This is a performance patch -- over in D94981, it was observed that LiveDebugVariables can behave quadratically if a lot of debug instructions are inserted back into the same place. We would:
- Pick out a slot index,
- Step through all the debug instructions after it,
- Then insert a debug instruction there
And if you have ~10,000 debug instructions to insert in one place (which can regrettably happen with lots of inlining), this becomes expensive very quickly.
This problem affects instruction referencing too. To get around it, whenever we insert a debug instruction at a slot index, check whether there are more debug instructions to insert at this point, and insert them too. That avoids the repeated lookup and stepping through. It relies on the container for unlinked debug instructions being recorded in-order, which is how LiveDebugVariables currently does it.