boltzgen

…a framework for symbolically generating optimized implementations of the Lattice Boltzmann Method.

At the moment this is a more structured and cleaned up version of the OpenCL kernel generator employed by symlbm_playground. The goal is to build upon this foundation to provide an easy way to generate efficient code using a high level description of various collision models and boundary conditions. This should allow for easy comparision between various streaming patterns and memory layouts.

Features

BGK collision step
D2Q9, D3Q7, D3Q19 and D3Q27 lattices with automatic weight inference
momenta and bounce back boundary conditions
equilibrilization and moment collection utility functions
optimization via common subexpression elimination
array-of-structures and structure-of-arrays memory layouts
configurable cell indexing sequence
static resolution of memory offsets
AB, AA and SSS streaming patterns
C++, OpenCL and CUDA targets
simple CLI frontend

Usage

The development and execution environment is described using a Nix expression in shell.nix.

> nix-shell
> ./boltzgen.py --help
usage: boltzgen.py [-h] --lattice LATTICE [--model MODEL] --precision
                   PRECISION --layout LAYOUT [--index INDEX] --streaming
                   STREAMING --geometry GEOMETRY --tau TAU [--disable-cse]
                   [--functions FUNCTIONS [FUNCTIONS ...]]
                   [--extras EXTRAS [EXTRAS ...]]
                   target

Generate LBM kernels in various languages using a symbolic description.

positional arguments:
  target                Target language (currently either "cl" or "cpp")

optional arguments:
  -h, --help            show this help message and exit
  --lattice LATTICE     Lattice type ("D2Q9", "D3Q7", "D3Q19", "D3Q27")
  --model MODEL         LBM model (currently only "BGK")
  --precision PRECISION
                        Floating precision ("single" or "double")
  --layout LAYOUT       Memory layout ("AOS" or "SOA")
  --index INDEX         Cell indexing ("XYZ" or "ZYX")
  --streaming STREAMING
                        Streaming pattern ("AB" or "AA")
  --geometry GEOMETRY   Size of the block geometry ("x:y(:z)")
  --tau TAU             BGK relaxation time
  --disable-cse         Disable common subexpression elimination
  --functions FUNCTIONS [FUNCTIONS ...]
                        Function templates to be generated
  --extras EXTRAS [EXTRAS ...]
                        Additional generator parameters

> ./boltzgen.py cpp --lattice D2Q9
\                   --geometry 128:128
\                   --precision double
\                   --streaming AB
\                   --layout AOS
\                   --tau 0.52
\                   --functions default bounce_back_boundary | tee kernel.h
void collide(double* f_next,
             const double* f_prev,
             std::size_t gid)
{
          double* preshifted_f_next = f_next + gid*9;
    const double* preshifted_f_prev = f_prev + gid*9;

    const double f_curr_0 = preshifted_f_prev[1161];
    const double f_curr_1 = preshifted_f_prev[1153];
    const double f_curr_2 = preshifted_f_prev[1145];
    const double f_curr_3 = preshifted_f_prev[12];
    const double f_curr_4 = preshifted_f_prev[4];
[...]

For some examples on how to actually run the generated code see boltzgen_examples.