Others have pointed out adding bits to identify instruction types eats into your instruction length, so let's go stupid big time: what if you had the instructions as described here, without any instruction length being part of the instruction, but have that stored separately? (3 bits would be plenty per word) You might put it 1. as a contiguous bit string somewhere, or you might 2. put it at the bottom of the cache line that holds the instructions (the cache line being 512 bits I assume).

Okay, for 1. you'd have to do two fetches to get stuff into the I-cache (but not if it's part of the same cache line, option 2.) and of course you're going to reduce instruction density because you're using up cache, but there's nothing you can do about that, but at least it would allow n-bit instructions to be genuinely n-bits long which is a big advantage.

That this hasn't been done before to my knowledge is proof that it's a rotten idea, but can the experts here please explain why – thanks