Code generation

The generation of a code snippet is done by a CodeGenerator class. The templates are stored in the CodeObject.templater attribute, which is typically implemented as a subdirectory of templates. The compilation and running of code is done by a CodeObject. See the sections below for each of these.

Code path

The following gives an outline of the key steps that happen for the code generation associated to a NeuronGroup StateUpdater. The items in grey are Brian core functions and methods and do not need to be implemented to create a new code generation target or device. The parts in yellow are used when creating a new device. The parts in green relate to generating code snippets from abstract code blocks. The parts in blue relate to creating new templates which these snippets are inserted into. The parts in red relate to creating new runtime behaviour (compiling and running generated code).

../_images/codegen_code_paths.png

In brief, what happens can be summarised as follows. Network.run will call BrianObject.before_run on each of the objects in the network. Objects such as StateUpdater, which is a subclass of CodeRunner use this spot to generate and compile their code. The process for doing this is to first create the abstract code block, done in the StateUpdater.update_abstract_code method. Then, a CodeObject is created with this code block. In doing so, Brian will call out to the currently active Device to get the CodeObject and CodeGenerator classes associated to the device, and this hierarchy of calls gives several hooks which can be changed to implement new targets.

Code generation

To implement a new language, or variant of an existing language, derive a class from CodeGenerator. Good examples to look at are the NumpyCodeGenerator, CPPCodeGenerator and CythonCodeGenerator classes in the brian2.codegen.generators package. Each CodeGenerator has a class_name attribute which is a string used by the user to refer to this code generator (for example, when defining function implementations).

The derived CodeGenerator class should implement the methods marked as NotImplemented in the base CodeGenerator class. CodeGenerator also has several handy utility methods to make it easier to write these, see the existing examples to get an idea of how these work.

Syntax translation

One aspect of writing a new language is that sometimes you need to translate from Python syntax into the syntax of another language. You are free to do this however you like, but we recommend using a NodeRenderer class which allows you to iterate over the abstract syntax tree of an expression. See examples in brian2.parsing.rendering.

Templates

In addition to snippet generation, you need to create templates for the new language. See the templates directories in brian2.codegen.runtime.* for examples of these. They are written in the Jinja2 templating system. The location of these templates is set as the CodeObject.templater attribute. Examples such as CPPCodeObject show how this is done.

Code objects

To allow the final code block to be compiled and run, derive a class from CodeObject. This class should implement the placeholder methods defined in the base class. The class should also have attributes templater (which should be a Templater object pointing to the directory where the templates are stored) generator_class (which should be the CodeGenerator class), and class_name (which should be a string the user can use to refer to this code generation target.

Default functions

You will typically want to implement the default functions such as the trigonometric, exponential and rand functions. We usually put these implementations either in the same module as the CodeGenerator class or the CodeObject class depending on whether they are language-specific or runtime target specific. See those modules for examples of implementing these functions.

Code guide

  • brian2.codegen: everything related to code generation
  • brian2.codegen.generators: snippet generation, including the CodeGenerator classes and default function implementations.
  • brian2.codegen.runtime: templates, compilation and running of code, including CodeObject and default function implementations.
  • brian2.core.functions, brian2.core.variables: these define the values that variable names can have.
  • brian2.parsing: tools for parsing expressions, etc.
  • brian2.parsing.rendering: AST tools for rendering expressions in Python into different languages.
  • brian2.utils: various tools for string manipulation, file management, etc.

Additional information

For some additional (older, but still accurate) notes on code generation: