Optimizing std::sort in libc++ Aditya Kumar Divya Shanmughan - PowerPoint PPT Presentation

Optimizing std::sort in libc++ Aditya Kumar Divya Shanmughan

Issues with std::sort (libc++) • Worst case – clang-libc++ O(N^2) vs. gcc-libstdc++ O(NlogN) Time complexity (worst case*) 40 gcc-libstdc++ 35 clang-libcxx 30 25 Time (seconds) 20 15 10 5 0 32 64 128 256 512 1024 2048 4096 8192 16384 32768 Data Size (Bytes) * https://bugs.llvm.org/show_bug.cgi?id=20837

Sorting Algorithm in libc++ • Sorting algorithm currently implemented in libc++ uses quicksort • Worst Case Complexity – O(N^2) • Recursion Stack Space – O(logN)

Modifications done • Convert to introsort* – Sorting technique, which begins with quicksort and switches to heapsort after recursion reaches a threshold – Worst case complexity of O(NlogN) • Eliminate recursion – Replaced memory Intensive recursive calls with stack – std::stack uses std::deque, which uses std::algorithm :( • Improved worst case time complexity by a factor of 10 – https://reviews.llvm.org/D36423 https://en.wikipedia.org/wiki/Introsort

Sorting Results Plot (With std-benchmark) * https://github.com/hiraditya/std-benchmark

Thank You