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Making the CELT fixed-point decoder a bit more robust to extreme signals

denormalise_bands() can now produce signals close to the max MDCT amplitude.

--- a/celt/bands.c

+++ b/celt/bands.c

@@ -214,7 +214,9 @@

          j=M*eBands[i];

          band_end = M*eBands[i+1];

          lg = ADD16(bandLogE[i+c*m->nbEBands], SHL16((opus_val16)eMeans[i],6));

-#ifdef FIXED_POINT

+#ifndef FIXED_POINT

+         g = celt_exp2(lg);

+#else

          /* Handle the integer part of the log energy */

          shift = 16-(lg>>DB_SHIFT);

          if (shift>31)

@@ -225,9 +227,23 @@

             /* Handle the fractional part. */

             g = celt_exp2_frac(lg&((1<<DB_SHIFT)-1));

          }

-#else

-         g = celt_exp2(lg);

+         /* Handle extreme gains with negative shift. */

+         if (shift<0)

+         {

+            /* For shift < -2 we'd be likely to overflow, so we're capping

+               the gain here. This shouldn't happen unless the bitstream is

+               already corrupted. */

+            if (shift < -2)

+            {

+               g = 32767;

+               shift = -2;

+            }

+            do {

+               *f++ = SHL32(MULT16_16(*x++, g), -shift);

+            } while (++j<band_end);

+         } else

 #endif

+         /* Be careful of the fixed-point "else" just above when changing this code */

          do {

             *f++ = SHR32(MULT16_16(*x++, g), shift);

          } while (++j<band_end);