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Stencil Computations (View Offsets)

Key RAJA features shown in the following example:

  • RAJA::Kernel loop execution template
  • RAJA kernel execution policies
  • RAJA::View multi-dimensional data access
  • RAJA:make_offset_layout method to apply index offsets

This example applies a five-cell stencil sum to the interior cells of a two-dimensional square lattice and stores the resulting sums in a second lattice of equal size. The five-cell stencil accumulates values from each interior cell and its four neighbors. We use RAJA::View and RAJA::Layout constructs to simplify the multi-dimensional indexing so that we can write the stencil operation as follows:

output(row, col) = input(row, col) +
                   input(row - 1, col) + input(row + 1, col) +
                   input(row, col - 1) + input(row, col + 1)

A lattice is assumed to have N_r \times N_c interior cells with unit values surrounded by a halo of cells containing zero values for a total dimension of (N_r + 2) \times (N_c + 2). For example, when N_r = N_c = 3, the input lattice and values are:

0 0 0 0 0
0 1 1 1 0
0 1 1 1 0
0 1 1 1 0
0 0 0 0 0

After applying the stencil, the output lattice and values are:

0 0 0 0 0
0 3 4 3 0
0 4 5 4 0
0 3 4 3 0
0 0 0 0 0

For this (N_r + 2) \times (N_c + 2) lattice case, here is our (row, col) indexing scheme.

(-1, 3) (0, 3) (1, 3) (2, 3) (3, 3)
(-1, 2) (0, 2) (1, 2) (2, 2) (3, 2)
(-1, 1) (0, 1) (1, 1) (2, 1) (3, 1)
(-1, 0) (0, 0) (1, 0) (2, 0) (3, 0)
(-1, -1) (0, -1) (1, -1) (2, -1) (3, -1)

Notably [0, N_r) \times [0, N_c) corresponds to the interior index range over which we apply the stencil, and [-1,N_r] \times [-1, N_c] is the full lattice index range.

RAJA Offset Layouts

We use the RAJA::make_offset_layout method to construct a RAJA::OffsetLayout object that defines our two-dimensional indexing scheme. Then, we create two RAJA::View objects for each of the input and output lattice arrays.

.. literalinclude:: ../../../../examples/tut_offset-layout.cpp
   :start-after: _offsetlayout_views_start
   :end-before: _offsetlayout_views_end
   :language: C++

Here, the row index range is [-1, N_r], and the column index range is [-1, N_c]. The first argument to each call to the RAJA::View constructor is a pointer to an array that holds the data for the view; we assume the arrays are properly allocated before these calls.

The offset layout mechanics of RAJA allow us to write loops over data arrays using non-zero based indexing and without having to manually compute the proper offsets into the arrays. For more details on the RAJA::View and RAJA::Layout concepts we use in this example, please refer to :ref:`view-label`.

RAJA Kernel Implementation

For the RAJA implementations of the example computation, we use two RAJA::RangeSegment objects to define the row and column iteration spaces for the interior cells:

.. literalinclude:: ../../../../examples/tut_offset-layout.cpp
   :start-after: _offsetlayout_ranges_start
   :end-before: _offsetlayout_ranges_end
   :language: C++

Here, is an implementation using RAJA::kernel multi-dimensional loop execution with a sequential execution policy.

.. literalinclude:: ../../../../examples/tut_offset-layout.cpp
   :start-after: _offsetlayout_rajaseq_start
   :end-before: _offsetlayout_rajaseq_end
   :language: C++

Since the stencil operation is data parallel, any parallel execution policy may be used. The file RAJA/examples/tut_offset-layout.cpp contains a complete working example code with various parallel implementations. For more information about using the RAJA::kernel interface, please see :ref:`loop_elements-kernel-label`.