looking deeper into what count_ones actually does, it calls the llvm.ctpop intrinsic—as a result, I'm pretty sure it's target-specific as to what the performance characteristics look like.

Regardless, probably safe to just use that in an impl, and if a target architecture somehow includes bit-counting, then we get that speed. It looks like -C target-cpu=native might optimize it down to popcnt, but by default it's just a shift-add chain.

Remember to use the result in the Display traits and anywhere else where CIDR checking is necessary.

[rye]

Triage: still not done. Re-titling and adding some information to the OP.