Kokkos::View

Kokkos::View

Header File: Kokkos_Core.hpp

Usage:

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.

Interface

template <class DataType [, class LayoutType] [, class MemorySpace] [, class MemoryTraits]>
class View;

Parameters

Template parameters other than DataType are optional, but ordering is enforced. That means for example that LayoutType can be omitted but if both MemorySpace and MemoryTraits are specified, MemorySpace must come before MemoryTraits.

• 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.
• LayoutType: determines the mapping of indices into the underlying 1D memory storage. Custom Layouts can be implemented, but Kokkos comes with some built-in ones:
  • 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 ([][][]) 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. If omitted the default memory space of the default execution space is used (i.e. Kokkos::DefaultExecutionSpace::memory_space)
• MemoryTraits: Sets access properties via enum parameters for the templated Kokkos::MemoryTraits<> class. Enums can be combined bit combined. Posible values:
  • Unmanaged: The View will not be reference counted. The allocation has to be provided to the constructor.
  • Atomic: All accesses to the view will use atomic operations.
  • RandomAccess: Hint that the view is used in a random access manner. If the view is also const this will trigger special load operations on GPUs (i.e. texture fetches).
  • Restrict: There is no aliasing of the view by other data structures in the current scope.

Requirements:

Public Class Members

Enums

• rank: rank of the view (i.e. the dimensionality).
• rank_dynamic: number of runtime determined dimensions.
• reference_type_is_lvalue_reference: whether the reference type is a C++ lvalue reference.

Typedefs

Data Types

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

• const_data_type: Const version of DataType, same as data_type if that is already const.

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

• 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.

• const_scalar_array_type: Const version of scalar_array_type, same as scalar_array_type if that is already const

• 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

• 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.
• const_value_type: const version of value_type.
• non_const_value_type: non-const version of value_type.

Spaces

• execution_space: Execution Space associated with the view, will be used for performing view initialization, and certain deep_copy operations.
• memory_space: Data storage location type.
• device_type: the compound type defined by Device<execution_space,memory_space>
• memory_traits: The memory traits of the view.
• host_mirror_space: Host accessible memory space used in HostMirror.

ViewTypes

• non_const_type: this view type with all template parameters explicitly defined.
• const_type: this view type with all template parameters explicitly defined using a const data type.
• HostMirror: compatible view type with the same DataType and LayoutType stored in host accessible memory space.

Data Handles

• reference_type: return type of the view access operators.
• pointer_type: pointer to scalar type.

Other

• array_layout: The Layout of the View.
• size_type: index type associated with the memory space of this view.
• dimension: An integer array like type, able to represent the extents of the view.
• specialize: A specialization tag used for partial specialization of the mapping construct underlying a Kokkos View.

Constructors

• View(): Default Constructor. No allocations are made, no reference counting happens. All extents are zero and its data pointer is NULL.
• View( const View<DT, Prop...>& rhs): Copy constructor with compatible view. Follows View assignment rules.
• View( View&& rhs): Move constructor
• View( const std::string& name, const IntType& ... indices): Standard allocating constructor. The initialization is executed on the default instance of the execution space corresponding to MemorySpace and fences it.
  • name: a user provided label, which is used for profiling and debugging purposes. Names are not required to be unique,
  • indices: Runtime dimensions of the view.
  • Requires: sizeof(IntType...)==rank_dynamic()
  • Requires: 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 MemorySpace and fences it.
  • 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.
• View( const AllocProperties& prop, , const IntType& ... indices): Allocating constructor with allocation properties. 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 MemorySpace and fences it.
  • An allocation properties object is returned by the view_alloc function.
  • indices: Runtime dimensions of the view.
  • Requires: sizeof(IntType...)==rank_dynamic()
  • Requires: array_layout::is_regular == true.
• View( const AllocProperties& prop, const array_layout& layout): Allocating constructor with allocation properties 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 MemorySpace and fences it.
  • An allocation properties object is returned by the view_alloc function.
  • layout: an instance of a layout class.
• View( const pointer_type& ptr, const IntType& ... indices): Unmanaged data wrapping constructor.
  • ptr: pointer to a user provided memory allocation. Must provide storage of size View::required_allocation_size(n0,...,nR)
  • indices: Runtime dimensions of the view.
  • Requires: sizeof(IntType...)==rank_dynamic()
  • Requires: array_layout::is_regular == true.
• View( const std::string& name, const array_layout& layout): Unmanaged data wrapper constructor.
  • ptr: pointer to a user provided memory allocation. Must provide storage of size View::required_allocation_size(layout) (NEEDS TO BE IMPLEMENTED)
  • layout: an instance of a layout class.
• View( const ScratchSpace& space, const IntType& ... indices): Constructor which acquires memory from a Scratch Memory handle.
  • space: scratch memory handle. Typically returned from team_handles in TeamPolicy kernels.
  • indices: Runtime dimensions of the view.
  • Requires: sizeof(IntType...)==rank_dynamic()
  • Requires: array_layout::is_regular == true.
• View( const ScratchSpace& space, const array_layout& layout): Constructor which acquires memory from a Scratch Memory handle.
  • space: scratch memory handle. Typically returned from team_handles in TeamPolicy kernels.
  • layout: an instance of a layout class.
• View( const View<DT, Prop...>& rhs, Args ... args): Subview constructor. See subview function for arguments.

Data Access Functions

• reference_type operator() (const IntType& ... indices) const

  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. See notes on reference_type for properties of the return type.

  • Requires: sizeof(IntType...)==rank_dynamic()

• reference_type access (const IntType& i0=0, ... , const IntType& i7=0) const

  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. See notes on reference_type for properties of the return type.

Data Layout, Dimensions, Strides

• constexpr array_layout layout() const

  Returns the layout object. Can be used to to construct other views with the same dimensions.

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

  Return the extent of the specified dimension. iType must be an integral type, and dim must be smaller than rank.

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

  Return the extent of the specified dimension as an int. iType must be an integral type, and dim must be smaller than rank. 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.

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

  Return the stride of the specified dimension. iType must be an integral type, and dim must be smaller than rank. Example: a.stride(3) == (&a(i0,i1,i2,i3+1,i4)-&a(i0,i1,i2,i3,i4))

• constexpr size_t stride_0() const

  Return the stride of dimension 0.

• constexpr size_t stride_1() const

  Return the stride of dimension 1.

• constexpr size_t stride_2() const

  Return the stride of dimension 2.

• constexpr size_t stride_3() const

  Return the stride of dimension 3.

• constexpr size_t stride_4() const

  Return the stride of dimension 4.

• constexpr size_t stride_5() const

  Return the stride of dimension 5.

• constexpr size_t stride_6() const

  Return the stride of dimension 6.

• constexpr size_t stride_7() const

  Return the stride of dimension 7.

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

  Sets strides[r] to stride(r) for all r with 0<=r<rank. Sets strides[rank] to span(). iType must be an integral type, and strides must be an array of length rank+1.

• constexpr size_t span() const

  Returns 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

  Return the pointer to the underlying data allocation.

• bool span_is_contiguous() const

  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 N8 = 0);

  Returns the number of bytes necessary for an unmanaged view of the provided dimensions. This function is only valid if array_layout::is_regular == true.

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

  Returns the number of bytes necessary for an unmanaged view of the provided layout.

Other

• int use_count() const;

  Returns the current reference count of the underlying allocation.

• const char* label() const;

  Returns the label of the View.

• const bool is_assignable(const View<DT, Prop...>& rhs);

  Returns true if the View can be assigned to rhs. See below for assignment rules.

• void assign_data(pointer_type arg_data);

  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.

NonMember Functions

• template<class ViewDst, class ViewSrc>
  bool operator==(ViewDst, ViewSrc);

  Returns true if value_type, array_layout, memory_space, rank, data() and extent(r), for 0<=r<rank, match.

• template<class ViewDst, class ViewSrc>
  bool operator!=(ViewDst, ViewSrc);

  Returns true if any of value_type, array_layout, memory_space, rank, data() and extent(r), for 0<=r<rank don't match.

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.:

ScrType 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<SrcType::value_type>::value == true than std::is_const<DstType::value_type>::value == true.
• MemorySpaceAccess<DstType::memory_space,SrcType::memory_space>::assignable == true
• If DstType::dynamic_rank != DstType::rank and SrcType::dynamic_rank != SrcType::rank than for each dimension k which 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::dynamic_rank != DstType::rank than 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 than 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

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();
}