[libcxx-commits] [PATCH] D61878: [libc++] Optimize unordered_{multiset, multimap} equality comparison

[Louis Dionne via Phabricator via libcxx-commits]

ldionne updated this revision to Diff 556969.
ldionne added a comment.

Apply the optimization to non-multi containers too. Long story short, I am not convinced that these changes are a real improvement.

Indeed, map lookup starts by computing the hash of the key. If computing the hash is cheaper than fully comparing the key for equality, it can happen that using `std::mismatch` actually pessimizes the algorithm, since it always checks for full key equality.

Consequently, I am going to drop this patch and only cherry-pick some of the changes in this patch (e.g. testing improvements).

Before patch:

  ----------------------------------------------------------------------------------------------------
  Benchmark                                                          Time             CPU   Iterations
  ----------------------------------------------------------------------------------------------------
  BM_Compare_same_container/unordered_set_string/1024           155440 ns       155422 ns         4495
  BM_Compare_same_container/unordered_set_int/1024                2501 ns         2501 ns       269041
  BM_Compare_different_containers/unordered_set_string/1024       67.7 ns         67.7 ns     10428771
  BM_Compare_different_containers/unordered_set_int/1024          1.33 ns         1.33 ns    528206212
  BM_Compare_shared_prefix/unordered_set_string/1024              68.1 ns         68.1 ns     10355336
  BM_Compare_shared_prefix/unordered_set_int/1024                 1.33 ns         1.33 ns    527314912

After patch:

  ----------------------------------------------------------------------------------------------------
  Benchmark                                                          Time             CPU   Iterations
  ----------------------------------------------------------------------------------------------------
  BM_Compare_same_container/unordered_set_string/1024           153609 ns       153589 ns         4515
  BM_Compare_same_container/unordered_set_int/1024                2390 ns         2390 ns       279170
  BM_Compare_different_containers/unordered_set_string/1024       70.7 ns         70.7 ns      9918807
  BM_Compare_different_containers/unordered_set_int/1024          3.11 ns         3.11 ns    173664520
  BM_Compare_shared_prefix/unordered_set_string/1024              69.7 ns         69.7 ns     10042753
  BM_Compare_shared_prefix/unordered_set_int/1024                 1.44 ns         1.44 ns    486236047


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D61878/new/

https://reviews.llvm.org/D61878

Files:
  libcxx/benchmarks/ContainerBenchmarks.h
  libcxx/benchmarks/unordered_set_operations.bench.cpp
  libcxx/include/unordered_map
  libcxx/include/unordered_set
  libcxx/test/std/containers/unord/unord.multimap/eq.pass.cpp
  libcxx/test/std/containers/unord/unord.multiset/eq.pass.cpp

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D61878.556969.patch
Type: text/x-patch
Size: 12658 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/libcxx-commits/attachments/20230918/be9ac1e9/attachment.bin>