More info here: https://www.amazon.com/dp/1502958082

I bought a nice little booklet of small x86 gems a few years back[1][2] and this one is my favorite:

  .loop:
      xadd     rax,rdx
      loop     .loop

Fibonacci with two instructions.

[1] https://www.amazon.com/dp/1502958082

[2] https://www.xorpd.net/pages/xchg_rax/snip_00.html