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.

1 files changed, 19 insertions, 3 deletions

diff --git a/celt/bands.c b/celt/bands.c
index f22951b5..cce56e2f 100644
--- a/celt/bands.c
+++ b/celt/bands.c
@@ -214,7 +214,9 @@ void denormalise_bands(const CELTMode *m, const celt_norm * OPUS_RESTRICT X,
          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 @@ void denormalise_bands(const CELTMode *m, const celt_norm * OPUS_RESTRICT X,
             /* 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);