annotate constant-q-cpp/src/ext/kissfft/TIPS @ 372:af71cbdab621 tip

Update bqvec code

Speed:
    * If you want to use multiple cores, then compile with -openmp or -fopenmp (see your compiler docs).
	Realize that larger FFTs will reap more benefit than smaller FFTs. This generally uses more CPU time, but
	less wall time.

    * experiment with compiler flags
        Special thanks to Oscar Lesta. He suggested some compiler flags 
        for gcc that make a big difference. They shave 10-15% off
        execution time on some systems.  Try some combination of:
                -march=pentiumpro
                -ffast-math
                -fomit-frame-pointer

    * If the input data has no imaginary component, use the kiss_fftr code under tools/.
      Real ffts are roughly twice as fast as complex.

    * If you can rearrange your code to do 4 FFTs in parallel and you are on a recent Intel or AMD machine,
    then you might want to experiment with the USE_SIMD code.  See README.simd


Reducing code size:
    * remove some of the butterflies. There are currently butterflies optimized for radices
        2,3,4,5.  It is worth mentioning that you can still use FFT sizes that contain 
        other factors, they just won't be quite as fast.  You can decide for yourself 
        whether to keep radix 2 or 4.  If you do some work in this area, let me 
        know what you find.

    * For platforms where ROM/code space is more plentiful than RAM,
     consider creating a hardcoded kiss_fft_state. In other words, decide which 
     FFT size(s) you want and make a structure with the correct factors and twiddles.

    * Frank van der Hulst offered numerous suggestions for smaller code size and correct operation 
    on embedded targets.  "I'm happy to help anyone who is trying to implement KISSFFT on a micro"

    Some of these were rolled into the mainline code base:
        - using long casts to promote intermediate results of short*short multiplication
        - delaying allocation of buffers that are sometimes unused.
    In some cases, it may be desirable to limit capability in order to better suit the target:
        - predefining the twiddle tables for the desired fft size.