`View`¶

Header File: <Kokkos_Core.hpp>

Class Interface¶

template<class DataType, class ...Properties> class View¶

Kokkos View is a potentially reference counted multi dimensional array with compile time layouts and memory space. Its semantics are similar to that of std::shared_ptr.

Template Parameters:

DataType –
Defines the fundamental scalar type of the View and its dimensionality.

The basic structure is ScalarType STARS BRACKETS where the number of STARS denotes the number of runtime length dimensions and the number of BRACKETS defines the compile time dimensions. Due to C++ type restrictions runtime dimensions must come first. Examples:
- double**: 2D View of double with 2 runtime dimensions
- const int***[5][3]: 5D View of int with 3 runtime and 2 compile dimensions.
  The data is const.
- Foo[6][2]: 2D View of a class Foo with 2 compile time dimensions.
Properties... –
Defines various properties of the View, including layout, memory space, and memory traits.

View’s template parameters after DataType are variadic and optional, but must be specified in order. That means for example that LayoutType can be omitted but if both MemorySpace and MemoryTraits are specified, MemorySpace must come before MemoryTraits.
The ordering of View template parameters.¶
```
template <class DataType [, class LayoutType] [, class MemorySpace] [, class MemoryTraits]>
class View;
```
LayoutType –
Determines the mapping of indices into the underlying 1D memory storage.

Kokkos comes with some built-in layouts:
- LayoutRight: strides increase from the right most to the left most dimension.
  The last dimension has a stride of one. This corresponds to how C multi dimensional arrays (e.g foo[][][]) are laid out in memory.
- LayoutLeft: strides increase from the left most to the right most dimension.
  The first dimension has a stride of one. This is the layout Fortran uses for its arrays.
- LayoutStride: strides can be arbitrary for each dimension.
MemorySpace –
Controls the storage location of the View.

If omitted the default memory space of the default execution space is used (i.e. DefaultExecutionSpace::memory_space)
MemoryTraits –
Sets access properties via enum parameters for the struct template MemoryTraits. Possible template parameters are bitwise OR of the following flags:
- Unmanaged
- RandomAccess
- Atomic
- Restrict
- Aligned
See the sub-section on memory access traits in the Programming Guide also for further information.

Public Constants¶

static constexpr bool reference_type_is_lvalue_reference¶: whether the reference type is a C++ lvalue reference.

Data Types¶

type data_type¶: The DataType of the View, note data_type contains the array specifiers (e.g. int**[3])

type const_data_type¶: const version of DataType, same as data_type if that is already const.

type non_const_data_type¶: Non-const version of DataType, same as data_type if that is already non-const.

type scalar_array_type¶: If DataType represents some properly specialised array data type such as Sacado FAD types, scalar_array_type is the underlying fundamental scalar type.

type const_scalar_array_type¶: const version of scalar_array_type, same as scalar_array_type if that is already const

type non_const_scalar_array_type¶: Non-const version of scalar_array_type, same as scalar_array_type if that is already non-const.

Scalar Types¶

type value_type¶: The data_type stripped of its array specifiers, i.e. the scalar type of the data the view is referencing (e.g. if data_type is const int**[3], value_type is const int).

type const_value_type¶: const version of value_type.

type non_const_value_type¶: non-const version of value_type.

Spaces¶

type execution_space¶: The execution space associated with the view, will be used for performing view initialization, and certain deep_copy operations.

type memory_space¶: The memory space where the View data is stored.

type device_type¶: the compound type defined by Device<execution_space, memory_space>

type memory_traits¶: The memory traits of the view.

type host_mirror_space¶: Host accessible memory space used in HostMirror.

View Types¶

type non_const_type¶: this View type with non_const_data_type passed as the DataType template parameter

type const_type¶: this View type with const_data_type passed as the DataType template parameter

type HostMirror¶: compatible view type with the same data_type and array_layout stored in host accessible memory space.

Data Handles¶

type reference_type¶: return type of the view access operators.

See also

operator()()

access()

type pointer_type¶: pointer to value_type.

Other Types¶

type array_layout¶: The LayoutType of the View.

type size_type¶: index type associated with the memory space of this View.

type dimension¶: An integer array like type, able to represent the extents of the View.

type specialize¶: A specialization tag used for partial specialization of the mapping construct underlying a View.

Constructors¶

View()¶: Default Constructor. No allocations are made, no reference counting happens. All extents are zero and its data pointer is nullptr.

template<class DT, class ...Prop> View(const View<DT, Prop...> &rhs)¶: Copy constructor with a compatible view. Follows View assignment rules.

See also

Assignment Rules

View(View &&rhs)¶: Move constructor

template<class IntType> View(const std::string &name, const IntType&... extents)¶

Standard allocating constructor. The initialization is executed on the default instance of the execution space corresponding to memory_space and fences it.

Template Parameters:

IntType – an integral type

Parameters:

name – a user provided label, which is used for profiling and debugging purposes. Names are not required to be unique.
extents – Extents of the View.

Requirements:

sizeof(IntType...) == rank_dynamic() or sizeof(IntType...) == rank().
In the latter case, the extents corresponding to compile-time dimensions must match the View type’s compile-time extents.
array_layout::is_regular == true.

View(const std::string &name, const array_layout &layout)¶

Standard allocating constructor. The initialization is executed on the default instance of the execution space corresponding to memory_space and fences it.

Parameters:

name – a user provided label, which is used for profiling and debugging purposes. Names are not required to be unique.
layout – an instance of a layout class. The number of valid extents must either match the rank_dynamic() or rank(). In the latter case, the extents corresponding to compile-time dimensions must match the View type’s compile-time extents.

template<class IntType> View(const ALLOC_PROP &prop, const IntType&... extents)¶

Allocating constructor with allocation properties (created by a call to view_alloc()). If an execution space is specified in prop, the initialization uses it and does not fence. Otherwise, the View is initialized using the default execution space instance corresponding to memory_space and fences it.

Template Parameters:

IntType – an integral type

Parameters:

prop – An allocation properties object that is returned by view_alloc().
extents – Extents of the View.

Requirements:

sizeof(IntType...) == rank_dynamic() or sizeof(IntType...) == rank().
In the latter case, the extents corresponding to compile-time dimensions must match the View type’s compile-time extents.
array_layout::is_regular == true.

View(const ALLOC_PROP &prop, const array_layout &layout)¶

Allocating constructor with allocation properties (created by a call to view_alloc()) and a layout object. If an execution space is specified in prop, the initialization uses it and does not fence. Otherwise, the View is initialized using the default execution space instance corresponding to memory_space and fences it.

Parameters:

prop – An allocation properties object that is returned by view_alloc().
layout – an instance of a layout class. The number of valid extents must either match the rank_dynamic() or rank(). In the latter case, the extents corresponding to compile-time dimensions must match the View type’s compile-time extents.

template<class IntType> View(pointer_type ptr, const IntType&... extents)¶

Unmanaged data wrapping constructor.

Template Parameters:

IntType – an integral type

Parameters:

ptr – pointer to a user provided memory allocation. Must provide storage of size required_allocation_size(extents...)
extents – Extents of the View.

Requirements:

sizeof(IntType...) == rank_dynamic() or sizeof(IntType...) == rank().
In the latter case, the extents corresponding to compile-time dimensions must match the View type’s compile-time extents.
array_layout::is_regular == true.

View(pointer_type ptr, const array_layout &layout)¶

Unmanaged data wrapper constructor.

Parameters:

ptr – pointer to a user provided memory allocation. Must provide storage of size View::required_allocation_size(layout)
layout – an instance of a layout class. The number of valid extents must either match the dynamic rank or the total rank. In the latter case, the extents corresponding to compile-time dimensions must match the View type’s compile-time extents.

template<class IntType> View(const ScratchSpace &space, const IntType&... extents)¶

Constructor which acquires memory from a Scratch Memory handle.

Template Parameters:

IntType – an integral type

Parameters:

space – scratch memory handle. Typically returned from team_shmem(), team_scratch(), or thread_scratch() in TeamPolicy kernels.
extents – Extents of the View.

Requirements:

sizeof(IntType...) == rank_dynamic() or sizeof(IntType...) == rank().
In the latter case, the extents corresponding to compile-time dimensions must match the View type’s compile-time extents.
array_layout::is_regular == true.

View(const ScratchSpace &space, const array_layout &layout)¶

Constructor which acquires memory from a Scratch Memory handle.

Parameters:

space – scratch memory handle. Typically returned from team_shmem(), team_scratch(), or thread_scratch() in TeamPolicy kernels.
layout – an instance of a layout class. The number of valid extents must either match the dynamic rank or the total rank. In the latter case, the extents corresponding to compile-time dimensions must match the View type’s compile-time extents.

template<class DT, class ...Prop> View(const View<DT, Prop...> &rhs, Args... args)¶

Parameters:

rhs – the View to take a subview of
args... – the subview slices as specified in subview()

Subview constructor.

Data Access Functions¶

template<class IntType> reference_type operator()(const IntType&... indices) const¶

Template Parameters:: IntType – an integral type
Parameters:: indices – the indices of the element to get a reference to
Returns:: a reference to the element at the given indices

Returns a value of reference_type which may or not be referenceable itself. The number of index arguments must match the rank() of the view.

Requirements:

sizeof(IntType...) == rank_dynamic()

template<class IntType> reference_type access(const IntType &i0 = 0, const IntType &i1 = 0, const IntType &i2 = 0, const IntType &i3 = 0, const IntType &i4 = 0, const IntType &i5 = 0, const IntType &i6 = 0, const IntType &i7 = 0) const¶

Template Parameters:: IntType – an integral type
Parameters:: i0, i1, i2, i3, i4, i5, i6, i7 – the indices of the element to get a reference to
Returns:: a reference to the element at the given indices

Returns a value of reference_type which may or not be referenceable itself. The number of index arguments must be equal or larger than the rank() of the view. Index arguments beyond rank() must be 0, which will be enforced if KOKKOS_DEBUG is defined.

Data Layout, Dimensions, Strides¶

static constexpr size_t rank()¶

Returns:: the rank of the view.

Added in version 4.1.

static constexpr size_t rank_dynamic()¶

Returns:: the number of runtime determined dimensions.

Added in version 4.1.

Note

In practice, rank() and rank_dynamic() are not actually implemented as static member functions but rank and rank_dynamic underlying types have a nullary member function (i.e. callable with no argument).

Changed in version 4.1: rank() and rank_dynamic() are static member constants that are convertible to size_t. Their underlying types are unspecified, but equivalent to std::integral_constant with a nullary member function callable from host and device side. Users are encouraged to use rank() and rank_dynamic() (akin to a static member function call) instead of relying on implicit conversion to an integral type.

The actual type of rank() and rank_dynamic() as they were defined until Kokkos 4.1 was left up to the implementation (that is, up to the compiler not to Kokkos) but in practice it was often int which means this change may yield warnings about comparing signed and unsigned integral types. It may also break code that was using the type of rank(). Furthermore, it appears that MSVC has issues with the implicit conversion to size_t in certain constexpr contexts. Calling rank() or rank_dynamic() will work in those cases.

constexpr array_layout layout() const¶

Returns:: the layout object that can be used to to construct other views with the same dimensions.

template<class iType> constexpr size_t extent(const iType &dim) const¶

Template Parameters:: iType – an integral type
Parameters:: dim – the dimension to get the extent of
Returns:: the extent of dimension dim

Preconditions:

dim must be smaller than rank().

template<class iType> constexpr int extent_int(const iType &dim) const¶

Template Parameters:: iType – an integral type
Parameters:: dim – the dimension to get the extent of
Returns:: the extent of dimension dim as an int

Compared to extent() this function can be useful on architectures where int operations are more efficient than size_t. It also may eliminate the need for type casts in applications which otherwise perform all index operations with int.

Preconditions:

dim must be smaller than rank().

template<class iType> constexpr size_t stride(const iType &dim) const¶

Template Parameters:: iType – an integral type
Parameters:: dim – the dimension to get the stride of
Returns:: the stride of dimension dim

Example: a.stride(3) == (&a(i0, i1, i2, i3 + 1, i4) - &a(i0, i1, i2, i3, i4))

Preconditions:

dim must be smaller than rank().

constexpr size_t stride_0() const¶

Returns:: the stride of dimension 0.

constexpr size_t stride_1() const¶

Returns:: the stride of dimension 1.

constexpr size_t stride_2() const¶

Returns:: the stride of dimension 2.

constexpr size_t stride_3() const¶

Returns:: the stride of dimension 3.

constexpr size_t stride_4() const¶

Returns:: the stride of dimension 4.

constexpr size_t stride_5() const¶

Returns:: the stride of dimension 5.

constexpr size_t stride_6() const¶

Returns:: the stride of dimension 6.

constexpr size_t stride_7() const¶

Returns:: the stride of dimension 7.

template<class iType> void stride(iType *strides) const¶

Template Parameters:: iType – an integral type
Parameters:: strides – the output array of length rank() + 1

Sets strides[r] to stride(r) for all \(r\) with \(0 \le r \lt \texttt{rank()}\). Sets strides[rank()] to span().

Preconditions:

strides must be an array of length rank() + 1

constexpr size_t span() const¶

Returns:: the size of the span of memory between the element with the lowest and highest address

Obtains the memory span in elements between the element with the lowest and the highest address. This can be larger than the product of extents due to padding, and or non-contiguous data layout as for example LayoutStride allows.

constexpr size_t size() const¶

Returns:: the product of extents, i.e. the logical number of elements in the View.

constexpr pointer_type data() const¶

Returns:: the pointer to the underlying data allocation.

Warning

Calling any function that manipulates the behavior of the memory (e.g. memAdvise) on memory managed by Kokkos results in undefined behavior.

bool span_is_contiguous() const¶

Returns:: whether the span is contiguous (i.e. whether every memory location between in span belongs to the index space covered by the View).

static constexpr size_t required_allocation_size(size_t N0 = 0, size_t N1 = 0, size_t N2 = 0, size_t N3 = 0, size_t N4 = 0, size_t N5 = 0, size_t N6 = 0, size_t N7 = 0);¶

Parameters:: N0, N1, N2, N3, N4, N5, N6, N7 – the dimensions to query
Returns:: the number of bytes necessary for an unmanaged View of the provided dimensions.

Requirements:

array_layout::is_regular == true.

static constexpr size_t required_allocation_size(const array_layout &layout);¶

Parameters:: layout – the layout to query
Returns:: the number of bytes necessary for an unmanaged View of the provided layout.

Other Utility Methods¶

int use_count() const;¶

Returns:: the current reference count of the underlying allocation.

const std::string label() const;¶

Returns:: the label of the View.

void assign_data(pointer_type arg_data);¶

Parameters:: arg_data – the pointer to set the underlying View data pointer to

Decrement reference count of previously assigned data and set the underlying pointer to arg_data. Note that the effective result of this operation is that the view is now an unmanaged view; thus, the deallocation of memory associated with arg_data is not linked in anyway to the deallocation of the view.

constexpr bool is_allocated() const;¶

Returns:: true if the view points to a valid memory location.

This function works for both managed and unmanaged views. With the unmanaged view, there is no guarantee that referenced address is valid, only that it is a non-null pointer.

Conversion to mdspan¶

template<class OtherElementType, class OtherExtents, class OtherLayoutPolicy, class OtherAccessor> constexpr operator mdspan<OtherElementType, OtherExtents, OtherLayoutPolicy, OtherAccessor>()¶

Template Parameters:

OtherElementType – the target mdspan element type
OtherExtents – the target mdspan extents
OtherLayoutPolicy – the target mdspan layout
OtherAccessor – the target mdspan accessor

Constraints:

View‘s natural mdspan must be assignable to mdspan<OtherElementType, OtherExtents, OtherLayoutPolicy, OtherAccessor>

Returns:

an mdspan with extents and a layout converted from the View’s natural mdspan.

template<class OtherAccessorType = default_accessor<typename traits::value_type>> constexpr auto to_mdspan(const OtherAccessorType &other_accessor = OtherAccessorType{})¶

Template Parameters:: OtherAccessor – the target mdspan accessor
Constraints:: typename OtherAccessorType::data_handle_type must be assignable to value_type*
Returns:: View‘s natural mdspan, but with an accessor policy constructed from other_accessor

Non-Member Functions¶

template<class ...ViewTDst, class ...ViewTSrc> bool is_assignable(const View<ViewTDst...> &dst, const View<ViewTSrc...> &src)¶

Returns:: true if src can be assigned to dst.

Assignment Rules¶

Assignment rules cover the assignment operator as well as copy constructors. We aim at making all logically legal assignments possible, while intercepting illegal assignments if possible at compile time, otherwise at runtime. In the following we use DstType and SrcType as the type of the destination view and source view respectively. dst_view and src_view refer to the runtime instances of the destination and source views, i.e.:

SrcType src_view(...);
DstType dst_view(src_view);
dst_view = src_view;

The following conditions must be met at and are evaluated at compile time:

DstType::rank() == SrcType::rank()
DstType::non_const_value_type is the same as SrcType::non_const_value_type
If std::is_const_v<SrcType::value_type> == true then std::is_const_v<DstType::value_type> must also be true.
MemorySpaceAccess<DstType::memory_space,SrcType::memory_space>::assignable == true
If DstType::rank_dynamic() != DstType::rank() and SrcType::rank_dynamic() != SrcType::rank() then for each dimension k that is compile time for both it must be true that dst_view.extent(k) == src_view.extent(k)

Additionally the following conditions must be met at runtime:

If DstType::rank_dynamic() != DstType::rank() then for each compile time dimension k it must be true that dst_view.extent(k) == src_view.extent(k).

Furthermore there are rules which must be met if DstType::array_layout is not the same as SrcType::array_layout. These rules only cover cases where both layouts are one of LayoutLeft, LayoutRight or LayoutStride

If neither DstType::array_layout nor SrcType::array_layout is LayoutStride:
- If DstType::rank > 1 then DstType::array_layout must be the same as SrcType::array_layout.
If either DstType::array_layout or SrcType::array_layout is LayoutStride
- For each dimension k it must hold that dst_view.extent(k) == src_view.extent(k)

Assignment Examples¶

 View<int*>       a1 = View<int*>("A1",N);     // OK
 View<int**>      a2 = View<int*[10]>("A2",N); // OK
 View<int*[10]>   a3 = View<int**>("A3",N,M);  // OK if M == 10 otherwise runtime failure
 View<const int*> a4 = a1;                     // OK
 View<int*>       a5 = a4;                     // Error: const to non-const assignment
 View<int**>      a6 = a1;                     // Error: Ranks do not match
 View<int*[8]>    a7 = a3;                     // Error: compile time dimensions do not match
 View<int[4][10]> a8 = a3;                     // OK if N == 4 otherwise runtime failure
 View<int*, LayoutLeft>    a9  = a1;           // OK since a1 is either LayoutLeft or LayoutRight
 View<int**, LayoutStride> a10 = a8;           // OK
 View<int**>               a11 = a10;          // OK
 View<int*, HostSpace> a12 = View<int*, CudaSpace>("A12",N); // Error: non-assignable memory spaces
 View<int*, HostSpace> a13 = View<int*, CudaHostPinnedSpace>("A13",N); // OK

Natural mdspans¶

Added in version 4.4.0.

C++23 introduces mdspan, a non-owning multidimensional array view. View is compatible with std::mdspan and can be implicitly converted from and to valid mdspans. These conversion rules are dictated by the natural mdspan of a view. For an mdspan m of type M that is the natural mdspan of a View v of type V, the following properties hold:

M::value_type is V::value_type
M::index_type is std::size_t.
M::extents_type is std::extents<M::index_type, Extents...> where
- sizeof(Extents...) is V::rank()
- and each element at index r of Extents... is V::static_extents(r) if V::static_extents(r) != 0, otherwise std::dynamic_extent
M::layout_type is
- std::layout_left_padded<std::dynamic_extent> if V::array_layout is LayoutLeft
- std::layout_right_padded<std::dynamic_extent> if V::array_layout is LayoutRight
- std::layout_stride if V::array_layout is LayoutStride
M::accessor_type is std::default_accessor<V::value_type>

Additionally, the natural mdspan is constructed so that m.data() == v.data() and for each extent r, m.extents().extent(r) == v.extent(r).

Examples¶

#include<Kokkos_Core.hpp>
#include<cstdio>

int main(int argc, char* argv[]) {
    Kokkos::initialize(argc,argv);

    int N0 = atoi(argv[1]);
    int N1 = atoi(argv[2]);

    Kokkos::View<double*> a("A",N0);
    Kokkos::View<double*> b("B",N1);

    Kokkos::parallel_for("InitA", N0, KOKKOS_LAMBDA (const int& i) {
        a(i) = i;
    });

    Kokkos::parallel_for("InitB", N1, KOKKOS_LAMBDA (const int& i) {
        b(i) = i;
    });

    Kokkos::View<double**,Kokkos::LayoutLeft> c("C",N0,N1);
    {
        Kokkos::View<const double*> const_a(a);
        Kokkos::View<const double*> const_b(b);
        Kokkos::parallel_for("SetC", Kokkos::MDRangePolicy<Kokkos::Rank<2,Kokkos::Iterate::Left>>({0,0},{N0,N1}),
            KOKKOS_LAMBDA (const int& i0, const int& i1) {
            c(i0,i1) = a(i0) * b(i1);
        });
    }

    Kokkos::finalize();
}