[all-commits] [llvm/llvm-project] e4eccd: [libc++] Optimize __hash_table::erase(iterator, it...

[Nikolas Klauser via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e4eccd6a3c2415c10bb8217c247d7aca76cc9ad5
      https://github.com/llvm/llvm-project/commit/e4eccd6a3c2415c10bb8217c247d7aca76cc9ad5
  Author: Nikolas Klauser <nikolasklauser at berlin.de>
  Date:   2025-08-25 (Mon, 25 Aug 2025)

  Changed paths:
    M libcxx/docs/ReleaseNotes/22.rst
    M libcxx/include/__hash_table
    M libcxx/test/std/containers/unord/unord.multimap/unord.multimap.modifiers/erase_range.pass.cpp

  Log Message:
  -----------
  [libc++] Optimize __hash_table::erase(iterator, iterator) (#152471)

Instead of just calling the single element `erase` on every element of
the range, we can combine some of the operations in a custom
implementation. Specifically, we don't need to search for the previous
node or re-link the list every iteration. Removing this unnecessary work
results in some nice performance improvements:
```
-----------------------------------------------------------------------------------------------------------------------
Benchmark                                                                                             old           new
-----------------------------------------------------------------------------------------------------------------------
std::unordered_set<int>::erase(iterator, iterator) (erase half the container)/0                    457 ns        459 ns
std::unordered_set<int>::erase(iterator, iterator) (erase half the container)/32                   995 ns        626 ns
std::unordered_set<int>::erase(iterator, iterator) (erase half the container)/1024               18196 ns       7995 ns
std::unordered_set<int>::erase(iterator, iterator) (erase half the container)/8192              124722 ns      70125 ns
std::unordered_set<std::string>::erase(iterator, iterator) (erase half the container)/0            456 ns        461 ns
std::unordered_set<std::string>::erase(iterator, iterator) (erase half the container)/32          1183 ns        769 ns
std::unordered_set<std::string>::erase(iterator, iterator) (erase half the container)/1024       27827 ns      18614 ns
std::unordered_set<std::string>::erase(iterator, iterator) (erase half the container)/8192      266681 ns     226107 ns
std::unordered_map<int, int>::erase(iterator, iterator) (erase half the container)/0               455 ns        462 ns
std::unordered_map<int, int>::erase(iterator, iterator) (erase half the container)/32              996 ns        659 ns
std::unordered_map<int, int>::erase(iterator, iterator) (erase half the container)/1024          15963 ns       8108 ns
std::unordered_map<int, int>::erase(iterator, iterator) (erase half the container)/8192         136493 ns      71848 ns
std::unordered_multiset<int>::erase(iterator, iterator) (erase half the container)/0               454 ns        455 ns
std::unordered_multiset<int>::erase(iterator, iterator) (erase half the container)/32              985 ns        703 ns
std::unordered_multiset<int>::erase(iterator, iterator) (erase half the container)/1024          16277 ns       9085 ns
std::unordered_multiset<int>::erase(iterator, iterator) (erase half the container)/8192         125736 ns      82710 ns
std::unordered_multimap<int, int>::erase(iterator, iterator) (erase half the container)/0          457 ns        454 ns
std::unordered_multimap<int, int>::erase(iterator, iterator) (erase half the container)/32        1091 ns        646 ns
std::unordered_multimap<int, int>::erase(iterator, iterator) (erase half the container)/1024     17784 ns       7664 ns
std::unordered_multimap<int, int>::erase(iterator, iterator) (erase half the container)/8192    127098 ns      72806 ns
```



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