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Merge inverse mdct post-rotate and de-shuffle loops

Saves some memory access and gives a tiny speedup

Signed-off-by: Timothy B. Terriberry <[email protected]>

--- a/celt/mdct.c

+++ b/celt/mdct.c

@@ -253,35 +253,26 @@

    /* Inverse N/4 complex FFT. This one should *not* downscale even in fixed-point */

    opus_ifft(l->kfft[shift], (kiss_fft_cpx *)f2, (kiss_fft_cpx *)f);

-   /* Post-rotate */

+   /* Post-rotate and de-shuffle */

    {

       kiss_fft_scalar * OPUS_RESTRICT fp = f;

+      kiss_fft_scalar * OPUS_RESTRICT yp0 = f2;

+      kiss_fft_scalar * OPUS_RESTRICT yp1 = f2+N2-1;

       const kiss_twiddle_scalar *t = &l->trig[0];

       for(i=0;i<N4;i++)

       {

          kiss_fft_scalar re, im, yr, yi;

-         re = fp[0];

-         im = fp[1];

+         re = *fp++;

+         im = *fp++;

          /* We'd scale up by 2 here, but instead it's done when mixing the windows */

          yr = S_MUL(re,t[i<<shift]) - S_MUL(im,t[(N4-i)<<shift]);

          yi = S_MUL(im,t[i<<shift]) + S_MUL(re,t[(N4-i)<<shift]);

          /* works because the cos is nearly one */

-         *fp++ = yr - S_MUL(yi,sine);

-         *fp++ = yi + S_MUL(yr,sine);

-      }

-   }

-   /* De-shuffle the components for the middle of the window only */

-   {

-      const kiss_fft_scalar * OPUS_RESTRICT fp1 = f;

-      const kiss_fft_scalar * OPUS_RESTRICT fp2 = f+N2-1;

-      kiss_fft_scalar * OPUS_RESTRICT yp = f2;

-      for(i = 0; i < N4; i++)

-      {

-         *yp++ =-*fp1;

-         *yp++ = *fp2;

-         fp1 += 2;

-         fp2 -= 2;

+         *yp0 = -(yr - S_MUL(yi,sine));

+         *yp1 = yi + S_MUL(yr,sine);

+         yp0 += 2;

+         yp1 -= 2;

       }

    }

    out -= (N2-overlap)>>1;