# Get Started with Parallel STL

### Contents

Prerequisites

Using Parallel STL

Examples

Implemented Algorithms

Known limitations

Legal Information

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 P1001R1.

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.

Parallel STL is available as a part of Intel® Parallel Studio XE and Intel® System Studio.

## 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
- Intel® Threading Building Blocks (Intel® TBB) 2019

The latest version of the Intel® C++ Compiler is recommended for better performance of Parallel STL algorithms, comparing to previous compiler versions.

To build an application that uses Parallel STL on the command line, you need to set the environment variables for compilation and linkage. You can do this by calling suite-level environment scripts such as `compilervars.{sh|csh|bat}`, or you can set just the Parallel STL environment variables by running `pstlvars.{sh|csh|bat}` in `<install_dir>/{linux|mac|windows}/pstl/bin`.

`<install_dir>` is the installation directory, by default, it is:

For Linux* and macOS*:

- For super-users: `/opt/intel/compilers_and_libraries_<version>`
- For ordinary users: `$HOME/intel/compilers_and_libraries_<version>`

For Windows*:

- `<Program Files>\IntelSWTools\compilers_and_libraries_<version>`

## Using Parallel STL

Follow these steps to add Parallel STL to your application:

1. Add the `<install_dir>/include` folder to the compiler include paths. You can do this by calling the `pstlvars` script.

2. Add `#include "pstl/execution"` to your code. Then add a subset of the following set of lines, depending on the algorithms you intend to use:

   - `#include "pstl/algorithm"`
   - `#include "pstl/numeric"`
   - `#include "pstl/memory"`
3. When using algorithms and execution policies, specify the namespaces `std::execution` in case of there is no vendor implementation of C++17 standard library or `pstl::execution` otherwise. See the 'Examples' section below.

4. For any of the implemented algorithms, pass one of the values `seq`, `unseq`, `par` or `par_unseq` as the first parameter in a call to the algorithm to specify the desired execution policy. The policies have the following meaning:

   Execution policy | Meaning
   --- | ---
   `seq` | Sequential execution.
   `unseq` | Unsequenced SIMD execution. This policy requires that all functions provided are SIMD-safe.
   `par` | Parallel execution by multiple threads.
   `par_unseq` | Combined effect of `unseq` and `par`.

5. Compile the code as C++11 (or later) and using compiler options for vectorization:

   - For the Intel® C++ Compiler:
     - For Linux* and macOS*: `-qopenmp-simd`
     - For Windows*: `/Qopenmp-simd`
   - For other compilers, find a switch that enables OpenMP* 4.0 SIMD constructs.

   To get good performance, specify the target platform. For the Intel® C++ Compiler, some of the relevant options are:

   - For Linux* and macOS*: `-xHOST`, `-xSSE4.1`, `-xCORE-AVX2`, `-xMIC-AVX512`.
   - For Windows*: `/QxHOST`, `/QxSSE4.1`, `/QxCORE-AVX2`, `/QxMIC-AVX512`.
   If using a different compiler, see its documentation.

6. Link with the Intel TBB dynamic library for parallelism. For the Intel® C++ Compiler, use the options:

   - For Linux* and macOS*: `-tbb`
   - For Windows*: `/Qtbb` (optional, this should be handled by `#pragma comment(lib, <libname>)`)

#### Version Macros

Macros related to versioning, as described below. You should not redefine these macros.

`PSTL_VERSION`

Current Parallel STL version. The value is a decimal numeral of the form xyy where x is the major version number and yy is the minor version number.

`PSTL_VERSION_MAJOR`

PSTL_VERSION/100; that is, the major version number.

`PSTL_VERSION_MINOR`

PSTL_VERSION - PSTL_VERSION_MAJOR * 100; that is, the minor version number.

#### Macros

`PSTL_USE_PARALLEL_POLICIES`

This macro controls the use of parallel policies.

When set to 0, it disables the par and par_unseq policies, making their use a compilation error. It's recommended for code that only uses vectorization with unseq policy, to avoid dependency on the TBB runtime library.

When the macro is not defined (default) or evaluates to a non-zero value all execution policies are enabled.

`PSTL_USE_NONTEMPORAL_STORES`

This macro enables the use of `#pragma vector nontemporal` in the algorithms `std::copy`, `std::copy_n`, `std::fill`, `std::fill_n`, `std::generate`, `std::generate_n`, `std::move`, `std::rotate`, `std::rotate_copy`, `std::swap_ranges` with the unseq policy. For further details about the pragma, see the User and Reference Guide for the Intel® C++ Compiler at https://software.intel.com/en-us/node/524559.

If the macro evaluates to a non-zero value, the use of `#pragma vector nontemporal` is enabled.

When the macro is not defined (default) or set to 0, the macro does nothing.

`PSTL_USAGE_WARNINGS`

This macro enables Parallel STL to emit compile-time messages, such as warnings about an algorithm not supporting a certain execution policy.

When set to 1, the macro allows the implementation to emit usage warnings. When the macro is not defined (default) or evaluates to zero, usage warnings are disabled.

## Examples

Example 1

The following code calls vectorized `copy`:

    #include "pstl/execution"
    #include "pstl/algorithm"
    void foo(float* a, float* b, int n) {
        std::copy(pstl::execution::unseq, a, a+n, b);
    }

Example 2

This example calls the parallelized version of `fill_n`:

    #include <vector>
    #include "pstl/execution"
    #include "pstl/algorithm"

    int main()
    {
        std::vector<int> data(10000000);
        std::fill_n(pstl::execution::par_unseq, data.begin(), data.size(), -1);  // Fill the vector with -1

        return 0;
    }

## Algorithms

The following table specifies whether parallel and unsequenced execution are supported for each of the C++17 algorithms accepting execution policies. Using an unsupported combination of algorithm and execution policy will result in sequential execution.

Algorithm | Algorithm page at cppreference.com | Implementation
--- | --- | ---
`adjacent_difference` | http://en.cppreference.com/w/cpp/algorithm/adjacent_difference | parallel, unsequenced
`adjacent_find` | http://en.cppreference.com/w/cpp/algorithm/adjacent_find | parallel, unsequenced
`all_of` | http://en.cppreference.com/w/cpp/algorithm/all_any_none_of | parallel, unsequenced
`any_of` | http://en.cppreference.com/w/cpp/algorithm/all_any_none_of | parallel, unsequenced
`copy` | http://en.cppreference.com/w/cpp/algorithm/copy | parallel, unsequenced
`copy_if` | http://en.cppreference.com/w/cpp/algorithm/copy | parallel, unsequenced
`copy_n` | http://en.cppreference.com/w/cpp/algorithm/copy_n | parallel, unsequenced
`count` | http://en.cppreference.com/w/cpp/algorithm/count | parallel, unsequenced
`count_if` | http://en.cppreference.com/w/cpp/algorithm/count | parallel, unsequenced
`destroy` | http://en.cppreference.com/w/cpp/memory/destroy | parallel, unsequenced
`destroy_n` | http://en.cppreference.com/w/cpp/memory/destroy_n | parallel, unsequenced
`equal` | http://en.cppreference.com/w/cpp/algorithm/equal | parallel, unsequenced
`exclusive_scan` | http://en.cppreference.com/w/cpp/algorithm/exclusive_scan | parallel, unsequenced
`fill` | http://en.cppreference.com/w/cpp/algorithm/fill | parallel, unsequenced
`fill_n` | http://en.cppreference.com/w/cpp/algorithm/fill_n | parallel, unsequenced
`find` | http://en.cppreference.com/w/cpp/algorithm/find | parallel, unsequenced
`find_end` | http://en.cppreference.com/w/cpp/algorithm/find_end | parallel, unsequenced
`find_first_of` | http://en.cppreference.com/w/cpp/algorithm/find_first_of | parallel, unsequenced
`find_if` | http://en.cppreference.com/w/cpp/algorithm/find | parallel, unsequenced
`find_if_not` | http://en.cppreference.com/w/cpp/algorithm/find | parallel, unsequenced
`for_each` | http://en.cppreference.com/w/cpp/algorithm/for_each | parallel, unsequenced
`for_each_n` | http://en.cppreference.com/w/cpp/algorithm/for_each_n | parallel, unsequenced
`generate` | http://en.cppreference.com/w/cpp/algorithm/generate | parallel, unsequenced
`generate_n` | http://en.cppreference.com/w/cpp/algorithm/generate_n | parallel, unsequenced
`includes` | http://en.cppreference.com/w/cpp/algorithm/includes | parallel
`inclusive_scan` | http://en.cppreference.com/w/cpp/algorithm/inclusive_scan | parallel, unsequenced
`inplace_merge` | http://en.cppreference.com/w/cpp/algorithm/inplace_merge | parallel
`is_heap` | http://en.cppreference.com/w/cpp/algorithm/is_heap | parallel, unsequenced
`is_heap_until` | http://en.cppreference.com/w/cpp/algorithm/is_heap_until | parallel, unsequenced
`is_partitioned` | http://en.cppreference.com/w/cpp/algorithm/is_partitioned | parallel, unsequenced
`is_sorted` | http://en.cppreference.com/w/cpp/algorithm/is_sorted | parallel, unsequenced
`is_sorted_until` | http://en.cppreference.com/w/cpp/algorithm/is_sorted_until | parallel, unsequenced
`lexicographical_compare` | http://en.cppreference.com/w/cpp/algorithm/lexicographical_compare | parallel, unsequenced
`max_element` | http://en.cppreference.com/w/cpp/algorithm/max_element | parallel, unsequenced
`merge` | http://en.cppreference.com/w/cpp/algorithm/merge | parallel
`min_element` | http://en.cppreference.com/w/cpp/algorithm/min_element | parallel, unsequenced
`minmax_element` | http://en.cppreference.com/w/cpp/algorithm/minmax_element | parallel, unsequenced
`mismatch` | http://en.cppreference.com/w/cpp/algorithm/mismatch | parallel, unsequenced
`move` | http://en.cppreference.com/w/cpp/algorithm/move | parallel, unsequenced
`none_of` | http://en.cppreference.com/w/cpp/algorithm/all_any_none_of | parallel, unsequenced
`nth_element` | http://en.cppreference.com/w/cpp/algorithm/nth_element | parallel
`partial_sort` | http://en.cppreference.com/w/cpp/algorithm/partial_sort | parallel
`partial_sort_copy` | http://en.cppreference.com/w/cpp/algorithm/partial_sort_copy | parallel
`partition` | http://en.cppreference.com/w/cpp/algorithm/partition | parallel
`partition_copy` | http://en.cppreference.com/w/cpp/algorithm/partition_copy | parallel, unsequenced
`reduce` | http://en.cppreference.com/w/cpp/algorithm/reduce | parallel, unsequenced
`remove` | http://en.cppreference.com/w/cpp/algorithm/remove | parallel, unsequenced
`remove_copy` | http://en.cppreference.com/w/cpp/algorithm/remove_copy | parallel, unsequenced
`remove_copy_if` | http://en.cppreference.com/w/cpp/algorithm/remove_copy | parallel, unsequenced
`remove_if` | http://en.cppreference.com/w/cpp/algorithm/remove | parallel, unsequenced
`replace` | http://en.cppreference.com/w/cpp/algorithm/replace | parallel, unsequenced
`replace_copy` | http://en.cppreference.com/w/cpp/algorithm/replace_copy | parallel, unsequenced
`replace_copy_if` | http://en.cppreference.com/w/cpp/algorithm/replace_copy | parallel, unsequenced
`replace_if` | http://en.cppreference.com/w/cpp/algorithm/replace | parallel, unsequenced
`reverse` | http://en.cppreference.com/w/cpp/algorithm/reverse | parallel, unsequenced
`reverse_copy` | http://en.cppreference.com/w/cpp/algorithm/reverse_copy | parallel, unsequenced
`rotate` | http://en.cppreference.com/w/cpp/algorithm/rotate | parallel, unsequenced
`rotate_copy` | http://en.cppreference.com/w/cpp/algorithm/rotate_copy | parallel, unsequenced
`search` | http://en.cppreference.com/w/cpp/algorithm/search | parallel, unsequenced
`search_n` | http://en.cppreference.com/w/cpp/algorithm/search_n | parallel, unsequenced
`set_difference` | http://en.cppreference.com/w/cpp/algorithm/set_difference | parallel
`set_intersection` | http://en.cppreference.com/w/cpp/algorithm/set_intersection | parallel
`set_symmetric_difference` | http://en.cppreference.com/w/cpp/algorithm/set_symmetric_difference | parallel
`set_union` | http://en.cppreference.com/w/cpp/algorithm/set_union | parallel
`sort` | http://en.cppreference.com/w/cpp/algorithm/sort | parallel
`stable_sort` | http://en.cppreference.com/w/cpp/algorithm/stable_sort | parallel
`stable_partition` | http://en.cppreference.com/w/cpp/algorithm/stable_partition | parallel
`swap_ranges` | http://en.cppreference.com/w/cpp/algorithm/swap_ranges | parallel, unsequenced
`transform` | http://en.cppreference.com/w/cpp/algorithm/transform | parallel, unsequenced
`transform_exclusive_scan` | http://en.cppreference.com/w/cpp/algorithm/transform_exclusive_scan | parallel, unsequenced
`transform_inclusive_scan` | http://en.cppreference.com/w/cpp/algorithm/transform_inclusive_scan | parallel, unsequenced
`transform_reduce` | http://en.cppreference.com/w/cpp/algorithm/transform_reduce | parallel, unsequenced
`uninitialized_copy` | http://en.cppreference.com/w/cpp/memory/uninitialized_copy | parallel, unsequenced
`uninitialized_copy_n` | http://en.cppreference.com/w/cpp/memory/uninitialized_copy_n | parallel, unsequenced
`uninitialized_default_construct` | http://en.cppreference.com/w/cpp/memory/uninitialized_default_construct | parallel, unsequenced
`uninitialized_default_construct_n` | http://en.cppreference.com/w/cpp/memory/uninitialized_default_construct_n | parallel, unsequenced
`uninitialized_fill` | http://en.cppreference.com/w/cpp/memory/uninitialized_fill | parallel, unsequenced
`uninitialized_fill_n` | http://en.cppreference.com/w/cpp/memory/uninitialized_fill_n | parallel, unsequenced
`uninitialized_move` | http://en.cppreference.com/w/cpp/memory/uninitialized_move | parallel, unsequenced
`uninitialized_move_n` | http://en.cppreference.com/w/cpp/memory/uninitialized_move_n | parallel, unsequenced
`uninitialized_value_construct` | http://en.cppreference.com/w/cpp/memory/uninitialized_value_construct | parallel, unsequenced
`uninitialized_value_construct_n` | http://en.cppreference.com/w/cpp/memory/uninitialized_value_construct_n | parallel, unsequenced
`unique` | http://en.cppreference.com/w/cpp/algorithm/unique | parallel
`unique_copy` | http://en.cppreference.com/w/cpp/algorithm/unique_copy | parallel, unsequenced

## Known 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 most algorithms if random access iterators are provided, while for other iterator types the execution will remain serial, excepting `for_each` and `transform` which support parallel execution with forward iterators as well.
In case of forward iterators an execution of the invoked function should have enough work for the parallel execution to be effective.

Semantics of the following algorithms does not allow unsequenced execution: `includes, inplace_merge, merge, set_difference, set_intersection, set_symmetric_difference, set_union, stable_partition, unique`

The initial value type for `exclusive_scan, inclusive_scan, transform_exclusive_scan, transform_inclusive_scan` shall satisfy the `DefaultConstructible` requirements. A default-constructed instance of the initial value type shall be the identity element for `binary_op`.

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`.

## Legal Information

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