Backends

Faust backends

The faust compiler can produce various languages on output. Support for these languages is provided using backends that may or may not be embedded into the compiler or into the faust libraries. This is intended to simplify the compilation process: some backends (like LLVM) proved to be a bit complex to compile, some others are not supported by all compilers (like the interpreter backend). In addition, selecting only the set of backends to be used, can reduce significantly the size of the resulting binary.

Selecting your backends

The backends selection is described using backends files which are actually cmake files that simply populate the cmake cache. These files are located in the backends folder. They consist in a matrix where each line corresponds to a language support and where the columns select (or discard) the corresponding backend for each binary output i.e.:

• the faust compiler,
• the libfaust static library,
• the libfaust dynamic library,
• the libfaust wasm library

For example, the following selects the cpp backend for the compiler and the faust static and dynamic libraries and discards the interpreter backend.

set ( CPP_BACKEND    COMPILER STATIC DYNAMIC  CACHE STRING  "Include CPP backend"       FORCE )
set ( INTERP_BACKEND OFF                      CACHE STRING  "Include INTERPRETER backend" FORCE )

A BACKENDS option is provided to select a backend file using make e.g.:

$ make BACKENDS=backends.cmake

By default the selected backends are taken from backends.cmake Note that make always looks for the backend files in the backends folder.

You can get similar results using direct cmake invocation:

$ cd faustdir
$ cmake -C ../backends/backends.cmake ..

The -C file option instructs cmake to populate the cache using the file given as argument.

Note that once the backends have been selected, they won't change unless you specify another backend file.

Review compiled backends

On output of the project generation, cmake prints a list of all the backends that will be compiled for each component. Below you have an example of this output:

-- In target faust: include C backend
-- In target faust: include CPP backend
-- In target faust: include OCPP backend
-- In target faust: include WASM backend
-- In target staticlib: include C backend
-- In target staticlib: include CPP backend
-- In target staticlib: include OCPP backend
-- In target staticlib: include WASM backend
-- In target staticlib: include LLVM backend
-- In target wasmlib: include WASM backend

Note also that the command faust -v prints the list of embedded backends only e.g.:

FAUST Version 2.57.7
Embedded backends: 
   DSP to C
   DSP to C++
   DSP to Cmajor
   DSP to CSharp
   DSP to DLang
   DSP to Java
   DSP to JAX
   DSP to Julia
   DSP to old C++
   DSP to Rust
   DSP to WebAssembly (wast/wasm)
Copyright (C) 2002-2023, GRAME - Centre National de Creation Musicale. All rights reserved. 

Example of specific targets

• make cmake BACKENDS=interp.cmake TARGETS=interp.cmake allows to build the libfaust library only with the Interp backend only