std::execution::unsequenced_policy

3. std::execution::parallel_unsequenced_policy

Performance Comparison between Execution Policies

This policy specifies that the execution of the algorithm may be vectorized, i.e, executed on a single thread using instructions that operate on multiple data items.

Syntax of unsequenced_policy

stlFunction (std::execution::unseq, ...other_arguments...);

Example of unsequenced_policy

C++

#include <algorithm>
#include <iostream>
#include <vector>
#include <execution>
 
int main()
{
    std::vector<int> v = { 1, 2, 3, 4, 5 };
    std::for_each(std::execution::unseq, v.begin(),
                  v.end(),
                  [](int x) { std::cout << x << " "; });
    return 0;
}

Output

1 2 3 4 5

Advantages of unsequenced_policy

Fast Execution on a single thread
Avoids Race Conditions

Disadvantages of unsequenced_policy

Some Hardware may not support vectorization.
Non-Deterministic execution sequence.

Execution Policy of STL Algorithms in Modern C++

C++ algorithms are a set of pre-defined functions that can perform various operations on containers, such as arrays, vectors, and lists. These algorithms have a defined execution policy that determines how they execute and how they interact with the underlying hardware.

The C++ 17 standard introduces three new execution policies and one policy was introduced in C++20. These execution policies in C++ allow algorithms to be executed in different ways depending on the requirements of the task and the hardware available. They are as follows:

std::execution::sequenced_policy
std::execution::parallel_policy
std::execution::parallel_unsequenced_policy
std::execution::unsequenced_policy