Texas Instruments TMS320C64X - Manual (Page 41)

DSP_fft16x16_imre
4-13 C64x+ DSPLIB Reference
To vectorize the FFT, it is desirable to access twiddle factor array using double
word wide loads and fetch the twiddle factors needed. To do this, a modified
twiddle factor array is created, in which the factors WN/4, WN/2, W3N/4 are
arranged to be contiguous. This eliminates the separation between twiddle
factors within a butterfly. However, this implies that we maintain a redundant
version of the twiddle factor array as the loop is traversed from one stage to
another. Hence, the size of the twiddle factor array increases as compared to
the normal Cooley Tukey FFT. The modified twiddle factor array is of size
“2 * N”, where the conventional Cooley Tukey FFT is of size “3N/4”, where N
is the number of complex points to be transformed. The routine that generates
the modified twiddle factor array was presented earlier. With the above
transformation of the FFT, both the input data and the twiddle factor array can
be accessed using double-word wide loads to enable packed data processing.
The final stage is optimized to remove the multiplication as w0 = 1. This stage
also performs digit reversal on the data, so the final output is in natural order.
In addition, if the number of points to be transformed is a power of 2, the final
stage applies a DSP_radix2 pass instead of a radix 4. In any case, the outputs
are returned in normal order.
The code performs the bulk of the computation in place. However, because
digit-reversal cannot be performed in-place, the final result is written to a
separate array, y[].
Benchmarks Cycles (6 * nx/8 + 19) * ceil[log
4
(nx) − 1] + 8*nx/8 + 30
Codesize 864 bytes