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In-place Parallel Super Scalar Samplesort (IPS⁴o)

This is the implementation of the algorithm presented in the eponymous paper, which contains an in-depth description of its inner workings, as well as an extensive experimental performance evaluation. Here's the abstract:

We present a sorting algorithm that works in-place, executes in parallel, is cache-efficient, avoids branch-mispredictions, and performs work O(n log n) for arbitrary inputs with high probability. The main algorithmic contributions are new ways to make distribution-based algorithms in-place: On the practical side, by using coarse-grained block-based permutations, and on the theoretical side, we show how to eliminate the recursion stack. Extensive experiments show that our algorithm IPS⁴o scales well on a variety of multi-core machines. We outperform our closest in-place competitor by a factor of up to 3. Even as a sequential algorithm, we are up to 1.5 times faster than the closest sequential competitor, BlockQuicksort.

This repository is deprecated! The current version of IPS⁴o is published at You may also want to check out our prototypical implementation of In-place Parallel Super Scalar Radix Sort (IPS²Ra) at IPS²Ra applies the in-place approach of IPS⁴o to radix sort. The new repositories support the CMake build system.


#include "ips4o.hpp"

// sort sequentially
ips4o::sort(begin, end[, comparator])

// sort in parallel (uses OpenMP if available, std::thread otherwise)
ips4o::parallel::sort(begin, end[, comparator])

Make sure to compile with C++14 support. Currently, the code does not compile on Windows.

For the parallel algorithm, you need to enable either OpenMP (-fopenmp) or C++ threads (e.g., -pthread). You also need a CPU that supports 16-byte compare-and-exchange instructions. If you get undefined references to __atomic_fetch_add_16, either set your CPU correctly (e.g., -march=native), enable the instructions explicitly (-mcx16), or try linking against GCC's libatomic (-latomic).


IPS⁴o is free software provided under the BSD 2-Clause License described in the LICENSE file. If you use IPS⁴o in an academic setting please cite the eponymous paper using the BibTeX entry

  author =	{Michael Axtmann and
                Sascha Witt and
                Daniel Ferizovic and
                Peter Sanders},
  title =	{{In-Place Parallel Super Scalar Samplesort (IPSSSSo)}},
  booktitle =	{25th Annual European Symposium on Algorithms (ESA 2017)},
  pages =	{9:1--9:14},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  year =	{2017},
  volume =	{87},
  publisher =	{Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik},
  doi =		{10.4230/LIPIcs.ESA.2017.9},


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