llvm-project

c4d820ce - [DenseMap] Replace tombstone deletion with TAOCP 6.4 Algorithm R (#199615)

[DenseMap] Replace tombstone deletion with TAOCP 6.4 Algorithm R (#199615) DenseMap uses quadratic probing with lazy deletion: an erased entry becomes a tombstone, a third bucket state alongside empty and live that every find/insert must inspect. Switch to linear probing with backward-shift deletion (Knuth TAOCP 6.4 Algorithm R), similar to the SmallPtrSet change #197637. This removes the tombstone state entirely. In exchange, erase now relocates the following live entries to close the hole, so it invalidates iterators and references other than the erased one. For callers that cache pointers into the bucket array, erase(Key, OnMoved) and erase(iterator, OnMoved) fire a callback once per shifted bucket, so fix-ups cost O(cluster) rather than O(NumEntries). ValueHandleBase::RemoveFromUseList uses this to refresh each moved handle's PrevPtr. Linear probing is more vulnerable to primary clustering than quadratic probing, so this relies on the stronger DenseMapInfo<T*>::getHashValue mixer from #197390. Operation distribution when compiling CGExpr.cpp/ScalarEvolution.cpp: 62.8% lookups, 34.3% inserts, 2.9% erases. The heaviest DenseMap specializations have pointer keys and 16-byte key/value pairs. Alternatives such as Robin Hood hashing, Verstable, and Boost's unordered_flat_map were evaluated; they are slower and have a larger code footprint. I believe the current in-band sentinel value approach, despite the pain (#146595), is the best, or at least very difficult to beat. Non-core cleanups aided by Claude Opus 4.7.