Brick Library 0.1
Performance-portable stencil datalayout & codegen
Vectorization in Bricks

One of main target of the code generation is to handle discontinuity using vectorization. The process largely depends on which vectorization ISA is used.

Vectorization Abstraction

Vector is data abstraction derived from single instruction, multiple data (SIMD) processing, where one instruction can express operation applied to multiple elements in one or more vectors. Each element of vector may also be referred to as a lane of the vector units. The vector abstraction we used can be based on bits or fixed number of elements due to global definition of bElem.

Each individual brick is a collection of whole vectors. Each vector is logically multi-dimensional even when physically the vector only have one dimension. The size of the vectors is specified with the second template argument to the Brick<Dim<...>,Dim<...>> structure.

Note that the code generator can place some extra restriction on the vector dimensions due to ISA support. For example, AVX2 support efficient suffling between 128 bits and within 128 bits. For AVX2 it is recommended to view the physical vector as 2x128bits.

Due to the size of vectors, it may place some restrictions on the size of the brick. However, because the dimensions of the vector is rather flexible from balancing between the dimensions, this restriction on brick dimensions is generally irrelevant.

The underlying code generator assumes that vector ISA supports the following operations:

  • Aligned load of vectors
  • Element-wise computation between vectors
  • Element-wise selection from two vectors
  • Shuffling across lanes

Using the vectorizing code generator

See stencil expression on expressing stencils for the code generator and see integration on integrating the generated code in C++ program.

Internal structure of the code generator

The code generator is located in codegen, note that the vectorscatter executable is used to process C++ files which is usually what's of the most interests.

  • st/*.py defines the AST elements. These elements are used to define the stencil expression.
  • st/codegen/backend/* defines different backend ISA. See asimd.py for a basic example.
  • st/codegen/*.py defines the code generation process.
  • st/codegen/base.py is the entry point to the codegen. See CodeGen::gencode.