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Iterators

Iterators#

Requirements For Iterator Use With Device Policies#

Iterators and iterator-like types may or may not refer to content accessible within a SYCL kernel on a device. The term indirectly device accessible refers to a type that represents content accessible on a device. An indirectly device accessible iterator is such a type that can also be dereferenced within a SYCL kernel.

An example of indirectly device accessible iterators include SYCL USM shared pointers which can inherently be used in a SYCL kernel. An example of an iterator type that is not indirectly device accessible is a random access iterator referring to host allocated data because dereferencing it within a SYCL kernel would result in undefined behavior.

Buffer position objects returned by oneapi::dpl::begin and oneapi::dpl::end are not iterators. However, they are indirectly device accessible because they represent data accessible on the device.

When passed to oneDPL algorithms with a device_policy, indirectly device accessible iterator types that are also random access iterators and satisfy SYCL device-copyable must not cause unnecessary data movement beyond what is required by the algorithm’s semantics and what would be required to use the type directly within a SYCL kernel. Indirectly device accessible buffer position objects must not cause unnecessary data movement beyond what is required by the algorithm’s semantics and what would be required by using an accessor to the buffer within a SYCL kernel.

Indirect Device Accessibility Type Trait#

The following class template and variable template are defined in <oneapi/dpl/iterator> inside the namespace oneapi::dpl:

template <typename T>
struct is_indirectly_device_accessible{ /* see below */ };

template <typename T>
inline constexpr bool is_indirectly_device_accessible_v = is_indirectly_device_accessible<T>::value;

template <typename T> oneapi::dpl::is_indirectly_device_accessible is a template which has the base characteristic of std::true_type if T is indirectly device accessible. Otherwise, it has the base characteristic of std::false_type.

oneDPL Iterators#

The oneDPL iterators are defined in the <oneapi/dpl/iterator> header, in namespace oneapi::dpl.

Let us define a named requirement, AdaptingIteratorSource, to describe valid random access iterator-like types that can be used as source for oneDPL iterators as described below. The type Iter satisfies the AdaptingIteratorSource named requirement if it is any of the following:

  • A random access iterator

  • The unspecified iterator-like type returned by oneapi::dpl::begin or oneapi::dpl::end

  • A permutation_iterator

  • A transform_iterator

  • A counting_iterator

  • A discard_iterator

  • A zip_iterator

template <typename Integral>
class counting_iterator
{
  public:
    using difference_type = /* a signed integer type of the same size as Integral */;
    using value_type = Integral;
    using reference = Integral;

    counting_iterator();
    explicit counting_iterator(Integral init);

    reference operator*() const;
    reference operator[](difference_type i) const;

    difference_type operator-(const counting_iterator& it) const;

    counting_iterator operator+(difference_type forward) const;
    counting_iterator operator-(difference_type backward) const;

    counting_iterator& operator+=(difference_type forward);
    counting_iterator& operator-=(difference_type backward);

    counting_iterator& operator++();
    counting_iterator& operator--();
    counting_iterator& operator++(int);
    counting_iterator& operator--(int);

    bool operator==(const counting_iterator& it) const;
    bool operator!=(const counting_iterator& it) const;
    bool operator<(const counting_iterator& it) const;
    bool operator>(const counting_iterator& it) const;
    bool operator<=(const counting_iterator& it) const;
    bool operator>=(const counting_iterator& it) const;
};

counting_iterator is a random access iterator-like type that represents an integer counter. When dereferenced, counting_iterator provides an Integral rvalue equal to the value of the counter; dereference operations cannot be used to modify the counter. The arithmetic and comparison operators of counting_iterator behave as if applied to the values of Integral type representing the counters of the iterator instances passed to the operators.

counting_iterator is SYCL device-copyable, and is an indirectly device accessible iterator.

class discard_iterator
{
  public:
    using difference_type = std::ptrdiff_t;
    using value_type = /* unspecified */;
    using reference = /* unspecified */;

    discard_iterator();
    explicit discard_iterator(difference_type init);

    reference operator*() const;
    reference operator[](difference_type) const;

    difference_type operator-(const discard_iterator& it) const;

    discard_iterator operator+(difference_type forward) const;
    discard_iterator operator-(difference_type backward) const;

    discard_iterator& operator+=(difference_type forward);
    discard_iterator& operator-=(difference_type backward);

    discard_iterator& operator++();
    discard_iterator& operator--();
    discard_iterator operator++(int);
    discard_iterator operator--(int);

    bool operator==(const discard_iterator& it) const;
    bool operator!=(const discard_iterator& it) const;
    bool operator<(const discard_iterator& it) const;
    bool operator>(const discard_iterator& it) const;
};

discard_iterator is a random access iterator-like type that, when dereferenced, provides an lvalue that may be assigned an arbitrary value. The assignment has no effect on the discard_iterator instance; the write is discarded. The arithmetic and comparison operators of discard_iterator behave as if applied to integer counter values maintained by the iterator instances to determine their position relative to each other.

discard_iterator is SYCL device-copyable, and is an indirectly device accessible iterator.

template <typename SourceIterator, typename IndexMap>
class permutation_iterator
{
  public:
    using difference_type =
        typename std::iterator_traits<SourceIterator>::difference_type;
    using value_type = typename std::iterator_traits<SourceIterator>::value_type;
    using pointer = typename std::iterator_traits<SourceIterator>::pointer;
    using reference = typename std::iterator_traits<SourceIterator>::reference;

    permutation_iterator(const SourceIterator& input1, const IndexMap& input2,
                         std::size_t index = 0);

    SourceIterator base() const;

    reference operator*() const;
    reference operator[](difference_type i) const;

    difference_type operator-(const permutation_iterator& it) const;

    permutation_iterator operator+(difference_type forward) const;
    permutation_iterator operator-(difference_type backward) const;

    permutation_iterator& operator+=(difference_type forward);
    permutation_iterator& operator-=(difference_type forward);

    permutation_iterator& operator++();
    permutation_iterator& operator--();
    permutation_iterator operator++(int);
    permutation_iterator operator--(int);

    bool operator==(const permutation_iterator& it) const;
    bool operator!=(const permutation_iterator& it) const;
    bool operator<(const permutation_iterator& it) const;
    bool operator>(const permutation_iterator& it) const;
    bool operator<=(const permutation_iterator& it) const;
    bool operator>=(const permutation_iterator& it) const;
};

permutation_iterator is a random access iterator-like type whose dereferenced value set is defined by the source iterator provided, and whose iteration order over the dereferenced value set is defined by either another iterator or a functor that maps the permutation_iterator index to the index of the source iterator. The arithmetic and comparison operators of permutation_iterator behave as if applied to integer counter values maintained by the iterator instances to determine their position in the index map.

permutation_iterator is SYCL device-copyable if both the SourceIterator and the IndexMap are SYCL device-copyable. permutation_iterator is indirectly device accessible if both the SourceIterator and the IndexMap are indirectly device accessible.

SourceIterator must satisfy AdaptingIteratorSource. When using permutation_iterator in combination with an algorithm with a device_policy, SourceIterator must be indirectly device accessible.

The type IndexMap must be one of the following:

  • A random access iterator

  • The unspecified iterator-like type returned by oneapi::dpl::begin or oneapi::dpl::end

  • A permutation_iterator

  • A transform_iterator

  • A counting_iterator

  • A functor with a signature equivalent to T operator()(const T&) const where T is a std::iterator_traits<SourceIterator>::difference_type

permutation_iterator::operator* uses the counter value of the instance on which it is invoked to index into the index map. The corresponding value in the map is then used to index into the value set defined by the source iterator. The resulting lvalue is returned as the result of the operator.

permutation_iterator::operator[] uses the parameter i to index into the index map. The corresponding value in the map is then used to index into the value set defined by the source iterator. The resulting lvalue is returned as the result of the operator.

template <typename SourceIterator, typename IndexMap>
permutation_iterator<SourceIterator, IndexMap>
make_permutation_iterator(SourceIterator source, IndexMap map);

make_permutation_iterator constructs and returns an instance of permutation_iterator using the source iterator and index map provided.

template <typename Iterator, typename UnaryFunc>
class transform_iterator
{
  public:
    using difference_type = typename std::iterator_traits<Iterator>::difference_type;
    using reference = typename std::invoke_result<UnaryFunc,
                          typename std::iterator_traits<Iterator>::reference>::type;
    using value_type = typename std::remove_reference<reference>::type;
    using pointer = typename std::iterator_traits<Iterator>::pointer;

    Iterator base() const;

    transform_iterator(Iterator it, UnaryFunc unary_func);
    transform_iterator(const transform_iterator& input);
    transform_iterator& operator=(const transform_iterator& input);

    reference operator*() const;
    reference operator[](difference_type i) const;

    difference_type operator-(const transform_iterator& it) const

    transform_iterator operator+(difference_type forward) const;
    transform_iterator operator-(difference_type backward) const;

    transform_iterator& operator+=(difference_type forward);
    transform_iterator& operator-=(difference_type backward);

    transform_iterator& operator++();
    transform_iterator& operator--();
    transform_iterator operator++(int);
    transform_iterator operator--(int);

    bool operator==(const transform_iterator& it) const;
    bool operator!=(const transform_iterator& it) const;
    bool operator<(const transform_iterator& it) const;
    bool operator>(const transform_iterator& it) const;
    bool operator<=(const transform_iterator& it) const;
    bool operator>=(const transform_iterator& it) const;
};

transform_iterator is a random access iterator-like type whose dereferenced value set is defined by the unary function and source iterator provided. When dereferenced, transform_iterator provides the result of the unary function applied to the corresponding element of the source iterator; dereference operations cannot be used to modify the elements of the source iterator unless the unary function result includes a reference to the element. The arithmetic and comparison operators of transform_iterator behave as if applied to the source iterator itself. The template type Iterator must satisfy AdaptingIteratorSource.

transform_iterator is SYCL device-copyable if the source iterator is SYCL device-copyable, and is indirectly device accessible if the source iterator is indirectly device accessible.

template <typename UnaryFunc, typename Iterator>
transform_iterator<UnaryFunc, Iterator>
make_transform_iterator(Iterator, UnaryFunc);

make_transform_iterator constructs and returns an instance of transform_iterator using the source iterator and unary function object provided.

template <typename... Iterators>
class zip_iterator
{
  public:
    using difference_type = typename std::make_signed<std::size_t>::type;
    using value_type =
        std::tuple<typename std::iterator_traits<Iterators>::value_type...>;
    using reference = /* unspecified tuple of reference types */;
    using pointer =
        std::tuple<typename std::iterator_traits<Iterators>::pointer...>;

    std::tuple<Iterators...> base() const;

    zip_iterator();
    explicit zip_iterator(Iterators... args);
    zip_iterator(const zip_iterator& input);
    zip_iterator& operator=(const zip_iterator& input);

    reference operator*() const;
    reference operator[](difference_type i) const;

    difference_type operator-(const zip_iterator& it) const;
    zip_iterator operator-(difference_type backward) const;
    zip_iterator operator+(difference_type forward) const;

    zip_iterator& operator+=(difference_type forward);
    zip_iterator& operator-=(difference_type backward);

    zip_iterator& operator++();
    zip_iterator& operator--();
    zip_iterator operator++(int);
    zip_iterator operator--(int);

    bool operator==(const zip_iterator& it) const;
    bool operator!=(const zip_iterator& it) const;
    bool operator<(const zip_iterator& it) const;
    bool operator>(const zip_iterator& it) const;
    bool operator<=(const zip_iterator& it) const;
    bool operator>=(const zip_iterator& it) const;
};

zip_iterator is an iterator-like type defined over one or more iterators. When dereferenced, the value returned from zip_iterator is a tuple of the values returned by dereferencing the source iterators over which the zip_iterator is defined. The arithmetic operators of zip_iterator update the source iterators of a zip_iterator instance as though the operation were applied to each of these iterators. The types T within the template pack Iterators... must satisfy AdaptingIteratorSource.

zip_iterator is SYCL device-copyable if all the source iterators are SYCL device-copyable, and is indirectly device accessible if all the source iterators are indirectly device accessible.

template <typename... Iterators>
zip_iterator<Iterators...>
make_zip_iterator(Iterators...);

make_zip_iterator constructs and returns an instance of zip_iterator using the set of source iterators provided.

Other Iterators#

Pointers are assumed to be USM shared or device memory pointers when used in combination with an algorithm with a device_policy and are indirectly device accessible. Pointers are trivially copyable and therefore SYCL device-copyable.

It is implementation defined whether other iterators are indirectly device accessible, including iterator types from the C++ Standard.

Customization For User Defined Iterators#

oneDPL provides a mechanism to indicate whether custom iterators are indirectly device accessible.

Applications may define a free function is_onedpl_indirectly_device_accessible(T), which accepts an argument of type T and returns a type with the base characteristic of std::true_type if T is indirectly device accessible. Otherwise, it returns a type with the base characteristic of std::false_type. The function must be discoverable by argument-dependent lookup (ADL). It may be provided as a forward declaration only, without defining a body.

The return type of is_onedpl_indirectly_device_accessible is examined at compile time to determine if T is indirectly device accessible. The function overload to use must be selected with argument-dependent lookup. [Note: Therefore, according to the rules in the C++ Standard, a derived type for which there is no function overload will match its most specific base type for which an overload exists. – end note]

Once is_onedpl_indirectly_device_accessible(T) is defined, the public trait template<typename T> oneapi::dpl::is_indirectly_device_accessible[_v] will return the appropriate value. This public trait can also be used to define the return type of is_onedpl_indirectly_device_accessible(T) by applying it to any source iterator component types. Refer to the example below.

Example#

The following example shows how to define a customization for is_indirectly_device_accessible trait for a simple user defined iterator. It also shows a more complicated example where the customization is defined as a hidden friend of the iterator class.

namespace usr
{
    struct accessible_it
    {
        /* user definition of an indirectly device accessible iterator */
    };

    std::true_type
    is_onedpl_indirectly_device_accessible(accessible_it);

    struct inaccessible_it
    {
        /* user definition of an iterator which is not indirectly device accessible */
    };

    // The following could be omitted, as returning std::false_type matches the default behavior.
    std::false_type
    is_onedpl_indirectly_device_accessible(inaccessible_it);
}

static_assert(oneapi::dpl::is_indirectly_device_accessible<usr::accessible_it> == true);
static_assert(oneapi::dpl::is_indirectly_device_accessible<usr::inaccessible_it> == false);

// Example with base iterators and ADL overload as a hidden friend
template <typename It1, typename It2>
struct it_pair
{
    It1 first;
    It2 second;
    friend auto
    is_onedpl_indirectly_device_accessible(it_pair) ->
        std::conjunction<oneapi::dpl::is_indirectly_device_accessible<It1>,
                         oneapi::dpl::is_indirectly_device_accessible<It2>>
    {
        return {};
    }
};

static_assert(oneapi::dpl::is_indirectly_device_accessible<
                                it_pair<usr::accessible_it, usr::accessible_it>> == true);
static_assert(oneapi::dpl::is_indirectly_device_accessible<
                                it_pair<usr::accessible_it, usr::inaccessible_it>> == false);
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