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DSP_fft16x16t
4-110
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* Set up offsets to access ”N/4”, ”N/2”, ”3N/4” complex point or */
/* ”N/2”, ”N”, ”3N/2” half word */
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
h2 = stride>>1;
l1 = stride;
l2 = stride + (stride >> 1);
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* Reset ”x” to point to the start of the input data array. */
/* ”tw_offset” starts off at 0, and increments by ”6 * stride” */
/* The stride quarters with every iteration of the outer loop */
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
x = ptr_x;
w = ptr_w + tw_offset;
tw_offset += fft_jmp;
stride >>= 2;
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* The following loop iterates through the different butterflies, */
/* within a given stage. Recall that there are logN to base 4 */
/* stages. Certain butterflies share the twiddle factors. These */
/* are grouped together. On the very first stage there are no */
/* butterflies that share the twiddle factor, all N/4 butter− */
/* flies have different factors. On the next stage two sets of */
/* N/8 butterflies share the same twiddle factor. Hence, after */
/* half the butterflies are performed, j the index into the */
/* factor array resets to 0, and the twiddle factors are reused. */
/* When this happens, the data pointer ’x’ is incremented by the */
/* fft_jmp amount. In addition, the following code is unrolled to */
/* perform ”2” radix4 butterflies in parallel. */
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
for (i = 0; i < npoints; i += 8)
{
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* Read the first 12 twiddle factors, six of which are used */
/* for one radix 4 butterfly and six of which are used for */
/* next one. */