LLVM SmallPtrSet is a hash table for small pointer keys and rare deletions. Using plain linear probing with Knuth's Algorithm R (TAOCP Vol 3 §6.4) for deletions is brilliant. Tiny code footprint, fast inserts/lookups, very slow deletions. https://llvm-compile-time-tracker.com/compare.php?from=5d5220c53848ef86de57946acda4bc7d8b91880c&to=7b227a281d29861c959db0d08c02fc97b55709e0&stat=instructions:u