[libcxx-commits] [PATCH] D113413: Add introsort to avoid O(n^2) behavior and a benchmark for adversarial quick sort input.

[Nilay Vaish via Phabricator via libcxx-commits]

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Fix compilation issues with cxx03


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Updated the commit message.


  There are two commits in this request.  The first one adds a benchmark that tests std::sort on an adversarial inputs.  The second 
   commit adds intro sort to the std::sort.  Inputs where partitions are unbalanced even after 2 log(n) pivots have been selected, the 
   algorithm switches to heap sort to avoid the possibility of spending O(n^2) time on sorting the input.  Benchmark results show that
   the intro sort implementation does significantly better.
  
  Benchmarking results before this change.  Time represents the sorting time
  required per element.
  
  ----------------------------------------------------------------------------------------------------------
  Benchmark                                                                Time             CPU   Iterations
  ----------------------------------------------------------------------------------------------------------
  BM_Sort_uint32_QuickSortAdversary_1                                   3.75 ns         3.74 ns    187432960
  BM_Sort_uint32_QuickSortAdversary_4                                   3.05 ns         3.05 ns    231211008
  BM_Sort_uint32_QuickSortAdversary_16                                  2.45 ns         2.45 ns    288096256
  BM_Sort_uint32_QuickSortAdversary_64                                  32.8 ns         32.8 ns     21495808
  BM_Sort_uint32_QuickSortAdversary_256                                  132 ns          132 ns      5505024
  BM_Sort_uint32_QuickSortAdversary_1024                                 498 ns          497 ns      1572864
  BM_Sort_uint32_QuickSortAdversary_16384                               3846 ns         3845 ns       262144
  BM_Sort_uint32_QuickSortAdversary_262144                             61431 ns        61400 ns       262144
  BM_Sort_uint64_QuickSortAdversary_1                                   3.93 ns         3.92 ns    181141504
  BM_Sort_uint64_QuickSortAdversary_4                                   3.10 ns         3.09 ns    222560256
  BM_Sort_uint64_QuickSortAdversary_16                                  2.50 ns         2.50 ns    283639808
  BM_Sort_uint64_QuickSortAdversary_64                                  33.2 ns         33.2 ns     21757952
  BM_Sort_uint64_QuickSortAdversary_256                                  132 ns          132 ns      5505024
  BM_Sort_uint64_QuickSortAdversary_1024                                 478 ns          477 ns      1572864
  BM_Sort_uint64_QuickSortAdversary_16384                               3932 ns         3930 ns       262144
  BM_Sort_uint64_QuickSortAdversary_262144                             61646 ns        61615 ns       262144
  
  Benchmarking results after this change
  
  ----------------------------------------------------------------------------------------------------------
  Benchmark                                                                Time             CPU   Iterations
  ----------------------------------------------------------------------------------------------------------
  BM_Sort_uint32_QuickSortAdversary_1                                   15.0 ns         15.0 ns     45875200
  BM_Sort_uint32_QuickSortAdversary_4                                   4.56 ns         4.55 ns    156237824
  BM_Sort_uint32_QuickSortAdversary_16                                  3.04 ns         3.04 ns    233832448
  BM_Sort_uint32_QuickSortAdversary_64                                  44.9 ns         44.9 ns     15728640
  BM_Sort_uint32_QuickSortAdversary_256                                 69.8 ns         69.8 ns     10223616
  BM_Sort_uint32_QuickSortAdversary_1024                                 119 ns          119 ns      6029312
  BM_Sort_uint32_QuickSortAdversary_16384                                174 ns          174 ns      4194304
  BM_Sort_uint32_QuickSortAdversary_262144                               212 ns          212 ns      3407872
  BM_Sort_uint64_QuickSortAdversary_1                                   15.1 ns         15.1 ns     47448064
  BM_Sort_uint64_QuickSortAdversary_4                                   4.54 ns         4.53 ns    149684224
  BM_Sort_uint64_QuickSortAdversary_16                                  3.01 ns         3.01 ns    233832448
  BM_Sort_uint64_QuickSortAdversary_64                                  44.9 ns         44.8 ns     15466496
  BM_Sort_uint64_QuickSortAdversary_256                                 70.3 ns         70.2 ns     10223616
  BM_Sort_uint64_QuickSortAdversary_1024                                 121 ns          121 ns      6029312
  BM_Sort_uint64_QuickSortAdversary_16384                                176 ns          176 ns      4194304
  BM_Sort_uint64_QuickSortAdversary_262144                               212 ns          212 ns      3145728


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D113413

Files:
  libcxx/benchmarks/algorithms.bench.cpp
  libcxx/include/__algorithm/sort.h
  libcxx/test/std/algorithms/alg.sorting/alg.sort/sort/sort.pass.cpp

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