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# 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](https://tree.kummerlaender.eu/projects/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
* static resolution of memory offsets
* AB streaming pattern
* C++ and OpenCL 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 --layout LAYOUT --precision
PRECISION --geometry GEOMETRY --tau TAU [--disable-cse]
[--functions FUNCTIONS [FUNCTIONS ...]]
[--extras EXTRAS [EXTRAS ...]]
language
Generate LBM kernels in various languages using a symbolic description.
positional arguments:
language 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)
--layout LAYOUT Memory layout ("AOS" or "SOA")
--precision PRECISION
Floating precision ("single" or "double")
--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
\ --layout AOS
\ --precision double
\ --tau 0.52
\ --functions default bounce_back_boundary example
\ --extras omp_parallel_for moments_vtk | tee kernel.h
void collide_and_stream( 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];
[...]
```
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