Parallel STL Overview

Introduction to 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 the final draft for the C++ 20 standard (N4860). For more details see the standard execution policies.

Parallel STL offers efficient support for both parallel and vectorized execution of algorithms for Intel® processors. For sequential execution, it relies on an available implementation of the C++ standard library.

The implementation supports the DPC++ execution policies used to run the massive parallel computational model for heterogeneous systems. The policies are specified in the oneDPL Spec.

For any of the implemented algorithms, pass one of the execution policies values as the first argument in a call to the algorithm to specify the desired execution policy. The policies have the following meaning:

Execution policy value



Sequential execution.


Unsequenced SIMD execution. This policy requires that all functions provided are SIMD-safe.


Parallel execution by multiple threads.


Combined effect of unseq and par.


Massive parallel execution on devices using DPC++.


Massive parallel execution on FPGA devices.

Implementation is based on Parallel STL from LLVM.


C++11 is the minimal version of the C++ standard, which oneDPL requires. That means, any use of oneDPL requires at least a C++11 compiler. Some uses of the library may require a higher version of C++. To use Parallel STL with the C++ standard policies, you must have the following software installed:

  • C++ compiler with support for OpenMP* 4.0 (or higher) SIMD constructs

  • oneAPI Threading Building Blocks (oneTBB) or Threading Building Blocks (TBB) 2019 and later

To use Parallel STL with the DPC++ execution policies, you must have the following software installed:

  • C++ compiler with support for SYCL 2020