ISO/IEC TS 19570:2015
(Main)Programming Languages — Technical Specification for C++ Extensions for Parallelism
Programming Languages — Technical Specification for C++ Extensions for Parallelism
Langages de programmation — Spécification technique pour les extensions C++ relatives au parallélisme
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TECHNICAL ISO/IEC
SPECIFICATION TS
19570
First edition
2015-07-01
Programming Languages — Technical
Specification for C++ Extensions for
Parallelism
Langages de programmation — Spécification technique pour les
extensions C++ relatives au parallélisme
Reference number
ISO/IEC TS 19570:2015(E)
©
ISO/IEC 2015
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ISO/IEC TS 19570:2015(E)
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ISO/IEC TS 19570:2015(E)
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are members of
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technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the different types of
document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights. Details of any
patent rights identified during the development of the document will be in the Introduction and/or on the ISO
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Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity assessment,
as well as information about ISO's adherence to the WTO principles in the Technical Barriers to Trade (TBT)
see the following URL: Foreword - Supplementary information
ISO/IEC TS 19570 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 22, Programming languages, their environments and system software interfaces.
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Contents
1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Normative references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Namespaces and headers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Terms and definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 Feature-testing recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Execution policies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 In general . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Header synopsis . . . . . . . . . . . . . . . . . 6
2.3 Execution policy type trait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4 Sequential execution policy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5 Parallel execution policy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.6 Parallel+Vector execution policy . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.7 Dynamic execution policy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.7.1 execution_policy construct/assign . . . . . . . . . . . . . . . . . . . . . . . 8
2.7.2 execution_policy object access . . . . . . . . . . . . . . . . . . . . . . . . 9
2.8 Execution policy objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 Parallel exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Exception reporting behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2 Header synopsis . . . . . . . . . . . . . . . . . . . 10
4 Parallel algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1 In general . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1.1 Requirements on user-provided function objects . . . . . . . . . . . . . . . 12
4.1.2 Effect of execution policies on algorithm execution . . . . . . . . . . . . . . 12
4.1.3 ExecutionPolicy algorithm overloads . . . . . . . . . . . . . . . . . . . . . 14
4.2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3 Non-Numeric Parallel Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3.1 Header synopsis . . . . . . . . . . . . . . . . . 15
4.3.2 For each . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.4 Numeric Parallel Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.4.1 Header synopsis . . . . . . . . . . . . . . . . . . 17
4.4.2 Reduce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.4.3 Exclusive scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.4.4 Inclusive scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.4.5 Transform reduce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.4.6 Transform exclusive scan . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.4.7 Transform inclusive scan . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3
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1 General [parallel.general]
1.1 Scope [parallel.general.scope]
1
This Technical Specification describes requirements for implementations of an interface that
computer programs written in the C++ programming language may use to invoke algorithms
with parallel execution. The algorithms described by this Technical Specification are realizable
across a broad class of computer architectures.
2
This Technical Specification is non-normative. Some of the functionality described by this
Technical Specification may be considered for standardization in a future version of C++, but it
is not currently part of any C++ standard. Some of the functionality in this Technical
Specification may never be standardized, and other functionality may be standardized in a
substantially changed form.
3
The goal of this Technical Specification is to build widespread existing practice for parallelism
in the C++ standard algorithms library. It gives advice on extensions to those vendors who wish
to provide them.
1.2 Normative references [parallel.general.references]
1
The following referenced document is indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
1
— ISO/IEC 14882:— , Programming Languages — C++
2
ISO/IEC 14882:— is herein called the C++ Standard. The library described in ISO/IEC 14882:—
clauses 17-30 is herein called the C++ Standard Library. The C++ Standard Library
components described in ISO/IEC 14882:— clauses 25, 26.7 and 20.7.2 are herein called the
C++ Standard Algorithms Library.
3
Unless otherwise specified, the whole of the C++ Standard's Library introduction (C++14 §17)
is included into this Technical Specification by reference.
1.3 Namespaces and headers [parallel.general.namespaces]
1
Since the extensions described in this Technical Specification are experimental and not part of
the C++ Standard Library, they should not be declared directly within namespace std. Unless
otherwise specified, all components described in this Technical Specification are declared in
namespace std::experimental::parallel::v1.
[ Note: Once standardized, the components described by this Technical Specification are
expected to be promoted to namespace std. — end note ]
2
Unless otherwise specified, references to such entities described in this Technical Specification
are assumed to be qualified with std::experimental::parallel::v1, and references to entities
described in the C++ Standard Library are assumed to be qualified with std::.
3
Extensions that are expected to eventually be added to an existing header are provided
inside the header, which shall include the standard contents of as if by
1. To be published. Section references are relative to N3937.
§ 1.3 4
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#include
1.4 Terms and definitions [parallel.general.defns]
1
For the purposes of this document, the terms and definitions given in the C++ Standard and
the following apply.
2
A parallel algorithm is a function template described by this Technical Specification declared in
namespace std::experimental::parallel::v1 with a formal template parameter named
ExecutionPolicy.
3
Parallel algorithms access objects indirectly accessible via their arguments by invoking the
following functions:
— All operations of the categories of the iterators that the algorithm is instantiated with.
— Functions on those sequence elements that are required by its specification.
— User-provided function objects to be applied during the execution of the algorithm, if
required by the specification.
— Operations on those function objects required by the specification. [ Note: See clause
25.1 of C++ Standard Algorithms Library. — end note ]
These functions are herein called element access functions.
[ Example: The sort function may invoke the following element access functions:
— Methods of the random-access iterator of the actual template argument, as per 24.2.7,
as implied by the name of the template parameters RandomAccessIterator.
— The swap function on the elements of the sequence (as per 25.4.1.1 [sort]/2).
— The user-provided Compare function object.
— end example ]
1.5 Feature-testing recommendations [parallel.general.features]
1
An implementation that provides support for this Technical Specification shall define the
feature test macro(s) in Table 1.
Table 1 — Feature Test Macro(s)
Name Value Header
__cpp_lib_experimental_parallel_algorithm 201505
§ 1.5 5
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2 Execution policies [parallel.execpol]
2.1 In general [parallel.execpol.general]
1
This clause describes classes that are execution policy types. 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.
[ Example:
std::vector v = .
// standard sequential sort
std::sort(v.begin(), v.end());
using namespace std::experimental::parallel;
// explicitly sequential sort
sort(seq, v.begin(), v.end());
// permitting parallel execution
sort(par, v.begin(), v.end());
// permitting vectorization as well
sort(par_vec, v.begin(), v.end());
// sort with dynamically-selected execution
size_t threshold = .
execution_policy exec = seq;
if (v.size() > threshold)
{
exec = par;
}
sort(exec, v.begin(), v.end());
— end example ]
[ Note: Because different parallel architectures may require idiosyncratic parameters for
efficient execution, implementations of the Standard Library may provide additional execution
policies to those described in this Technical Specification as extensions. — end note ]
2.2 Header synopsis [parallel.execpol.synopsis]
namespace std {
namespace experimental {
namespace parallel {
inline namespace v1 {
// 2.3, Execution policy type trait
template struct is_execution_policy;
template constexpr bool is_execution_policy_v = is_execution_policy::value;
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// 2.4, Sequential execution policy
class sequential_execution_policy;
// 2.5, Parallel execution policy
class parallel_execution_policy;
// 2.6, Parallel+Vector execution policy
class parallel_vector_execution_policy;
// 2.7, Dynamic execution policy
class execution_policy;
}
}
}
}
2.3 Execution policy type trait [parallel.execpol.type]
template struct is_execution_policy { see below };
1
is_execution_policy can be used to detect parallel execution policies for the purpose of excluding
function signatures from otherwise ambiguous overload resolution participation.
2
is_execution_policy shall be a UnaryTypeTrait with a BaseCharacteristic of true_type if T is the
type of a standard or implementation-defined execution policy, otherwise false_type.
[ Note: This provision reserves the privilege of creating non-standard execution policies to the
library implementation. — end note ]
3
The behavior of a program that adds specializations for is_execution_policy is undefined.
2.4 Sequential execution policy [parallel.execpol.seq]
class sequential_execution_policy{ unspecified };
1
The class sequential_execution_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.
2.5 Parallel execution policy [parallel.execpol.par]
class parallel_execution_policy{ unspecified };
1
The class parallel_execution_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.
2.6 Parallel+Vector execution policy [parallel.execpol.vec]
class parallel_vector_execution_policy{ unspecified };
1
The class class parallel_vector_execution_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 and parallelized.
§ 2.6 7
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2.7 Dynamic execution policy [parallel.execpol.dynamic]
class execution_policy
{
public:
// 2.7.1, execution_policy construct/assign
template execution_policy(const T& exec);
template execution_policy& operator=(const T& exec);
// 2.7.2, execution_policy object access
const type_info& type() const noexcept;
template T* get() noexcept;
template const T* get() const noexcept;
};
1
The class execution_policy is a container for execution policy objects. execution_policy allows
dynamic control over standard algorithm execution.
[ Example:
std::vector sort_me = .
using namespace std::experimental::parallel;
execution_policy exec = seq;
if(sort_me.size() > threshold)
{
exec = std::par;
}
std::sort(exec, std::begin(sort_me), std::end(sort_me));
— end example ]
2
Objects of type execution_policy shall be constructible and assignable from objects of type T for
which is_execution_policy::value is true.
2.7.1 execution_policy construct/assign [parallel.execpol.con]
1
template execution_policy(const T& exec);
2
Effects: Constructs an execution_policy object with a copy of exec's state.
3
Remarks: This constructor shall not participate in overload resolution unless
is_execution_policy::value is true.
4
template execution_policy& operator=(const T& exec);
5
Effects: Assigns a copy of exec's state to *this.
6
Returns: *this.
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2.7.2 execution_policy object access [parallel.execpol.access]
1
const type_info& type() const noexcept;
2
Returns: typeid(T), such that T is the type of the execution policy object contained by *this.
3
template T* get() noexcept;
template const T* get() const noexcept;
4
Returns: If target_type() == typeid(T), a pointer to the stored execution policy object;
otherwise a null pointer.
5
Requires: is_execution_policy::value is true.
2.8 Execution policy objects [parallel.execpol.objects]
constexpr sequential_execution_policy seq{};
constexpr parallel_execution_policy par{};
constexpr parallel_vector_execution_policy par_vec{};
1
The header declares a global object associated with each type of
execution policy defined by this Technical Specification.
§ 2.8 9
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3 Parallel exceptions [parallel.exceptions]
3.1 Exception reporting behavior [parallel.exceptions.behavior]
1
During the execution of a standard parallel algorithm, if temporary memory resources are
required and none are available, the algorithm throws a std::bad_alloc exception.
2
During the execution of a standard parallel algorithm, if the invocation of an element access
function exits via an uncaught exception, the behavior of the program is determined by the type
of execution policy used to invoke the algorithm:
— If the execution policy object is of type class parallel_vector_execution_policy,
std::terminate shall be called.
— If the execution policy object is of type sequential_execution_policy or
parallel_execution_policy, the execution of the algorithm exits via an exception. The
exception shall be an exception_list containing all uncaught exceptions thrown during
the invocations of element access functions, or optionally the uncaught exception if
there was only one.
[ Note: For example, when for_each is executed sequentially, if an invocation of the user-
provided function object throws an exception, for_each can exit via the uncaught
exception, or throw an exception_list containing the original exception. — end note ]
[ Note: These guarantees imply that, unless the algorithm has failed to allocate memory
and exits via std::bad_alloc, all exceptions thrown during the execution of the algorithm
are communicated to the caller. It is unspecified whether an algorithm implementation
will "forge ahead" after encountering and capturing a user exception. — end note ]
[ Note: The algorithm may exit via the std::bad_alloc exception even if one or more user-
provided function objects have exited via an exception. For example, this can happen
when an algorithm fails to allocate memory while creating or adding elements to the
exception_list object. — end note ]
— If the execution policy object is of any other type, the behavior is implementation-
defined.
3
3.2 Header synopsis
[parallel.exceptions.synopsis]
namespace std {
namespace experimental {
namespace parallel {
inline namespace v1 {
class exception_list : public exception
{
public:
typedef unspecified iterator;
size_t size() const noexcept;
iterator begin() const noexcept;
iterator end() const noexcept;
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const char* what() const noexcept override;
};
}
}
}
}
1
The class exception_list owns a sequence of exception_ptr objects. The parallel algorithms may
use the exception_list to communicate uncaught exceptions encountered during parallel
execution to the caller of the algorithm.
2
The type exception_list::iterator shall fulfill the requirements of ForwardIterator.
3
size_t size() const noexcept;
4
Returns: The number of exception_ptr objects contained within the exception_list.
5
Complexity: Constant time.
6
iterator begin() const noexcept;
7
Returns: An iterator referring to the first exception_ptr object contained within the
exception_list.
8 iterator end() const noexcept;
9
Returns: An iterator that is past the end of the owned sequence.
10
const char* what() const noexcept override;
11
Returns: An implementation-defined NTBS.
§ 3.2 11
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4 Parallel algorithms [parallel.alg]
4.1 In general [parallel.alg.general]
This clause describes components that C++ programs may use to perform operations on
containers and other sequences in parallel.
4.1.1 Requirements on user-provided function objects [parallel.alg.general.user]
1
Function objects passed into parallel algorithms as objects of type BinaryPredicate, Compare, and
BinaryOperation shall not directly or indirectly modify objects via their arguments.
4.1.2 Effect of execution policies on algorithm execution [parallel.alg.general.exec]
1
Parallel algorithms have template parameters named ExecutionPolicy which describe the manner
in which the execution of these algorithms may be parallelized and the manner in which they
apply the element access functions.
2
The invocations of element access functions in parallel algorithms invoked with an execution
policy object of type sequential_execution_policy execute in sequential order in the calling thread.
3
The invocations of element access functions in parallel algorithms invoked with an execution
policy object of type parallel_execution_policy are permitted to execute in an unordered fashion
in either the invoking thread or in a thread implicitly created by the library to support parallel
algorithm execution. Any such invocations executing in the same thread are indeterminately
sequenced with respect to each other. [ Note: It is the caller's responsibility to ensure
correctness, for example that the invocation does not introduce data races or deadlocks.
— end note ]
[ Example:
using namespace std::experimental::parallel;
int a[] = {0,1};
std::vector v;
for_each(par, std::begin(a), std::end(a), [&](int i) {
v.push_back(i*2+1);
});
The program above has a data race because of the unsynchronized access to the container v.
— end example ]
[ Example:
using namespace std::experimental::parallel;
std::atomic x = 0;
int a[] = {1,2};
for_each(par, std::begin(a), std::end(a), [&](int n) {
x.fetch_add(1, std::memory_order_relaxed);
// spin wait for another iteration to change the value of x
while (x.load(std::memory_order_relaxed) == 1) { }
});
The above example depends on the order of execution of the iterations, and is therefore
undefined (may deadlock). — end example ]
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[ Example:
using namespace std::experimental::parallel;
int x=0;
std::mutex m;
int a[] = {1,2};
for_each(par, std::begin(a), std::end(a), [&](int) {
m.lock();
++x;
m.unlock();
});
The above example synchronizes access to object x ensuring that it is incremented correctly.
— end example ]
4
The invocations of element access functions in parallel algorithms invoked with an execution
policy of type parallel_vector_execution_policy are permitted to execute in an unordered fashion
in unspecified threads, and unsequenced with respect to one another within each thread.
[ Note: This means that multiple function object invocations may be interleaved on a single
thread. — end note ]
[ Note: This overrides the usual guarantee from the C++ standard, Section 1.9 [intro.execution]
that function executions do not interleave with one another. — end note ]
Since parallel_vector_execution_policy allows the execution of element access functions to be
interleaved on a single thread, synchronization, including the use of mutexes, risks deadlock.
Thus the synchronization with parallel_vector_execution_policy is restricted as follows:
A standard library function is vectorization-unsafe if it is specified to synchronize with another
function invocation, or another function invocation is specified to synchronize with it, and if it is
not a memory allocation or deallocation function. Vectorization-unsafe standard library
functions may not be invoked by user code called from parallel_vector_execution_policy
algorithms.
[ Note: Implementations must ensure that internal synchronization inside standard library
routines does not induce deadlock. — end note ]
5
[ Example:
using namespace std::experimental::parallel;
int x=0;
std::mutex m;
int a[] = {1,2};
for_each(par_vec, std::begin(a), std::end(a), [&](int) {
m.lock();
++x;
m.unlock();
});
The above program is invalid because the applications of the function object are not guaranteed
to run on different threads. — end example ]
[ Note: The application of the function object may result in two consecutive calls to m.lock on
the same thread, which may deadlock. — end note ]
[ Note: The semantics of the parallel_execution_policy or the parallel_vector_execution_policy
invocation allow the implementation to fall back to sequential execution if the system cannot
parallelize an algorithm invocation due to lack of resources. — end note ]
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6
Algorithms invoked with an execution policy object of type execution_policy execute internally as
if invoked with the contained execution policy object.
7
The semantics of parallel algorithms invoked with an execution policy object of implementation-
defined type are implementation-defined.
4.1.3 ExecutionPolicy algorithm overloads [parallel.alg.overloads]
1
The Parallel Algorithms Library provides overloads for each of the algorithms named in Table 1,
corresponding to the algorithms with the same name in the C++ Standard Algorithms Library.
For each algorithm in Table 2, if there are overloads for corresponding algorithms with the
same name in the C++ Standard Algorithms Library, the overloads shall have an additional
template type parameter named ExecutionPolicy, which shall be the first template parameter. In
addition, each such overload shall have the new function parameter as the first function
parameter of type ExecutionPolicy&&.
2
Unless otherwise specified, the semantics of ExecutionPolicy algorithm overloads are identical to
their overloads without.
3
Parallel algorithms shall not participate in overload resolution unless
is_execution_policy>::value is true.
Table 2 — Table of parallel algorithms
adjacent_difference adjacent_find all_of any_of
copy copy_if copy_n count
count_if equal exclusive_scan fill
fill_n find find_end find_first_of
find_if find_if_not for_each for_each_n
generate generate_n includes inclusive_scan
inner_product inplace_merge is_heap is_heap_until
is_partitioned is_sorted is_sorted_until lexicographical_compare
max_element merge min_element minmax_element
mismatch move none_of nth_element
partial_sort partial_sort_copy partition partition_copy
reduce remove remove_copy remove_copy_if
remove_if replace replace_copy replace_copy_if
replace_if reverse reverse_copy rotate
rotate_copy search search_n set_difference
set_intersection set_symmetric_difference set_union sort
stable_partition stable_sort swap_ranges transform
transform_exclusive_scan transform_inclusive_scan transform_reduce uninitialized_copy
uninitialized_copy_n uninitialized_fill uninitialized_fill_n unique
unique_copy
[ Note: Not all algorithms in the Standard Library have counterparts in Table 2. — end note ]
4.2 Definitions [parallel.alg.defns]
1
Define GENERALIZED_SUM(op, a1, ., aN) as follows:
— a1 when N is 1
— op(GENERALIZED_SUM(op, b1, ., bK), GENERALIZED_SUM(op, bM, ., bN)) where
— b1, .,
...
Questions, Comments and Discussion
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