This takes me back to the early 80s when "Personal Computer World" (UK) used to have a letters column called "Subset", where people would share short assembler routines for the 8 bit chips of the time. People would share optimisations, hidden opcodes, quirks about the chips etc. Often there'd be posts that shaved a byte or two or a few t-states off a previous months post. It's amazing how inventive people can be with limited resources.

It seems that as processors are so fast and memory so abundant now that squeezing the last drop of efficiency from a routine is a dying art.

https://en.wikipedia.org/wiki/Personal_Computer_World

Many of the Z80 and 6502 routines have been collected together into a couple of books, mentioned here:

http://mycodehere.blogspot.com/2011/10/my-work-in-print-1985...