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[execpol] Move [execpol] to the end of subclause [algorithms.parallel]
Part of the C++26 clause restructuring (#5315).
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‎source/algorithms.tex‎

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Parallel algorithms shall not participate in overload resolution unless
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\tcode{is_execution_policy_v<remove_cvref_t<ExecutionPolicy>>} is \tcode{true}.
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\rSec2[execpol]{Execution policies}
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\rSec3[execpol.general]{General}
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\pnum
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Subclause~\ref{execpol} describes classes that are \defn{execution policy} types. An
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object of an execution policy type indicates the kinds of parallelism allowed
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in the execution of an algorithm and expresses the consequent requirements on
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the element access functions.
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Execution policy types are declared in header \libheaderref{execution}.
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\begin{example}
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\begin{codeblock}
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using namespace std;
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vector<int> v = @\commentellip@;
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// standard sequential sort
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sort(v.begin(), v.end());
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// explicitly sequential sort
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sort(execution::seq, v.begin(), v.end());
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// permitting parallel execution
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sort(execution::par, v.begin(), v.end());
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// permitting vectorization as well
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sort(execution::par_unseq, v.begin(), v.end());
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\end{codeblock}
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\end{example}
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\begin{note}
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Implementations can provide additional execution policies
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to those described in this document as extensions
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to address parallel architectures that require idiosyncratic
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parameters for efficient execution.
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\end{note}
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\rSec3[execpol.type]{Execution policy type trait}
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\indexlibraryglobal{is_execution_policy}%
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\begin{itemdecl}
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template<class T> struct is_execution_policy { @\seebelow@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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\tcode{is_execution_policy} can be used to detect execution policies for the
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purpose of excluding function signatures from otherwise ambiguous overload
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resolution participation.
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\pnum
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\tcode{is_execution_policy<T>} is a \oldconcept{UnaryTypeTrait} with a
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base characteristic of \tcode{true_type} if \tcode{T} is the type of a standard
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or \impldef{additional execution policies supported by parallel algorithms}
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execution policy, otherwise \tcode{false_type}.
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\begin{note}
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This provision reserves the privilege of creating non-standard execution
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policies to the library implementation.
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\end{note}
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\pnum
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The behavior of a program that adds specializations for
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\tcode{is_execution_policy} is undefined.
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\end{itemdescr}
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\rSec3[execpol.seq]{Sequenced execution policy}
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\indexlibraryglobal{execution::sequenced_policy}%
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\begin{itemdecl}
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class execution::sequenced_policy { @\unspec@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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The class \tcode{execution::sequenced_policy} is an execution policy type used
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as a unique type to disambiguate parallel algorithm overloading and require
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that a parallel algorithm's execution may not be parallelized.
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\pnum
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During the execution of a parallel algorithm with
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the \tcode{execution::sequenced_policy} policy,
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if the invocation of an element access function exits via an exception,
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\tcode{terminate} is invoked\iref{except.terminate}.
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\end{itemdescr}
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\rSec3[execpol.par]{Parallel execution policy}
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\indexlibraryglobal{execution::parallel_policy}%
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\begin{itemdecl}
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class execution::parallel_policy { @\unspec@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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The class \tcode{execution::parallel_policy} is an execution policy type used as
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a unique type to disambiguate parallel algorithm overloading and indicate that
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a parallel algorithm's execution may be parallelized.
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\pnum
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During the execution of a parallel algorithm with
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the \tcode{execution::parallel_policy} policy,
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if the invocation of an element access function exits via an exception,
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\tcode{terminate} is invoked\iref{except.terminate}.
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\end{itemdescr}
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\rSec3[execpol.parunseq]{Parallel and unsequenced execution policy}
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\indexlibraryglobal{execution::parallel_unsequenced_policy}%
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\begin{itemdecl}
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class execution::parallel_unsequenced_policy { @\unspec@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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The class \tcode{execution::parallel_unsequenced_policy} is an execution policy type
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used as a unique type to disambiguate parallel algorithm overloading and
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indicate that a parallel algorithm's execution may be parallelized and
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vectorized.
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\pnum
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During the execution of a parallel algorithm with
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the \tcode{execution::parallel_unsequenced_policy} policy,
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if the invocation of an element access function exits via an exception,
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\tcode{terminate} is invoked\iref{except.terminate}.
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\end{itemdescr}
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\rSec3[execpol.unseq]{Unsequenced execution policy}
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\indexlibraryglobal{execution::unsequenced_policy}%
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\begin{itemdecl}
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class execution::unsequenced_policy { @\unspec@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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The class \tcode{unsequenced_policy} is an execution policy type
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used as a unique type to disambiguate parallel algorithm overloading and
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indicate that a parallel algorithm's execution may be vectorized,
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e.g., executed on a single thread using instructions
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that operate on multiple data items.
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\pnum
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During the execution of a parallel algorithm with
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the \tcode{execution::unsequenced_policy} policy,
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if the invocation of an element access function exits via an exception,
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\tcode{terminate} is invoked\iref{except.terminate}.
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\end{itemdescr}
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\rSec3[execpol.objects]{Execution policy objects}
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\indexlibraryglobal{seq}%
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\indexlibraryglobal{par}%
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\indexlibraryglobal{par_unseq}%
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\indexlibrarymember{execution}{seq}%
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\indexlibrarymember{execution}{par}%
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\indexlibrarymember{execution}{par_unseq}%
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\begin{itemdecl}
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inline constexpr execution::sequenced_policy execution::seq{ @\unspec@ };
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inline constexpr execution::parallel_policy execution::par{ @\unspec@ };
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inline constexpr execution::parallel_unsequenced_policy execution::par_unseq{ @\unspec@ };
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inline constexpr execution::unsequenced_policy execution::unseq{ @\unspec@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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The header \libheaderref{execution} declares global objects associated with each type of execution policy.
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\end{itemdescr}
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\rSec1[algorithm.syn]{Header \tcode{<algorithm>} synopsis}
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\indexheader{algorithm}%
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‎source/utilities.tex‎

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\ref{bitset} & Fixed-size sequences of bits & \tcode{<bitset>} \\ \rowsep
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\ref{function.objects} & Function objects & \tcode{<functional>} \\ \rowsep
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\ref{type.index} & Type indexes & \tcode{<typeindex>} \\ \rowsep
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\ref{execpol} & Execution policies & \tcode{<execution>} \\ \rowsep
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\ref{charconv} & Primitive numeric conversions & \tcode{<charconv>} \\ \rowsep
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\ref{format} & Formatting & \tcode{<format>} \\ \rowsep
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\ref{bit} & Bit manipulation & \tcode{<bit>} \\
@@ -15216,172 +15215,6 @@
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\tcode{hash<type_index>()(index)} shall evaluate to the same result as \tcode{index.hash_code()}.
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\end{itemdescr}
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\rSec1[execpol]{Execution policies}
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\rSec2[execpol.general]{General}
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\pnum
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Subclause~\ref{execpol} describes classes that are \defn{execution policy} types. An
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object of an execution policy type indicates the kinds of parallelism allowed
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in the execution of an algorithm and expresses the consequent requirements on
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the element access functions.
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Execution policy types are declared in header \libheaderref{execution}.
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\begin{example}
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\begin{codeblock}
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using namespace std;
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vector<int> v = @\commentellip@;
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// standard sequential sort
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sort(v.begin(), v.end());
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// explicitly sequential sort
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sort(execution::seq, v.begin(), v.end());
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// permitting parallel execution
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sort(execution::par, v.begin(), v.end());
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// permitting vectorization as well
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sort(execution::par_unseq, v.begin(), v.end());
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\end{codeblock}
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\end{example}
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\begin{note}
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Implementations can provide additional execution policies
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to those described in this document as extensions
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to address parallel architectures that require idiosyncratic
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parameters for efficient execution.
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\end{note}
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\rSec2[execpol.type]{Execution policy type trait}
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\indexlibraryglobal{is_execution_policy}%
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\begin{itemdecl}
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template<class T> struct is_execution_policy { @\seebelow@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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\tcode{is_execution_policy} can be used to detect execution policies for the
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purpose of excluding function signatures from otherwise ambiguous overload
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resolution participation.
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\pnum
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\tcode{is_execution_policy<T>} is a \oldconcept{UnaryTypeTrait} with a
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base characteristic of \tcode{true_type} if \tcode{T} is the type of a standard
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or \impldef{additional execution policies supported by parallel algorithms}
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execution policy, otherwise \tcode{false_type}.
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\begin{note}
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This provision reserves the privilege of creating non-standard execution
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policies to the library implementation.
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\end{note}
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\pnum
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The behavior of a program that adds specializations for
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\tcode{is_execution_policy} is undefined.
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\end{itemdescr}
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\rSec2[execpol.seq]{Sequenced execution policy}
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\indexlibraryglobal{execution::sequenced_policy}%
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\begin{itemdecl}
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class execution::sequenced_policy { @\unspec@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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The class \tcode{execution::sequenced_policy} is an execution policy type used
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as a unique type to disambiguate parallel algorithm overloading and require
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that a parallel algorithm's execution may not be parallelized.
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\pnum
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During the execution of a parallel algorithm with
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the \tcode{execution::sequenced_policy} policy,
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if the invocation of an element access function exits via an exception,
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\tcode{terminate} is invoked\iref{except.terminate}.
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\end{itemdescr}
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\rSec2[execpol.par]{Parallel execution policy}
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\indexlibraryglobal{execution::parallel_policy}%
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\begin{itemdecl}
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class execution::parallel_policy { @\unspec@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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The class \tcode{execution::parallel_policy} is an execution policy type used as
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a unique type to disambiguate parallel algorithm overloading and indicate that
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a parallel algorithm's execution may be parallelized.
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\pnum
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During the execution of a parallel algorithm with
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the \tcode{execution::parallel_policy} policy,
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if the invocation of an element access function exits via an exception,
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\tcode{terminate} is invoked\iref{except.terminate}.
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\end{itemdescr}
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\rSec2[execpol.parunseq]{Parallel and unsequenced execution policy}
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\indexlibraryglobal{execution::parallel_unsequenced_policy}%
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\begin{itemdecl}
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class execution::parallel_unsequenced_policy { @\unspec@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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The class \tcode{execution::parallel_unsequenced_policy} is an execution policy type
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used as a unique type to disambiguate parallel algorithm overloading and
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indicate that a parallel algorithm's execution may be parallelized and
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vectorized.
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\pnum
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During the execution of a parallel algorithm with
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the \tcode{execution::parallel_unsequenced_policy} policy,
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if the invocation of an element access function exits via an exception,
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\tcode{terminate} is invoked\iref{except.terminate}.
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\end{itemdescr}
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\rSec2[execpol.unseq]{Unsequenced execution policy}
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\indexlibraryglobal{execution::unsequenced_policy}%
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\begin{itemdecl}
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class execution::unsequenced_policy { @\unspec@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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The class \tcode{unsequenced_policy} is an execution policy type
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used as a unique type to disambiguate parallel algorithm overloading and
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indicate that a parallel algorithm's execution may be vectorized,
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e.g., executed on a single thread using instructions
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that operate on multiple data items.
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\pnum
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During the execution of a parallel algorithm with
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the \tcode{execution::unsequenced_policy} policy,
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if the invocation of an element access function exits via an exception,
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\tcode{terminate} is invoked\iref{except.terminate}.
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\end{itemdescr}
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\rSec2[execpol.objects]{Execution policy objects}
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\indexlibraryglobal{seq}%
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\indexlibraryglobal{par}%
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\indexlibraryglobal{par_unseq}%
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\indexlibrarymember{execution}{seq}%
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\indexlibrarymember{execution}{par}%
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\indexlibrarymember{execution}{par_unseq}%
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\begin{itemdecl}
15374-
inline constexpr execution::sequenced_policy execution::seq{ @\unspec@ };
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inline constexpr execution::parallel_policy execution::par{ @\unspec@ };
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inline constexpr execution::parallel_unsequenced_policy execution::par_unseq{ @\unspec@ };
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inline constexpr execution::unsequenced_policy execution::unseq{ @\unspec@ };
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\end{itemdecl}
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\begin{itemdescr}
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\pnum
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The header \libheaderref{execution} declares global objects associated with each type of execution policy.
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\end{itemdescr}
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\rSec1[charconv]{Primitive numeric conversions}
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\rSec2[charconv.syn]{Header \tcode{<charconv>} synopsis}

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