There has been renewed interest in having instructions track their own order in basic blocks to help make dominance queries fast. I have a very simple naive implementation of this here:
Essentially, every instruction will carry an integer order number, and inserting new instructions invalidates the ordering. I know there are better algorithms for maintaining the ordering in the face of random insertions, but I wanted to focus on the simple case of making dominance queries amortized O(1) when no insertion is occuring. My personal belief is that most transforms are 80% analysis, 20% transformation, so most of the benefits can be delivered with simple heuristics.
MLIR already uses this technique:
With the renewed interest in the patch, I believe there is broad consensus that we should do this, but I wanted to re-open this RFC that I started back in 2018 to give anyone a chance to say “no”.