| # Parallel STL |
| |
| Parallel STL is an implementation of the C++ standard library algorithms with support for execution policies, |
| as specified in ISO/IEC 14882:2017 standard, commonly called C++17. The implementation also supports the unsequenced |
| execution policy specified in Parallelism TS version 2 and proposed for the next version of the C++ standard in the |
| C++ working group paper [P1001](https://wg21.link/p1001). |
| Parallel STL offers efficient support for both parallel and vectorized execution of algorithms. For sequential |
| execution, it relies on an available implementation of the C++ standard library. |
| |
| ## Prerequisites |
| |
| To use Parallel STL, you must have the following software installed: |
| * C++ compiler with: |
| * Support for C++11 |
| * Support for OpenMP* 4.0 SIMD constructs |
| * Threading Building Blocks (TBB) which is available for download at https://github.com/01org/tbb/ |
| |
| ## Known issues and limitations |
| |
| * `unseq` and `par_unseq` policies only have effect with compilers that support `#pragma omp simd` or `#pragma simd`. |
| * Parallel and vector execution is only supported for the algorithms if random access iterators are provided, |
| while for other iterator types the execution will remain serial. |
| * The following algorithms do not allow efficient SIMD execution: `includes`, `inplace_merge`, `merge`, `nth_element`, |
| `partial_sort`, `partial_sort_copy`, `set_difference`, `set_intersection`, `set_symmetric_difference`, `set_union`, |
| `sort`, `stable_partition`, `stable_sort`, `unique`. |
| * The initial value type for `exclusive_scan`, `inclusive_scan`, `transform_exclusive_scan`, `transform_inclusive_scan` |
| shall be DefaultConstructible. A default constructed-instance of the initial value type shall be the identity element |
| for the specified binary operation. |
| * For `max_element`, `min_element`, `minmax_element`, `partial_sort`, `partial_sort_copy`, `sort`, `stable_sort` |
| the dereferenced value type of the provided iterators shall be DefaultConstructible. |
| * For `remove`, `remove_if`, `unique` the dereferenced value type of the provided iterators shall be MoveConstructible. |
| * The following algorithms require additional O(n) memory space for parallel execution: `copy_if`, `inplace_merge`, |
| `partial_sort`, `partial_sort_copy`, `partition_copy`, `remove`, `remove_if`, `rotate`, `sort`, `stable_sort`, |
| `unique`, `unique_copy`. |
| |