Speed Up Fast Fourier Transforms in Generated Standalone Code by Using FFTW Library Calls - MATLAB & Simulink (original) (raw)

This example shows how to produce calls to a specific installed FFTW library when you generate standalone code (static library, dynamically linked library, or executable program). For more information about FFTW, see www.fftw.org.

When you generate a MEX function from MATLAB® code that includes fast Fourier transform (FFT) functions, the code generator uses the library that MATLAB uses for FFT algorithms. If you generate standalone C/C++ code, by default, the code generator produces code for the FFT algorithms instead of producing FFT library calls. To increase the speed of fast Fourier transforms in generated standalone code, specify that the code generator produce calls to a specific installed FFTW library.

The code generator produces FFTW library calls when all of these conditions are true:

FFTW Planning Considerations

The planning method, coder.fftw.StandaloneFFTW3Interface.getPlanMethod, must be set according to your workflow to optimize the performance of the FFTW library. Set the method to one of the following values as required by your workflow:

For more information about other planning method types, see https://www.fftw.org/fftw3_doc/Planner-Flags.html.

Consider using a previously called FFT for inputs of the same size and type. Reusing the same FFT operation on inputs of identical size and type prompts the planner to use a cached version of the FFT which has previously been called. The execution time of the FFT call also depends on the size of inputs. FFT algorithms whose inputs size is a power of two executes faster than an FFT call whose inputs sizes are non power of two. See https://www.fftw.org/fftw2_doc/fftw_3.html.

Install FFTW Library

If you do not have access to an installed FFTW library, version 3.2 or later, then you must install one. For a Linux® platform or a Mac platform, consider using a package manager to install the FFTW library. For a Windows® platform, in addition to .dll files, you must have.lib import libraries, as described in the Windows installation notes on the FFTW website.

See the installation instructions for your platform on the FFTW website.

Write an FFT Callback Class

To specify your installation of the FFTW library, write an FFT callback class. Share the callback class with others who want to use this FFTW library for FFTW calls in standalone code.

The callback class must derive from the abstract classcoder.fftw.StandaloneFFTW3Interface. Use this example callback class as a template.

% copyright 2017 The MathWorks, Inc.

classdef useMyFFTW < coder.fftw.StandaloneFFTW3Interface

methods (Static)
    function th = getNumThreads
        coder.inline('always');
        th = int32(coder.const(1));
    end
            
    function updateBuildInfo(buildInfo, ctx)
        fftwLocation = '/usr/lib/fftw';
        includePath = fullfile(fftwLocation, 'include');
        buildInfo.addIncludePaths(includePath);
        libPath = fullfile(fftwLocation, 'lib');
        
        %Double
        libName1 = 'libfftw3-3';
        [~, libExt] = ctx.getStdLibInfo();
        libName1 = [libName1 libExt];
        addLinkObjects(buildInfo, libName1, libPath, 1000, true, true);
        
        %Single
         libName2 = 'libfftw3f-3';
        [~, libExt] = ctx.getStdLibInfo();
        libName2 = [libName2 libExt];
        addLinkObjects(buildInfo, libName2, libPath, 1000, true, true);
    end
end

end

Implement the updateBuildInfo andgetNumThreads methods. In theupdateBuildInfo method, set fftwLocation to the full path for your installation of the library. SetincludePath to the full path of the folder that containsfftw3.h. Set libPath to the full path of the folder that contains the library files. If your FFTW installation uses multiple threads, modify the getNumThreads method to return the number of threads that you want to use.

Optionally, you can implement these methods:

Generate FFTW Library Calls by Specifying an FFT Library Callback Class

To generate FFTW library calls in standalone C code:

  1. Write a MATLAB function that calls a MATLAB fast Fourier transform function. For example, write a function myfft that calls the MATLAB function fft.
    function y = myfft()
    %#codegen
    t = 0:1/50:10-1/50;
    x = sin(2pi15t) + sin(2pi20t);
    y = fft(x);
    end
  2. Define a code generation configuration object for a static library, dynamically linked library, or executable program. For example, define a configuration object for a dynamically linked library.
    cfg = coder.config('dll');
  3. Specify the FFTW callback classuseMyFFTW.
    cfg.CustomFFTCallback = 'useMyFFTW';
    The callback class must be on the MATLAB path.
  4. Generate code.
    codegen myfft -config cfg -report

Locate FFTW Library in Execution Environment

The FFTW library must be available in your execution environment. If the FFTW library is shared, use environment variables or linker options to specify the location of the library.

To specify the rpath linker option, you can use the build information addLinkFlags method in theupdateBuildInfo method of yourcoder.fftw.StandaloneFFTW3Interface class. For example, for a GCC compiler:

buildInfo.addLinkFlags(sprintf('-Wl,-rpath,"%s"',libPath));

Clean Up Memory Used By FFTW Library Functions

To clean up memory that the FFTW library functions use while the calling process is still running, you must incorporate the generated code into your own project and manually call the cleanup function after executing the generated code. By contrast, when the calling process terminates, FFTW library functions automatically free the memory and you do not need to call these cleanup functions.

The available cleanup functions are:

Follow these rules to decide which cleanup function to use:

See Also

coder.fftw.StandaloneFFTW3Interface

Topics

External Websites