tips

AD系列fft变换

Speed:
    * 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.

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 
        these 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.