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cppdraft/execpol.md

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+[execpol]
+
+# 26 Algorithms library [[algorithms]](./#algorithms)
+
+## 26.3 Parallel algorithms [[algorithms.parallel]](algorithms.parallel#execpol)
+
+### 26.3.6 Execution policies [execpol]
+
+#### [26.3.6.1](#general) General [[execpol.general]](execpol.general)
+
+[1](#general-1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L636)
+
+Subclause [execpol] describes classes that are [*execution policy*](#def:execution_policy "26.3.6.1 General [execpol.general]") types[.](#general-1.sentence-1)
+
+An
+object of an execution policy type indicates the kinds of parallelism allowed
+in the execution of an algorithm and expresses the consequent requirements on
+the element access functions[.](#general-1.sentence-2)
+
+Execution policy types are declared in header [<execution>](execution.syn#header:%3cexecution%3e "33.4 Header <execution> synopsis [execution.syn]")[.](#general-1.sentence-3)
+
+[*Example [1](#general-example-1)*: using namespace std;
+vector<int> v = /* ... */;
+
+// standard sequential sort sort(v.begin(), v.end());
+
+// explicitly sequential sort sort(execution::seq, v.begin(), v.end());
+
+// permitting parallel execution sort(execution::par, v.begin(), v.end());
+
+// permitting vectorization as well sort(execution::par_unseq, v.begin(), v.end()); — *end example*]
+
+[*Note [1](#general-note-1)*:
+
+Implementations can provide additional execution policies
+to those described in this document as extensions
+to address parallel architectures that require idiosyncratic
+parameters for efficient execution[.](#general-1.sentence-4)
+
+— *end note*]
+
+#### [26.3.6.2](#type) Execution policy type trait [[execpol.type]](execpol.type)
+
+[🔗](#lib:is_execution_policy)
+
+`template<class T> struct is_execution_policy;
+`
+
+[1](#type-1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L675)
+
+is_execution_policy can be used to detect execution policies for the
+purpose of excluding function signatures from otherwise ambiguous overload
+resolution participation[.](#type-1.sentence-1)
+
+[2](#type-2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L680)
+
+is_execution_policy<T> is a *Cpp17UnaryTypeTrait* with a
+base characteristic of true_type if T is the type of a standard
+or implementation-defined
+execution policy, otherwise false_type[.](#type-2.sentence-1)
+
+[*Note [1](#type-note-1)*:
+
+This provision reserves the privilege of creating non-standard execution
+policies to the library implementation[.](#type-2.sentence-2)
+
+— *end note*]
+
+[3](#type-3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L691)
+
+The behavior of a program that adds specializations foris_execution_policy is undefined[.](#type-3.sentence-1)
+
+#### [26.3.6.3](#seq) Sequenced execution policy [[execpol.seq]](execpol.seq)
+
+[🔗](#lib:execution::sequenced_policy)
+
+`class execution::sequenced_policy { unspecified };
+`
+
+[1](#seq-1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L704)
+
+The class execution​::​sequenced_policy is an execution policy type used
+as a unique type to disambiguate parallel algorithm overloading and require
+that a parallel algorithm's execution may not be parallelized[.](#seq-1.sentence-1)
+
+[2](#seq-2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L709)
+
+During the execution of a parallel algorithm with
+the execution​::​sequenced_policy policy,
+if the invocation of an element access function exits via an exception,terminate is invoked ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#seq-2.sentence-1)
+
+#### [26.3.6.4](#par) Parallel execution policy [[execpol.par]](execpol.par)
+
+[🔗](#lib:execution::parallel_policy)
+
+`class execution::parallel_policy { unspecified };
+`
+
+[1](#par-1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L724)
+
+The class execution​::​parallel_policy is an execution policy type used as
+a unique type to disambiguate parallel algorithm overloading and indicate that
+a parallel algorithm's execution may be parallelized[.](#par-1.sentence-1)
+
+[2](#par-2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L729)
+
+During the execution of a parallel algorithm with
+the execution​::​parallel_policy policy,
+if the invocation of an element access function exits via an exception,terminate is invoked ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#par-2.sentence-1)
+
+#### [26.3.6.5](#parunseq) Parallel and unsequenced execution policy [[execpol.parunseq]](execpol.parunseq)
+
+[🔗](#lib:execution::parallel_unsequenced_policy)
+
+`class execution::parallel_unsequenced_policy { unspecified };
+`
+
+[1](#parunseq-1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L744)
+
+The class execution​::​parallel_unsequenced_policy is an execution policy type
+used as a unique type to disambiguate parallel algorithm overloading and
+indicate that a parallel algorithm's execution may be parallelized and
+vectorized[.](#parunseq-1.sentence-1)
+
+[2](#parunseq-2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L750)
+
+During the execution of a parallel algorithm with
+the execution​::​parallel_unsequenced_policy policy,
+if the invocation of an element access function exits via an exception,terminate is invoked ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#parunseq-2.sentence-1)
+
+#### [26.3.6.6](#unseq) Unsequenced execution policy [[execpol.unseq]](execpol.unseq)
+
+[🔗](#lib:execution::unsequenced_policy)
+
+`class execution::unsequenced_policy { unspecified };
+`
+
+[1](#unseq-1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L765)
+
+The class unsequenced_policy is an execution policy type
+used as a unique type to disambiguate parallel algorithm overloading and
+indicate that a parallel algorithm's execution may be vectorized,
+e.g., executed on a single thread using instructions
+that operate on multiple data items[.](#unseq-1.sentence-1)
+
+[2](#unseq-2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L772)
+
+During the execution of a parallel algorithm with
+the execution​::​unsequenced_policy policy,
+if the invocation of an element access function exits via an exception,terminate is invoked ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#unseq-2.sentence-1)
+
+#### [26.3.6.7](#objects) Execution policy objects [[execpol.objects]](execpol.objects)
+
+[🔗](#lib:seq)
+
+`inline constexpr execution::sequenced_policy            execution::seq{ unspecified };
+inline constexpr execution::parallel_policy             execution::par{ unspecified };
+inline constexpr execution::parallel_unsequenced_policy execution::par_unseq{ unspecified };
+inline constexpr execution::unsequenced_policy          execution::unseq{ unspecified };
+`
+
+[1](#objects-1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L795)
+
+The header [<execution>](execution.syn#header:%3cexecution%3e "33.4 Header <execution> synopsis [execution.syn]") declares global objects associated with each type of execution policy[.](#objects-1.sentence-1)