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Saturate MDCT output post-TDAC rather than pre-

Gives us a tighter bound on the pitch postfilter input to avoid overflows

--- a/celt/celt_decoder.c

+++ b/celt/celt_decoder.c

@@ -345,6 +345,12 @@

             clt_mdct_backward(&mode->mdct, &freq[b], out_syn[c]+NB*b, mode->window, overlap, shift, B, arch);

       } while (++c<CC);

    }

+   /* Saturate IMDCT output so that we can't overflow in the pitch postfilter

+      or in the */

+   c=0; do {

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

+         out_syn[c][i] = SATURATE(out_syn[c][i], SIG_SAT);

+   } while (++c<CC);

    RESTORE_STACK;

 }

--- a/celt/mdct.c

+++ b/celt/mdct.c

@@ -306,8 +306,8 @@

          /* We swap real and imag because we're using an FFT instead of an IFFT. */

          re = yp1[1];

          im = yp1[0];

-         yp0[0] = SATURATE(yr, SIG_SAT);

-         yp1[1] = SATURATE(yi, SIG_SAT);

+         yp0[0] = yr;

+         yp1[1] = yi;

          t0 = t[(N4-i-1)];

          t1 = t[(N2-i-1)];

@@ -314,8 +314,8 @@

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

          yr = ADD32_ovflw(S_MUL(re,t0), S_MUL(im,t1));

          yi = SUB32_ovflw(S_MUL(re,t1), S_MUL(im,t0));

-         yp1[0] = SATURATE(yr, SIG_SAT);

-         yp0[1] = SATURATE(yi, SIG_SAT);

+         yp1[0] = yr;

+         yp0[1] = yi;

          yp0 += 2;

          yp1 -= 2;

       }